Mad cow disease

or bovine spongiform encephalopathy (BSE), a neurological disease that primarily affects mature cattle. The first suspected case of BSE occurred in Great Britain in April 1985. A specific diagnosis was arrived at in 1986. By June of 1990 there were 14,324 confirmed cases out of an estimated population of 10 million cattle in Great Britain.

Cases of BSE in British cattle in 1995 totaled more than 146,000. Additional cases were reported in other countries including Switzerland (about 200 cases), Ireland (about 120 cases), and Portugal (about 30 cases). BSE is one of a group of transmissible and fatal spongiform encephalopathies (TSEs) affecting both animals and humans. This has raised concern regarding possible risks to human health. The disease has crossed the species barrier to infect at least 18 other species. Some researchers suggested a possible link between scrapie or BSE and Creutzfeldt-Jakob disease (CJD) in humans.

This suggestion gained more support in 1996 when British scientists reported a series of related findings. A new variant of Creutzfeldt-Jakob disease (nvCJD) was identified as causing a form of CJD that affects younger people. In another finding, the brain proteins most affected by BSE in cows and CJD in humans are much more similar than was previously thought, given the evolutionary distance between the two species. A third report in 1996 announced research showing that not only is nvCJD different from CJD, but also that nvCJD is probably associated with BSE.

Clinical signs of mad cow disease include abnormally stilted gaits, high stepping, heightened sensory perception, itching, anorexia, and excessive licking, ending in death. Abnormal motor nerve control coupled with aggressiveness have earned the disease the common name of mad, or raging, cow disease. Although all types of cattle are susceptible, most cases have been reported in the Holstein-Friesian breed. The first clinical signs, or onset, of the disease has been observed in cattle at an age of 1 year 10 months to 15 years. From onset, the disease course varies from less than 2 weeks to 14 months usually resulting in death or humane destruction within 4 months.

The causative agent of BSE is not known, however, histopathological studies and clinical signs indicate a strong resemblance to that of scrapie, a neurologic disease of sheep. Brain extracts of suspect cattle have produced disease-specific structures known as scrapie-associated fibrils (SAF). SAF are attributed to infectious type agents called prions. Prions are associated with a unique protein found in brain tissue called prion protein (PrP).

In 1979 studies were conducted by the United States Department of Agriculture (USDA) to attempt experimental transmission of scrapie to cattle. Approximately 30 percent of the inoculated cattle developed progressive neurological signs of the disease. Upon initial histological examination scrapie could not be confirmed. However, a more recent examination of the same tissues, coupled with new information, have shown PrP structures to be present.

As of the mid-1990s there was no evidence of animal-to-animal transmission of BSE. BSE is considered a “common source” epidemic, meaning that animals contract the disease from a common element in their environment. Evidence has ruled out such possible sources of the disease as semen, chemicals, inheritance, chemicals, and pharmaceuticals. The primary suspected sources of the disease are diets fed to cattle containing ruminant-based meat and bone protein. Scrapie- or BSE-contaminated carcasses that are rendered for ruminant diets may account for the presence of SAF in the brains of affected cattle. The feeding of animal protein specifically derived from ruminants was ceased in the United Kingdom as of July 1988.

As of 1996, there were no reported cases of BSE in the United States, but scrapie and other forms of spongiform encephalopathy were present, hence the intense interest in BSE. A transmissible form of spongiform encephalopathy found in ranched mink, transmissible mink encephalopathy (TME), in the United States has been primarily attributed to the feeding of scrapie-infected sheep and goat carcasses to minks.

To prevent BSE from entering the United States, USDA's Animal and Plant Health Inspection Service (APHIS) has taken the following steps:

Beginning in July 1989, APHIS banned the importation of live ruminants and ruminant products from countries where BSE is known to exist.

Since 1991, there has been a voluntary ban in place on using products rendered from adult sheep in animal feeds.

In 1986, APHIS established a program for BSE surveillance in the United States and provided specialized training for 250 APHIS veterinarians who conduct field investigations involving animals with any suspicious symptoms.

APHIS veterinary pathologists and field investigators have received training from British counterparts for diagnosing BSE.

More than 60 veterinary diagnostic laboratories throughout the United States are participating in the BSE Surveillance Program, along with the National Veterinary Services Laboratories in Ames, Iowa.

APHIS veterinarians are tracing 499 head of cattle imported from Great Britain between 1981 and 1989 (before the ban on imports went into effect) to check their health status. As of Jan. 22, 1996, no signs of BSE have been found.

Between 1986 and Dec. 31, 1995, approximately 2,660 brain specimens from cattle exhibiting possible neurological problems had been studied by APHIS. All samples submitted were negative.