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Susceptibility of Domestic Cattle to Chronic Wasting Disease Following Oral Inoculation-- 10 Year Study


The research objectives are to determine if domestic cattle can be infected with chronic wasting disease (CWD) by an oral route. CWD is a fatal transmissible spongiform encephalopathy (TSE) that infects free-ranging and farmed deer and elk in Wyoming and Colorado, as well as elsewhere in North America. It is specific to cervids but is related to scrapie of sheep and goats, and to BSE, which infects cattle. Ten cattle housed in isolation at the Wyoming State Veterinary Laboratory that were each given a 45 gram oral dose of a pool of brain material from CWD-infected mule deer in late August 1997 will be euthanized and necropsied by veterinary pathologists at the WSVL at the end of the ten year incubation period. Fixed and frozen tissues from brain and spinal cord, lymphoid and endocrine systems, alimentary tract, peripheral nervous system and viscera will be examined and assayed by immunohistochemistry and Western blot for the presence of disease-associated prion protein (PrPd). Untreated control cattle will also be tested. This project is being carried out in conjunction with a separately funded similar analysis of cattle that have been maintained for ten years in contact with CWD-infected deer and elk, or with premises contaminated by infected animals. Evidence from intracerebral inoculations of cattle with CWD indicates they are susceptible by that route, but it is not known if the natural route will produce similar results. Both BSE in cattle and CWD in cervids are known to be experimentally induced by ingestion of brain macerate from infected animals, and the oral route is considered the natural means of transmission by which CWD is maintained as an endemic disease in our wildlife populations, but it is not known if cattle will become infected by CWD after oral exposure. Because the implications for loss of market and public health concerns are potentially major, CWD could be agriculturally very important if it were naturally transmissible to domestic livestock, specifically cattle, but it has not been established if this is or is not the case.

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NON-TECHNICAL SUMMARY: This research seeks to determine if domestic cattle became infected with chronic wasting disease (CWD) after being given a large oral dose of macerated brain tissues from CWD-infected mule deer ten years ago. CWD is a transmissible spongiform encephalopathy that infects wild and farmed deer and elk. It is related to the prion diseases scrapie of sheep and BSE of cattle. It has been shown that some cattle can develop a BSE-like disease 2 to 5 years afterwards when this CWD brain material is injected intracerebrally, but it is not known if a more natural route of exposure to the infectious agent (i.e., oral ingestion) can transmit the disease to cattle. Cervid carcasses are not normally rendered and used in feed supplements for livestock, but cattle can encounter CWD-infected deer and/or elk, or the long-lasting infectious prion shed by them, on shared grazing range. The BSE epidemic in the UK in the 1980's and subsequent transmission of the BSE prion to humans through infected beef products created an awareness of the possible economic and public health impacts of a similar occurrence in the U.S. This research project will analyze brain and other tissues from the CWD-exposed cattle, none of which have died or developed clinical signs of disease, to determine if transmission has occurred. A separate project will examine cattle that have been kept in physical contact with CWD-infected deer and elk or with contaminated premises for ten years. Whether CWD in cattle would become infectious for humans is not known, but it is a risk factor that would need to be evaluated.
APPROACH: Ten orally dosed cattle that are ten years post-exposure and untreated controls will be sequentially euthanized for necropsy and collection of tissue samples. Approximately 20-30 sections will be examined primarily from nervous and lymphoid system tissues for histopathology and for PrP deposition by immunohistochemistry using conditions previously published for CWD, scrapie, and BSE. Approximately 12 specific areas of brain will be examined. Based on results published to date for PrPd deposition in central nervous system of cattle orally dosed with BSE and in CNS of cattle intracerebrally inoculated with CWD, the most relevant tissue samples to analyze for deposition of PrP in the CNS, peripheral nervous system, and lymphoreticular system are brainstem at the level of the obex and the lymphoid and enteric nervous system cells of the distal ileum. In addition, an assortment of other tissues in which disease-associated prions are most commonly found in cervids with CWD will be sampled, such as palatine tonsil, retropharyngeal, submandibular and mesenteric lymph nodes, cervical, thoracic and lumbar spinal cord sections, retina, and adrenal gland. Replicate tissue samples will be frozen; Western blots on samples of brainstem and tonsil or lymph node will be performed, with additional frozen tissues held in reserve for tests depending on IHC results. Genomic DNA will be extracted from spleen tissue from each animal and cellular prion protein coding sequences will be amplified by polymerase chain reaction (PCR) using previously published conditions and PCR primers. Products will be purified and submitted for DNA sequencing. An important aspect of this project is the availability for analysis of animals that have survived without loss to accidents or other diseases for the full ten year period. To date also, none of the principals have shown clinical signs of neurologic disease. We will ultimately determine infection of cattle with CWD to be positive or negative based on the detection of disease-associated prions in brainstem sections by immunohistochemistry and Western immunoblot. Based on observations of age-related degenerative changes including neuronal vacuoles in normal cattle brains, we anticipate such changes will be observed in most or all of our ten-year-old animals. The untreated control animals will be important for distinguishing non-specific lesions, but primary identification of disease associated prions will be based on immunological detection methods.

Jewell, Jean
University of Wyoming
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