New studies on scrapie (prions) fatal brain diseases found in six USA sheep

July 7, 1:17 PMSacramento Nutrition ExaminerAnne Hart

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This article also has been picked up by Wildlife Disease News Digest listed under "Domestic Animal Diseases News," July 13, 2009. Also, according to another article based on a 2005 study, "A newly identified type of scrapie agent can naturally infect sheep with resistant PrP genotypes," Scrapie in small ruminants belongs to transmissible spongiform encephalopathies (TSEs), or prion diseases, a family of fatal neurodegenerative disorders that affect humans and animals and can transmit within and between species by ingestion or inoculation.

The question is can scrapie be passed to humans eating sheep infected with TSEs, known as prion diseases? The fatal neurodegenerative disorder affects humans and animals. The study noted the prion diseases can be transmitted between species.

Conversion of the host-encoded prion protein (PrP), normal cellular PrP (PrPc), into a misfolded form, abnormal PrP (PrPSc), plays a key role in TSE transmission and pathogenesis. The intensified surveillance of scrapie in the European Union, together with the improvement of PrPSc detection techniques, has led to the discovery of a growing number of so-called atypical scrapie cases.

According to the study's abstract, "These include clinical Nor98 cases first identified in Norwegian sheep on the basis of unusual pathological and PrPSc molecular features and “cases” that produced discordant responses in the rapid tests currently applied to the large-scale random screening of slaughtered or fallen animals."

The study noted, Worryingly, a substantial proportion of such cases involved sheep with PrP genotypes known until now to confer natural resistance to conventional scrapie. "Here we report that both Nor98 and discordant cases, including three sheep homozygous for the resistant PrPARR allele (A136R154R171), efficiently transmitted the disease to transgenic mice expressing ovine PrP, and that they shared unique biological and biochemical features upon propagation in mice. These observations support the view that a truly infectious TSE agent, unrecognized until recently, infects sheep and goat flocks and may have important implications in terms of scrapie control and public health."

For updated information, see the blog article posted July 1, 2009, titled, "NOR-98 Atypical Scrapie USA Update as of Oct. 2007." Basically, the blog refers readers to a more recent study, "Nor98 scrapie identified in the United States," Journal of Veterinary Diagnosis and Investigation, 21:454–463 (2009)  

The six case histories of the infected sheep in the USA are described in detail on the blog. For example, "The 6 U.S. cases were identified in a variety of testing scenarios that ranged from routine slaughter surveillance of healthy appearing animals to surveillance testing in an aged animal diagnosed with thoracic neoplasia and secondary pneumonia. None of the 6 cases had a clinical history that suggested the presence of a TSE."

The blog reporting the findings of the study noted that, "This is consistent with the findings in many European countries that have found Nor98 disease on a regular basis through slaughter surveillance of apparently healthy animals and fallen stock.1 Three of the sheep (cases 1, 4, and 6) were identified through the RSSS program. Of the remaining 3 animals, 2 sheep (cases 2 and 5) were found in flocks in which classic scrapie had been previously diagnosed, and the last case (case 3, a 16-year-old California wether) was presented for a routine diagnostic necropsy in which scrapie testing was included. Clinical signs are uncommon in the Nor98 strain of scrapie but most often include ataxia without pruritis, which is more commonly associated with classic scrapie.2"

None of the 6 sheep described in the present study were reported to have had clinical signs, but it might be that subtle signs were not detected. This is a possibility with cases 1, 4, 5, and 6, because they were each part of large flocks with high rates of turnover. With regard to case 3, signs, including ataxia, may have been masked by other clinical signs associated with the unrelated disease processes of pneumonia and an obstructing tumor."

The blog's case histories noted that, "Clinical signs in case 2 were unlikely to be missed, because the animal was closely scrutinized for scrapie signs as part of a scrapie study flock. Both Nor98 and classic scrapie can be diagnosed in a wide variety of age groups, but Nor98 appears to be more commonly found in older animals. In contrast to classic scrapie, which was diagnosed most commonly in 3–5-year-old sheep, Nor98 scrapie was found most frequently in animals more than 5 years of age.17 The U.S. cases described in the current report ranged in age from approximately 3 to 16 years, with a mean age of 7.8 years."

The abstract of the study noted, "A distinct strain of scrapie identified in sheep of Norway in 1998 has since been identified in numerous countries throughout Europe. The disease is known as Nor98 or Nor98-like scrapie, among other names. Distinctions between classic scrapie and Nor98 scrapie are made based on histopathology and immunodiagnostic results."

 What changes have been found by the updated study? "There are also differences in the epidemiology, typical signalment, and likelihood of clinical signs being observed. In addition, sheep that have genotypes associated with resistance to classic scrapie are not spared from Nor98 disease. The various differences between classic and Nor98 scrapie have been consistently reported in the vast majority of cases described across Europe. The current study describes in detail the pathologic changes and diagnostic results of the first 6 cases of Nor98 scrapie disease diagnosed in sheep of the United States."

Just as mad cow disease occurs in bovines and can be transmitted to humans, the question is whether scrapie from sheep is a similar prion disease that also can be transmitted to humans and other species. The only way to find out is to keep reading the latest research. For further information see the blog, "NOR-98 Atypical Scrapie USA Update as of Oct. 2007."  Or read the study and/or abstract titled, "Nor98 scrapie identified in the United States,"J Vet Diagn Invest 21:454–463 (2009). Authors of the study are: Christie M. Loiacono,1 Bruce V. Thomsen, S. Mark Hall, Matti Kiupel, Diane Sutton, Katherine O’Rourke, Bradd Barr, Lucy Anthenill, and Delwyn Keane.

Photo credits: flickr.com.