Establishment and application of an indirect ELISA for Getah virus E2 antibody detection

Getah virus (GETV) is a mosquito-transmitted disease that affects animals, causing fever, aseptic meningitis, and abortion. Its prevalence in China poses risks to both animal health and public well-being. Currently, there is a scarcity of seroepidemiological data on GETV due to the absence of commercial antibody detection kits for pigs. The aim of this study is to develop a rapid, accurate, and sensitive ELISA, providing a reliable tool for GETV seroepidemiology and laying the foundation for future commercial assay development. In this study, we removed specific hydrophobic domains and intracellular structures from E2 proteins and constructed the recombinant plasmid pCold-TF-E2. The recombinant protein was expressed using a prokaryotic expression system, and efficient purification of the rE2 protein was achieved using a nickel affinity column. The purified rE2 protein is suitable for the development of an indirect ELISA (rE2 ELISA). Following the optimization of reaction conditions for the rE2-ELISA, the cut-off value was 0.356. Additionally, the rE2-ELISA method showed a positive rate of 37.1% for IgG antibodies against GETV when testing 986 pig clinical serum samples collected from pigs in Sichuan between May 2022 and September 2022. The rE2-ELISA method displayed a 95.1% overall agreement with VNT, boasting a sensitivity of 98.2% and a specificity of 92.6%. These results indicate that IgG ELISA based on rE2 protein is an efficient and economical method for the detection of GETV antibodies in pigs, facilitating the diagnosis and prevention of GETV.

Recombinantly expressed virus-like particles (VLPs) of canine circovirus for development of an indirect ELISA

Canine circovirus (CanineCV) is an emerging pathogen in domestic dogs, detected in multiple countries in association with varying clinical and pathological presentations including diarrhoea, vasculitis, granulomatous inflammation, and respiratory signs. Understanding the pathology of CanineCV is confounded by the fact that it has been detected in asymptomatic dogs as well as in diseased dogs concurrently infected with known pathogens. Recombinantly expressed self-assembling Virus-like particles (VLPs) lack viral genomic material but imitate the capsid surface conformations of wild type virion, allowing arrays of biological applications including subunit vaccine development and immunodiagnostics. In this study, full length CanineCV capsid gene was expressed in Escherichia coli followed by two-step purification process to yield soluble capsid protein in high concentration. Transmission electron microscopy (TEM) confirmed the capsid antigen self-assembled into 17-20 nm VLPs in glutathione S-transferase (GST) buffer, later utilised to develop an indirect enzyme-linked immunosorbent assay (iELISA). The respective sensitivity and specificity of the proposed iELISA were 94.10% and 88.40% compared with those obtained from Western blot. The mean OD450 value for western blot positive samples was 1.22 (range 0.12-3.39) and negative samples was 0.21 (range 0.07-0.41). An optimal OD450 cut-off of 0.35 was determined by ROC curve analysis. Median inter-assay and intra-assay validation revealed that the iELISA test results were reproducible with coefficients of variation 7.70 (range 5.6-11.9) and 4.21 (range 1.2-7.4). Our results demonstrated that VLP-based iELISA is a highly sensitive method for serological diagnosis of CanineCV infections in dogs, suitable for large-scale epidemiological studies.

Development and application of an indirect ELISA for detection of antibodies against emerging atypical porcine pestivirus

BACKGROUND

Atypical porcine pestivirus (APPV) is a newly discovered swine pestivirus, which can cause congenital tremor and high mortality in newborn piglets and subclinical infection in adult pigs, leading to significant impacts on the pig industry. Currently, there is no approved serological method to assess APPV infection status in pig farms.

METHODS

In this study, the envelope glycoprotein E2 of APPV was highly expressed in suspension HEK293 cells, and further an indirect enzyme-linked immunosorbent assay based on the recombinant E2 protein (E2-iELISA) was developed and evaluated.

RESULTS

The reaction parameters of the E2-iELISA were optimized, and the cutoff value was determined to be 0.2 by analyzing S/P values of 165 negative sera against APPV that were confirmed by virus neutralization test (VNT). Specificity test showed that the method had no cross-reaction with other common swine viruses. The E2-iELISA was evaluated using a panel of swine sera, and showed high sensitivity (113/120, 94.2%) and specificity (65/70, 92.9%), and the agreement rate with VNT was 93.7% (178/190). Subsequently, the E2-iELISA was utilized to investigate the seroprevalence of APPV in pig herds of China. When detecting 1368 pig serum samples collected from nine provinces in China, the overall seroprevalence of APPV was 73.9% (1011/1368).

CONCLUSION

Our findings suggest that the E2-iELISA is specific and sensitive, and could be a valuable tool for serological surveillance of APPV infection in pigs.

Subclinical circulation of Chicken infectious anaemia virus- A Sero-molecular study

Chicken infectious anaemia-an important immunosuppressive viral disease of chicken-gained much attention in the recent past. Based on huge mortality and production loss observed in the fast-growing poultry sector, the present study aimed to find out the current status of the chicken infectious anaemia virus (CIAV), among chicken flocks in the Punjab state of India by sero-molecular study. The sera from the blood samples were tested for anti-CIAV antibodies by indirect ELISA and also compared with haematological parameters. DNA from sero-positive samples underwent PCR amplification, sequencing and phylogenetic analysis of the most conserved genomic region (VP3 gene) to detect viraemia in asymptomatic birds. The serological study using indirect ELISA showed a high sero-positivity of 77.27% in chicken flocks. Additionally, the present study also revealed the high molecular evidence (72.54%) of CIAV in apparently healthy birds. Genetic analysis showed that all CIAVs have conserved VP3 genes without any nucleotide substitutions, indicating presence of CIAV and its subclinical circulation among apparently healthy flocks. The wide distribution of CIAV among birds may be the reason for huge mortality and production loss. Further, it is suggested that studies be conducted to find out the co-involvement of CIAV with other immunosuppressive microbial agents and the immunosuppressive effect of CIAV in apparently healthy birds. Also, its role in vaccine failure and outbreaks of various other avian diseases needs to be explored.

A colorimetric immunoassay for the detection of human vascular endothelial growth factor 165 (VEGF165) based on anti-VEGF-iron oxide nanoparticle conjugation

Vascular endothelial growth factor (VEGF) is an indispensable element in many physiological processes, while alterations in its level in the circulating system are signs of pathology-associated diseases. Therefore, its precise and selective detection is critical for clinical applications to monitor the progression of the pathology. In this study, an optical immunoassay biosensor was developed as a model study for detecting recombinant VEGF165. The VEGF165 sample was purified from recombinant Kluyveromyces lactis GG799 yeast cells. Indirect ELISA was used during the detection, wherein iron oxide nanoparticles (FeNPs) were utilized to obtain optical signals. The FeNPs were synthesized in the presence of lactose p-amino benzoic acid (LpAB). VEGF165 antibody was conjugated to the LpAB-FeNPs through EDC/NHS chemistry to convert the iron oxide nanoparticles into VEGF165 specific probes. The specificity of the prepared system was tested in the presence of potential serum-based interferents (i.e., glucose, urea, insulin, C-reactive protein, and serum amyloid A), and validation studies were performed in a simulated serum sample. The proposed immunoassay showed a wide detection range (0.5 to 100 ng/mL) with a detection limit of 0.29 ng/mL. These results show that the developed assay could offer a sensitive, simple, specific, reliable, and high-throughput detection platform that can be used in the clinical diagnostics of VEGF.

An evaluation of techniques to diagnose Dioctophyme renale in dogs

Dioctophyme renale is a nematode with zoonotic potential that affects the kidneys of carnivorous, wild, and domestic mammals. In this study, we sought to evaluate the indirect ELISA method against routine methods used to diagnose dioctophimosis. Hence, 38 dogs parasitized by D. renale, as confirmed by surgery, were selected. The dogs were evaluated by abdominal ultrasound and urinalysis, and their sera were tested by indirect ELISA using D. renale adult secretion and excretion antigen (DES). Five dogs were followed up with serum collections on day 0 (day of surgery) and 30, 60, and 90 days after surgery to evaluate antibody kinetics. Abdominal ultrasound and indirect ELISA successfully diagnosed 37 dogs parasitized by D. renale, while urinalysis diagnosed 29 animals. The positive animals were parasitized with 1-7 parasites; 17 dogs were infected by male and female parasites, 15 only by female parasites, and six were parasitized only by male parasites. When assessing specificity and sensitivity, all techniques showed 100% specificity and 81.6%, 97.4%, and 97.4% sensitivity for urinalysis, ultrasound, and ELISA, respectively (p < 0.001). The five positive dogs that were followed up after surgery showed a progressive decrease in mean absorbances in indirect ELISA (0.644, 0.516, 0.511, and 0.440, respectively). This study demonstrated that the indirect ELISA using the DE antigen could diagnose dioctophimosis regardless of the number, sex, and location of the parasites, with the potential to be used in epidemiological research and implementing immunological and molecular studies, opening new lines of research on D. renale.

Identification of cocoonase and cocoonase like protein using polyclonal antibody of Antheraea mylitta cocoonase

BACKGROUND

Cocoonase is a proteolytic enzyme released by silk moths during pupal adult emergence. Without damaging the silk fibroin, this enzyme dissolves the shell of the tasar cocoon by exclusively targeting the protein sericin. Prior to this study, there was no available antibody against Antheraea mylitta cocoonase to identify or screen out similar variants or cocoonase like protein.

