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An epidemiological review of a fatal neurodegenerative disease outbreak: quantification of genetic risk and the impact of lateral transmission in the outbreak.

Scrapie in sheep and goats, a fatal neurodegenerative disease, is a member of the family of transmissible spongiform encephalopathies (TSEs), which includes bovine spongiform encephalopathy (BSE) in cattle, chronic wasting disease in deer and elk, and Creutzfeldt-Jakob disease in humans. Common to all TSE diseases is the accumulation of the abnormal PrPSc form of the normal prion protein PrPC, predominantly in the brain and nervous tissue in late stage disease. This infectious protein, PrPSc, is thought to be the causative agent of TSE.

Susceptibility to scrapie is primarily controlled by polymorphisms in the priori protein gene (PRNP). Three of these polymorphisms are strongly linked to the occurrence of natural and experimental scrapie. These are valine (V) or alanine (A) at codon 13 6, arginine (R) or histidine (H) at codon 154, and glutamine (Q), arginine (R), or histidine (H) at codon 171. In the United States, diploid genotypes are determined by a number of commercially available assays and typically shown as the results at codon 136 and 171 (AAQQ, AAQR, AARR, VVQQ, AVQQ or AVQR). Relative susceptibility and incubation time are a function of PRNP genotype and scrapie strain.

The objective of this retrospective study was to characterize an outbreak of valine- associated scrapie in a United States flock, quantify the risk of scrapie infection based on allele frequencies at codon 136 and 171 and provide evidence of lateral transmission.

One thousand six sheep, with genotypes at codon 171 (n=164) or codons 136 and 171 (n=842) and scrapie diagnostic testing (n=190), with 44 sheep considered positive for the disease were studied. Genotype was determined using commercial testing, with presence or absence of PrPSc through immunohistochemical localization in either neural or lymphatic tissue used to determine scrapie status.

Animals with at least one [V.sub.136] (n=160) allele were 52.3 times more likely to be positive than sheep homozygous for [A.sub.136]. Homozygous [V.sub.136] sheep (n=72) were 1.7 times more likely to be positive than heterozygotes. For homozygous [Q.sub.171] animals (n=127), those at least heterozygous for [V.sub.136] were 36.4 times more likely to be positive than those homozygous for [A.sub.136]. Of animals (72) with at least one [V.sub.136], AVQR sheep were 1.96 times more likely to be negative than AVQQ and/or VVQQ sheep, despite the [V.sub.136] at codon 136. Exposure route was estimated in several cases based on average incubation times for AVQQ and VVQQ sheep. Using the date of transmission, which is based on the death date, we provide evidence of lateral transmission, (transmission via the environment or by close contact with infected individuals).

In conclusion, this flock had an outbreak of valine-associated scrapie, associated with a relatively high frequency of sheep with valine at codon 136, introduction of a valine-dependent scrapie strain, and the occurrence of lateral transmission. Our findings are uncharacteristic of most scrapie outbreaks in the United States, in which the alanine-associated scrapie is most prevalent. Further study of this rare form of this disease will provide important information regarding the transmission and etiology of scrapie in sheep and TSE diseases in general.

Supported in part by USDA, ARS, ADRU.

Jessica M. Evoniuk (1) Charles L. Stoltenow (1), Katherine I. O'Rourke (2), Dale A. Redmer (1), Bert L. Moore (1)

(1) Department of Animal and Range Sciences, North Dakota State University, Fargo, ND 58105-5727 (2) USDA Agricultural Research Service, Animal Disease Research Unit, Pullman, WA 99164-6630
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Title Annotation:Collegiate Communications--Graduate
Author:Evoniuk, Jessica M.; Stoltenow, Charles L.; O'Rourke, Katherine I.; Redmer, Dale A.; Moore, Bert L.
Publication:Proceedings of the North Dakota Academy of Science
Date:Apr 1, 2005
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