Surveillance and diagnosis of transmissible spongiform encephalopathies in the United States
Date
2007
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Abstract
Since limited knowledge of transmissible spongiform encephalopathies (TSEs) restricts treatment and successful control interventions, and since some may cause fatal food-borne disease in humans, the United States (U.S.) has established TSE surveillance programs to support control efforts and to protect agriculture-based economy. The enhanced BSE surveillance system was conducted to characterize the extent of the presence of BSE in the U.S. cattle population in order to reassure consumers and trading partners of the U.S. BSE status. Given the level of importance and the cost of the enhanced BSE surveillance program, surveillance system evaluation was conducted to provide feedback for improving future surveillance and to determine the extent to which the system had met its objectives. Recommendations were made to improve efficiency and quality of future BSE surveillance systems. The enhanced BSE surveillance certainly met its stated objectives. Surveillance interests in the U.S. were subsequently re-directed towards efficiently assuring that BSE control measures remain effective, and to maintain assurance of trading partners of the U.S. BSE status. A plan for ongoing BSE surveillance was constructed using the standards and guidelines for animal health surveillance established by the National Surveillance Unit (NSU). Results derived from the enhanced BSE surveillance system and its evaluation prompted appriopriate adaptations for maintenance surveillance methods. Conditions which naturally degrade prions need to be elucidated to facilitate disposal of prion-contaminated biowastes. In order to determine whether long-term heating could destroy prions, the immunodetection of protease-resistant, disease-associated prion protein (PrPres) was evaluated in brain from chronic wasting disease (CWD)-affected elk. Using 3 diagnostic assays for CWD, progressive loss of PrPres immunodetectability, which increased with incubation temperature, was demonstrated when brain homogenates were incubated at 37, 55, and 80° C over a period of 200 days. Disposal systems which use heat over time may effectively degrade prions. Furthermore, the validity of test results derived from tissues which have been exposed to such conditions is questionable. In the U.S., scrapie surveillance uses PrPres immunohistochemistry (IHC) applied to tissues collected postmortem. The only live animal test available, PrPres IHC applied to third eyelid biopsy, is limited by comparatively lower sensitivity, high frequency of inconclusive test results, and the limited amount of tissue available for repeat testing. A study evaluated PrP res IHC applied to recto-anal mucosa associated lymphoid tissue (RAMALT) biopsy for scrapie diagnosis in live sheep. Biopsy-related complications were rare. The sensitivity of RAMALT biopsy PrPres IHC ranged from 87.5-89.3%, and approximated or exceeding that applied to third eyelid biopsy. The use of PrPres IHC applied to RAMALT biopsies for scrapie diagnosis in live high-risk sheep is expected to improve the surveillance activities that support the success of the U.S. National Scrapie Eradication Program.
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Subject
bovine spongiform encephalopathy
chronic wasting disease
degradation
immunohistochemistry
prions
scrapie
pathology
epidemiology