Mad Cow Disease
A new cattle disease which can be transmitted in meat to human beings
It is a new disease of cattle. The first cases of BSE were seen in cattle in the United Kingdom in 1984. It has now been found in other European countries, although it is rare. Recently, two cases have been reported from Japan.
BSE affects adult cattle of both sexes, typically first appearing when the animal is five or six years old. At first, it was regarded as a rare and rather strange disease of cattle. The big shock came several years later, when we learned that the disease can cross the species barrier and infect people.
How Is It Transmitted?
The agent responsible for transmitting the disease is smaller than the smallest known virus. We are still not sure what it is. Some scientists believe it is an unusual kind of virus. Others have suggested it is a deformed protein known as a "prion".
Whatever the agent, it is extremely resistant to heat, and can survive normal sterilization procedures. It does not seem to evoke any immune response in host animals. There is no treatment, and there is no vaccine to prevent the disease.
Animals and human beings become infected by eating diseased animal tissue. Even small amounts may be enough to transmit the disease if they are from the brain or spinal cord.
Since cows eat grass, not meat, how did they get infected? The outbreak of mad cow disease seems to have been caused by forcing cows to become cannibals. The carcasses of dead cows, and by-products from meat processing, were rendered down into livestock feed and fed to cattle. Since this was done on a large scale, a single infected animal could infect the feed of hundreds or thousands of cows. The disease can also be transmitted by a pregnant dam to her unborn offspring.
The Symptoms
Unfortunately, there is no test to detect the disease in a live animal. Even experienced veterinarians find the disease difficult to diagnose. About 20% of the live cattle they diagnose with the disease turn out to be false positives.
Diagnostic tests use brain tissue from dead animals. This is injected into mice and other animals. Mouse inoculation studies take a long time. It may be up to 700 days (nearly two years!) before symptoms begin to appear. A positive result proves that the animal has BSE. However, a negative result does not prove that it is free of BSE. It may only show that the diagnostic methods available are not very reliable.
The disease incubates in both animals and humans for 2-8 years or more, before the first symptoms begin to appear. Once symptoms start to show, they develop gradually over weeks or even months. The illness is always fatal. ?
Those infected with BSE show a progressive degeneration of the nervous system.
In cattle, affected animals are likely to show some (but not all) of the following symptoms.
- - They are apprehensive and nervous;
- - They are reluctant to turn corners or go through gates;
- - They become aggressive, to people or other cows;
- - They are uncoordinated, and find it difficult to rise;
- - They lose condition and lose weight.
How Many People Are Infected?
Nobody knows. So far, more than 100 people have died of the disease, most of them in the United Kingdom. Only one person in Asia is known to have BSE, a woman living in Hong Kong. She was probably infected when she lived in the United Kingdom for several years during the 1980s.
Nobody can predict how many people will die of the disease. This is because nobody knows how long it remains undetected in humans before the symptoms begin. People who carry the infection may be blood donors. A diagnostic test is urgently needed for both cattle and humans.
Making Beef Safe from Bse
People who develop the disease in the United Kingdom today were probably infected years ago. There are unlikely to be new infections. The country's rules today about what materials can be used in livestock feed, and what parts of the animal can be sold as meat, are among the strictest in the world. No meat or bone meal from mammals can now be included in the feed of any animals killed for meat.
In many other countries cattle are still being fed recycled cattle carcasses. However, some countries, including USA, have made it illegal to use "downers" in livestock feed (i.e. cattle which are sick or have died naturally). Other countries have taken even stronger measures. Although Australia and New Zealand have never had BSE, in both countries it is now illegal to feed cattle any tissues derived from mammals or their blood.
Japan has begun a program of screening every cow slaughtered for human food, after a few cases of BSE were found on dairy farms. Spain has begun a massive slaughter of 180,000 cattle, after finding a few cases of BSE in Spanish cattle. In both countries, the outbreaks seem to have been caused by imported cattle feed containing contaminated meat and bone meal.
Index of Images
Figure 1 Symptoms of Bse May Be Found in Other Diseases of the Nervous System. so Far, There Is No Test to Detect the Disease in a Live Animal.
