Epidemiology and laboratory diagnosis of very virulent infectious bursal disease virus in vaccinated chickens in Khartoum, Sudan

BACKGROUND

Infectious Bursal Disease (IBD, Gumboro disease) has become more severe than in early outbreaks in the 1980s. The present research aims to study the epidemiology of IBD in Khartoum state and compare some commonly used laboratory techniques for diagnosis.

METHOD

We collected epidemiological data from 30 farms that showed signs suggestive of IBD, estimated the morbidity and mortality rates, and interviewed the owners about the type and the doses of the used vaccines. We collected bursas of Fabricius for virus assays and histopathology. Samples positive in the agar gel immunodiffusion (AGID) test were inoculated onto chicken embryo fibroblast cell culture and embryonated chicken eggs. Twenty-two-day-old chicks were infected experimentally with three selected isolates, and morbidity and mortality rates were compared.

RESULTS

The results showed that 70% of outbreaks occurred between 6 and 8 weeks of age, and the mean mortality rate was 51%. Epidemiologic, clinical, gross, and histopathological findings were characteristic of the severe disease caused by the very virulent IBDvirus (vvIBDV). The farms that used intermediate or the intermediate plus vaccines had lowered mortality compared with the farms that used intermediate vaccines. The AGID was found more sensitive than the counter-immuno-electrophoresis (CIEP) since it detected 83.4% of the IBDV antigen in the samples while the CIEP detected 66.7% of the samples. The reverse transcriptase polymerase chain reaction (RT-PCR) was found to be rapid, specific, and was more sensitive detecting 100% of the tested samples. Virus isolation in embryonated eggs and cell culture was not successful.

CONCLUSION

A vvIBDV is responsible for the recent outbreaks of the disease in Sudan, resulting in a mean high mortality rate of 51%, even in vaccinated flocks. The RT-PCR and AGID are the best methods for laboratory confirmation.

Design and validation of a RT-qPCR procedure for diagnosis and quantification of most types of infectious pancreatic necrosis virus using a single pair of degenerated primers

Infectious pancreatic necrosis virus (IPNV) is an important virus which affects the salmonid aquaculture industry worldwide; therefore, it is important to develop rapid and reliable methods of diagnosis to detect the disease at early stages. Nowadays, RT-qPCR is replacing other methods because it provides additional information on the viral load, which is important to have a better understanding of the virus replication level and of the stage of the infection and its risk level. The main problem stems from the high diversity of this virus, which can compromise the reliability of the diagnosis. In this study, we have designed an RT-qPCR procedure for diagnosis and quantification of IPNV based on a single pair of primers targeted to segment B. The procedure has been validated, in vitro and in vivo, testing two different types of standards against seven reference strains and 23 field isolates from different types. The procedure is reliable for the detection of any type, with a detection limit of 31 TCID₅₀  mL^-1 , 50 pfu mL^-1 or 66 RNA copies mL^-1 , depending on the standard. All the standard curves showed high reliability (R²  > 0.95). The results support the high reliability of this new procedure for the diagnosis and quantification of IPNV.

[IDENTIFICATION OF THE INFECTIOUS PANCREATIC NECROSIS VIRUS (IPNV) USING THE ENZYME IMMUNOASSAY]

The infectious pancreatic necrosis (IPN) caused by a non-enveloped virus of the Birnaviridae family is one of the most important loss factors in the salmonid aquaculture. Virus isolation in the sensitive cell cultures has been approved in the Russian Federation as the diagnostic method for determination of IPNV antigen. This work gives the results of the development of the diagnostic test to reveal IPNV using the antigen-bound ELISA (sandwich ELISA). The developed test supplements a new diagnostic method and verifies some disputable results obtained with classical methods.

Tissue distribution of infectious pancreatic necrosis virus serotype Sp in naturally infected cultured rainbow trout, Oncorhynchus mykiss (Walbaum): an immunohistochemical and nested-PCR study

This study investigates the occurrence and distribution pattern of infectious pancreatic necrosis virus (IPNV) within the pancreas, liver, kidney and spleen of naturally infected cultured rainbow trout, Oncorhynchus mykiss (Walbaum), using immunohistochemistry (IHC). A nested PCR was also employed to confirm the presence of the virus in the pooled tissues of the specimens. All the examined tissues except spleen were immunohistochemically positive for IPNV, but staining intensity and distribution pattern varied. The kidney tubules had the most intense and widespread staining by IHC, indicating a specific tissue tropism at least for this particular serotype. The nucleotide sequence had the greatest identity with the Sp serotype confirming the presence of the nucleic acid of IPNV in the pooled tissues. Based on the present findings, it could be concluded that the absence of lesions consistent with infectious pancreatic necrosis (IPN) disease in the H&E-stained sections cannot rule out the presence of the IPNV, and the use of an alternative rapid confirmatory method such as IHC with formalin-fixed, paraffin-embedded tissue sections is helpful for the final diagnosis of IPN in rainbow trout.

One-step reverse-transcription loop-mediated isothermal amplification for detection of infectious bursal disease virus

A fast, sensitive, and specific reverse-transcription (RT) loop-mediated isothermal amplification (RT-LAMP) assay was developed that involved a single tube and a 1-step reaction for detecting infectious bursal disease virus (IBDV). Four specific primers were used for amplification of the VP2 gene of IBDV. The amplified LAMP products were detected by DNA electrophoresis and by direct observation with the naked eye in the presence of SYBR Green I. The sensitivity of RT-LAMP was determined to be 0.01 fg of IBDV viral RNA. This assay for IBDV is more sensitive than the conventional RT-polymerase chain reaction assay, which has a detection limit of 1 ng. The LAMP assay was also assessed for specificity and was found to precisely discriminate between positive and negative test samples. This newly established LAMP assay, combined with RT, is a practical diagnostic tool because IBDV-infected and uninfected clinical samples collected from an experimental farm could be discriminated. Full verification of a sample's IBDV status was obtained within 40 min of extraction of the viral RNA, which could then be directly added to the RT-LAMP reaction mixture.

Detection and characterization of infectious bursal disease viruses in broilers at processing

The presence of infectious bursal disease virus (IBDV) in broilers entering processing plants was examined. The dissemination of IBDV and the introduction of non-native strains for example very virulent (vv) IBDV have had a negative economic impact on poultry production in many countries. Restrictions have been placed on the import and export of poultry products by some countries. There is a perceived risk that IBDV can be spread through transportation and contamination of processing equipment, poultry protein products and processing plant personnel. This risk, however, is fundamentally unknown because scientific studies have not been conducted on the presence of IBDV in birds entering processing plants or the variables that may affect this risk during and post-harvest. The goal of this study was to determine if infectious IBDV was present in broilers entering processing plants. A total of 47 pooled bursa samples from 26 processing plants in the Eastern U.S. were examined. Molecular testing indicated that an IBDV specific RT-PCR was positive in 12 (25.5%) of the samples from 11 different processing plants. Nucleotide sequence analysis was conducted on the 12 RT-PCR positive samples and indicated the IBDV was not commercially available attenuated vaccine strains. Most of the sequences were unique and a phylogenic analysis indicated they were most closely related to variant strains of IBDV. Five RT-PCR positive samples were selected at random for testing in specific-pathogen-free chickens. All five samples contained infectious IBDV as evidenced by macroscopic lesions and bursa/body weight ratios that were significantly lower in infected birds than in the non-inoculated controls. The five viruses were re-identified in bursa tissue from chickens in their respective groups at necropsy using RT-PCR and nucleotide sequencing. The results indicate that infectious and pathogenic IBDV are entering processing plants in this geographic region of the U.S. Additional studies are needed on post-harvest variables that could increase or decrease the risk that these viruses are being disseminated during this process.