RESULTS

In the present study, naturally secreted A. mylitta cocoonase was purified and used to immunize New Zealand white rabbits. The developed polyclonal antibody of cocoonase was purified and its specific interaction with cocoonase was determined using Indirect ELISA. The confirmation of its specificity and immuno-reactivity was evaluated by western blot using native cocoonase of tasar silkworm A. mylitta. The efficacy and specificity of the polyclonal antibody were further verified and confirmed by western blot which was performed to detect ten different ecotypes of A. mylitta cocoonase.

CONCLUSION

The developed antibody successfully detected the cocoonase of different ecotypes. Thus, in future this antibody can serve as one of the molecular detection method for cocoonase and cocoonase-like proteins.

Establishment and characterization of a novel indirect ELISA method based on ASFV antigenic epitope-associated recombinant protein

African Swine Fever (ASF) is an acute and highly lethal disease in pigs caused by African Swine Fever Virus (ASFV). Viral proteins have been commonly used as antigenic targets for the development of ASF diagnostic methods. However, the prokaryotic expression of viral proteins has deficiencies such as instability, insolubility, and high cost in eukaryotic situations. This study screened and verified ASFV-encoded p72, p54, and p30 protein antigenic epitopes. Subsequently, a novel antigenic epitope-associated recombinant protein was designed based on an ideal structural protein and expressed in Escherichia coli (E. coli). Western blot analysis indicated that the recombinant protein could specifically react with the monoclonal antibody (mAb) of p72 and polyclonal antibodies of p54 and p30, respectively. Next, an ASF indirect ELISA (iELISA) method was established based on the recombinant protein, which has no specific reaction with sera of other important pig viral diseases. Meanwhile, it shows a sensitivity to detecting dilutions of ASF-positive reference serum up to 1:6400. The clinical sample detection results showed a high coincidence rate of 98 % with a commercial competition ELISA kit. In conclusion, we established a novel specific, and sensitive ASF serologic detection method that opens new avenues for ASF serodiagnostic method development.

Prokaryotic expression of the V protein of the peste des petits ruminants virus and development of an indirect ELISA

In this study, we recombinantly expressed the V protein of the peste des petits ruminants virus (PPRV) and evaluated its diagnostic value for PPRV infection using an indirect ELISA (i-ELISA). The optimal concentration of the coated antigen of V protein was 15 ng/well at a serum dilution of 1:400, and the optimal positive threshold value was 0.233. A cross-reactivity assay showed that the V protein-based i-ELISA was specific to PPRV with consistent reproducibility and showed a specificity of 82.6% and a sensitivity of 100% with a virus neutralization test. Using the recombinant V protein as an antigen in ELISA is useful for seroepidemiological studies of PPRV infections.

Identification of specific antigens between Toxoplasma gondii and Neospora caninum and application of potential diagnostic antigen TgGRA54

Toxoplasma gondii is a zoonotic parasite that is very common in livestock. Meat products from livestock infected with T. gondii are one of the important transmission routes of toxoplasmosis. Rapid and reliable diagnosis is a prerequisite for the prevention and control of toxoplasmosis. Neospora caninum and T. gondii are similar in morphology and life history, and there are a large number of cross antigens between them, making clinical diagnosis of toxoplasmosis more difficult. In this study, immunoprecipitation-mass spectrometry (IP-MS) was used to screen for T. gondii-specific antigens, and the specific antigen was cloned and expressed in Escherichia coli. The specific antigen was then used to establish an indirect ELISA diagnostic method. A total of 241 specific antigens of T. gondii and 662 cross antigens between T. gondii and N. caninum were screened by IP-MS. Through bioinformatics analysis and homology comparison, seven proteins were selected for gene cloning and prokaryotic expression, and the most suitable antigen, TgGRA54, was selected to establish an indirect ELISA for T. gondii. Compared with the indirect immunofluorescent antibody test (IFAT), the positive coincidence rate of the ELISA based on rTgGRA54 was 100% (72/72) and the negative coincidence rate was 80.95% (17/21). The indirect ELISA method based on TgGRA54 recombinant protein was established to detect T. gondii antibodies in bovine sera, and the recombinant protein reacted well with T. gondii positive sera from sheep, mouse, and swine, indicating that the recombinant protein is a good diagnostic antigen for T. gondii.

Development and diagnostic evaluation of indirect enzyme linked immunosorbent assay for Epidermophyton floccosum infection in humans

BACKGROUND

Epidermophyton floccosum is an anthrophophilic dermatophyte widely distributed in the tropics and subtropics. It can invade keratinized tissues of humans and cause superficial mycoses called dermatophytosis (ringworm).

OBJECTIVE

The main objective of this study was to develop an in-house indirect enzyme-linked immunosorbent assay (ELISA) and to evaluate its performance for the immunological diagnosis of E. floccosum infection in humans.

METHODS

An in-house indirect ELISA was developed using partially purified E. floccosum antigens, pre immunized rabbit serum as negative control, immunized rabbit polyclonal antibodies as positive control, enzyme labeled goat anti rabbit antibodies and goat anti human antibodies. A total of 50 serum samples from E. floccosum infected patients as confirmed by direct microscopy and culture and 30 samples from humans without history of dermatophyte infection that served as controls were used to evaluate the performance of an in-house indirect ELISA developed in this study. Analytical and diagnostic performance characteristics were determined to evaluate its diagnostic value.

RESULTS

The diagnostic sensitivity, specificity, positive and negative predictive values of E. floccosum indirect ELISA were 90.00 %, 83.33 %, 90.83 %, 83.83 % respectively. The performance of indirect ELISA assay was compared with gold standard diagnostic tests such as KOH hydrolysis test and fungal culture. The correlation coefficients of E. floccosum indirect ELISA with KOH hydrolysis and fungal culture method were 0.612** and 0.826** (P < 0.01) respectively indicating significant correlation between these tests.

CONCLUSION

This revealed the great potentiality of E. floccosum indirect ELISA in early, specific and precise detection of E. floccosum infection in humans.

Serodiagnosis of human brucellosis by an indirect ELISA test using recombinant outer membrane protein 19 kDa (rOMP19) as an antigen

BACKGROUND

Brucellosis remains one of the global health concerns that reemerges in recent years. Delayed or inaccurate diagnosis end to a long treatment duration and financial burden; therefore, finding a good antigen for detection of specific anti-Brucella antibodies is crucial. We intended to evaluate the serodiagnosis value of recombinant Brucella outer membrane protein 19 kDa (rOMP19) using indirect ELISA system compared with Rose Bengal test.

RESULTS

The OMP19 sequence was successfully cloned into pET-28a and produced in E. coli cells (DE3). After extraction and purification of rOMP19, this protein was used for designing indirect ELISA to detect anti-Brucella antibodies in 73 human sera, including 6 brucellosis-positive and 67 brucellosis-negative samples. The accuracy of rOMP19 ELISA was evaluated by receiver operating characteristic (ROC) curve and then compared with Rose Bengal plate test and a commercial anti-IgG Brucella ELISA kit. In comparison with Rose Bengal plate test, the area under the ROC curve was 0.985 (95% CI, 0.96-1.00). From coordinates of the curve, the optimal cut-off value was selected at 0.147, in which the diagnostic sensitivity was 100%, and the specificity was 94%. At this cut-off point, 10 samples were diagnosed as positive (6 true positives and 4 false positives), while negative samples were all correctly diagnosed. The results of our designed rOMP19 ELISA was the same as data obtained from commercial ELISA kit, which applied LPS as an antigen.

CONCLUSIONS

We concluded that OMP19 is an efficient antigen for the serodiagnosis of human brucellosis.

Development of a novel method for diagnosis of fasciolosis based on cathepsin L7 in ruminants

Fasciolosis is a widely distributed zoonosis reported over 81 countries around the world. Good and early diagnostic method is critical in controlling this disease and prevention of injury to the liver and bile ducts. In this study, we identified a novel member (cathepsin L7) of cathepsin family from Fasciola spp.. Firstly, the biological character of CL7 was analyzed according to the information of cathepsin L family, and then rCL7 was expressed and purified, a new iELISA based on CL7 was developed. The results exhibited CL7 iELISA had 100% sensitivity 100% specificity in sheep (cut-off 1.329) and 100% sensitivity 93.75% specificity in cattle (cut-off 0.756). Moreover, anti-Fasciola CL7 antibodies could be detected in early Fasciola gigantica infected buffaloes, as early as 3 week-post-infection (WPI). In conclusion, it is suggested that CL7 with low cost, early detection, good specificity and sensitivity could be used as a candidate antigen for detection of ruminant fasciolosis.

