HYPOTHESIS ON SCRAPIE TRANSMISSION FROM THE EPIDEMIOLOGIC ANALYSIS OF15 INFECTED SHEEP FARMS

An epidemiological investigation was carried out on 15 French sheep fl ocks infected with scrapie. The case study was based on the data alrea dy collected in the course of the technical follow-up of the farm, on the interview of the farmer and the veterinary practitioner who invest igated the scrapie outbreak, and on the observation of farming conditi on. It consisted in describing the chronicle of scrapie within the far m, and trying to establish the source of the flock contamination and t he circumstances of the spread of the disease among the sheep. Farms w ere located in 7 counties (French 'departements'); 9 flocks produced m ilk, the others lambs. The source of the flock contamination does not seem to be the introduction of females in 12 of the 15 farms; but all of these farms have introduced rams, and some of the flocks have been in contact with other flocks during the transhumance (4 flocks), and/o r have an infected flock in their neighborhood (2 flocks). Consequentl y,we have to consider the eventuality of a spreading of scrapie throug h rams or through infected contact with animals. For 9 of the 15 farms , the chronological pattern of the scrapie cases cannot be explained w ithout an hypothesis on an asymptomatic carriage of the disease (healt hy carriers or incubatory carriers). Moreover even in the absence of c linically affected animals, the contamination can be massive, can take place outside the lambing period, and can affect not only young anima ls but also adults. Sometimes, the horizontal transmission seems to be important, for young as well as for adult animals. In some cases, the disease apparently disappeared from the farm since 4 or 5 years, due to different fighting measures depending on the farms (stamping out; r econstitution of a safe flock from female lambs removed from their mot her at lambing; replacement of the rams together with a quick removing of sick animals and placentas). The study was not designed for statis tical inference and results generalization. However, it shows the nece ssity to reconsider the current knowledge on scrapie transmission, in terms of sources of contagion and ways of transmission; these hypothes es have to be taken into account in later research projects.