[A real-time polymerase chain reaction-based test system for quantitation of Gumboro disease virus]

A real-time polymerase chain reaction-based test system for quantitation of infectious bursal disease (Gumboro disease) virus was developed. The reaction parameters were analyzed, which affected the linear relationship of a C1 depentanizer to the quantity of cDNA. The use of specific primers for reverse transcription was shown tohave some advantage over that of random hexanucleotides.

Detection of infectious pancreatic necrosis virus (IPNV) from asymptomatic redbanded seabream, Pagrus auriga Valenciennes, and common seabream, Pagrus pagrus (L.), using a non-destructive procedure

A non-destructive procedure based on nested RT-PCR and dot-blot hybridization has been developed for the detection of asymptomatic IPNV-carrier fish. The pair of primers designed for RT-PCR amplified a 599-bp fragment of the pVP2 region within the polyprotein gene, resulting in the detection of IPNV genotype III.1. The use of a nested RT-PCR allowed the amplification of IPNV genotypes III.1 and I.2. In addition, a 191-bp probe was designed for hybridization studies used in combination with the nested RT-PCR. The application of the nested RT-PCR to analyse blood samples from asymptomatic redbanded seabream, Pagrus auriga, and common seabream, P. pagrus, specimens showed a 53.1% and 77.8% prevalence of IPNV-carriers, respectively. The combination of nested RT-PCR and dot-blot hybridization increased the detection rates up to 100% for redbanded seabream and 94.4% for common seabream. Therefore, the protocol described in this study is highly sensitive and specific for the detection of IPNV in asymptomatic carrier fish, and, in addition, the results demonstrate the carrier state in two newly cultured sparid species in southern Spain.

Development and validation of a short-time cell culture and multiplex reverse transcriptase polymerase chain reaction assay for infectious pancreatic necrosis virus in Mexican farm-sampled rainbow trout

The infectious pancreatic necrosis virus (IPNV) affects several species of freshwater and marine fish. In Mexico, IPNV has an important impact on farming of rainbow trout Oncorhynchus mykiss; however, IPNV distribution in Mexico is unclear. The diagnosis of IPNV is laborious; usually it is based on isolation tests in cell culture followed by immunological identification using techniques of serum neutralization, immunofluorescence, or enzyme-linked immunosorbent assay. It has recently been demonstrated that reverse transcriptase polymerase chain reaction (RT-PCR) is an adequate method for the detection of aquatic birnaviruses. However, its diagnostic use is still limited because very low titers of viable virus cannot be easily detected. In this study, a combination of short-time cell culture and multiplex RT-PCR was established for the diagnosis of IPNV in rainbow trout obtained from farms in the state of Mexico. Three primer sets were used in a single reaction in the multiplex RT-PCR to increase the probability of identifying all serotypes of IPNV serogroup A as well as to help prevent a false-negative result. This approach was able to identify samples with an IPNV concentration of just 0.01 tissue culture infective dose with 50% endpoint (TCID50)/mL, and it identified more infected fish than RT-PCR alone or first-passage cell culture alone. Moreover, this technique made the same identifications as second-passage cell culture but in approximately 30% of the time needed for second-passage cell culture. Consequently, the time and cost efficiency of IPNV diagnosis were greatly reduced.

Development of reverse-transcription loop-mediated isothermal amplification for the detection of infectious bursal disease virus

To establish a reverse-transcription loop-mediated isothermal amplification (RT-LAMP) method for rapid detection of infectious bursal disease virus (IBDV), four primers specific to six regions of the VP3 gene were designed; the VP3 region was selected because it is a conserved part of the IBDV genome. After amplification in an isothermal water bath for 70 min, samples containing IBDV generated the expected ladder-like products while other viruses generated no product. The sensitivity and specificity of the RT-LAMP assay were evaluated by comparison with reverse-transcription polymerase chain reaction (RT-PCR) and virus isolation. The assay was significantly more sensitive than normal gel-based RT-PCR. Because it is specific and simple, the RT-LAMP assay can be widely applied in clinical laboratories for rapid detection of IBVD.

An improved in situ hybridization method for the detection of fish pathogens

A fluorescent in situ hybridization (FISH) method was developed for detection of infectious pancreatic necrosis virus (IPNV) in paraffin-embedded tissues of Atlantic salmon, Salmo salar L. Several methods of probe labelling and detection were evaluated and found unsuitable for FISH because of tissue autofluorescence. Likewise, the use of avidin to detect biotin-labelled probe was obviated by the presence of endogenous biotin. An existing approach, using digoxigenin (DIG)-labelled probes and detection by anti-DIG antibody-labelled with alkaline phosphatase, was modified to use a fluorescent substrate, 2-hydroxy-3-naphthoic acid-2'-phenylanilide phosphate/4-chloro-2-methylbenzene diazonium hemi-zinc chloride salt (HNPP/Fast Red TR). This improved method allowed sensitive detection of IPNV target, without interference from autofluorescence or endogenous alkaline phosphatase. Furthermore, the reporter produces a discrete, non-fading signal, which is particularly suitable for analysis by confocal microscopy.

A comparison between non-destructive and destructive testing of Atlantic salmon, Salmo salar L., broodfish for IPNV--destructive testing is still the best at time of maturation

Two populations of Atlantic salmon broodstock, previously identified as infectious pancreatic necrosis virus (IPNV) carriers, were screened for IPNV at the time of stripping. Four hundred and ten broodfish were individually sampled of which 91 were detected as IPNV positive by virus culture of sonicated kidney homogenates combined with gonadal fluid, but none tested positive by the blood leucocyte assay. Thirty fish identified as IPNV carriers prior to maturation by the blood leucocyte assay were used in a separate study to compare non-destructive vs. destructive testing methods at stripping. IPNV was not detected using the blood leucocyte method at the time of stripping. RT-PCR and real-time PCR assays failed to detect IPNV from 13 blood samples, the virus was not isolated from milt (0/14) or sonicated ovarian fluid cell pellets (0/16) and only three fish tested positive by the standard culture of kidney homogenates. A third study of Atlantic salmon broodfish compared the IPNV isolation rates prior to maturation with the isolation rates at spawning during 1999-2001. In each year the percentage of IPNV-positive broodfish was significantly lower than in the pre-broodstock sample. While in pre-broodfish samples IPNV was detected by the blood leucocyte assay, no culture isolations or PCR positives were detected from non-destructive samples of the same individual broodfish at stripping. A consistent finding was that even for the kidney assay, the percentage of IPNV-positive fish in carrier populations was higher in pre-broodstock than in broodfish at stripping. These results indicate that destructive kidney sampling is still the most sensitive method for detecting IPNV carrier Atlantic salmon broodfish and that a change in IPNV carrier-status occurs during the maturation period.

