Submitted to: American College of Veterinary Internal Medicine
Publication Type: Proceedings
Publication Acceptance Date: April 1, 2007
Publication Date: June 6, 2007
Citation: Richt, J. 2007. BSE: description of typical and atypical cases. In: Proceedings of the American College of Veterinary Internal Medicine. 2007 ACVIM Forum, June 6-9, 2007, Seattle, Washington. Paper No. 159. Technical Abstract: Introduction Transmissible spongiform encephalopathy (TSE) agents or prions induce fatal neurodegenerative diseases in humans and in other mammalian species. They are transmissible among their species of origin, but they can also cross the species barrier and induce infection and/or disease in other species. Human TSEs include Creutzfeldt–Jakob disease (CJD), Gerstmann–Sträussler–Scheinker syndrome (GSS), Kuru and fatal familial insomnia (FFI) (1). In animals, four distinct TSE diseases are recognized: scrapie in sheep and goats, transmissible mink encephalopathy (TME) in mink, chronic wasting disease (CWD) in cervids, and bovine spongiform encephalopathy (BSE) in cattle. Although considerable research has been undertaken, the precise nature of the causative agent remains controversial. A number of theories describe the etiology, however the "protein only" theory has emerged to dominate the literature. Bovine Spongiform Encephalopathy (BSE) Widely referred to as "mad cow disease", BSE was first identified as a TSE of cattle in the mid 1980s in the U.K. and more than 180,000 positive cases have been diagnosed in the U.K. to date. BSE is also transmissible via BSE-contaminated feed to cats (feline spongiform encephalopathy, FSE) and exotic ungulates (exotic ungulate encephalopathy, EUE) (2, 3). BSE is a chronic degenerative disease affecting the central nervous system of cattle. Affected animals display changes in temperament, abnormal posture, incoordination and difficulty in rising, decreased milk production, and/or loss of body weight despite continued appetite (4). The average incubation period is about 4-6 years and all affected animals succumb to the disease (5). Following the onset of clinical signs, the animal's condition deteriorates until it either dies or is destroyed. This process usually takes from 2 weeks to 6 months. Most cases in the U.K. occurred in dairy cows between 3 and 6 years of age with the highest susceptibility to infection being in the first 6 months of life; adult cattle appear to be at relatively low risk of infection (6). Using epidemiological surveillance programs, many European and non-European countries have discovered BSE-positive animals within the last decade (7) (8). BSE has been reported in native-born cattle in twenty-four countries with a geographic distribution that includes Europe, the Middle East, North America, and Asia. These outbreaks, caused by the consumption of infected meat and bone meal containing a malformed prion protein, have resulted in the destruction of thousands of cattle and have caused significant economic losses. All currently validated diagnostic tests for BSE require brain tissue. There are no validated ante mortem tests for BSE available at present. "Typical" versus "Atypical" BSE cases Molecular characterization of the abnormal form of the prion protein, called PrPres, has allowed the identification of "atypical" cases of BSE cases in cattle. BSE in cattle was considered to be a disease with unique features (9) and the majority of BSE cases so far have been defined as "typical" BSE cases. However, unusual or "atypical" cases of BSE have been reported in the past 3 years by investigators from several countries. Most of these animals were greater than 8 years of age and of various breeds. There have been two molecular types of "atypical" BSE isolates described in the literature so far: (i) a type with a lower molecular mass of the unglycosylated isoform also called the L-type and (ii) a type with a higher molecular mass of the unglycosylated isoform, also called the H-type. The L-type has been found in cattle in Italy (10), Japan (11), Germany (12) and Belgium (13). So far, the H-type has been described in cattle from France (14), Germany (12) and the United States (15). The U.S. cases were animals born and raised in the U.S. (Texas, Alabama). Unusual cases of BSE are an unexpected finding since it was previously believed that BSE disease in cattle is caused by a single strain of infectious agent, which has been shown to be very consistent and uniform in appearance, even after transmission to other species. The reports of unusual phenotypes of BSE in cattle suggest that different PrPSc phenotypes exist in cattle with BSE. There are several hypotheses which can explain these findings: (i) there are variants of the BSE agent with different molecular features in cattle; (ii) cattle may have been infected by another source of an infectious prion agent (e.g. scrapie or CWD); or (iii) a rare sporadic or genetic form of TSE disease could exist in cattle as described for humans.