Prokaryotic expression of porcine deltacoronavirus S gene truncated segment and establishment of indirect ELISA detection method

Porcine deltacoronavirus (PDCoV) is an emerging discovered coronavirus that causes significant losses in the global swine industry. This study aimed to establish an indirect ELISA method for detecting PDCoV antibodies using the truncated gene of PDCoV spike protein (S). The purified S protein was used as the coating antigen for the polyclonal antibody. The conditions were optimized to establish an indirect ELISA detection method for PDCoV based on the S protein, which showed good specificity and no cross-reaction with SVV-VP1, ASFV-P72, GETV-E2, PRV-gE, etc. The method has high repeatability, with coefficients of variation within and between batches less than 10%. Compared with the commercial kit, the positive coincidence rate is 86.40%, the negative coincidence rate is 89.43%, and the total coincidence rate is 91.76%. This ELISA can be used for PDCoV serological investigation and antibody evaluation. It can also lay the foundation for further research and development of PDCoV S protein ELISA antibody detection kit.

Establishment of an indirect ELISA for Mycobacterium tuberculosis MTB39A protein antibody

The MTB39A protein is a member of the unique Mycobacterium tuberculosis (MTB) PE/PPE protein family and is the main candidate for tuberculosis (TB) diagnosis. The aim of this study was to establish a novel indirect ELISA (iELISA) method that uses antibodies against MTB. The MTB39A gene sequence was synthesized according to the MTB39A nucleotide sequence of the MTB H37Rv strain (GenBank accession number: NC_000962.3) and cloned into the pET28a( +) vector. After correct sequencing, it was transferred to Escherichia coli BL21 (DE3) receptor cells for expression and purification, and the purified recombinant protein was identified by SDS-PAGE and western blotting. The purified MTB39A protein was used as the capture antibody, and a rabbit polyclonal antibody against the MTB MTB39A protein was used as the detection antibody to establish an indirect ELISA method. The ELISA conditions were optimized, and the optimal coating concentration of the MTB39A antigen was determined to be 0.5 μg/mL. The optimal dilution of MTB39A rabbit polyclonal antibody was 1:4096, and the optimal dilution of HRP-goat anti-rabbit IgG was 1:4000. The results showed that this indirect ELISA method has high sensitivity, specificity and efficacy for MTB39A protein detection. Moreover, this indirect ELISA method has optimal stability and can be used for the initial detection of MTB antibodies in clinical human and bovine serum samples. The establishment of this assay provides a new method for the rapid diagnosis of MTB and technical support for the prevention and control of tuberculosis. KEY POINTS: • MTB MTB39A protein was expressed in a prokaryotic expression system. • Rabbit polyclonal antibody against MTB39A was prepared. • To establish an iELISA based on the MTB39A protein for the detection of MTB antibodies.

Evaluation of humoral immune responses against C. perfringens epsilon toxin in Iranian sheep and goats after vaccination

Clostridium perfringens is a common cause of death in domestic animals worldwide. However, vaccination on a regular basis is an economically beneficial means for controlling clostridial contamination.The objective of the current investigation was to evaluate the humoral immune responses using iELISA in Iranian sheep and goats following the vaccination programs administered by the bacterin-toxoid polyvalent entrotoxemia vaccine. A total of one-hundred-and-twenty animals, consisting of sixty sheep and sixty goats, were randomly divided into three groups. These animals were vaccinated with clostridial vaccine on days 0 and 14 using two different dosages. Blood samples were collected on day zero, 15, 30, 45, 60, 75, and 90 following vaccination. The sera samples were then separated and antibody titers were measured using an in-house enzyme-linked immunosorbent assay (ELISA) against C. perfringens epsilon toxin. The titers of antibodies in sheep were notably higher than those in goats, particularly after receiving the booster dose. No statistically significant variations were identified in the immune responses of Iranian sheep and goat breeds. (p>0.05). Overall, the duration of the humoral immune response in goats upon administration of the clostridial vaccine was relatively brief, requiring multiple booster injections.

Localization and identification of denatured antigenic sites of glycinin A3 subunit after using two processing technologies

Glycinin is an important allergen in soybeans. In this study, molecular cloning and recombinant phage construction were performed to explore the antigenic sites of the glycinin A3 subunit that were denatured during processing. Next, the A-1-a fragment was located as the denatured antigenic sites by indirect ELISA. The combined UHP heat treatment showed better denaturation of this subunit than the single heat treatment assay. In addition, identification of the synthetic peptide showed that the A-1-a fragment was an amino acid sequence containing a conformational and linear IgE site, in which the first synthetic peptide (P1) being both an antigenic and allergenic site. The results of alanine-scanning showed that the key amino acids affecting antigenicity and allergenicity of A3 subunit were S²⁸, K²⁹, E³², L³⁵ and N¹³. Our results could provide the basis for further development of more efficient methods to reduce the allergenicity of soybeans.

Production of virus-like particles of porcine circovirus 2 in baculovirus expression system and its application for antibody detection

BACKGROUND

Porcine circovirus 2 (PCV-2) is one of the pathogens that leads to a growing and persistent threat in pigs. Thus, the development of serological detection methods for PCV-2 is of great necessity for clinical diagnosis as well as epidemiological investigations. This study aimed to establish an indirect enzyme-linked immunosorbent assay (ELISA) to examine antibodies against PCV-2 based on virus-like particles (VLPs).

RESULTS

Recombinant PCV-2 Cap protein was expressed in the baculovirus-insect cells system and PCV-2 VLPs were observed over transmission electron microscopy (TEM). The PCV-2 VLPs were shown to have good immunogenicity in mice and stimulated a high level of PCV-2 antibody titers. Using PCV-2 VLPs as coating antigen, the indirect ELISA can detect PCV-2 antibodies in animals with diagnostic sensitivity and specificity of 98.33% and 93.33% compared to immunofluorescence assay (IFA), respectively. The intra- and inter-assay coefficient variations (CVs) were < 10% in a batch, and < 15% in different batches, indicating good repeatability. There was no cross-reaction of this ELISA with antibodies against other porcine viruses. A total of 170 serum samples collected from different pig farms in China were tested for PCV-2 antibodies, and 151 (88.8%) samples were PCV-2 antibody positive.

CONCLUSION

Our findings suggest that this ELISA was rapid, specific, and reproducible and can be used for large-scale serological investigations of PCV-2 antibodies in pigs.

Development of silver nano-based indirect ELISA and dot-ELISA methods for serological diagnosis of a bacterial fish pathogen Aeromonas veronii

Rapid and accurate detection of bacterial pathogens is critical in controlling disease outbreaks affecting farmed fish. The present study aimed to develop a novel serological diagnostic approach using nano‑silver based Enzyme-linked immunosorbent assay (ELISA) for speedy detection of Aeromonas veronii infections in Nile tilapia. A. veronii isolates used in ELISA assays were recovered from moribund Nile tilapia during a disease outbreak in a private fish farm in Egypt. A. veronii isolates were identified based on alignment analysis of the gyrB and 16S rRNA gene sequences. A. veronii antisera used in ELISA assays were prepared in tilapia, and the bacterial antigens were formalin-killed. The cut-off values were 0.46 and 0.48 in traditional and nano-based ELISA. There were no cross-reactions with bacterial isolates (Aeromonas hydrophila, Aeromonas caviae, Aeromonas sobria, Pseudomonas fluorescens, and Vibrio vulnificus). The lowest antigen concentration that produced positive results after checkerboard titration in indirect-ELISA (i-ELISA) and dot ELISA was 15 μg and 250 ng of prepared antigen, respectively. Nano-ELISA and nano-based dot-ELISA antigen concentration was 10 μg and 100 ng, respectively. Sera concentration was 1:100 in indirect-ELISA and dot-ELISA, while it was 1:50 in nano-based ELISA and nano dot-ELISA. The i-ELISA successfully detected anti-Aeromonas IgG antibodies with 83.33% sensitivity and 66.67% specificity, while in the dot-ELISA, the sensitivity and specificity were 83.33% and 100%, respectively. Nano dot-ELISA had 100% sensitivity, specificity, and accuracy. Nano dot-ELISA assays have higher specificity, sensitivity, and accuracy than traditional ELISAs in detecting A. veronii. Further studies are needed to develop a rapid test kit for on-site field diagnosis.

Differentiation of Salmonella vaccinated and infected animals by serological detection of antibody to T3SS effector SsaK in an indirect ELISA

The differentiation of animals that are vaccinated and those that are naturally infected with Salmonella is difficult by conventional serological tests. We have shown here an indirect Enzyme-linked immunosorbent assay for detection of Salmonella infection based on the presence of a Type III secretory effector SsaK in the sera.

Development of an indirect ELISA test for the detection of Tilapia lake virus (TiLV) in fish tissue and mucus samples

A serological test for screening of TiLV in Oreochromis niloticus would be useful for the epidemiological investigations. Using polyclonal antisera against TiLV (TiLV-Ab), an indirect enzyme-linked immune sorbent assay (iELISA) was developed for the detection of TiLV antigen in fish tissue and mucus. After a cutoff value was established and antigen and antibody concentrations were optimized, the iELISA's sensitivity and specificity were assessed. We found the ideal dilutions of TiLV-Ab as 1: 4000 and secondary antibody as 1:65,000. High analytical sensitivity and moderate specificity were displayed by the developed iELISA. The Positive and Negative Likelihood Ratio (LR+, LR-) were 1.75 and 0.29, respectively. The estimated Positive and Negative Predictive Values (PPV and NPV) of the test were 76.19% and 65.62%, respectively. The accuracy of the developed iELISA was estimated as 73.28%. An immunological survey was performed using the developed iELISA with samples from the field and 155/195 fishes tested positive, indicating a 79.48% TiLV antigen positives. Among the pooled organs and mucus tested, the highest positive rate of 92.3% (36/39) is observed in mucus compared to other tissues, and least positive rate is found in liver of 46% (18/39). The newly designed iELISA proved sensitive and may be helpful for extensive examinations of TiLV infections and monitoring disease status even from apparently healthy samples using a non-invasive technology by collecting mucus as sample for iELISA.

