Evidence of effective scrapie transmission via colostrum and milk in sheep

Abstract

Background: Evidence for scrapie transmission from VRQ/VRQ ewes to lambs via milk was first reported in 2008 but in that study there were concerns that lateral transmission may have contributed to the high transmission rate observed since five control lambs housed with the milk recipients also became infected. This report provides further information obtained from two follow-up studies, one where milk recipients were housed separately after milk consumption to confirm the validity of the high scrapie transmission rate via milk and the second to assess any difference in infectivity from colostrum and subsequent milk. Protein misfolding cyclic amplification (PMCA) was also used to detect prion protein in milk samples as a comparison with the infectivity data and extended to milk samples from ewes without a VRQ allele.

Results: Seven pairs of lambs fed colostrum and milk individually from seven scrapie-affected sheep (pre-clinical or clinical) presented with disease-associated prion protein, PrPd, in rectal lymphoid tissue at 4-5 months of age. Five further pairs of lambs fed either colostrum or subsequent milk from five pre-clinical scrapie-affected sheep equally presented with PrPd in lymphoid tissue by 9 months of age. Nine sheep were lost due to intercurrent diseases but all remaining milk or colostrum recipients, including those in the original study with the lateral transmission controls, developed clinical signs of scrapie from 19 months of age and scrapie was confirmed by brain examination. Unexposed control sheep totalling 19 across all three studies showed no evidence of infection.Scrapie PrP was amplified repeatedly by PMCA in all tested milk samples from scrapie-affected VRQ/VRQ sheep, and in one scrapie-affected ARQ/ARQ sheep. By contrast, milk samples from five VRQ/VRQ and 11 ARQ/ARQ scrapie-free sheep did not have detectable scrapie PrP on repeated tests.

Conclusions: Feeding of milk from scrapie-affected sheep results in a high transmission rate in VRQ/VRQ sheep and both colostrum and milk transmit scrapie. Detection of scrapie prion protein in individual milk samples from scrapie-affected ewes confirms PMCA as a valuable in vitro test.

Figure 1

Incubation periods in days of scrapie milk donor, milk recipient sheep and lateral transmission controls. The median incubation periods for milk donors were 808.5 (range: 683–1436) days in Study 1, 657 (623–679) days in Study 2 and 666 (639–784) days in Study 3. The median incubation periods for milk recipients were 713 (588–816) days in Study 1, 729 (682–755) in Study 2 and 696.5 (694–738) days in Study 3. Lateral transmission controls in Study 1 had a median incubation period of 889 (range: 746–1095) days and colostrum recipients in Study 3 730 (693–749) days.

Figure 2

PrP^d accumulation in colostrum recipient 09–1447 and milk recipient 09–1426 in Study 3 (donor: 07–1288). Immunolabelling with antibody R145. A) RAMALT of 09–1447 at 5 months of age. B) RAMALT of 09–1426 at 5 months of age. C) Obex of 09–1447, culled at 24 months of age. D) Obex of 09–1426, culled at 24 months of age. PrP^d immunolabelling is evident in the lymphoid follicles of the rectal mucosa, which is suggestive of infection with the scrapie agent following ingestion of colostrum (A) and milk (B), from scrapie sheep 07–1288. After cull with clinical signs of scrapie, both sheep presented with PrP^d immunolabelling in the brain [here: parasympathetic nucleus of the vagus nerve: C (colostrum recipient) and D (milk recipient)] consistent with the diagnosis of scrapie.