Detection of infectious bursal disease virus from formalin-fixed paraffin-embedded tissue by immunohistochemistry and real-time reverse transcription-polymerase chain reaction

Formalin fixed paraffin embedded tissues blocks are used routinely to diagnose the economically important immunosuppressive infectious bursal disease virus (IBDV) in chickens. Immunohistochemical detection of viruses in tissue blocks has been done with varying results between laboratories. Extraction of IBDV RNA from tissue blocks allows IBDV strain identification at a molecular level. This allows correlation between virus identity and histological lesions present in the tissue. Experimentally reverse transcription-polymerase chain reaction (RT-PCR) detectable IBDV RNA could always be extracted from tissue blocks with acute +3 or higher histological lesion scores. However, many blocks from diagnostic field cases did not yield detectable IBDV RNA, in spite of having severe IBDV histological lesion scores. The reason for this can be the effect different formalin fixation conditions have on RNA detection from tissue blocks. To study the effect of various fixation parameters on RNA extraction and immunohistochemical detection of IBDV, bursas with maximum histological lesion score of 4 for IBDV were fixed in formalin under various conditions (different pH levels, temperatures, concentrations of formalin, and fixation duration). Only tissues fixed in formalin with a pH of 7.0, concentration of 5 or 10% formaldehyde, storage temperature of 25 degrees C or less, and kept for up to 2 weeks in formalin yielded detectable IBDV RNA upon extraction. No RNA could be detected from tissues fixed under extreme temperature, pH, or formalin concentrations. Optimal fixation conditions for IHC detection of IBDV were 10% formalin concentration, pH 7.0, and temperature of 4 degrees C, where maximum intensity of immunostaining was observed.

Diagnostic potential of recombinant protein of hexahistidine tag and infectious bursal disease virus VPX expressed in Escherichia coli

The current method to detect antibody titre against infectious bursal disease virus (IBDV) in chickens is based on enzyme-linked immunosorbent assay (ELISA) using whole virus as coating antigen. Coating the ELISA plates requires a purified or at least semi-purified preparation of virus as antigen, which needs special skills and techniques. In this study, instead of using whole virus, recombinant protein of hexahistidine tag (His 6 tag) and VPX protein of IBDV expressed in E. coli was used as an alternative antigen to coat the ELISA plates. There was a good correlation coefficient (R2 = 0.972) between the results of the ELISA using plates coated with monoclonal antibody against His 6 tag and those of the commercial IBDV ELISA kit. Hence, His 6 tag and VPX recombinant protein expressed in E. coli has the potential for the development of ELISA for the measurement of IBDV-specific antibody.

Molecular characteristics of infectious bursal disease viruses from asymptomatic broiler flocks in Europe

Infectious bursal disease virus (IBDV) exists in several different antigenic and pathogenic forms. The immune suppression caused by this virus in young chickens is not always associated with clinical signs of disease. The antigenic variant viruses originally described in the United States typically do not cause clinical signs of disease but can cause a marked immune suppression via the destruction of B lymphocytes. Using a reverse transcription-polymerase chain reaction (RT-PCR) assay we conducted a survey of asymptomatic broiler chicken flocks in Europe for IBDV. Restriction fragment length polymorphisms in the viral protein 2 (VP2) gene of four isolates from Spain and four isolates from France indicated they may be different from the classic and very virulent (vv) IBDV strains found throughout Europe. Nucleotide sequence and phylogenetic analysis of the hypervariable region of the VP2 gene indicated that all eight viruses were more similar to U.S. variant viruses than classic viruses. In two viruses, one from France and one from Spain, threonine was observed at amino acid position 222 and serine was found at position 254. These two substitution mutations are characteristic of Delaware variant viruses. In addition, all eight viruses had mutated amino acid position 318 from glycine to aspartic acid, another substitution mutation commonly found in U.S. variant viruses. Although importation restrictions prevented us from directly testing the antigenicity of these viruses, their nucleotide and predicted amino acid sequences suggest they could be antigenically distinctive compared to classic and vvIBDV commonly found in Europe. Confirmation of the presence of antigenic variant IBDV strains in Europe requires additional immunologic studies to elucidate the exact nature of the viral epitopes. Our data support the need for these immunologic studies.

A Practical Tissue Sampling Method Using Ordinary Paper for Molecular Detection of Infectious Bursal Disease Virus RNA by RT-PCR

A practical sampling method for bursal tissue using ordinary paper for molecular diagnosis of infectious bursal disease (IBD) was established. IBD virus-infected bursa was directly smeared on chromatography paper, filter paper, or stationery copy paper and was then fixed with absolute ethanol, Tris-HCl-saturated phenol, or phenol:chloroform:isoamyl alcohol (25:24:1). Flinders Technology Associates (FTA) card, which is designed for the collection of biological samples for molecular detection, was also used. After storage at 37 C for up to 30 days, total RNA directly extracted from the tissue fixed on the papers and FTA card were subjected to reverse transcriptase-polymerase chain reaction (RT-PCR) for detection of IBD virus (IBDV) RNA. In addition, the ability of each chemical used in the fixation and the FTA card to inactivate IBDV was evaluated. Regardless of the paper quality, storage period, and fixation method, IBDV RNA was consistently detected in all of the samples. IBDV in the bursal tissue was inactivated with phenol but not with ethanol or the unknown chemicals in FTA card. These results show that ordinary papers sustain the viral RNA, as does FTA card, but phenol fixation is superior to FTA card in inactivating IBDV. The new sampling method using ordinary paper with phenol fixation is safe, inexpensive, simple, and easy, and is thus suitable for conducting a global survey of IBD even where laboratory resources are limited. This practical method should contribute to the control of IBD worldwide.

Molecular analysis of infectious bursal disease virus from bursal tissues collected on FTA filter paper

We investigated the feasibility of using FTA filter cards for the storage of bursas of Fabricius containing infectious bursal disease virus (IBDV) and for IBDV detection by reverse transcriptase (RT)-polymerase chain reaction (PCR), and characterization by restriction fragment length polymorphism (RFLP) or nucleotide sequencing. The FTA card is a cotton-based cellulose membrane containing lyophilized chemicals that lyses many types of bacteria and viruses. IBDV was inactivated upon contact with the FTA as shown by the inability of the virus to be propagated in embryonating chicken eggs. Viral RNA in minced bursas or stamped bursas could be amplified by RT-PCR (VP2 gene fragment, 248 base pairs) after storage on FTA for at least 15 days at room temperature or 8 mo at -20 C. Analytical sensitivity of the test was between 0.5-5 ng of RNA template or 5 x 10(1) mean tissue culture infective dose (TCID50)/FTA spot. Detection rate of IBDV in domestic clinical samples collected on FTA or collected by the non-FTA standard procedure was 36.7% and 41.7%, respectively, which represents 88% agreement. Detection of IBDV from FTA cards inoculated with bursal tissues in the laboratory or in the field was 36.7% and 37.1%, respectively. Detection of IBDV from FTA samples when the cards were inoculated with bursal tissues and sent through customs into the United States was 32.9%. Analysis of the amplified products showed that molecular characterization of IBDV by RFLP or nucleotide sequencing is feasible in bursas stored on FTA at 25 C for 1-3 mo or at -20 C for at least 8 mo. The use of FTA for the collection of bursal tissues and simultaneous inactivation of IBDV allows the movement of specimens within the United States and also from outside the United States in compliance with federal regulations and in a manner adequate for molecular characterization.