EgSeverin and Eg14-3-3zeta from Echinococcus granulosus are potential antigens for serological diagnosis of echinococcosis in dogs and sheep

Cystic echinococcosis (CE) is a zoonotic parasitic disease caused by the metacestode larva of Echinococcus granulosus. In this study, two-dimensional gel electrophoresis (2-DE) coupled with immunoblot analysis revealed that E. granulosus severin and 14-3-3zeta proteins (named EgSeverin and Eg14-3-3zeta, respectively) might be two potential biomarkers for serological diagnosis of echinococcosis. The recombinant EgSeverin (rEgSeverin, 45 kDa) and Eg14-3-3zeta (rEg14-3-3zeta, 35 kDa) were administered subcutaneously to BALB/c mice to obtain polyclonal antibodies for immunofluorescence analyses (IFAs). And IFAs showed that both proteins were located on the surface of protoscoleces (PSCs). Western blotting showed that both proteins could react with sera from E. granulosus-infected sheep, dog, and mice. Indirect ELISAs (rEgSeverin- and rEg14-3-3zeta-iELISA) were developed, respectively, with sensitivities and specificities ranging from 83.33% to 100% and a coefficient of variation (CV %) of less than 10%. The rEgSeverin-iELISA showed cross-reaction with both E. granulosus and E. multilocularis, while the rEg14-3-3zeta-iELISA showed no cross-reaction with other sera except for the E. granulosus-infected ones. The field sheep sera from Xinjiang and Qinghai were analyzed using rEgSeverin-iELISA, rEg14-3-3zeta-iELISA, and a commercial kit respectively, and no significant differences were found among the three methods (p > 0.05). However, the CE positive rates in sheep sera from Qinghai were significantly higher than those from Xinjiang (p < 0.01). Overall, the results suggest that EgSeverin and Eg14-3-3zeta could be promising diagnostic antigens for E. granulosus infection.

Seroprevalence, associated risk factors, and molecular detection of bovine brucellosis in rural areas of Egypt

The study was carried out on six villages in northern Egypt to evaluate the epidemiological situation of bovine brucellosis among 989 unvaccinated household cattle by Rose Bengal Plate Test (RBPT) and indirect ELISA (iELISA) and to investigate the existence of Brucella DNA using real-time PCR in 100 milk and 100 sera from seropositive cattle and 50 sera from seronegative cattle. The overall seroprevalence was 20.7% and 23.7% by RBPT and iELISA, respectively. Based on the iELISA results, the seroprevalence was significantly (P < 0.05) higher in the village II (34.7%) and cattle > 7 years (30.1%). More males than females were non-significant seropositive (P = 0.6). There was 95% agreement between RBPT and iELISA, although iELISA showed a higher positivity rate (23.7%, 95% CI: 0.21-0.26) than RBPT (20.7%, 95% CI: 0.18-0.24). DNA of Brucella was confirmed in 16 milk samples by IS711 qPCR from seropositive cattle, however, no Brucella DNA was detected in serum samples tested positive and negative. Brucella abortus was the only species detected based on the alkB gene. Prevalence is highly related to the sampling site and the age of the animals. In conclusion, although qPCR is more accurate and commonly used in the diagnosis of most infectious diseases but in this situation iELISA is preferred and recommended for continuous screening and animal movement restriction and vaccination protocols, especially in high-risk areas.

Development and application of an indirect ELISA for the serological detection of bovine viral diarrhea virus infection based on the protein E2 antigen

BACKGROUND

Bovine viral diarrhea virus (BVDV) causes continuous economic losses to the livestock industry. Monitoring antibodies with enzyme-linked immunosorbent assay (ELISA) is a valuable tool to ensure the purification of BVDV in cattle. However, currently available ELISA kits based on the whole BVDV virion are both costly and time-consuming. The E2 protein has good immunogenicity, induces the secretion of neutralizing antibodies and is an essential immunogen for serological detection.

METHODS AND RESULTS

We developed a novel recombinant E2 protein-based indirect ELISA (rE2-iELISA) and conducted a serological survey for BVDV antibodies in 2021-2022 in Beijing, China. The results showed that E2 protein was successfully expressed with high immunogenicity and the optimal rE2-iELISA displayed high sensitivity, reproducibility and specificity. Clinical testing of 566 serum specimens indicated that 318 BVDV positive samples and 194 BVDV negative samples were tested by rE2-iELISA and the IDEXX BVDV ELISA-Ab kit, with a positive coincidence rate of 93.3%, a negative coincidence rate of 86.3%, and an overall coincidence rate of 90.5%.

CONCLUSION

This study established an rE2-iELISA method, which is a highly sensitive, specific and robust ELISA-test validated to detect anti-BVDV antibodies. These findings indicate that the newly developed rE2-iELISA method has the potential to be used as a rapid, reliable and cost-effective screening tool for BVDV infection and provides technical support for the evaluation of vaccine efficacy in cattle herds in the future.

Enzyme-Linked Immunosorbent Assay: Types and Applications

Enzyme-linked immunosorbent assay (ELISA) is an immunological assay widely used in basic science research, clinical application studies, and diagnostics. The ELISA technique relies on the interaction between the antigen (i.e., the target protein) versus the primary antibody against the antigen of interest. The presence of the antigen is confirmed through the enzyme-linked antibody catalysis of the added substrate, the products of which are either qualitatively detected by visual inspection or quantitatively using readouts from either a luminometer or a spectrophotometer. ELISA techniques are broadly classified into direct, indirect, sandwich, and competitive ELISA-all of which vary based on the antigens, antibodies, substrates, and experimental conditions. Direct ELISA relies on the binding of the enzyme-conjugated primary antibodies to the antigen-coated plates. Indirect ELISA introduces enzyme-linked secondary antibodies specific to the primary antibodies bound to the antigen-coated plates. Competitive ELISA involves a competition between the sample antigen and the plate-coated antigen for the primary antibody, followed by the binding of enzyme-linked secondary antibodies. Sandwich ELISA technique includes a sample antigen introduced to the antibody-precoated plate, followed by sequential binding of detection and enzyme-linked secondary antibodies to the recognition sites on the antigen. This review describes ELISA methodology, the types of ELISA, their advantages and disadvantages, and a listing of some multifaceted applications both in clinical and research settings, including screening for drug use, pregnancy testing, diagnosing disease, detecting biomarkers, blood typing, and detecting SARS-CoV-2 that causes coronavirus disease 2019 (COVID-19).

Determination of Proinflammatory and Antiinflammatory Cytokines by ELISA Technique

The main objective of this chapter is to determine inflammatory and anti-inflammatory cytokines in patients with or without preeclampsia by means of the enzyme-linked immunosorbent assay (ELISA) technique. In this chapter, 16 cell cultures were obtained from different patients admitted to the hospital with term vaginal delivery or cesarean section. Here, we describe the ability to quantify the presence of cytokines in cell culture supernatants. The supernatants of the cell cultures were collected and concentrated. The concentration of IL-6 and VEGF-R1 was measured by ELISA to determine the prevalence of alterations in the samples studied. We observed that the sensitivity of the kit allowed the detection of several cytokines in a range between 2 and 200 pg/mL. The test was performed using the ELISpot method (5×), which allowed a higher level of precision.

Serological testing of an equal-volume milk sample - a new method to estimate the seroprevalence of small ruminant lentivirus infection?

BACKGROUND

In cattle attempts to evaluate within-herd prevalence of various infectious and parasitic diseases by bulk-tank milk (BTM) testing with ELISA have been made with moderate success. The fact that BTM is composed of variable and unknown volumes of milk from individual lactating animals weakens the relationship between numerical result of the ELISA and the within-herd prevalence. We carried out a laboratory experimental study to evaluate if a pooled milk sample created by mixing an equal volume of individual milk samples from seropositive and seronegative goats, henceforth referred to as an equal-volume milk sample (EVMS), would allow for accurate estimation of within-herd seroprevalence of caprine arthritis-encephalitis (CAE) using 3 different commercial ELISAs. By mixing randomly selected milk samples from seronegative and seropositive goats, 193 EVMS were created - 93 made of seronegative samples and 100 with the proportion of seropositive individual milk samples (EVMS_%POS) ranging from 1 to 100%. EVMS_%POS could be considered as a proxy for the within-herd seroprevalence. Then, OD of EVMS (OD_EVMS) of the 193 EVMS was measured using 3 commercial ELISAs for CAE - 2 indirect and 1 competitive.

RESULTS

The cut-off values of OD_EVMS indicating SRLV infection were determined. The regression functions were developed to link OD_EVMS with EVMS_%POS. A significant monotonic relationship between OD_EVMS measured with 2 commercial indirect ELISAs and EVMS_%POS was identified. Two regression models developed on this basis described approximately 90% of variability and allowed to estimate EVMS_%POS, when it was below 50%. High OD_EVMS indicated EVMS_%POS of > 50%.

