Surveillance for lesions of bovine spongiform encephalopathy in US cattle.

Abstract

In October 1987, bovine spongiform encephalopathy (BSE) was described in Great Britain as a novel disease of cattle with neurologic lesions comparable to those of scrapie in sheep and other spongiform encephalopathies of animals and humans. Cases in adult cattle occurred with increasing frequency primarily in southern England during the next several months, and there are approximately 600 new cases each week in Great Britain? Epidemiologic studies incriminated the feeding of rations containing ruminant-derived protein, especially to calves. The prolonged incubation period was similar to that observed in scrapie. The presence in the United States of scrapie and other animal spongiform encephalopathies (transmissible mink encephalopathy and chronic wasting disease of mule deer and elk) provided impetus to survey US cattle for lesions of BSE. A cooperative project between the US Department of Agriculture (USDA), Animal Plant Health Inspection Service (APHIS), Science and Technology, National Veterinary Services Laboratories (NVSL), and the Iowa State University, Department of Veterinary Pathology, was initiated in May 1990 with the goal of examining brain specimens from 250 suspect cattle. Beginning in May 1990, formalin-fixed brain specimens or microscope slides from BSE-suspect cattle were requested, and in April 1991, the project was expanded to include referred cases submitted for rabies examination and found not to be rabies. The criteria used to classify cases as BSE suspects are cattle that (1) are ≥2 years old, (2) have documented signs of neurologic disease, and (3) have received protein supplement as a substantial part of the ration. Specimens are shipped to the NVSL, Ames, Iowa, in standard cartons via US Postal Service or commercial transport systems. Fixed brain specimens are referred to the Iowa State University, Department of Veterinary Pathology, for further processing. Blocks of tissue from 12 areas, if available, are selected as follows: frontal cortex, corpus striatum, temporal cortex, optic chiasm, occipital cortex with hippocampus, thalamus, mesencephalon, pons, cerebellum, rostra1 medulla, obex, and caudal medulla. Tissue blocks are processed by routine methods, embedded in paraffin, sectioned at 8 μm, mounted on glass microscope slides, and stained with hematoxylin and eosin. Special stains, such as Brown-Hopps Gram stain, luxol fast blue-cresyl echt violet, and glial fibrillary acidic protein, are used as needed. Evaluation of the morphology is based on light microscopy.