Moving molecular diagnostics from laboratory to clinical application: a case study using infectious pancreatic necrosis virus serotype A

AIMS

Using an RT-PCR method for detection of infectious pancreatic necrosis virus (IPNV) in Atlantic salmon as a model, this study examined the optimization and validation required to provide a method suitable for IPNV detection from fish tissue.

METHODS AND RESULTS

IPNV-positive Atlantic salmon kidney samples that had been titred or kidney spiked with IPNV were used. The amount of RNA in the reverse transcription (RT), RT denaturation temperature and incubation time, PCR annealing temperature and number of cycles were optimized. The optimized RT-PCR was able to detect IPNV in Atlantic salmon kidney calculated to have a titre of ten infectious units.

CONCLUSIONS

Extensive optimization is required to produce a PCR for detection of fish pathogens from methods designed in the laboratory.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrated some of the many variables that should be optimized before a fully validated assay can be claimed and illustrates the extensive validation required to fulfil requirements of the OIE and is of relevance to laboratories carrying out clinical testing.

Detection of antibodies against serotypes 1 and 2 infectious bursal disease virus by commercial ELISA kits

Two distinct serotypes of infectious bursal disease virus (IBDV) are recognized in chicken and turkey flocks in the United States. Serologic testing of chicken flocks for serotype 1 viruses is routinely performed to monitor disease status and vaccination. Earlier studies indicated that enzyme-linked immunosorbent assay (ELISA) test detects antibodies to both serotypes of the virus, while the virus neutralization (VN) test is serotype specific. It is useful to evaluate currently available commercial ELISA kits for their ability to differentiate between antibodies elicited by the two serotypes. Three trials were performed in which chickens were orally inoculated with either a high or a low dose of serotype 1 STC or serotype 2 OH strains of IBDV. Sera collected at 0, 7, 14, and 21 days from these chickens and antisera procured from naturally infected broiler (n=20) and layer (n=30) flocks were tested with five different commercial ELISA kits and by VN. All ELISA kits detected different levels of antibodies elicited against serotype 1 of the virus and moderate and high levels of antibodies against serotype 2 virus. A correlation existed between the ELISA and the VN titers of experimentally infected chickens. All serum samples tested from the commercial layer flocks and 65% of the broiler flocks had antibodies against the OH strain. However, no correlation between the VN titers and ELISA titers was observed for the commercial broilers and layers sera by the majority of the kits. The results indicated that currently available commercial ELISA kits detect antibodies elicited by the two serotypes of IBDV. Hence, the prevalence of serotype 2 antibodies in the flocks should be considered while determining antibody profiles of the flocks against serotype 1 viruses.

Detection of vvIBDV in vaccinated SPF chickens

The purpose of our experiment was to investigate, if apparently healthy, vaccinated chickens may be involved in maintaining and spreading infectious bursal disease virus (IBDV) in poultry environments. We aimed at simultaneous detection and identification of very virulent field strain IBDV (vvIBDV) as well as vaccine strain IBDV in experimentally infected chickens. Two groups of specific pathogen free (SPF) chickens were vaccinated using the intermediate infectious bursal disease (IBD) vaccine D78. Group 1 was vaccinated at the age of one week and group 2 at the age of three weeks. Both groups were challenged with vvIBDV at the age of four weeks. A third, vaccinated, non-challenged group served as negative control. No clinical symptoms were observed in any of these groups. The chickens were euthanised and submitted to autopsy and sample preparation in groups of three at fixed intervals from the age of 28 to 44 days. Gross pathological lesions were not observed. Lymphoid tissues from the bursa of Fabricius, bone marrow, spleen and thymus in addition to cloacal- and bursal swaps were analysed by one-step reverse transcription polymerase chain reaction (RT-PCR). Positive results were confirmed by two-step strain specific duplex (DPX) RT-PCR. The vaccine strain was detected in bursa tissues from all groups, while the challenge strain was detected in few bursal as well as non-bursal tissue samples.

The results indicate a possibility of replication of vvIBDV in vaccinated chickens.

Chicken recombinant antibodies specific for very virulent infectious bursal disease virus

A phage-displayed single chain variable fragment (scFv) antibody library was constructed from the immune spleen cells of chickens immunized with very virulent infectious bursal disease virus (vvIBDV) strain CS89. A library consisting of around 9.2 x 10(7) clones was subjected to 3 rounds of panning against captured CS89 virus. Analysis of individual clones by nucleotide sequencing revealed at least 22 unique scFv antibodies binding to vvIBDV in ELISA. Testing of the scFv antibody panel in ELISA against classical, variant or vaccine strains and a wide variety of vvIBDV isolates from the UK, China, France, Belgium, Africa, Brazil, Indonesia and the Netherlands identified one antibody, termed chicken recombinant antibody 88 (CRAb 88) that was specific for vvIBDV. CRAb 88 was capable of recognizing all vvIBDV strains tested regardless of their country of origin and showed no reactivity with classical, variant or vaccine strains, lending support to the use of this scFv as a powerful diagnostic tool for the differentiation of vvIBDV strains. Immunoprecipitation studies revealed that CRAb 88 was directed towards a highly conformational epitope located within the major neutralizing protein VP2. Sequence analysis of the hypervariable region of VP2 of the IBDV strains tested indicate that Ile(256) and Ile(294) may play roles in binding of CRAb 88. This is the first reagent of its type capable of positively distinguishing vvIBDV from other IBDV strains.

A reliable RT-PCR–ELISA method for the detection of infectious pancreatic necrosis virus (IPNV) in farmed rainbow trout

A new method, termed RT-PCR-ELISA, was evaluated for ease of use, reliability and sensitivity when detecting infectious pancreatic necrosis virus (IPNV) present in trout kidney tissue. The method had comparable sensitivity to existing PCR assays and could successfully detect 1.5 x 10(4) pfu IPNV in artificially contaminated trout kidney samples. The technique was easily established in a new laboratory and required no specialised equipment. The method had a high sample throughput capable of screening 96 samples per run, making the technique extremely time efficient. The RT-PCR-ELISA is a safe, quick, reliable technique, which has the potential for use as a standard virus detection method.

Infectious pancreatic necrosis virus in Atlantic salmon, Salmo salar L., post-smolts in the Shetland Isles, Scotland: virus identification, histopathology, immunohistochemistry and genetic comparison with Scottish mainland isolates

During mid-June 1999 peak mortalities of 11% of the total stock per week were seen at a sea cage site of Atlantic salmon, Salmo salar L., post-smolts in the Shetland Isles, Scotland. Virus was isolated on chinook salmon embryo (CHSE) cells in a standard diagnostic test and infectious pancreatic necrosis virus (IPNV) identified by enzyme-linked immunosorbent assay. IPNV was confirmed as serogroup A by a cell immunofluorescent antibody test using the cross-reactive monoclonal antibody AS-1. Four weeks after the main outbreak, virus titres in surviving moribund fish were assayed at >10(10) TCID50 g(-1) kidney. Histopathology of moribund fish was characterized by pancreatic acinar cell necrosis and a marked catarrhal enteritis of the intestinal mucosa. In the liver, necrosis, leucocytic infiltration and a generalized cell vacuolation were noted. IPNV-specific immunostaining was demonstrated in pancreas, liver, heart, gill and kidney tissue. The nucleotide sequence of the coding region of segment A was determined from the Shetland isolate. A 1180 bp fragment of the VP2 gene of this isolate was compared with a 1979 reference isolate from mainland Scottish Atlantic salmon, La/79 and another more recent mainland isolate, 432/00. Both A2 isolates were derived from carrier fish without signs of IPN and serotyped by a plaque neutralization test. The Shetland isolate shows a different nucleotide and amino acid sequence compared with the two isolates from carrier fish. These latter isolates showed identical amino acid sequences in the fragment examined, despite the 21 years separating the isolations. Sequence comparisons with other A2 (Sp) isolates on the database confirm all three Scottish isolates are A2 (Sp).

