The Scrapie Prevalence in a Goat Herd Is Underestimated by Using a Rapid Diagnostic Test

Abstract

Current European surveillance regulations for scrapie, a naturally occurring transmissible spongiform encephalopathy (TSE) or prion disease in sheep and goats, require testing of fallen stock or healthy slaughter animals, and outline measures in the case of confirmation of disease. An outbreak of classical scrapie in a herd with 2500 goats led to the culling of the whole herd, providing the opportunity to examine a subset of goats, take samples, and examine them for the presence of disease-associated prion protein (PrP Sc) to provide further information on scrapie test sensitivity, pathology, and association with prion protein genotype. Goats were examined clinically prior to cull, and the brains examined post mortem by Bio-Rad ELISA, a rapid screening test used for active surveillance in sheep and goats, and two confirmatory tests, Western blot and immunohistochemistry. Furthermore, up to 10 lymphoid tissues were examined by immunohistochemistry. Of 151 goats examined, three (2.0%) tested positive for scrapie by ELISA on brain, confirmed by confirmatory tests, and a further five (3.3%) were negative by ELISA but positive by at least one of the confirmatory tests. Only two of these, both positive by ELISA, displayed evident signs of scrapie. In addition, 10 (6.6%) goats, which also included two clinical suspects, were negative on brain examination but had detectable PrP Sc in lymphoid tissue. PrP Sc was detected most frequently in the medial retropharyngeal lymph node (LN; 94.4% of all 18 cases) and palatine tonsil (88.9%). Abnormal behavior and circling or loss of balance when blindfolded were the best clinical discriminators for scrapie status. None of the goats that carried a single allele in the prion protein gene associated with increased resistance to scrapie (Q 211 , K 222 , S 146) were scrapie-positive, and the percentage of goats with these alleles was greater than expected from previous surveys. Significantly more goats that were scrapiepositive were isoleucine homozygous at codon 142 (II 142). The results indicate that the sensitivity of the applied screening test is poor in goats compared to the confirmatory tests as gold standard, particularly for asymptomatic animals. Sensitivity of surveillance could be improved by testing retropharyngeal LN or palatine tonsil in addition to brain.

Classical Scrapie Did Not Re-occur in Goats After Cleaning and Disinfection of the Farm Premises

“…If re-infection has in fact re-occurred to a very low level, a larger sample size would have been required. We assumed a test sensitivity of 97.4% using IHC for brain and RPLN based on the only study available at the time that used comparable tests ( 17 ), although it may be less (94.4%) as reported for a more recent study using the same methods ( 15 ). It would have been preferable to test predominantly fallen stock, which represent the population most likely to have scrapie confirmed.…”

“…There was no evidence, however, that infection had since re-occurred in the new population. Goats in an earlier stage of disease should have been identified by examination of RPLN, which was the most frequently affected lymph node in studies of naturally occurring classical scrapie in goats where multiple lymph nodes were examined ( 14 , 15 ). Some goats may have detectable PrP Sc only in brain without evidence of peripheral distribution, which has been demonstrated in four goats on this farm at the time of herd cull ( 30 ), but these goats were relatively young with a median age of 44.5 months, so should have been detected in the present study if it occurred.…”

“…As confirmatory tests were superior in their diagnostic sensitivity compared to the rapid TSE test currently used in the UK ( 14 , 15 ), only immunohistochemistry (IHC) to detect scrapie-associated prion protein (PrP Sc ) was used. The following tissues were collected from euthanized goats for examination by IHC: retropharyngeal lymph node (RPLN), obex and cerebellum.…”

“…The lymph node from one side, half of the obex cut sagittally and the whole cerebellum were fixed in 10% buffered formalin. Tissue processing and immunolabeling with monoclonal antibody R145 (APHA Weybridge, New Haw, UK) for IHC was carried out as described previously ( 15 ).…”

Annual Report of the Scientific Network on BSE‐TSE 2020

“…John Spiropoulos, senior veterinary pathologist of the Animal and Plant Health Agency (APHA) (UK), invited speaker, presented the results of two recently published scientific articles of interest, of which he is co-author: 'Transmissible spongiform encephalopathy in goats: is PrP rapid test sensitivity affected by genotype?' (Simmons et al, 2020), and 'The Scrapie Prevalence in a Goat Herd Is Underestimated by Using a Rapid Diagnostic Test' (Konold et al, 2020). The sensitivity of the Rapid test for the detection of TSE in goats (scrapie and BSE) is not 100%, and is influenced by the polymorphisms in certain codons which may alter epitope binding, the rapid test chosen, sampling issues and other still unknown factors.…”