The Status of Bovine Spongiform Encephalopathy in the United States
Dr. Jim Randolph
Remember the days when viruses were the scapegoat of every undiagnosed illness?
"We don't know what it is, ma'am, so it must be a virus."
Now, we don't just have bacteria, fungi and viruses to fear as disease-causing agents, prions now get into the picture. Prions are known to be the cause of the cow disease BSE (Bovine Spongiform Encephalopathy), a similar disease in sheep and goats called scrapie, transmissible mink encephalopathy and "chronic wasting disease" of mule deer and elk.
Human diseases caused by prions include CJD (Cruetzfeldt-Jakob), vCJD (variant CJD, in which the syndrome is similar but the cause is BSE prion, not CJD prion), GSS (Gerstmann-Straussler-Scheinker syndrome), kuru, FFI (fatal insomnia) and Alper's Syndrome.
What makes a prion a prion? It is a form of protein. Like viruses, prions attack cells, and do their damage inside the cell. Unlike viruses, they lack RNA and DNA, and, therefore cannot change the genetic code of the cell. What they can do, though, is just as damaging. They convert
normal protein molecules into dangerous ones just by inducing the normal molecules to bend their shape. Thus, they can cause disease by inheritance as well as by infection.
BSE is known to be transmitted by the consumption of infected body parts (specifically brain, spinal cord and intestine). For example, in the English epidemic that made headlines a few years ago, British cattle were fed scrapie-infected sheep leftovers, as well as cattle parts, from slaughterhouse processing until 1988. In New Guinea, the Fore Highlanders acquired kuru from cannibalism: They honored the dead by eating their brains. Both practices have since been stopped.
Why feed a sheep to a cow? Because protein is the most expensive ingredient in cattle feed, and the easy and inexpensive availability of material that would otherwise have been wasted after slaughtering could be used to cheaply improve quality of feed for both beef and dairy cows. It was a common practice in most developed countries until a ban on the use of ruminant protein in cattle feed was enacted in July of 1988. Because prion diseases have a very long incubation period, cases continued to appear for many years, culminating in the epidemic that lead to the slaughter of hundreds of thousands of British cattle in the 1990s.
BSE is not scrapie. It is believed that BSE is a unique bovine prion, whose infectivity was amplified by feeding leftovers from cattle slaughter back to cows.
Ingested prions may be absorbed across the gut wall at Peyers patches. These are a part of the MALT, or mucosal associated lymphoid tissue. It is thought that the MALT presents microorganisms to the immune system in a contained and ideal fashion, which then stimulates the immune system to protect the body from infection. Prions could be taken up in the same way. After lymph cells ingest a prion, it can be carried to lymph nodes, tonsils and spleen, where it replicates itself. Because nerves are present in many of these locations, prions may access the nervous system and finish their journey to the brain and spinal cord.
Given the fact that there has been one confirmed case of BSE in one American cow, what is the risk to humans in the United States?
First, recognize that importation of beef from any country in which BSE has ever been diagnosed was banned in 1989. That includes British beef.
It is generally accepted that beef and dairy products are safe, even from BSE-infected cows. The World Health Organization doesn't even consider BSE to be a human health hazard.
Studies of humans who work with BSE cows show no increase in the incidence of CJD. This includes veterinarians and butchers.
Over 60 veterinary diagnostic laboratories in the US, including Mississippi's, have participated in a BSE surveillance program since 1990. Brains specimens from 42 states, from over 2660 cattle with neurologic disease have been evaluated for BSE. None has tested positive.
While your pet's doctor takes care of your dogs, cats, birds, horses and pocket pets, thousands of veterinarians nationwide spend their entire careers ensuring the safety of America's food supply. Dr. Ron DeHaven is the veterinarian at the forefront of protecting your health through food safety in his job as the Head USDA veterinarian. Be assured that he will everything necessary to eliminate Americans' risk from BSE. Next Column