A user's inter-laboratory comparison of broodfish screening for infectious pancreatic necrosis virus using molecular and conventional diagnostic methods

Individual testing and subsequent removal of eggs from infectious pancreatic necrosis virus (IPNV)-positive parents is required for export of salmonid eggs to some farming countries. Testing by cell culture requires more than three weeks before the eggs can be released from quarantine and incurs significant logistic problems and costs. The feasibility of the RT-PCR testing method as offered by several laboratories was therefore evaluated during the current inter-laboratory comparison study. Frozen kidney sub-samples from 100 motherfish of Atlantic salmon (Salmo salar L.) were shipped to three diagnostic laboratories (A, B and C) for testing by RT-PCR and cell culture. Of the 100 examined samples, all proved IPNV-negative by cell culture. Thirty samples were positive by RT-PCR analyses, but only four of these samples were RT-PCR positive in two laboratories and none in any of the three laboratories. From a disease management point of view, the RT-PCR test outcomes gave no reasonable guidance as to which fish were truly infected and which batches of fertilised eggs should be discarded. This is clearly an unacceptable situation and calls for new research to standardise sample conservation, RNA extraction procedures and amplification techniques, to estimate method sensitivity and specificity, and to validate the method's performance and robustness to support disease control measures in salmonid aquaculture.

An ELISA for detection of infectious bursal disease virus and differentiation of very virulent strains based on single chain recombinant chicken antibodies

Two chicken single-chain variable antibody fragments (scFv) designated scFv154 and scFv88, previously shown to react with either all or very virulent (vv) infectious bursal disease virus (IBDV) strains, respectively, were evaluated for use in an enzyme-linked immunosorbent assay (ELISA) for differentiation of vvIBDV. Specificity and sensitivity of the vvIBDV ELISA was assessed when scFv154 and scFv88 were expressed as soluble antibodies (sAb), phage antibodies (pAb) or hyper-phage antibodies (hpAb). The highest test sensitivity and specificity was obtained using hpAb154 to detect all IBDV and pAb88 to differentiate vvIBDV strains. Such an ELISA was eight to 16 times more sensitive for IBDV antigen detection than the mouse monoclonal antibody ELISA. Using field samples, the scFv ELISA was able to differentiate between flocks infected with vvIBDV and those infected with classical or variant IBDV. In one instance IBDV was detected in a flock found to be negative by the monoclonal antibody ELISA. The results showed that scFv can be utilized as highly specific and sensitive ELISA reagents for the detection and discrimination of avian pathogens.

Total enteral nutrition as prophylactic therapy for pancreatic necrosis infection in severe acute pancreatitis

OBJECTIVE

To evaluate the capacity of enteral nutrition, in comparison with the total parenteral nutrition (TPN) plus antibiotic therapy, for avoiding pancreatic necrosis infection in the severe acute pancreatitis.

METHODS

In the period between October 1998 and September 2003, 87 patients met the inclusion criteria and took part in this research. Within the first week from their admission, 43 patients received TPN and 44 patients received total enteral nutrition (TEN). An adequate prophylactic antibiotic therapy was used in both groups. The severity of the manifestations was similar for both groups having a tomographic 'severity index' of 8 and an entry C-reactive protein of 208 and 203 mg/l, respectively.

RESULTS

The group that received TPN suffered an organ failure in 79% of the cases, while the percentage showed by the group that received TEN was 31%; 88 and 25% of the patients in each group requiring a surgical intervention, respectively (p < 0.001). There was decreased presence of pancreatic necrosis infection in the group of patients that was supplied with TEN (20%) than in the group receiving TPN, where it reached 74% (p < 0.001). The death rate was significantly higher among the patients who received TPN, (35%), while for the patients who received TEN it was only 5% (p < 0.001).

CONCLUSION

TEN could be used as a prophylactic therapy for infected pancreatic necrosis since it significantly diminished the necrosis infection as well as the mortality.

Recent trends in the molecular diagnosis of infectious bursal disease viruses

Infectious bursal disease virus (IBDV) causes an immunosuppressive disease in young chickens. Two serotypes of this double-stranded RNA virus exist but only serotype 1 viruses cause disease in chickens. Detection and strain identification of IBDV is important because antigenic subtypes found within serotype 1 make it necessary to tailor vaccination programs to the antigenic type found in the bird's environment. Because conventional virus isolation and characterization are not practical for routine diagnosis of IBDV, antigen-capture enzyme-linked immunosorbent assay (ELISA) and molecular assays based on reverse transcription-polymerase chain reaction (RT-PCR) technology were developed. Compared with antigen-capture ELISA, RT-PCR assays have greater versatility and are more sensitive and specific. Strain identification has been accomplished using a variety of post-RT-PCR assays, including restriction enzyme digestion of the RT-PCR products. The resulting restriction fragment length polymorphisms (RFLP) are used to differentiate viruses into molecular groups that correlate with antigenic and pathogenic types. Recently, two types of real-time RT-PCR have been used to identify and differentiate strains of IBDV. Both methods use distance-dependent interaction between two dye molecules, known as fluorescence resonance energy transfer (FRET). The dye molecules are attached to one or more nucleotide probes that detect specific nucleotide sequences of the virus. Our laboratory has used a two-probe assay to identify single-nucleotide mutations among IBDV strains. A mutation probe is used in this assay to detect substitution mutations in a region of the viral genome that encodes a neutralizing epitope of the virus. These assays are accurate, reliable and inexpensive compared with conventional RT-PCR because they do not require RFLP or other labor-intensive post-RT-PCR assays to distinguish viral strains.

Development of a One-Step Strip Test for the Diagnosis of Chicken Infectious Bursal Disease

A rapid diagnostic strip for chicken infectious bursal disease (IBD) was developed based on membrane chromatography using high-affinity monoclonal antibodies directed to chicken infectious bursal disease virus (IBDV). The diagnostic strip has high specificity for detection of chicken IBDV antigen and recognizes a variety of the virus isolates, including virulent and attenuated strains, with no cross-reactivity to other viruses, such as Newcastle disease virus, Marek's disease virus, infectious bronchitis virus, infectious laryngotracheitis virus, and egg-drop-syndrome virus. The results showed that its specificity was highly consistent with the agar-gel precipitation test (AGP). The diagnostic strip detected as low as 800 median egg lethal dose (ELD50) viruses in the IBDV BC6/85-infected sample, which was comparable with AC-ELISA (400 ELD50) and 32 times more sensitive than the AGP test (2.56 x 10(4) ELD50). In experimental infection, IBDV was detected in the bursa as early as 36 hr postinfection with the diagnostic strip before the clinical signs and gross lesions appeared. It takes only 1-2 min to do a strip test to detect chicken IBDV antigen after the specimen is grounded in a whirl pack with finger massage.

