Serologic diagnosis of caprine arthritis-encephalitis by ELISA with two recombinant proteins in a parallel testing format

The Prevalence of Histopathological Features of Pneumonia in Goats with Symptomatic Caprine Arthritis-Encephalitis

Chronic interstitial pneumonia (CIP) is a main pathology of sheep infected with small ruminant lentivirus (SRLV). Caprine arthritis-encephalitis (CAE) is caused by the same pathogen; however, the presence of CIP has been only occasionally reported in SRLV-infected goats. We carried out a cross-sectional study to determine the prevalence of histopathological lesions indicative of CIP in goats with symptomatic CAE, and to investigate whether CIP was associated with a higher prevalence of other types of pneumonia (purulent bronchopneumonia, fibrinous pleuropneumonia) or bacterial infections. Lung specimens and bronchial swabs were collected for histopathological and bacteriological examination, respectively, from 116 goats from a CAE-affected herd. All goats were euthanized due to severe clinical signs of CAE. The goats were seropositive for SRLV infection in two different ELISAs and the presence of SRLV antigen in the lung tissue was confirmed by immunohistochemistry. Histopathologically, pneumonia of any type was confirmed in 82 goats (70.7%) and CIP was present in 67 goats (57.8%). In most goats, the severity of the histopathological features of pneumonia was mild. Bacteria were detected in bronchial swabs from 73 goats (62.9%). CIP proved to be significantly positively linked to the occurrence of purulent bronchopneumonia (p < 0.001), fibrinous pleuropneumonia (p = 0.001), and of the infection of lungs with bacteria capable of causing pneumonia (p = 0.050). The causal character of these associations should be considered and warrants further investigation.

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.

The effect of the subclinical small ruminant lentivirus infection of female goats on the growth of kids

A longitudinal observational study was carried out to evaluate the influence of prenatal exposure to small ruminant lentivirus(SRLV)-infected does on the body weight (BWT) of young kids. The study was carried out in years 2001–2017 in the research dairy goat herd. Goats in the herd were regularly serologically tested and individuals showing clinical signs of caprine arthritis-encephalitis (CAE) were promptly culled. As a result all goats enrolled in the study were asymptomatic. Moreover, kids were weaned immediately after birth, fed on bovine colostrum and kept in strict separation from mothers to prevent SRLV lactogenic transmission. Kids were weighed immediately after birth, and then 1–3 times within the first 3 months of life. In total 620 goat kids were weighed at least once, excluding weighing at birth, providing 992 BWT records. The mixed linear model including four variables fitted as random effects (doe, kid, the year of kid’s birth and the exact age of a kid at weighing) and four potential confounders fitted as fixed effects (parity, kid’s sex, litter size and birth body weight) was developed and showed that BWT was not significantly associated with SRLV serological status of a doe, regardless of the time for which does had been infected before the delivery of the kid (p = 0.242). This study provides strong evidence that kids born to SRLV-infected does grow equally well as kids from uninfected does, provided that the lactogenic viral transmission is prevented by maintaining strict separation between the offspring and mothers. This observation is important for choosing the most optimal strategy of CAE control in a goat herd.

Impact of the subclinical small ruminant lentivirus infection of female goats on the litter size and the birth body weight of kids

A longitudinal study was conducted in a single dairy goat herd to investigate the relationship between subclinical small ruminant lentivirus (SRLV) infection in does and litter size (LS) or birth body weight of kids (BW). Each year kids born to seropositive and seronegative does were weighed before the first consumption of colostrum. LS and BW of each kid were recorded. BW was significantly negatively linked to LS (p = 0.006) - singletons weighed (mean ± SD) 4.20 ± 0.67 kg, twins - 3.75 ± 0.62 kg, and triplets and quadruplets - 3.38 ± 0.47 kg. Male kids were significantly heavier than female kids in twin litters (3.97 ± 0.53 kg vs. 3.52 ± 0.60 kg; p < 0.001) and triplet or quadruplet litters (3.62 ± 0.40 kg vs. 3.17 ± 0.43 kg; p < 0.001). However, BW of male and female kids from singleton litters did not differ (4.31 ± 0.71 kg vs. 4.07 ± 0.65 kg; p = 0.154). Then, two mixed models were developed to assess the relationship between LS (mixed Poisson log linear regression model) or BW (mixed linear model) and SRLV infection in the doe, controlling for potential confounders such as the effect of an individual doe, year in which the parturition took place, parity and kid's sex. Neither LS nor BW proved to be significantly associated with SRLV infection (p = 0.788 and p = 0.214, respectively). On this basis it was concluded that LS and BW were not affected by the subclinical SRLV infection of a doe.

