An advanced case of scrapie. 



Scrapie studies 



Each outbreak is carefully studied to estab- 

 lish the epidemiology of the disease. Particu- 

 lar attention is given to symptoms, laboratory 

 findings, age, sex, breed, history, and pedigree 

 of each infected animal, movements from in- 

 fected flocks, and the possible source of the 

 outbreak. 



During the 7 years the eradication program 

 has been in effect in this country, it has not 

 been possible to identify the source of several 

 outbreaks. The majority, however, fall into a 

 pattern and indicate spread from a limited 

 number of common foci. Such foci — usually 

 identified as source flocks — are now being 

 slaughtered. 



Approximately 80 percent of the infected 

 sheep in this countiy either were imported or 

 were the immediate progeny of sheep of for- 

 eign registry. The broadened Canadian pro- 

 gram should materially reduce the possibility 

 for further introduction of scrapie through 

 sheep imported from Canada. 



Exposed sheep and their immediate progeny 

 from 1,800 flocks have been slaughtered. Pres- 

 ently, some 1,500 flocks are receiving the re- 

 quired 6-months' inspection. In November 

 1956, more than 2,000 flocks were under inspec- 

 tion. These routine inspections have helped 

 locate infected animals. Almost 50 percent of 

 the 91 outbreaks were found in this manner. 



Sheep under 18 months of age seldom show 

 symptoms of natural scrapie. Only one of the 



IGl infected sheep in this country was 18 

 months old ; the others were older. Three were 

 more than 71a> years old. The great majority 

 were from 2\-^ to 4 years old with most being 

 about 3 years of age. 



Mortality rates are difiicult to determine 

 with certainty. When an outbreak occurs, the 

 entire flock is slaughtered before additional 

 cases have time to appear. 



Of the 161 confirmed cases in the United 

 States, 151 were Suffolk sheep and 10 were 

 Cheviots. Although the majority of cases have 

 occurred in one breed, it is generally believed 

 that sheep of any breed can develop the disease. 



The manner in which scrapie spreads is not 

 well understood. Both experimental and field 

 evidence suggest it can spread from the dam, 

 and perhaps the sire, to the progeny. The evi- 

 dence, however, does not explain how the dis- 

 ease spreads from one breed to another. 



Eradication and research 



The 91 outbreaks in the United States illus- 

 trate the potential danger of an insidious dis- 

 ease like scrapie in a country where lively trade 

 among growers results in wide distribution of 

 sheep and their diseases. 



We look forward to additional research re- 

 sults — particularly those designed to explain 

 natural transmission. Precise information 

 will permit moi'e exact identification of poten- 

 tially dangerous animals and may require 

 slaughter of fewer sheep. 



Experience is convincing that the elimination 

 of infected and exposed animals has effectively 

 controlled the disease, prevented it from be- 

 coming widely established in Cheviot and Suf- 

 folk sheep, and prevented spread into other 

 breeds. 



The chances for ultimate success appear 

 good if sound eradication procedures, sup- 

 ported by ample research, are continued. 



SCREWWORMS 



The screwworm, Callitroga hominivorax 

 (Cqrl.). is an obligatory parasite on warm- 

 blooded animals and causes myiasis. Myiasis 

 is defined as "any disease caused by the pres- 

 ence of the larvae of flies in or on the body of 

 an animal." Since screwworm attack, or 



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