Evidence of scrapie transmission via milk
Evidence of scrapie transmission via milk
Timm Konold , S JO Moore , Susan J Bellworthy and Hugh A Simmons
BMC Veterinary Research 2008, 4:14doi:10.1186/1746-6148-4-14
Published: 8 April 2008
The risk of scrapie infection increases with increased duration and proximity of contact between sheep at lambing. Scrapie infectivity has not been detected in milk but cellular prion protein, the precursor of disease-associated prion protein PrPd, has been found in milk from ruminants. To determine whether milk is able to transmit scrapie, 18 lambs with a prion protein genotype associated with high susceptibility to scrapie (VRQ/VRQ) were fed milk from twelve scrapie-affected ewes of the same genotype, and 15 VRQ/VRQ sheep reared on scrapie-free dams served as controls.
Three lambs fed milk from scrapie-affected ewes were culled due to intercurrent diseases at 43, 44 and 105 days of age respectively, and PrPd was detected in the distal ileum of the first two lambs, whilst PrPd was not found in lymphoreticular tissues in the third lamb. A control lamb, housed in a separate pen and culled at 38 days of age, was also negative for PrPd in a range of tissues. Samples of recto-anal mucosa associated lymphoid tissue collected from the remaining 15 live lambs at seven months of age (between five to seven months after mixing) were positive for PrPd in the scrapie milk recipients, whereas PrPd was not detected in the remaining 14 controls at that time. A subsequent sample collected from control lambs revealed PrPd accumulation in two of five lambs eight months after mixing with scrapie milk recipients suggestive of an early stage of infection via lateral transmission. By contrast, the control sheep housed in the same building but not mixed with the scrapie milk recipients were still negative for PrPd.
The presence of PrPd in distal ileum and rectal mucosa indicates transmission of scrapie from ewe to lamb via milk (or colostrum) although it is not yet clear if such cases would go on to develop clinical disease. The high level of infection in scrapie-milk recipients revealed by rectal mucosal testing at approximately seven months of age may be enhanced or supplemented by intra-recipient infection as these lambs were mixed together after feeding with milk from scrapie-affected ewes and we also observed lateral transmission from these animals to lambs weaned from scrapie-free ewes.
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INTERNATIONAL FORUM FOR TRANSMISSIBLE ANIMAL DISEASES AND FOOD SAFETY
a non-profit Swiss Foundation
(April 11, 2008)
TAFS1 STATEMENT ON TRANSMISSION OF SCRAPIE VIA MILK
With regards to:-
Konold et al.: Evidence of scrapie transmission via milk, BMC Veterinary Research 2008, 4:14;doi:10.1186/1746-6148-4-14; Published 8 April 2008; Available at:-
TAFS welcomes publication of these findings. They contribute to the understanding of the mechanisms of transmission of scrapie as a prion disease of sheep and goats that has been known for over 200 years.
Scrapie is not a highly contagious disease. It does not spread easily, but it is difficult to eradicate. It is known to spread between sheep, both from ewe to lamb and to other unrelated sheep and goats. The exact route of transmission has not been determined so far. There are several possible routes, which include contact with placenta of infected ewes, or possibly before birth while the lamb is still in the womb. Transmission via milk and/or uterine fluids after birth are additional possibilities. The study being reported attempted to assess the scope for transmission, under a worst-case scenario, by collecting milk from sheep of highly susceptible genetic makeup (high risk group), at a time when they were either about to die of scrapie, or when the first clinical signs were seen. Their milk was collected and fed to lambs that were born to uninfected mothers and kept in isolation while they received the milk. These lambs were also of the most susceptible genotype (VRQ/VRQ). The lambs have been shown to be infected by the testing of tissue samples collected either while still alive, by biopsy, or from some that had died of other diseases. None have yet reached the point of clinical disease themselves, and infectivity itself has not been demonstrated. Tests have revealed the presence of abnormal prion protein (PrPSc) that is normally recognised as a marker for the presence of infectivity.
The success of the study was dependent on having scrapie-free lambs to receive the milk. Despite having fully susceptible ewes and susceptible lambs, the ease with which the lambs were infected is a surprise. The experimental design anticipated transmission to smaller numbers of lambs. For this reason lambs received both milk and colostrum (the milk produced within the first 24-48 hours after lambing) from the same ewe in order to maximise the likelihood of transmission. As a result, it is not possible to conclude at this point whether transmission occurred via colostrum, milk or both. The study will therefore now be repeated, feeding lambs only on colostrum or milk.
This is important for several reasons.
• Firstly, colostrum would not be used for human consumption.
• Secondly, colostrum is rich in protein and antibodies that help to protect the lamb in the early days of life before its own immune system is fully functional. For that reason farmers sometimes collect and freeze colostrum to feed to other lambs, sometimes pooling it to feed to several lambs. This practice could increase the potential for infection of lambs at their most vulnerable time of life.
Scrapie is not recognised as a risk to humans, ***although this cannot be ruled out with certainty .The risk to humans from scrapie, and the scientific uncertainties that underpin any statement on risk, have been discussed at length in the two EFSA Opinions cited below. Since there is no established evidence to date that scrapie poses a risk to human health, the finding that it is transmissible from sheep to lambs via milk does not give any reason to change our view that ovine and caprine milk are safe for human consumption.
These results do not at the moment have any direct implications with respect to the risk from BSE in milk from cattle. Although an equivalent study has not been conducted in cattle, other studies attempting to find infectivity in bovine milk have not succeeded. Proving the total absence of infectivity is extremely difficult. The evidence for the absence of natural spread of BSE between cattle, from cow to calf or between unrelated cattle does however suggest that even in natural equivalent of this experiment, the feeding of calves on cows’ milk, transmission has not occurred, or does so only rarely. Consequently, cows’ milk is unlikely to carry BSE infectivity that might put consumers at risk. Furthermore, the control measures that have been put in place to eradicate BSE, and protect consumers in the interim, are succeeding in reducing numbers of infected cattle year by year. In conclusion, the study helps to better understand the epidemiology of scrapie, and may provide an impetus for additional measures to further strengthen animal or public health protections in regard to small ruminants TSEs. However, it does not put in question the safety of products derived from bovine milk destined for human consumption.
EFSA (2007). Opinion of the Scientific Panel on Biological Hazards on certain aspects related to the risk of Transmissible Spongiform Encephalopathies (TSEs) in ovine and caprine animals. 8 March 2007. The EFSA Journal. 466:1-10. Available at:-
EFSA (2008). Scientific and technical clarification in the interpretation and consideration of some facets of the conclusions of its Opinion of 8 March 2007 on certain aspects related to the risk of Transmissible Spongiform Encephalopathies (TSEs) in ovine and caprine animals. The EFSA Journal. 626:1-11. Available at:-
1 TAFS is an international platform created by a group of scientists, food industry experts, animal health regulators, epidemiologists, diagnosticians, food producers, and consumers. Its purpose is to establish and maintain lines of communication for the dissemination of reliable information to the public that can maintain confidence in the safety of food with regard to Transmissible Animal Diseases. TAFS 2
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