Use of multiple antigenic peptides related to antigenic determinants of infectious bursal disease virus (IBDV) for detection of anti-IBDV-specific antibody in ELISA--quantitative comparison with native antigen for their use in serodiagnosis

Multiple antigenic peptides (MAPs) prepared for the predicted antigenic determinants on the VP2 protein of infectious bursal disease virus (IBDV) were used as antigens in enzyme-linked immunosorbent assay (ELISA)--an alternative to whole viral antigen to detect anti-IBDV antibodies in the chicken sera. Two MAPs were synthesized, which could specifically detect the anti-IBDV antibodies in serum samples by ELISA. The optimum quantity of MAP1 and MAP2 required to coat the wells of the ELISA plate was 5 ng/ml, whereas the amount of purified IBDV whole viral antigen was 500 ng/ml, indicating the high efficiency of MAPs. In this study, we mainly focused on the antigenicity of two eight-branched MAPs to detect anti-IBDV antibodies in ELISA, which would serve as safe, chemically defined, noninfectious alternative antigens to whole virus in serodiagnosis. The specificity and sensitivity of both MAP1 and MAP2 were found to be relatively better than the whole viral antigen.

A non-destructive test for detection of IPNV-carriers in Atlantic halibut, Hippoglossus hippoglossus (L.)

Over 18 months after infectious pancreatic necrosis virus (IPNV) was first detected in fish (80 g-4 kg) on a halibut farm, the stocks were found to be still carrying the virus. This suggests that long-term persistence of IPNV occurs in farmed Atlantic halibut. A non-destructive test was applied to blood adherent leucocytes by placing 100 microL of whole blood collected in a heparinized tube into 96-well plates. After overnight incubation, the non-adherent cells were washed off, the remaining adherent cells lysed in a lysis buffer and inoculated onto CHSE-214 cells. The resulting cytopathic effect was confirmed as IPNV positive by enzyme-linked immunosorbent assay. In a sample of 10 fish tested by this method, all were positive for IPNV while only two were positive by the standard method for virus culture from sonicated kidney homogenates and only one fish, which was positive by the standard method, was positive by reverse transcription polymerase chain reaction on kidney tissue. The test on blood leucocytes is shown to be simple to perform on samples taken under field conditions.

A sensitive non-destructive method for detecting IPNV carrier Atlantic salmon, Salmo salar L., by culture of virus from plastic adherent blood leucocytes

In populations of Atlantic salmon in sea water, infectious pancreatic necrosis virus (IPNV) could be detected by standard virological culture methods in sonicated kidney homogenates and in mucus samples (gill, skin and rectum) from 14 and nine of 25 fish, respectively, but all fish were positive by virus culture from lysates of kidney macrophages and adherent blood leucocytes. In fish which tested negative for IPNV by the standard method of detection, the virus could be detected using adherent blood leucocytes isolated on a Percoll gradient from as little as 10 microL of blood. The blood sample could be stored for at least 3 days in a heparinized tube on ice before preparing the plastic adherent leucocytes. Furthermore, the latter could be prepared without prior fractionation on Percoll simply by incubating whole blood (33 microL) in cell culture medium (66 microL) in 96-well plates overnight and washing away the non-adherent cells before lysing the adherent cells and inoculation of the lysate onto CHSE-214 cells. This highly sensitive method for detecting IPNV-carriers is therefore very suitable for non-destructive sampling of fish in the field.

Real-time reverse transcriptase-polymerase chain reaction detection and analysis of nucleotide sequences coding for a neutralizing epitope on infectious bursal disease viruses

We used real-time reverse transcriptase (RT)-polymerase chain reaction (PCR) to detect infectious bursal disease virus (IBDV) strains. The LightCycler (Roche) and hybridization probe system (Roche, Molecular Biochemicals) were used. A mutation probe labeled with fluorescein and an anchor probe labeled with Red-640 dye were prepared for each of the STC, Del E, D78, and Bursine 2 viral sequences. The mutation probes were designed to hybridize to nucleotides that encode the hydrophilic B region of VP2 for each virus. The anchor probes were designed to a relatively conserved region immediately downstream from the mutation probes. When hybridized to the RT-PCR product, a mutation and anchor probe pair produced fluorescence resonance energy transfer that was detected by the LightCycler instrument. Because they were designed to have a lower melting temperature (Tm), the mutation probes dissociated from the template before the anchor probes. The Tm values of the four mutation probes for each of their homologous viruses (exact sequence match) were STC, 69.3 +/- 1.2 C; D78, 67.8 +/- 0.9 C; Del E, 65.5 +/- 0.6 C; and Bursine 2, 71.7 +/- 0.4 C. These values were compared with the Tm values observed for a particular probe and heterologous virus. If the Tm values observed for heterologous viruses were within two standard deviations of the Tm for the probe and its homologous virus, the nucleotide sequences of the viruses were considered to be similar. If they were below two standard deviations, they were considered to have one or more nucleotide mutations. The results indicated that the STC and Variant Vax BD viruses have similar genetic sequences at the hydrophilic B region. Likewise, Bursine 2, Bursine, Bursine+, BioBurs, BioBurs W, BioBurs AB, and IBDV Blen have similar nucleotide sequences in this region. The Tm values obtained for the D78 and Del E mutation probes with heterologous viruses indicated that none of the viruses tested had nucleotide sequences that matched these probes. Because the mutation probes were designed to bind to a region that encodes a neutralizing epitope, viruses with similar sequences were expected to have antigenically similar epitopes.

Detection of Infectious bursal disease virus by ELISA using an antipeptide antibody raised against VP3 region

Antigenic determinant analysis was carried out on VP3, one of major immunogenic proteins of Infectious bursal disease virus (IBDV) using computer algorithms. Altogether 17 peptides were synthesized for predicted putative regions and were tested for their reactivity with IBDV-positive polyclonal sera as well as with antisera to other common avian viruses to confirm specificity and to rule out cross reactivity. Of 17 peptides tested, three were selected and synthesized in multiple antigenic peptide (MAP) format. The immunization of rabbits with the three MAPs resulted in high humoral immune response. The purified antipeptide antibodies were screened against native IBDV antigen and the respective titers were determined. Out of the three antisera to MAPs that raised against the MAP3, spanning the amino acids (aa) 974-995 region on the VP3 protein had a very high titer (2048) and reacted specifically with IBDV. Thus, the antiserum to MAP3 detected native virus in enzyme-linked immunosorbent assay (ELISA), revealing the presence of a potential antigenic determinant on the C-terminus of the protein. This study proved that an antipeptide antibody could be used as a safe and specific tool for the diagnosis of IBD in chickens.

