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“Mad Cow Disease”:What You—and Your Patients—Need to Know Now

“Mad Cow Disease”:What You—and Your Patients—Need to Know Now

Q: What is bovine spongiform encephalopathy (BSE), or "mad cow disease," and how is it thought to infect cattle? Dr Lurie: BSE is a progressive neurologic disorder of cattle that is currently understood to be an infectious disease-- but not in the customary sense. It does not appear to be caused by a virus, a parasite, a bacterium, or a fungus. Instead, a misfolded protein, or prion, is believed to be the infectious particle that is transmitted between animals, or from an animal to a person. These misfolded proteins occur in high concentration in the diseased areas of the brain of animals with BSE and are highly resistant to heat, chemicals, irradiation, and other methods of sterilization. Q: What's the chief mode of transmission? Dr Lurie: It is not thought to be transmitted horizontally-- that is, from one cow to another through a mechanism such as sneezing. The chief mode of transmission appears to be ingestion of those parts of an infected animal that contain prions. The epidemic of BSE in Great Britain that peaked in the late 1980s has been linked to changes in the way cattle feed was produced in that country in the early 1980s. One of these changes led to an increase in the use of cattle parts in cattle feed. One theory of how that epidemic started is that parts of sheep infected with scrapie were included in cattle feed. Scrapie is another type of transmissible spongiform encephalopathy that for hundreds of years has been present at relatively low levels in the British sheep population. Alternatively, there might have been a spontaneous mutation among cattle. Either way, infected cattle were slaughtered, ground up, and included in the feed of a large number of other cattle. Some of these animals became infected and were then ground up and "recycled" yet again. This ongoing cycle of cannibalism eventually caused an epidemic of some 183,000 cases of BSE in the British cattle herd. Q: So humans contract the disease in the same way, through consumption of infected meat? Dr Lurie: When the British BSE epidemic first received public attention in the mid-1980s, scientists and public health officials worried whether the disease might cross the species barrier and become a problem for humans. Because it typically takes several years after infection for the symptoms and detectable brain changes of the disease to appear, there was no way to know at first. Then, in 1996, the first case of what is now called variant Creutzfeldt-Jakob disease (vCJD)--the human counterpart of BSE--was detected. As of December 1, 2003, a total of 153 cases of vCJD had been reported worldwide. The great majority of human cases have occurred in Britain. But there have been a modest number in France, and others have occurred in places to which people who lived for extensive periods in Britain have subsequently moved (including Florida, where vCJD in a young woman was attributed to her prior residence in Britain). It is assumed that all contracted the illness by eating parts of infected cattle. Q: What has been done to prevent the spread of BSE to American cattle? Dr Lurie: The major barriers against BSE were erected in 2 phases in this country. In 1989, a ban was placed on the import of ruminants and ruminant products (such as feed made from cattle parts) from any country that had reported a case of BSE. (Ruminants are animals with multi-chambered stomachs, which include goats, sheep, cows, and so forth.) Then, in 1997, a ban was instituted on the feeding of ruminants to other ruminants. This "feed ban" is probably as important as any safeguard we have in this country--probably more important than any testing regimen. Q: The fact that BSE was diagnosed in an American cow in December seems to indicate that these measures have not been sufficient to ensure the safety of American beef. So, where are the loopholes? Dr Lurie: The first problem is that the ban on foreign animals and feed products is far from airtight. Even if you ban animals from each country in which a case of BSE crops up, infected animals may have been imported before the detection of BSE. An example is the American cow in which BSE was diagnosed; it was brought in from Canada before the disease was detected there. Also, real questions exist about the extent to which foreign feed products can be excluded when the rate of inspection at our borders is very, very low--probably under 1%. Q: So, if it's still possible that infected foreign cattle are being imported into this country, can the ban on feeding ruminants to other ruminants at least prevent BSE from spreading? Dr Lurie: It is very helpful, but there have been worrisome exceptions to the ban. For example, you could feed cows to pigs and chickens. And then, any feed that chickens did