CONCLUSION

Our study introduces the concept of serological testing of EVMS as a method of detecting SRLV-infected herds and estimating the proportion of strongly seropositive goats. Further field studies are warranted to assess practical benefits of EVMS serological testing.

Development of in-house ELISA for detection of antibodies against lumpy skin disease virus in cattle and assessment of its performance using a bayesian approach

Lumpy skin disease (LSD) is a contagious disease among cattle and buffalo worldwide. Currently, an enzyme-linked immunosorbent assay (ELISA) has been recognized as an efficient diagnostic tool that is less time-consuming and easier than the viral neutralization test to measure the antibody levels. In the present study, an in-house method of indirect ELISA was developed to detect the bovine antibodies against Lumpy skin disease virus (LSDV) and its performance was assessed using field samples. This in-house method has been compared with the commercial ELISA test kit for detection of bovine antibodies against LSDV. The sensitivity (Se) and the specificity (Sp) of the test were estimated using a Bayesian latent class model. Checkerboard titration was performed using the naturally LSDV-infected bovine sera and colostrum-deprived calf sera. The LSDV antigen concentrations (1 TCID₅₀/mL), the sample serum (1:500), and goat anti-bovine immunoglobulin G (IgG) labeled with horseradish peroxidase (HRP) (1:10,000) were determined to be optimal for this assay. The calculated cut-off value was 0.067, and there were no differences in the results of tests that utilized positive and negative sera (p < 0.05). The characteristics of two diagnostic tests were evaluated using a conditional dependent and one-population Bayesian model. The Se value of an in-house indirect ELISA were almost similar to ELISA test kit. On the other hand, the Sp value of the in-house ELISA test was lower than that of the commercial ELISA test with the median values of 89% (95% PPI = 75.9-99.3%) and 91.4% (95% PPI = 85.3-95.5%), respectively. A posterior estimate for the prevalence was 66.9% (95% PPI = 60.8-83.3%) and higher than initially expected.

Detection of Serum Antibody Responses in Nipah Virus-Infected Pigs

Nipah virus (NiV) is an emerging, zoonotic paramyxovirus that is among the most pathogenic of viruses in humans. During the first reported outbreak of NiV in Malaysia and Singapore in the late 1990s, pigs served as an intermediate host, which enabled the transmission to humans. Although subsequent outbreaks in Asia only reported direct bat-to-human and human-to-human transmission, pigs are still considered a potential source for viral dissemination in the epidemiology of the disease. Thus, serological assays such as Enzyme-linked immunosorbent assay (ELISA) or virus neutralization test (VNT) represent powerful tools to characterize the serum antibody responses in NiV-infected pigs as well as to perform seroepidemiological surveillance studies on the potential circulation of NiV or NiV-related viruses among pig populations worldwide. This chapter describes both methods in detail. Furthermore, we discuss some of the major pitfalls and indicate how to avoid them.

Development of ELISA for the detection of antibodies against VP2 protein of bluetongue virus serotype-1

Serotype-specific diagnosis of bluetongue virus (BTV) is necessary for sero-surveillance and taking effective control measures. The VP2 is the major serotype determining protein and BTV-1 is the most predominant serotype in India. In the present study, an indirect ELISA (i-ELISA) was optimized for the detection of serotype-specific antibody against BTV-1 serotype. The VP2 protein of BTV-1 was expressed in a prokaryotic expression system and used to optimize i-ELISA to detect VP2 antibodies of BTV-1 in serum samples of both small and large ruminants. Serum samples (n = 363) classified as positive and negative for antibodies to BTV-1 by serum neutralization test (SNT) and also positive and negative for BTV antibodies by c-ELSIA kit (VMRD, USA) were used to determine the cut-off value, diagnostic sensitivity (DSn), and diagnostic specificity (D-Sp) using receiver operating characteristic (ROC) analysis. The percent positivity (PP) value >30.10% was accepted as the cut-off for i-ELISA at which DSn of 99.52% and D-Sp of 99.35% was observed with a 95% confidence interval. Further, there was no cross-reactivity with other available BTV serotypes in the country. The study indicated serotype-specific i-ELISA is sensitive, specific and suitable alternative to tedious SNT method for determining serotype. The assay will also help in the serotype-specific epidemiological studies and implementation of future control strategies including vaccination and selection of suitable serotype as a vaccine candidate.

Development of a diagnostic indirect ELISA test for detection of Brucella antibody using recombinant outer membrane protein 16 kDa (rOMP16)

Brucellosis is considered as one of the important global zoonotic diseases that causes medical as well as economic problems especially in tropical countries. The illness has no specific pathognomonic signs; therefore, the rapid and accurate diagnosis of the disease has a very important role in preventing the Brucella spillover and treatment. The purpose of this study was to design a new indirect ELISA test for detection of human brucellosis based on using recombinant Brucella abortus outer membrane protein 16 kDa (rOMP16) as an antigen. OMP16 gene of B. abortus was initially synthesized and cloned in pET-21d vector and then expressed in Escherichia coli cells. The expression was confirmed by the SDS-PAGE, western blotting and dot blotting. The purified protein was coated in ELISA plates and an indirect ELISA was performed on 70 human serum samples. The results were evaluated with a commercial IgG ELISA kit and Rose Bengal plate agglutination tests as reference tests. Diagnostic performance of designed OMP16 ELISA test in comparison with Rose Bengal plate test revealed 100% of sensitivity, 95.00% of specificity and good Fleiss kappa agreement, whereas, where it was compared to commercial ELISA kit, it revealed very good kappa agreement with 100% of sensitivity and 100% of specificity in cut-off value of 0.13. It was concluded that OMP 16 kDa could be acceptable alternative antigen for detecting Brucella IgG antibody with high accuracy.

Development of an indirect ELISA to detect PEDV specific IgA antibody based on a PEDV epidemic strain

Background

Porcine epidemic diarrhea (PED), a swine epidemic disease caused by porcine epidemic diarrhea virus (PEDV), is characterized by severe watery diarrhea, vomiting, dehydration and high mortality in piglets, and has caused serious economic losses to the global porcine industry. The level of PEDV IgA antibody is a key marker to assess the extent of passive immunity of the resistance against PEDV infection. However, current commercial structure proteins-based kits for detection of PEDV antibody are not affordable, and those kits require complicated antigen preparation procedures, which cannot meet the scope of economic benefits of many large-scale pig companies in China. Therefore, there is an urgent need to develop an accurate, simple, and economical method for IgA detection in clinical samples. In this study, an indirect ELISA (i-ELISA) method was developed based on a purified PEDV epidemic strain (NH-TA2020).

Results

The results show that optimal working dilution ratios of PEDV antigen and HRP anti-swine IgA are at 1: 1000 and 1:15000 respectively. The sensitivity of this method is high with the maximum dilution of samples up to 1:160, and coefficients of variation (CV) of both the intra assays and inter assays were no more than 15%. In addition, the relative sensitivities of the i-ELISA were above 90% compared with values from commercial kits in both serum and oral fluid samples.

Conclusions

Our results suggested that the i-ELISA developed in this study was an accurate, simple, and economical method for PEDV-IgA detection in clinical samples.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03419-w.

Enzyme-Linked Immunosorbent Assay (ELISA)

Enzyme-linked immunosorbent assay (ELISA) is one of the most specific and straightforward assays for detecting biomolecules in research and clinics. With advances in analytical methods, ELISA assay has been constantly optimized to improve its sensitivity, and different types of ELISA are now available to detect various biomarkers. This chapter provides an overall summary of the basic principle of ELISA, discusses different components of ELISA assay, and clearly outline protocols for different types of ELISA assays, including direct, indirect, sandwich, competitive, and nanoparticle-based ELISA.

Serological investigation of Gyrovirus homsa1 infections in chickens in China

BACKGROUND

Gyrovirus homsa1 (GyH1) (also known as Gyrovirus 3, GyV3) is a non-enveloped, small, single-stranded DNA virus, which was first identified in children with acute diarrhea, and was subsequently detected in marketed chickens, broilers with transmissible viral proventriculitis (TVP), and mammals. GyH1 is a pathogenic virus in chickens, causing aplastic anemia, immunosuppression, and multisystem damage. However, the seroepidemiology of GyH1 infection in chickens remains unclear. Here, we investigated the seroprevalence of GyH1 in chickens by ELISA to reveal the endemic status of GyH1 in China.

RESULTS

An indirect ELISA with high sensitivity and specificity was developed for investigation of seroepidemiology of GyH1 in chickens in China. The seropositive rate of GyH1 ranged from 0.6% to 7.7% in thirteen provinces, and ranged from 4.1% to 8.1% in eight species chickens. The seropositive rate of GyH1 in broiler breeders was significantly higher than that of in layers. There was a negative correlation between seropositive rate and age of chickens. The highest and lowest seropositive rate were present in chickens at 30-60 days and over 180 days, respectively.

CONCLUSIONS

The seroepidemiological investigation results demonstrated that natural GyH1 infection is widespread in chickens in China. Different species showed different susceptibility for GyH1. Aged chickens showed obvious age-resistance to GyH1. GyH1 has shown a high risk to the poultry industry and should be highly concerned.