Infectious Bursal Disease Virus and Proventriculitis in Broiler Chickens

Acute necrotic proventriculitis is a naturally occurring disease of broiler chickens that causes proventricular rupture during routine evisceration. Although infectious bursal disease virus (IBDV) has been implicated, it has not been proven to be a direct cause of this disease. To further study the role of IBDV in proventriculitis, proventriculi and bursas were collected during both acute and chronic phases of naturally occurring proventriculitis and from chickens experimentally infected with seven different [BDV strains. All tissues were examined for IBDV by light microscopy, immunohistochemistry (IHC), and real time reverse transcriptase(RT)-polymerase chain reaction (PCR) and for apoptosis by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling method (TUNEL). Tissues from naturally occurring proventriculitis had bursal and proventricular lesions. Two out of four bursas had no IHC-stainable IBDV antigen or RT-PCR detectable IBDV sequences. No proventriculus had IBDV detectable by any of these methods. Bursas from chickens experimentally infected with IBDV had microscopically evident lesions, IBDV was detectable by IHC and RT-PCR, and strong IHC staining for apoptosis was present. Proventriculi from these experimentally exposed chickens had no lesions, low levels of IBDV detectable by IHC or RT-PCR, and very little IHC-stainable apoptosis. We conclude that naturally occurring proventriculitis can occur in the absence of IBDV and that the IBDV strains tested do not directly produce proventriculitis or induce increased proventricular apoptosis.

Early stages of infectious bursal disease virus infection in chickens detected by in situ reverse transcriptase-polymerase chain reaction

Two infectious bursal disease virus (IBDV) strains were inoculated both intranasally and by eye drop into 5-week-old specific pathogen free chickens. The bursa, the liver, the kidney, the spleen, the thymus, the caecal tonsil and the thigh muscle were harvested at 4, 8, 16, 28, 40, 56, 72, 96 h post-inoculation (p.i.) for IBDV detection by in situ reverse transcriptase-polymerase chain reaction and, at the same time, the pathological changes in these tissues were investigated. A typical positive signal was detected in the liver, the kidney and the spleen of chickens inoculated with the very virulent IBDV H strain at 4 h p.i., but not in the thymus, the caecal tonsil or the thigh muscle until 8 h p.i. Virus was also found in the liver, the spleen, the kidney, the thymus, the caecal tonsils and the muscle of birds inoculated with the cell-adapted Ts strain at 4 h p.i. A positive signal was observed in the bursa later than in the other tissues. The signals increased markedly at 8 h p.i. A decrease in bursal lymphocytes was observed in haematoxylin and eosin stained sections at 28 h p.i. for the H strain and at 40 h p.i. for the Ts strain.

Evaluation of the immune response and detection of infectious bursal disease viruses by reverse transcriptase-polymerase chain reaction and enzyme-linked immunosorbent assay after in ovo vaccination of commercial broilers

Maternal immunity can interfere with infectious bursal disease virus (IBDV) vaccine efficacy. The effect of maternal antibody (MacAb) on the immune response and detection of two vaccine viruses, an IBDV immune complex (cx) and IBDV-2512, was investigated. Vaccines were administered in ovo at 100 mean embryo infectious dose to commerical broiler embryos derived from young (29 wk) or old (63 wk) flocks. Chickens with low MatAb were challenged with an antigenic standard IBDV strain at 21 and 35 days post in ovo vaccination (PIOV). At 28 and 42 days PIOV, birds were euthanatized and bursa weight: body weight ratios were determined. MatAb of progeny from the 29-wk-old flock was higher (P < 0.05) than that from progeny of the 63-wk-old flock. Progeny titers declined by day 21 PIOV. Birds with MatAb vaccinated with either the IBDV-2512 or IBDV-Icx vaccine did not mount an antibody response by 21 days PIOV. The IBDV-Icx vaccination protected chickens against bursal atrophy when they were challenged at 21 and 35 days PIOV (83% and 77%, respectively). Resistance to challenge in the absence of antibody implies that cellular immunity plays a role in IBDV protection. The IBDV-2512 vaccine caused atrophy of the bursae, and, therefore, we could not examine its protection by these criteria. Neither vaccine could be detected by antigen-capture chemiluminescent enzyme-linked immunosorbent assay. Therefore, total RNA was extracted and reverse transcription (RT)-polymerase chain reaction (PCR) and nested PCR were conducted to amplify the VP2 (hypervariable) region on viral RNA collected at 3, 6, 9, 15, and 21 days PIOV. Nested PCR detected the VP2 region of both IBDV vaccine viruses at day 3 PIOV. Only IBDV-2512 RNA was detected on day 6 PIOV. However, the IBDV-Icx RNA was detected on days 9 and 15 PIOV but not on day 21 PIOV. The IBDV-Icx RNA was evident in tissues and could induce protection against challenge. This work gives further insight into the mechanism of the IBDV-Icx vaccine.

The value of tissue imprint hybridization for rapid detection of infectious bursal disease virus from field outbreaks

The use of a peroxidase labelled PCR generated probe followed by enhanced chemiluminescence hybridization assay detected infectious bursal disease virus directly from bursal imprints on a nylon membrane. Tissue imprint hybridization proved to be a simple, rapid and safe means of detecting IBD virus for screening large numbers of field samples. The PCR generated probe was highly specific for IBD virus and did not hybridize with cellular nucleic acids in control imprints. Tissue imprint hybridization was found to be a more sensitive method than conventional antigen detection assays.

Detection of infectious bursal disease virus in different lymphoid organs by single-step reverse transcription polymerase chain reaction and microplate hybridization assay

A rapid and sensitive method for the detection of infectious bursal disease virus (IBDV) RNA in different chicken lymphoid organs was developed. The method is based on a single-step reverse transcription polymerase chain reaction (RT-PCR) procedure and the enzyme-linked immunosorbent assay (ELISA) detection method of amplified products. Vaccinal IBDV strain and field isolates were used for the optimization of RT-PCR and for the determination of conditions for microplate hybridization and colorimetric detection of the amplicons. With this method, viral RNA could be detected in various stages of infection in samples of different lymphoid organs. Bursas and cecal tonsils were suitable organs for viral RNA detection at different times during IBDV infection. The RT-PCR/ELISA method can be applied for IBDV detection in routine diagnostic tests, which are not usually carried out because of the difficulties involved in isolating the virus.

Pathogenesis and tissue distribution of a variant strain of infectious bursal disease virus in commercial broiler chickens

The detection of either infectious virus, viral antigen, and/or viral RNA in different tissues of commercial broilers inoculated at 1 day of age with E/Del variant strain of infectious bursal disease virus (IBDV) was investigated at 2, 4, and 6 wk postinoculation (PI). Virus was readily isolated from homogenates of bursa, cecal tonsils, and bone marrow at 2 and 4 wk PI. Virus isolation coupled with immunoperoxidase assay or reverse transcription-polymerase chain reaction for IBDV-specific RNA extended the window of IBDV detection in the bursa of Fabricius to 6 wk PI. Serology indicated an active early virus infection; however, viral pathology was observed later and beginning at 4 wk PI. This study indicates that variant strains of IBDV may be present in commercial broilers longer than previously thought, and cecal tonsils and bone marrow may serve as nonbursal lymphoid tissues supporting virus replication at later time points PI.

