TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHY

Bovine spongiform encepalopathy (BSE) is a transmissible, degenerative neurological disease of cattle that causes neither fever nor inflammation in the organs. Cattle infected with BSE experience a progressive degeneration of the nervous system. Symptoms include nervousness or aggression; abnormal posture, loss of coordination, and difficulty in standing; excessive itching or licking; decreased milk production; and loss of body weight despite continued appetite. The average age of symptom onset is five years, and death usually results within four months. The common name, "mad cow disease," is related to abnormal motor control and aggressiveness, which are also symptoms of rabies, which afflicts "mad dogs." Between 1986 and 2000, more than 170,000 cases of BSE were identified in cattle in the United Kingdom. The epidemic peaked in 1992–1993 at almost 1,000 cases per week.

The cause of BSE in British cattle was probably the use of commercial cattle feed containing meat and bone meal (MBM) derived from the rendered carcasses of sheep infected with scrapie, a degenerative disease of sheep. MBM is manufactured by rendering (melting) out the fat, and then drying the protein portion of by-products from the meat processing industry. Using MBM as a protein source in animal feed has been common for several decades. Apparently, the pre-1970 rendering methods, which included fat removal with solvents followed by a steam treatment, eliminated the infective agent in the rendered material before it was used in cattle feed. A change to "low temperature rendering" in the 1970s may have allowed the infective agent to remain in the protein portion that was dried and used as animal feed.

Natural transmission of BSE in cattle occurs when they eat infective material. The required oral dose for BSE transmission is small—five hundred to one thousand milligrams of BSE-infected brain tissue for calves. The BSE agent has been found only in the brain, spinal cord, retina, and the small intestines of cattle.

BSE and scrapie are transmissible spongiform encephalopathies, or TSEs. TSEs appear to be caused by an unconventional infectious agent known as a "prion" (proteinaceous infectious particle), an agent that contains no DNA or RNA. A prion is a normal protein (PrP) present on or in nerve cell membranes that can assume an abnormal (infective) physical shape referred to as PrP^sc. One PrP^sc molecule can induce normal PrP molecules to change shape into PrP^sc.

Mutations in the PrP gene may make the conversion of normal PrP to disease-causing PrP^sc more likely. There are at least twenty mutations in the PrP gene sequence resulting in "spontaneous" PrP^sc formation. Once a few PrP^sc molecules are formed, they rapidly convert other normal molecules to the infective form. Ingested PrP^sc can travel to the central nervous system where it converts normal PrP to PrP^sc.

The nerve cell attempts to break down the prion; however, PrP^sc is very resistant to the enzymes that normally break proteins down. The cell can be cleaved into fragments, which fill up and kill the cell (leaving holes known as "spongiform" damage). These fragments aggregate and precipitate the formation of plaques.

Creutzfeldt-Jakob disease (CJD) is a human TSE that occurs primarily in those over sixty-five years of age. It produces rapidly progressing neurological symptoms and dementia. In 1995, a new form of CJD, variant, or new variant, CJD (vCJD), was recognized. Patients have behavioral and psychiatric disturbances (e.g., depression, personality change), failure of muscular coordination, and memory impairment. The original ten cases occurred in people under forty-two years of age and were fatal within thirteen months. The hypothesized cause was consumption of BSE-infected food materials. As of March 2001, ninety-five cases of vCJD had been identified in the United Kingdom, with a few isolated cases occurring in France and Ireland.

TSEs occur more often in specific subgroups of populations (certain breeds of sheep, cattle, mink, and human families) suggesting a genetic component. Some gene combinations are very resistant, while some are particularly susceptible to the disease. In addition, a TSE in a specific species can exist in several strains, with each producing specific symptoms, times of onset and progression, and lesions in the brain that are distinctly different from those produced by other strains. Recent evidence indicates that the TSE strain occurring in vCJD is not different from the strain responsible for BSE in cattle. If the TSE strain that causes BSE also causes vCJD, the question arises of whether there is likely to be an epidemic of vCJD. The chances of such an epidemic are reduced by the fact that the incubation period for CJD is relatively long in humans, and because much of the population is likely genetically resistant to the PrP^sc that causes the disease. Further, while the infective dose in humans is not known, the dietary consumption of known infective tissues (brain, spinal cord, etc.) is low. However, it may be years before the full impact of this disease on the human population is known. As of 2001, no cases of BSE in cattle had been identified in the United States and no products had been imported that appear to pose potential risk to either human or animal health.

M. Susan Brewer

(see also: Bovine Spongiform Encephalopathy; Epidemics; Prions; Veterinary Public Health )

Bibliography

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