Performance of indirect enzyme-linked immunosorbent assay using Trichinella spiralis-derived Serpin as antigen for the detection of exposure to Trichinella spp. in swine

Indirect enzyme-linked immunosorbent assay (ELISA) utilizing excretory-secretory (E-S) antigens of Trichinella spiralis is currently the method of choice for testing pigs and wild boars for exposure to Trichinella spp. The E-S proteins are released by first-stage larvae (L1) of this parasitic nematode maintained in vitro. However, the production of these antigens is cumbersome and time-consuming. The process requires animals to be experimentally infected with the parasite as the source of L1. Antigen production using recombinant technology would be more time- and cost-effective. In this study, we produced a Serpin of T. spiralis as a recombinant protein secreted by the yeast Pichia pastoris. The diagnostic performance of indirect ELISA with purified Serpin antigen was compared to that of E-S ELISA. Both Serpin ELISA and E-S ELISA demonstrated 98 % diagnostic specificity in testing 1056 pigs from the Canadian Trichinella-free commercial herd. Twenty of 21 pigs with non-negative test results in E-S ELISA tested negative by the confirmatory Western blot (WB) assay. Therefore, the diagnostic specificity of combined E-S ELISA and WB was 99.9 %. Forty-five sera collected at or after six weeks from 34 pigs experimentally infected with various numbers of T. spiralis L1 produced positive results in both E-S and Serpin ELISA, resulting in 100 % diagnostic sensitivity. However, testing of sera serially collected from four pigs experimentally infected with various low doses of T. spiralis L1 demonstrated a delayed Serpin-specific antibody response compared to seroconversion detected by E-S ELISA in three animals. Moreover, Serpin ELISA demonstrated significantly lower sensitivity for detecting antibodies induced by experimental infections of pigs with T. britovi, T. nativa, Trichinella T6 and T. pseudospiralis, suggesting that it will not provide consistent detection of exposure to sylvatic Trichinella spp. The validation data support the application of Serpin ELISA in seroepidemiological surveys for detecting exposure to T. spiralis in swine.

Enhanced serodiagnosis of melioidosis by indirect ELISA using the chimeric protein rGroEL-FLAG300 as an antigen

Background

The accurate and rapid diagnosis of melioidosis is challenging. Several serological approaches have been developed using recombinant antigens to improve the diagnostic indices of serological tests for melioidosis.

Methods

Fusion proteins from Burkholderia pseudomallei (rGroEL-FLAG300) were evaluated as a potential target antigen for melioidosis antibodies. A total of 220 serum samples from 38 culture proven melioidosis patients (gold standard), 126 healthy individuals from endemic (n = 37) and non-endemic (n = 89) Thai provinces and 56 patients with other proven bacterial infections as negative controls were tested using indirect enzyme-linked immunosorbent assays (ELISA).

Results

Using an optical density (OD) cut-off of 0.299148, our assay had 94.74% sensitivity (95% confidence interval (CI) = 82.3–99.4%), 95.05% specificity (95% CI = 90.8–97.7%), and 95% accuracy, which was better than in our previous work (90.48% sensitivity, 87.14% specificity, and 87.63% accuracy).

Conclusion

Our results suggest that the application of chimeric antigens in ELISA could improve the serological diagnosis of melioidosis and should be reconfirmed with greater patient numbers.

Core antigenic advantage domain-based ELISA to detect antibody against novel goose astrovirus in breeding geese

Goose astrovirus (GAstV), the major causative agent of visceral and joint gout in goslings, is a novel pathogen greatly threatening waterfowl industry. Importantly, the horizontal and vertical transmissibility of GAstV posed a great challenge for disease prevention and control. Given the absence of commercial vaccine, restricting vertical transmission and protecting susceptible goslings must be a priority. Although many detection methods have been established, there is no serological method to detect GAstV-specific antibody, greatly limiting inspection and elimination of infected breeding bird. In this study, the B-cell epitopes of GAstV capsid protein were predicted, and its core antigenic advantage domain (shCAP) was expressed and purified. After authenticating the antigenicity, the recombinant shCAP protein was taken as the coating antigen, and an easily accessible indirect enzyme-linked immunosorbent assay (ELISA) was established to detect GAstV-specific antibody. The working conditions, including antigen concentration, serum dilution and incubation time, blocking buffer concentration, and color developing time, were gradually optimized by checkerboard titration. The cut-off OD₄₅₀ value of the indirect ELISA for positive sample was 0.379, and the analytical sensitivity was 1:800. There was no cross-reaction with sera against goose parvovirus (GPV), Tembusu virus (TUMV), H5 and H7 subtype avian influenza virus (AIV H5 + H7), and Newcastle disease virus (NDV). The assay was further applied to examine 73 breeding goose serum samples and shared excellent agreement of 93.5% (68/73) with western blot, which also suggested that GAstV is circulating in the goose population in China. In conclusion, the developed indirect ELISA is simple, specific, and sensitive, which will be greatly useful to screen GAstV infection and block vertical transmission. KEY POINTS: • B-cell epitopes of GAstV capsid protein were predicted and expressed as immunogen • A core antigenic advantage domain-based ELISA was established to detect GAstV-specific antibody • The established ELISA will contribute to inspection and elimination of infected breeding geese and provide a useful tool for large scale serological testing of GAstV in geese.

Bacterial coloration immunofluorescence strip for ultrasensitive rapid detection of bacterial antibodies and targeted antibody-secreting hybridomas

The indirect enzyme-linked immunosorbent assay (ELISA) is the gold standard method for monoclonal antibody (McAb) detection and plays a unique role in the preparation of bacterial antibodies. To solve the laborious issues associated with indirect ELISA, a novel bacterial coloration immunofluorescence strip (BCIFS) for antibody detection using colored bacteria instead of a labeled antibody as the antigen and tracer simultaneously and goat anti-mouse IgG as the test line was developed. The affinity range survey of BCIFS indicated that hybridoma cell cultures of E. coli O157:H7 (D3, E7) and Vibrio parahemolyticus (H7, C9) were detected, which complied with the results of indirect ELISA. Compared with the traditional indirect ELISA, the BCIFS sensitivity for E7 cell cultures, ascites, and purified antibodies was at least 4-fold more sensitive, and the BCIFS cross-reactivity for E7 cell cultures was almost consistent with that of indirect ELISA. In addition, the BCIFS isotypes for E. coli O157:H7 cell cultures and Vibrio parahemolyticus were IgG2a and IgG1, respectively, which were identical to the indirect ELISA. Furthermore, the BCIFS method was confirmed by McAb preparation, effective antibody use, and targeted antibody-secreted hybridoma preparation and screening, which showed excellent performance and substitution of the indirect ELISA method. Combined with methylcellulose semisolid medium, BCIFS offers a novel, easy to operate, rapid preparation method for antigen-specific hybridomas. This is the first report using BCIFS instead of indirect ELISA for bacterial antibody detection and application in different samples, which demonstrates a rapid and powerful tool for antibody engineering.

Occurrence of serum antibodies to Toxoplasma gondii and associated risk factors in donkeys from central Kenya

Toxoplasma gondii is an intracellular parasite of zoonotic concern and economic importance in humans and animals, respectively. This study was conducted to determine the occurrence of T. gondii and associated risk factors in domestic donkeys from Kirinyaga and Meru counties in Kenya. Blood samples were collected from 363 randomly selected donkeys for detection of antibodies to T. gondii using a commercial kit ID Screen® Toxoplasmosis Multi-species indirect enzyme-linked immunosorbent assay (ELISA). The data on risk factors were collected by interviewing donkey owners using epidemiological questionnaire. Serum antibodies to T. gondii were detected in 26.4% (95% CI: 22.2-31.3) of the donkeys. The analysis showed that age of donkeys (OR = 2.484, 95% CI: 1.315-4.693; p = 0.005) was associated with increased risk for T. gondii seroprevalence while county of origin of donkeys (OR = 0.182, 95% CI: 0.083-0.400; p = 0.000), residential place of donkeys (OR = 0.301, 95% CI: 0.136-0.665; p = 0.003), rearing chicken (OR = 0.203, 95% CI: 0.064-0.644; p = 0.007), and donkey production system (OR = 0.644, 95% CI: 0.456-0.909; p = 0.012) were associated with reduced risk of T. gondii seroprevalence. This is the first report to provide epidemiological information on T. gondii infection among donkeys in Kenya. The presence of antibodies to T. gondii in donkeys suggests the high potential of transmission to other animals and humans. Regular monitoring and control of T. gondii infection in donkeys were recommended in the study area.