Decline of maternal antibodies to small ruminant lentivirus in goat kids

We carried out this study to determine for how long small ruminant lentivirus (SRLV)-specific antibodies can be detected by three commercial ELISA kits in goat kids after suckling infected does in field conditions. Forty-one kids born to SRLV-seropositive asymptomatic does were blood sampled prior to colostrum consumption, and then weekly for 6 months in total. The sera were screened with three commercial ELISA kits: whole-virus ELISA (wELISA), recombinant transmembrane and capsid antigen ELISA (TM/CA-ELISA), and surface antigen ELISA (SU-ELISA). All but one kid were seronegative in all three ELISAs right after birth. At the age of 1 week all kids turned seropositive in wELISA, 39 kids (95%) in TM/CA-ELISA, and 35 kids (85%) in SU-ELISA. All seropositive kids turned seronegative in wELISA by the 15th week, and in SU-ELISA by the 19th week (median of 8 weeks in both ELISA), whereas in TM/CA-ELISA five kids (13% of 39 initially seropositive) were still seropositive at the age of 6 months (median of 11 weeks). Antibody levels at the age of 1 week proved significantly linked to the duration of maternal antibodies in all three ELISAs and could be employed to predict for how long maternal antibodies would remain detectable.

Use of two commercial caprine arthritis-encephalitis immunoenzymatic assays for screening of arthritic goats

Roughly one-fourth of goats infected with small ruminant lentivirus (SRLV) develop caprine arthritis-encephalitis (CAE). We compared the profile of antibody response to surface glycoprotein (SU), and combined transmembrane glycoprotein and capsid protein (TM/CA) in SRLV-infected arthritic and asymptomatic goats, and determined the ability of 2 commercial ELISAs to distinguish between arthritic and asymptomatic goats. We used sera from 312 SRLV-seropositive dairy goats in a whole-virus ELISA; 222 were collected from arthritic goats and 90 from apparently healthy goats. Sera were screened with a competitive inhibition ELISA based on SU antigen (SU-ELISA) and an indirect ELISA based on TM and CA antigens (TM/CA-ELISA). Receiver operating characteristic (ROC) curves were prepared for both ELISAs, and areas under the ROC curves (AUC) were compared. The proportion of goats with antibody response stronger to SU antigen than to TM/CA antigen was significantly higher among arthritic than asymptomatic goats (58.1% vs. 28.9%; p < 0.001). Antibody response to SU antigen was a good predictor of the arthritic form of CAE: AUC for SU-ELISA was 89.7% (95% CI: 85.2%, 94.2%), compared to 59.3% (95% CI: 51.9%, 66.8%) for TM/CA-ELISA ( p < 0.001). With the cutoff set at percentage of inhibition of 56%, SU-ELISA had sensitivity of 86.9% (95% CI: 81.9%, 90.7%) and specificity of 84.4% (95% CI: 75.6%, 90.5%) in discriminating between arthritic and asymptomatic goats.

Influence of true within-herd prevalence of small ruminant lentivirus infection in goats on agreement between serological immunoenzymatic tests

The study was conducted to evaluate influence of the true within-herd prevalence of small ruminant lentivirus (SRLV) infection on agreement beyond chance between three different types of commercial serological ELISAs. Blood samples were collected from 865 goats from 12 dairy goat herds. Serum samples were tested using three commercial ELISA kits: whole-virus indirect ELISA (wELISA), indirect ELISA based on recombined TM and CA antigens (TM/CA-ELISA), and competitive-inhibition ELISA based on SU antigen (SU-ELISA). Herds were classed into three prevalence strata of high (>50%), moderate (10-50%) and low (<10%) true within-herd prevalence of SRLV infection. The latter was estimated on the basis of results of wELISA adjusted by its sensitivity and specificity. Agreement beyond chance between the three ELISAs was assessed at two levels. First, the general agreement was determined using two coefficients corrected for chance-agreement: Cohen's kappa and Gwet's AC₁. Then, agreement between tests was evaluated using Gwet's AC₁ separately in the three prevalence strata and compared between them by computing 95% confidence intervals for differences with a Bonferroni correction for multiple comparisons. The general agreement between the three tests was very good: wELISA and TM/CA-ELISA - Cohen's kappa of 81.8% (CI 95%: 77.9% to 85.7%), Gwet's AC₁ of 82.7% (CI 95%: 79.0% to 86.4%); wELISA and SU-ELISA - Cohen's kappa of 83.2% (CI 95%: 79.4% to 86.9%), Gwet's AC₁ of 83.9% (CI 95%: 80.4% to 87.5%); TM/CA-ELISA and SU-ELISA - Cohen's kappa of 86.0% (CI 95%: 82.6% to 89.5%), Gwet's AC₁ of 86.9% (CI 95%: 83.6% to 90.1%). However, agreement between ELISAs was significantly related to the within-herd true prevalence - it was significantly lower (although still high) when within-herd true prevalence was moderate (Gwet's AC₁ between 67.2% and 78.7%), whereas remained very high, when true within-herd prevalence was either >50% (Gwet's AC₁ between 91.9% and 98.8%) or <10% (Gwet's AC₁ between 94.7% and 98.4%). Concluding, the three different commercial ELISAs for SRLV infection in goats available on the market yield highly consistent results. However, their agreement is affected by the true within-herd prevalence in a tested population, and the worse (although still high) agreement should be expected, when the percentage of infected goats is moderate.

