Scrapie

At the 1946 National Veterinary Medical Association of Great Britain and Ireland Annual Congress, W. S. Gordon, PhD, presented evidence of scrapie transmission by way of a vaccine for louping-ill. Louping-ill is a viral disease spread by the tick Ixodes ricinus L. Scrapie is considered the sheep equivalent of 'mad cow disease.' Dr. Gordon developed an effective vaccine to prevent louping-ill during 193 1-32. After four years of field trials, his vaccine was produced in three batches for widespread use in 1935. The vaccine was made from brain, spinal cord, and spleen tissues taken from sheep five days after they had received an intracerebral inoculation of louping-ill virus. Formalin was added to the 10% saline suspension to inactivate the virus.

During 1935 and 1936, no ill effects were noted in inoculated animals. Then two owners reported scrapie in their Blackface sheep who had been inoculated with louping-ill vaccine (batch 2) two and a half years earlier. Scrapie had not been seen in the Blackface breed before this. Upon investigation Dr. Gordon discovered that 8 lambs used to make batch 2 had been born to ewes who had been exposed to scrapie; some of the ewes developed scrapie in 1936-7. Dr. Gordon hypothesized that an "infective agent of scrapie" was present in the lambs' tissues used to make batch 2 and that this agent "could withstand a concentration of formalin...which inactivated the virus of louping ill; it could be transmitted by subcutaneous inoculation; it had an incubative period of two years or longer."

A four-and-a-half-year experiment involving '788 sheep was initiated by the Animal Disease Research Association in 1938. The researchers found that 60% of normal sheep inoculated intracerebrally with saline suspensions of brain and spinal cord tissue taken from sheep with scrapie developed scrapie within those four-and-a-half-years. The incubation period was seven months and up. Only 30% of the sheep receiving a subcutaneous inoculation of the suspension developed scrapie in that time, and the incubation period in this group was 15 months and up. The researchers concluded that the infective agent was probably a filtrable virus. Interestingly, Dr. Gordon reports that Cuille and Chelle of France published the results of a similar study in 1939. The French researchers found that sheep developed scrapie after receiving intracerebral, intraocular, and subcutaneous injections of spinal cord or brain tissue emulsions. Like the British experiment, this one also showed that incubation periods varied according to the type of injection: intracerebral infection took one year; intraocular took 15 months; and subcutaneous took 20 months.

Gordon, W.S., PhD. Advances in Veterinary Research. The Veterinary Record; 1946 November 23. Presented at the National Veterinary Medical Association of Great Britain and Ireland Annual Congress, 1946. Posted on http://www.vegsource.com/talk/lyman/messages/7634.html

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