Evaluation of two commercial, rapid, ELISA kits testing for scrapie in retro-pharyngeal lymph nodes in sheep

No evidence of uptake or propagation of reindeer CWD prions in environmentally exposed sheep

BACKGROUND

Chronic wasting disease (CWD) is a prion disease of cervids first reported in North America in the 1960s. In Europe, CWD was first diagnosed in 2016 in a wild reindeer in Norway. Detection of two more cases in the same mountain area led to the complete culling of this partially confined reindeer population of about 2400 animals. A total of 19 CWD positive animals were identified. The affected area is extensively used for the grazing of sheep during summers. There are many mineral licks intended for sheep in the area, but these have also been used by reindeer. This overlap in area use raised concerns for cross-species prion transmission between reindeer and sheep. In this study, we have used global positioning system (GPS) data from sheep and reindeer, including tracking one of the CWD positive reindeer, to investigate spatial and time-relevant overlaps between these two species. Since prions can accumulate in lymphoid follicles following oral uptake, samples of gut-associated lymphoid tissue (GALT) from 425 lambs and 78 adult sheep, which had grazed in the region during the relevant timeframe, were analyzed for the presence of prions. The recto-anal mucosa associated lymphoid tissue (RAMALT) from all the animals were examined by histology, immunohistochemistry (IHC) and enzyme-linked immunosorbent assay (ELISA), and the ileal Peyer's patch (IPP) from a subsample of 37 lambs were examined by histology and IHC, for the detection of prions.

RESULTS

GPS data showed an overlap in area use between the infected reindeer herd and the sheep. In addition, the GPS positions of an infected reindeer and some of the sampled sheep showed temporospatial overlap. No prions were detected in the GALT of the investigated sheep even though the mean lymphoid follicle number in RAMALT and IPP samples were high.

CONCLUSION

The absence of prions in the GALT of sheep that have shared pasture with CWD-infected reindeer, may suggest that transmission of this novel CWD strain to sheep does not easily occur under the conditions found in these mountains. We document that the lymphoid follicle rich RAMALT could be a useful tool to screen for prions in sheep.

Transmissible spongiform encephalopathy in goats: is PrP rapid test sensitivity affected by genotype?

Transmissible spongiform encephalopathy (TSE) surveillance in goats relies on tests initially approved for cattle, subsequently assessed for sheep, and approval extrapolated for use in "small ruminants." The current EU-approved immunodetection tests employ antibodies against various epitopes of the prion protein PrPSc, which is encoded by the host PRNP gene. The caprine PRNP gene is polymorphic, mostly at codons different from the ovine PRNP. The EU goat population is much more heterogeneous than the sheep population, with more PRNP-related polymorphisms, and with marked breed-related differences. The ability of the current tests to detect disease-specific PrPSc generated against these different genetic backgrounds is currently assumed, rather than proven. We examined whether common polymorphisms within the goat PRNP gene might have any adverse effect on the relative performance of EU-approved rapid tests. The sample panel comprised goats from the UK, Cyprus, France, and Italy, with either experimental or naturally acquired scrapie at both the preclinical and/or unknown and clinical stages of disease. Test sensitivity was significantly lower and more variable when compared using samples from animals that were preclinical or of unknown status. However, all of the rapid tests included in our study were able to correctly identify all samples from animals in the clinical stages of disease, apart from samples from animals polymorphic for serine or aspartic acid at codon 146, in which the performance of the Bio-Rad tests was profoundly affected. Our data show that some polymorphisms may adversely affect one test and not another, as well as underline the dangers of extrapolating from other species.