Seroprevalence and immunological characterization of Trypanosoma evansi infection in livestock of four agro-climatic zones of Himachal Pradesh, India

Trypanosoma evansi, a hemoflagellate protozoan parasite, causes wasting disease called surra in wide range of animals. Although the organism has been reported from various parts of India, data generated from organized epidemiological study is still in infancy in majority states of India. In the present study, livestock of Himachal Pradesh, India, was targeted for epidemiological investigation of T. evansi infections. A total of 440 equines and 444 cattle serum samples were collected from four agro-climatic zones. Furthermore, serum samples of 280 buffaloes from three different agro-climatic zones of Himachal Pradesh were also collected and evaluated for the presence of T. evansi infection by indirect ELISA. Data generated showed higher prevalence in buffalo (23.57%) followed by cattle (22.52%) and equines (1.82%). Disease was found to be more prevalent (P < 0.01) in cattle of lower altitude as compared to those of higher altitudes. No significant variation was seen in prevalence of disease on the basis of age and sex of the animals. Serum biochemical analysis revealed increased levels of BUN in T. evansi-infected equines. Levels of liver function enzymes such as ALT/GGT and AST were found to be significantly elevated (P < 0.01) in seropositive animals whereas glucose levels were significantly lower in surra-seropositive animals as compared to seronegative animals. Immunoblot analysis of whole cell lysate (WCL) antigen of T. evansi using surra-seropositive samples of equines showed immunodominant bands in the range of 100-25 kDa. Bovine-seropositive samples recognized polypeptide bands in the range of 85-32 kDa, including protein clusters of 52-55 and 48-46 kDa. Polypeptide cluster of 62-66 kDa was found common in seropositive samples of bovines and equines from all agro-climatic zones. T. evansi was found to be highly prevalent in livestock of Himachal Pradesh, and thus, there is dire need for designing of proper control strategies against surra.

Development and evaluation of an indirect ELISA based on recombinant structural protein σC to detect antibodies against avian orthoreovirus

The avian orthoreovirus (ARV) causes large economic losses to poultry industry around the whole world. ARV viral proteins can be classified into three sizes: λ (large), μ (medium) and σ (small). σC, one of the capsid protein of ARV, contains various specific neutralizing epitopes and can induce robust immune responses in infected chickens. An indirect enzyme-linked immunosorbent assay (I-ELISA) was established using the recombinant σC protein. The optimal dilutions of antigen, serum and conjugate goat anti-chicken IgY conjugate were 2.5 μg/ml, 1:80 and 1:4000, respectively. The optimal antigen coating condition was 4℃ for overnight. Hyperimmune serum of high-yielding hens of different age produced stronger reaction against σC. Chicken serum samples were randomly collected and total coincidence rate between commercial kit and σC-ARV-ELISA kit was 97.8%. Importantly, those discrepant serums were further verified using Western blot assay and three of them were determined to be positive. Thus, the method based on σC protein has higher sensitivity compared with the commercial kit and provide a rapid and simple method for serodiagnosis of ARV, contributing to convenient epidemiological monitoring of ARV.

Human and environmental factors driving Toxoplasma gondii prevalence in wild boar (Sus scrofa)

As one of the most relevant foodborne diseases, it is essential to know the factors related to the transmission, persistence and prevalence of Toxoplasma gondii infection. Eurasian wild boar (Sus scrofa) might play a relevant role in T.gondii's life cycle. This species is the most consumed big game animal in Spain and may act as a source of infection if the meat is eaten raw or undercooked or due to cross-contaminations. Additionally, wild boar can act as an excellent bioindicator of T.gondii circulation in the ecosystem, because its natural behaviour leads to exposure to oocysts from the soil when rooting and tissular bradyzoites when scavenging. A total of 1003 wild boar were sampled from 2010 to 2017 in Mediterranean Spain. Blood samples were tested with an indirect ELISA test giving a total of 14.1% (95% confidence interval 12.0-16.4%) positive results. The prevalence was not homogeneous in neither the animals nor the sampled districts. Significant differences were found regarding age, climatic conditions and human space occupancy. Human population aggregation, assessed by Demangeon's index, was identified as an influential factor in T.gondii infection risk. This multiple approach allows us to evaluate local risks for human and environmental contamination.

Detection of pseudorabies virus antibody in swine serum and oral fluid specimens using a recombinant gE glycoprotein dual-matrix indirect ELISA

Pseudorabies (Aujeszky disease) virus (PRV) was eliminated from domestic swine in many countries using glycoprotein E (gE)-deleted vaccines and serum antibody gE ELISAs, but PRV continues to circulate in some regions and in most feral swine populations in the world. We created a dual-matrix (serum and oral fluid) indirect IgG gE ELISA (iELISA) and evaluated its performance using samples from 4 groups of 10 pigs each: negative control (NC), vaccination (MLV), PRV inoculation (PRV), and vaccination followed by challenge (MLV-PRV). All serum and oral fluid samples collected before PRV challenge and all NC samples throughout the study were negative for gE antibodies by commercial blocking ELISA (bELISA) and our iELISA. Nasal swab samples from 9 of 10 animals in the PRV group were gB quantitative PRC (qPCR) positive at 2 days post-inoculation (dpi). The oral fluid iELISA detected a significant S/P response in the PRV (p = 0.03) and MLV-PRV (p = 0.01) groups by 6 dpi. ROC analyses of serum bELISA (n = 428), serum iELISA (n = 426), and oral fluid iELISA (n = 247) showed no significant differences in performance (p > 0.05). Our data support the concept of PRV surveillance based on oral fluid samples tested by an indirect gE ELISA.

Characterization and evaluation of a recombinant multiepitope peptide antigen MAG in the serological diagnosis of Toxoplasma gondii infection in pigs

Background

Toxoplasmosis caused by Toxoplasma gondii is a serious disease threatening human and animal health. People can be infected with T. gondii by ingesting raw pork contaminated with cysts or oocysts. Serological test is a sensitive and specific method usually used for large-scale diagnosis of T. gondii infection in humans and animals (such as pigs). Commercial pig Toxoplasma antibody ELISA diagnostic kits are expensive, which limits their use; moreover, the wide antigen composition used in these diagnostic kits is still unclear and difficult to standardize. The multiepitope peptide antigen is a novel diagnostic marker, and it has potential to be developed into more accurate and inexpensive diagnostic kits.

Methods

The synthetic multiepitope antigen (MAG) cDNA encoding a protein with epitopes from five T. gondii-dominant antigens (SAG1, GRA1, ROP2, GRA4, and MIC3) was designed, synthesized, and expressed in Escherichia coli BL21 (DE3) strain. The recombinant protein was detected through western blot with pig anti-T. gondii-positive and -negative serum, and then IgG enzyme-linked immunosorbent assay (ELISA) named MAG-ELISA was designed. The MAG-ELISA was evaluated in terms of specificity, sensitivity, and stability. The MAG-ELISA was also compared with a commercial PrioCHECK^®Toxoplasma Ab porcine ELISA (PrioCHECK ELISA). Finally, the trend of pig anti-T. gondii IgG levels after artificial infection with RH tachyzoites was evaluated using MAG-ELISA and two other ELISA methods (rMIC3-ELISA and PrioCHECK ELISA).

Results

MAG antigen could be specifically recognized by pig anti-T. gondii-positive but not -negative serum. MAG-ELISA showed high diagnostic performance in terms of specificity (88.6%) and sensitivity (79.1%). MAG-ELISA could be used for detecting anti-T. gondii IgG in the early stage of T. gondii infection in pigs (at least 7 days after artificial infection).

Conclusions

Our results suggest that MAG antigen can be applied to specifically recognize anti-T. gondii IgG in pig, and MAG-ELISA has the potential for large-scale screening tests of T. gondii infection in pig farms and intensive industries.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04917-w.

Comparison of recombinant and synthetic listeriolysin- O peptide- based indirect ELISA vis-à-vis cultural isolation for detection of listeriosis in caprine and ovine species

The present study evaluated the comparative serodiagnostic efficacy of recombinant listeriolysin-O (rLLO) and synthetic LLO- 2 peptide-based indirect ELISA vis-à-vis cultural isolation using samples (n = 1326; blood, sera, vaginal swabs, and rectal swabs) collected from caprines (n = 350) and ovines (n = 50) having reproductive and/or nervous system disorders and/or healthy animals. On screening the test sera by rLLO- based ELISA, the antibodies against LLO (ALLO) were observed in 17.71% of the caprines and 2% of the ovines, respectively, while synthetic LLO-2- based ELISA revealed ALLO in 6.86% of caprines and not in ovines. Moreover, the adsorption of positive test sera with streptolysin-O (SLO) resulted in a significant reduction (7.43%; p < 0.05) in the seropositivity with rLLO- based ELISA, whereas LLO-2- based ELISA revealed marginal reduction (4.29%; p > 0.05) in the seropositivity. Overall, the seropositivity with LLO-2 synthetic peptide revealed comparatively less cross-reactivity in comparison to rLLO. The cultural isolation yielded five pathogenic L. monocytogenes isolates and three non-pathogenic Listeria spp. from caprine samples; however, Listeria spp. could not be recovered from any of the ovine samples. Further, on comparing seropositivity with the isolation study results, it was found that two out of the five animals from which pathogenic L. monocytogenes isolated were also found seropositive in both the ELISAs even after adsorption with SLO. Interestingly, rLLO- based ELISA detected antibodies against unadsorbed caprine sera even in those samples from which non-pathogenic Listeria spp. were isolated, whereas antibodies were not detected in LLO-2 peptide-based ELISA. In conclusion, it could be inferred that the synthetic LLO-2 peptide serves as a non- cross-reactive, ideal diagnostic antigen in serodiagnosis of capro-ovine listeriosis.