Diagnostic performance of ID Screen® MVV-CAEV Indirect Screening ELISA in identifying small ruminant lentiviruses-infected goats

Diagnostic performance of ID Screen®MVV-CAEV Indirect Screening ELISA in identifying goats infected with small ruminant lentiviruses (SRLV) was evaluated. In total 299 serum samples from the collection of the Laboratory of Veterinary Epidemiology and Economics - 109 truly positive and 190 truly negative - were used. To be enrolled in the study a serum sample had to come from at least 2 year-old goat which had reacted identically in two serological surveys preceding sample collection and was kept in a herd of stable serological status confirmed at least twice during preceding 5 years. Moreover, in seropositive herds at least 20% of goats had to be serologically positive at the moment when the serum sample was collected for the study. The test proved to have high accuracy. Area under curve was 98.8% (95% CI: 97.5%, 100%). Diagnostic performance of the test was almost identical (Youlden’s index of 90%, sensitivity >90% and specificity >95%) within a fairly wide range of cut-off values - between 20% and 60%. At manufacturer’s cut-off of 50% sensitivity and specificity were 91.7% (95% CI: 85.0%, 95.6%) and 98.9% (95% CI: 96.2%, 99.7%), respectively. For this cut-off positive likelihood ratio was 87 (95% CI: 22, 346) and negative likelihood ratio was 0.08 (95% CI: 0.04, 0.16). In conclusion, the results of this study indicate that ID Screen®MVV-CAEV Indirect Screening ELISA is a highly accurate diagnostic test for SRLV infection.

Diagnostic tests for small ruminant lentiviruses

Maedi visna virus and caprine arthritis encephalitis virus are closely related retroviruses that cause chronic inflammatory disease in small ruminants. The infections are characterised by insidious onset and slow progression. Diagnosis of infection is usually by serological testing. A variety of assays are available for this purpose, though the relative sensitivity and specificity of these assays has not been compared systematically. Here we review recent developments in laboratory diagnostic methods and their use in field diagnosis. The results suggest that a combination of ELISA and PCR might afford optimal detection of SRLV infection.

Simple and rapid method for production of whole-virus antigen for serodiagnosis of caprine arthritis-encephalitis virus by enzyme-linked immunosorbent assay

Polyethylene glycol (PEG) was used to produce whole-virus antigen derived from tissue culture cells infected with a Canadian strain of caprine arthritis-encephalitis virus. PEG antigen batches were obtained after precipitation and concentration of infected tissue culture material with PEG 8000 and final treatment with sodium dodecyl sulfate. The optimum time of harvest of tissue culture extracted material to produce the maximum amount of viral proteins was determined in roller bottles, after cocultivation of infected and noninfected fetal lamb corneal cells. Samples from day 9 to day 25 postculture were collected and processed. By Western blotting, the optimum time of harvest was found to be day 25 following the coculture. Two large batches of PEG antigen were prepared at the optimum time of harvest. Both batches gave similar results when tested by Western blotting and enzyme-linked immunosorbent assay (ELISA), using reference control sera from infected and noninfected goats. For further testing in ELISA, cutoff values and ratios were determined for PEG batch 1, using 200 known serum samples from goats free of the disease. The PEG antigen batch was compared with an in-house ELISA antigen in a kinetic mode, using 498 serum samples from field goats. The in-house ELISA antigen was produced following two rounds of ultracentrifugation and treatment with sodium dodecyl sulfate (R. A. Heckert, W. B. McNab, S. M. Richardson, and M. R. Briscoe, Can. J. Vet. Res. 56:237-241, 1992). The PEG antigen batch was found suitable for ELISA, with a relative specificity of 100% and a relative sensitivity of 99.4% compared to the in-house ELISA antigen. This method of antigen production for ELISA was found to be rapid, inexpensive, and reliable for the diagnosis of caprine-arthritis encephalitis, without requiring the use of sophisticated laboratory equipment.

Recombinant Jembrana disease virus proteins as antigens for the detection of antibody to bovine lentiviruses

Jembrana disease virus (JDV) is a recently identified bovine lentivirus causing an acute severe disease syndrome in banteng cattle (Bos javanicus) and a milder disease syndrome in Bos taurus cattle in Indonesia. The virus is closely related genetically to the previously identified bovine lentivirus, bovine immunodeficiency virus (BIV). Recombinant clones were produced which contained the capsid (CA) and transmembrane (TM) subunits of the respective gag and env open reading frames of JDV. The proteins were expressed as fusions to the glutathione-s-transferase (GST) enzyme in Escherichia coli and purification was achieved using affinity chromatography via immobilized reduced glutathione. The soluble recombinant CA and TM antigens of JDV were reacted in western immunoblots with both serum antibodies from JDV-infected Bos javanicus cattle and Bos taurus cattle immunized with BIV. The recombinant CA protein of JDV reacted equally well with both the JDV and BIV antisera. The recombinant TM protein of JDV also reacted with antibody from the JDV infected cattle and with the BIV antisera. The results indicated conservation of immunogenic epitopes of the CA and TM proteins of the two viruses. The production of the recombinant proteins should enable the development of rapid and sensitive serological tests for JDV and BIV, and tools for further study of the immune response to JDV and the differential epidemiology of JDV infections in cattle.