By Roger Segelken
Maddening cow disease might be a better name, so frustrating is the causative agent with its apparent ability to move among species. Not to mention the public-health dilemmas facing authorities in Great Britain, where a cattle disease called bovine spongiform encephalopathy (BSE), or mad cow disease, may have infected humans.
"Whatever the agent is that's presumably responsible for transmissible spongiform encephalopathies, it has the most remarkable properties and it desperately needs more study," said Brian A. Summers, a board-certified veterinary pathologist and associate professor of pathology in the College of Veterinary Medicine.
The agent -- which is responsible for deadly brain diseases in victims ranging from cats to cannibals, Angus to antelope and men to mink -- is remarkably difficult to kill, Summers said. It is highly resistant to all the usual means that reliably inactivate conventional infectious organisms.
"It may not be an organism, in the usual sense of the word, but rather a kind of rogue protein," Summers said, referring to the "prion hypothesis." First proposed by University of California neurologist Dr. Stanley B. Prusiner, the prion hypothesis has gained widespread, if not universal, acceptance as the basis for the maladies that turn the brain's grey matter into a sponge-like mass riddled with empty vacuoles.
"Prion" comes from the transposition of letters in "infectious protein," and since the idea was proposed, it was learned that there are "normal," non-infectious prions in all animals and genes responsible for their manufacture in the brain. But if prions contain any nucleic acid at all, it has remained elusive, Summers said. The agent can't be grown in tissue cultures. It does not induce an immune response in the hosts, so there is no blood test for carriers of the diseases.
"Sheep that have developed the spongiform disease scrapie will react in a characteristic way when you scratch their backs. But for most animals, including humans, your presumptive diagnosis must await the appearance of characteristic neurological signs," Summers said. A definitive diagnosis can only be reached with brain tissue from a biopsy or post-mortem examination. Pathologists performing post-mortem histological tests look for the distinctive vacuole structures in tissue of the brain and other parts of the central nervous system and for amyloid plaques deposited in some forms of spongiform disease.
The neuropathologist described a BSE case he witnessed while visiting the University of Cambridge. The animal had lost its coordination, was swaying as it walked and was stumbling, kicking and bellowing.
Those signs spelled death for more than 100,000 British cattle, slaughtered under government orders since BSE was discovered in England in 1986. British government restriction on the use of certain parts of cow carcasses was generally thought to ensure the safety of British beef -- until the official announcement March 20 that new cases of human spongiform encephalopathy in England could be related to beef-eating.
Summers covers the human form, Creutzfeldt-Jakob Disease (CJD), and several others in his well-received new book,Veterinary Neuropathology, published in 1995 with co-authors John F. Cummings and Alexander de Lahunta, both professors of anatomy at the veterinary college.
One baffling aspect of the spongiform diseases is the extended lengths of incubation, Summers said. For example, CJD symptoms usually take decades to appear. However, some of the latest victims of the human form in Britain are adolescents. When spongiform diseases are experimentally transferred to laboratory animals, such as mice and hamsters, disease can occur in a matter of months.
"To get to the bottom of this, we really need to understand the nature of the infectious agent, whether it is a prion or a very small virus or whatever it turns out to be," Summers said. He is not optimistic that full knowledge of such a fundamentally different agent -- and how it acts to produce these neurological disorders -- will come soon.
"It may take another 20 or 30 years," he said, adding that "the matter is further complicated by the fact that some of the human conditions have a genetic basis."