Comparative evaluation of five serological methods and RT-PCR assay for the detection of IPNV in fish

In the present study, six diagnostic methods for the detection of infectious pancreatic necrosis virus (IPNV) (indirect immunofluorescence, flow cytometry, immunoperoxidase, immunodot blot, immunostaphylococcus-protein A, and RT-PCR) have been comparatively evaluated using the seroneutralization as the reference assay, and 83 Spanish isolates and 3 reference strains. The most reliable methods were flow cytometry and RT-PCR which could detect virus at titers of 1x10(2) and 1x10(3) TCID50/ml, respectively. At a multiplicity of infection of 50, both assays allowed the earliest detection of IPNV at 4 h post-inoculation. Indirect immunofluorescence and immunoperoxidase assays required at least 6 h post-inoculation to detect viral antigens. The immunodot blot assay possesses low sensitivity and the immunostaphylococcus-protein A test cannot be applied for routine examination of IPNV. Positive reactions were obtained in 100% of the samples tested by seroneutralization and RT-PCR, 90.4% by the flow cytometry, 80.7% by the indirect immunofluorescence assay, 67.5% by the immunoperoxidase, 62.6% by the immunodot blot, and only 27.7% by immunostaphylococcus-protein A test. Therefore, RT-PCR and flow cytometry were the most appropriate and sensitive methods for the routine detection of IPNV from affected fish.

One-step RT-PCR for the detection of infectious bursal disease virus in clinical samples

A single-tube, non-interrupted, one-step RT-PCR has been standardized to amplify the hypervariable region of the VP2 gene sequence of infectious bursal disease virus (IBDV). The technique standardized on purified viral RNA was successfully applied to the detection of the virus directly in clinical samples. The amplified products were confirmed to be IBDV specific by their size in ethidium bromide-stained agarose gel, nested PCR and restriction enzyme digestion. Digestion of the amplicons with StyI restriction enzyme also differentiated classical virus from six very virulent field isolates. The sensitivity of the one-step RT-PCR was found to be 0.2 pg of viral RNA.

Infectious bursal disease (Gumboro disease)

Infectious bursal disease (IBD) (Gumboro disease) has been described throughout the world, and the socio-economic significance of the disease is considerable world-wide. Various forms of the disease have been described, but typing remains unclear, since antigenic and pathotypic criteria are used indiscriminately, and the true incidence of different types is difficult to determine. Moreover, the infection, when not fatal, leads to a degree of immunosuppression which is often difficult to measure. Finally, the control measures used are subject to variations, and seldom follow a specific or standardised plan. In the context of expanding international trade, the authors provide an overview of existing knowledge on the subject to enhance available information on the epidemiology of IBD, the identification of reliable viral markers for diagnosis, and the implementation of specific control measures to ensure a global and co-ordinated approach to the disease.

Antigenic properties and diagnostic potential of baculovirus-expressed infectious bursal disease virus proteins VPX and VP3

The routine technique for detecting antibodies specific to infectious bursal disease virus (IBDV) is a serological evaluation by enzyme-linked immunosorbent assay (ELISA) with preparations of whole virions as the antigens. To avoid using complete virus in the standard technique, we have developed two new antigens through the expression of the VPX and VP3 genes in insect cells. VPX and especially VP3 were expressed at high levels in insect cells and simple to purify. The immunogenicity of both proteins was similar to that of the native virus. VPX was able to elicit neutralizing antibodies but VP3 was not. Purified VPX and VP3 were tested in an indirect ELISA with more than 300 chicken sera. There was an excellent correlation between the results of the ELISA using VPX and those of the two commercial kits. VP3 did not perform as well as VPX, and the linear correlation was significantly lower. A comparison with the standard reference technique, seroneutralization, showed that the indirect ELISA was more sensitive. Therefore, VPX-based ELISA is a good alternative to conventional ELISAs that use whole virions.

Rocket immunoelectrophoresis in the diagnosis of infectious bursal disease

The rocket immunoelectrophoresis (RIE) test was used for the qualitative detection and quantitative estimation of infectious bursal disease virus (IBDV) specific antigen in experimentally infected chickens and samples collected from suspected outbreaks. The IBDV specific antigen was detected in the bursae of experimentally inoculated chickens up to 5 days post infection (PI) by the agar gel precipitation (AGP) test and 7 days PI by the RIE test. The RIE detected IBDV specific antigen in a significantly greater number of samples collected from the field outbreaks than the conventional AGP test. Exudative bursae were found to have a higher antigen content than haemorrhagic bursae and are recommended as the material of choice for diagnosis of IBD. This test could also be used to quantify IBDV specific antigen in commercial killed vaccines.

Tissue print hybridisation and reverse transcriptase PCR in the detection of infectious bursal disease viruses in bursal tissues

The genome segments of infectious bursal disease viruses (IBDV) in the bursa of Fabricius from experimently infected chickens or field samples were detected by tissue print hybridization (TPH) with subsequent reverse transcriptase (RT)- polymerase chain reaction (PCR). Bursae were imprinted onto nylon membrane and then hybridized with a cloned digoxigenin (DIG)-labeled cDNA probe. Tissue prints on nylon membrane were readily distinguished from control prints by color development and differences in signal intensity. In order to verify the TPH test, RT - PCR was used to amplify a 643-base pair fragment on the VP 2 gene of IBDV in the bursa of Fabricius. With all isolates, a c DNA fragment of 643 bp long was generated as expected and further confirmed the specificity of TPH. Our results suggest that a large number of field samples or selected tissues can be rapidly examined by TPH technique when combined with a cloned DIG -labeled c DNA probe.

Multiplex Reverse Transcriptase PCR Assay for Simultaneous Detection of Three Fish Viruses

A multiplex reverse transcriptase (RT)-PCR assay was developed for the simultaneous detection of three different fish viruses: infectious pancreatic necrosis virus (IPNV), infectious hematopoietic necrosis virus (IHNV), and viral hemorrhagic septicemia virus (VHSV). The sensitivity levels of the multiplex RT-PCR assay were 100, 1, and 32 50% tissue culture infective doses/ml for IPNV, IHNV, and VHSV, respectively.

Development of a ssRNA internal control reagent for an infectious bursal disease virus reverse transcription/polymerase chain reaction-restriction fragment length polymorphism diagnostic assay

Infectious bursal disease virus (IBDV), family Birnaviradae, is the etiologic agent of a commercially important, globally distributed, contagious, immunosuppressive disease of young chickens. A restriction enzyme-compatible ssRNA internal control was developed for an IBDV reverse transcription/polymerase chain reaction-restriction fragment length polymorphism (RT/PCR-RFLP) diagnostic assay. An 841-bp bacteriophage-lambda DNA fragment was directionally ligated to 3' and 5' oligonucleotide linkers containing the IBDV RT/PCR target primer sequences. A pGEM-3Zf (+) transcription vector containing the internal control construct was used in an in vitro transcription reaction to produce ssRNA. After RT and PCR amplification, the transcripts produced an 882-bp cDNA product, larger than, co-amplifiable with, and free of the restriction sites used to prepare RFLP patterns of the 743-bp IBDV cDNA target product. The limit of detection of the transcripts in the RT/PCR test is 3.2 femtograms. With the internal control, a test inhibition rate of 7.7% (20/261) was determined for the IBDV RT/PCR assay. By identifying inhibited tests, the assay was improved through a reduction in the number of false-negative results.