Comparative analysis of the main outer membrane proteins of Brucella in the diagnosis of brucellosis

Brucellosis has placed a heavy economic burden on numerous countries and has consumed considerable medical resources worldwide. To improve the specificity and sensitivity of serological methods for diagnosing brucellosis, it is important to develop new diagnostic antigens. Brucella outer membrane proteins(omps) possess good immunogenicity, but there is a scarcity of comparative studies of these proteins in the clinical diagnosis of brucellosis. In this study, six recombinant Brucella outer membrane proteins, omp10, omp16, omp19, omp25, omp31 and BP26, were expressed in prokaryotic cells and utilized as diagnostic antigens. The clinical sera of humans, bovines and goats with brucellosis were analyzed by indirect ELISA using these proteins, lipopolysaccharide(LPS) and Rose Bengale Ag, served as positive-control antigens. In diagnosing human and goat serum, BP26 exhibited the highest diagnostic accuracy of 96.45% and 95.00%, respectively, while omp31 exhibited the strongest ability to detect Brucella in bovine serum with an accuracy of 84.03%. Cross-reaction experiments also confirmed that the diagnostic specificities of omp31 and BP26 were higher than those of the LPS and Rose Bengale Ag antigens. The results of this study indicate that omp31 and BP26 are candidate antigens with high potential application value in the clinical diagnosis of brucellosis.

Diagnostic accuracy of three commercial immunoenzymatic assays for small ruminant lentivirus infection in goats performed on individual milk samples

Caprine arthritis-encephalitis (CAE) caused by small ruminant lentivirus (SRLV) infection is one of the most widespread and devastating diseases of goats. Serological methods, mainly immunoenzymatic assays (ELISA), are the mainstay of CAE diagnostics. Even though blood is still the most commonly tested material, animal welfare issues and increasing costs of veterinary service prompt the development of serological methods based on milk testing. Several different types of ELISAs for CAE are available on the market. All of them perform well on serum, however their diagnostic accuracy for testing milk has not been so far compared. Therefore, we carried out the study in 5 dairy goat herds in Poland whose previous epidemiological situation regarding CAE was known. Paired serum and milk samples were collected from all adult females (n = 420) and tested with 3 commercial ELISAs - indirect ELISA based on the whole-virus antigen (wELISA), indirect ELISA based on the recombined transmembrane and capsid protein (TM/CA-ELISA), and competitive ELISA based on the surface glycoprotein (SU-ELISA). Milk was tested as lactoserum at dilution of 1/2 in wELISA and TM/CA-ELISA, and undiluted in SU-ELISA. The true status of goats was based on the composite reference standard comprising the results of all three ELISAs done on serum and the true prevalence of SRLV infection in the herd of origin. 243 (57.9 %) goats were classified as truly positive and 177 (42.1 %) goats as truly negative. Diagnostic accuracy was evaluated using the area under the ROC curve (AUROC) as well as sensitivity (Se) and specificity (Sp) for a range of cut-off values. AUROC was 98.8 % (CI 95 %: 97.5 %, 100 %) for wELISA, 97.9 % (CI 95 %: 96.5 %, 99.2 %) for TM/CA-ELISA, and 91.7 % (CI 95 %: 88.9 %, 94.5 %) for SU-ELISA. At the cut-off values recommended by the manufacturers both indirect ELISAs were highly sensitive (89.3 % and 91.4 %, respectively) and highly specific (98.3 % and 95.5 %, respectively), whereas SU-ELISA had only moderate Se (71.2 %) at comparably high Sp (96.6 %). Nevertheless, the optimal cut-off values were lower than those recommended by manufacturers for serum - sample-to-positive control serum ratio (S/P%) of 10 % for wELISA, S/P% of 80 % for TM/CA-ELISA, and percentage inhibition of 23 % for SU-ELISA. Concluding, the study shows that wELISA and TM/CA-ELISA may be interchangeably used for testing individual goat milk samples for SRLV infection. Diagnostic sensitivity and specificity of these ELISAs appear not to be lower on milk than on serum. SU-ELISA is considerably less sensitive on milk samples than indirect ELISAs.

A novel antigen immunochromatography fluorometric strip for rapid detection and application of pathogenic bacterial high-quality antibody

Unlike traditional immunoassay strips, a novel antigen immunechromatography fluorometric strip (AICFS) using inactivated bacterial antigen instead of an antibody as a test line and goat anti-mouse IgG-FITC as a tracer was developed. The applicability survey of AICFS indicated that E. coli O157:H7 (D3) and Acidovorax citrulli (6F) hybridoma cell cultures could be detected, but Vibrio parahemolyticus (H7, C9) hybridoma cell cultures were missed compared with the indirect enzyme-linked immunosorbent assay (ELISA). The four antibody affinity constants (Ka) were measured and compared, and AICFS could be suitable for high-affinity antibody detection. Compared with the traditional indirect ELISA, the AICFS sensitivity for D3 cell cultures, ascites, and purified antibodies was at least 2-fold more sensitive, the AICFS specific for D3 cell cultures by comparative interpretation was compliant except for the strain ATCC 43895, and the indirect ELISA missed it. More importantly, the AICFS method was confirmed by various real samples that it could be used in different scenarios regarding the antibody, including McAb preparation, the effective antibody use, and high-affinity antibody-secreted hybridoma auxiliary preparation and screening. It could be an excellent alternative method with less than 5% corresponding processing time for indirect ELISA method for pathogenic bacterial high-quality antibody detection. This is the first report of using AICFS for bacterial high-quality antibody detection and application in different samples, which demonstrates a rapid auxiliary tool for high-affinity antibody secreted-hybridoma screening and an excellent alternative method for high-quality antibody application.

Development of an indirect ELISA for detection of anti-Mycoplasma hyopneumoniae IgG in naturally infected pathogen-induced convalescent sera

BACKGROUND

Immunization of pigs with an inactivated Mycoplasma hyopneumoniae vaccine (bacterin) generates hyperimmune serum that contains high concentrations of anti-M. hyopneumoniae IgG. Commercially available IgG-ELISA kits cannot distinguish between anti-M. hyopneumoniae IgG in inactivated bacterin-induced hyperimmune sera and convalescent sera resulting from natural M. hyopneumoniae infection. Establishment of an ELISA to detect anti-M. hyopneumoniae IgG in convalescent sera will facilitate the evaluation of the M. hyopneumoniae status of pig farms.

RESULTS

In this study, we expressed and purified recombinant Mhp366-N protein, which contains an epitope recognized by M. hyopneumoniae convalescent sera but not hyperimmune sera, for use as a coating antigen. For the M. hyopneumoniae convalescent serum IgG-ELISA, the optimal antigen concentration, blocking buffer, blocking time, dilution of serum, incubation time with serum, secondary antibody dilution, secondary antibody incubation time and colorimetric reaction time were 0.25 µg/mL, 2.5 % skim milk, 1 h, 1:500, 0.5 h, 1:10,000, 1 h and 15 min, respectively. Validation of the M. hyopneumoniae convalescent serum IgG-ELISA showed a cut-off value of 0.323, the intra-assay CV ranged from 3.27 to 7.26 %, the inter-assay CV ranged from 3.46 to 5.93 %, and the assay was able to differentiate convalescent sera from antibodies to 7 other porcine respiratory pathogens. The convalescent serum IgG-ELISA detected no anti-M. hyopneumoniae IgG in hyperimmune serum samples while a commercial IgG-ELISA identified 95/145 of these sera as positive. The accuracy of the M. hyopneumoniae convalescent serum IgG-ELISA was comparable to the sIgA-ELISA but better than the commercial IgG-ELISA.

CONCLUSIONS

The convalescent serum IgG-ELISA is a reproducible, sensitive, and specific indirect ELISA to detect anti-M. hyopneumoniae IgG in naturally infected pathogen-induced convalescent sera. This ELISA could be used to carry out large scale surveillance of M. hyopneumoniae infection in pig farms regardless of vaccination status.

Molecular characterization and serodiagnostic potential of Echinococcus granulosus hexokinase

Background

Cystic echinococcosis (CE), caused by the larval stage of Echinococcus granulosus (sensu stricto), is a life-threatening but neglected zoonosis. Glycolytic enzymes are crucial molecules for the survival and development of E. granulosus. The aim of this study was to investigate the molecular characterization, immunogenicity, tissue distribution and serodiagnostic potential of E. granulosus hexokinase (EgHK), the first key enzyme in the glycolytic pathway.

Methods

EgHK was cloned and expressed in Escherichia coli. Specific serum antibodies were evaluated in mice immunized with recombinant EgHK (rEgHK). The location of EgHK in the larval stage of E. granulosus was determined using fluorescence immunohistochemistry, and the potential of rEgHK as a diagnostic antigen was investigated in patients with CE using indirect enzyme-linked immunosorbent assay (ELISA).

Results

Recombinant EgHK could be identified in the sera of patients with CE and in mouse anti-rEgHK sera. High titers of specific immunoglobulin G were induced in mice after immunization with rEgHK. EgHK was mainly located in the tegument, suckers and hooklets of protoscoleces and in the germinal layer and laminated layer of the cyst wall. The sensitivity and specificity of the rEgHK-ELISA reached 91.3% (42/46) and 87.8% (43/49), respectively.

Conclusions

We have characterized the sequence, structure and location of EgHK and investigated the immunoreactivity, immunogenicity and serodiagnostic potential of rEgHK. Our results suggest that EgHK may be a promising candidate for the development of vaccines against E. granulosus and an effective antigen for the diagnosis of human CE.