not totally consume--so-called chicken litter--was allowed in turn to be fed to cattle. Another permitted practice was the feeding of "plate waste" to cattle. Plate waste is food and even packaging that is left over at the end of a meal. These leftovers were collected from large institutional eating places, treated in various ways, and pressed into pellets for use as cattle feed. These could easily have contained leftover hamburger. The FDA has indicated that it will eliminate the foregoing 2 exceptions to the ruminant feed ban. However, an even more serious concern than these obvious loopholes is lack of proper enforcement of the current feed ban. Historically, the ban has not been carefully enforced. It took the FDA many years to finally inspect all the registered feed mills for the first time. Yet some have not been reinspected in years--and there are thousands of feed mills that are not registered. Q: Can you tell me about some of the things that are being done to minimize the risk of transmission to humans? Dr Lurie: For years there have been regulations to prevent animals with a high possibility of infection, such as those with neurologic disease, from entering the human food supply. Since January, the United States Department of Agriculture (USDA) has also excluded all cows that are unable to ambulate ("downer" cows); these are thought to be at higher risk for BSE than healthy animals. The Canadian cow in which BSE was diagnosed last year was reported to have been a downer cow. However, because downer cattle are much less common than cattle that appear normal, most of the risk of BSE is actually among "non-downers." Thus, the elimination of downers was a good move, but its impact is limited. Q: Wouldn't testing of cattle be a simpler and more effective way to keep infected animals out of the food supply? Dr Lurie: I think one has to have modest expectations about what testing can accomplish. There is no blood or cerebrospinal fluid test, either for animals or humans. The only way to diagnose BSE is by brain biopsy, which in cows is done at slaughter. Also, evidence suggests that while animals (and humans) are infected at a fairly young age, symptoms of the disease and detectable brain changes develop only as they get older. Thus, younger animals will test negative for the prion even if they have already been infected. Worldwide, only a small number of the cattle known to have been infected have been younger than 30 months. Last year, about 20,000 cows were tested. This represents a tiny fraction of the 40 million or so cattle that are slaughtered in this country each year. Q: Are there parts of a cow that are more risky to eat than others? Dr Lurie: Fortunately, the infectious prions are primarily confined to neurologic tissue. Thus, to the extent that you can avoid eating neurologic tissue, you can dramatically-- if not wholly--eliminate your risk of contracting the disease. USDA regulations now prohibit the use, in food for humans, of skull, brain, trigeminal ganglia, eyes, vertebral column, spinal cord, and dorsal root ganglia of cattle aged 30 months or older, as well as the tonsils and small intestines of all cattle. It may also be wise to avoid cuts of meat that have bone on them, such as T-bone steak, because they are more likely to contain neurologic tissue. Even ribs could be risky, because ribs attach to the vertebrae. However, most authorities feel that consumption of a cut of beef that is pure muscle meat represents minimal risk--if any. Although infectious material has recently been detected even in skeletal muscle, this was only with a highly sensitive assay. Q: What about ground meat? Dr Lurie: This can be somewhat worrisome, because of the use of a process called "advanced meat recovery." When an animal is slaughtered, the meat is trimmed from the bones with a circular saw. Still, little scraps adhere to the bone. Advanced meat recovery is the process by which these bones are either crushed in powerful machines or exposed to belts that shave off the last scraps of muscle. The material that results can be called "meat," according to US regulations, and can then be processed and used in hamburger, pizza toppings, or hot dogs. However, a number of tests have demonstrated that small amounts of neurologic tissue do get into the advanced meat recovery end product. The government has now banned advanced meat recovery for animals older than 30 months. We think it should just be banned entirely. More fundamental, the new regulations do not actually ban the inclusion of spinal cord in advanced meat recovery product; they only prevent any such product from being labeled as "meat." But it can still be labeled as "beef" and used in beef extract, beef flavoring, and beef stock. Q: Does it matter how well the meat is cooked? Dr Lurie: No, that's irrelevant. You can inactivate the infectious agent only at extremely high temperatures and pressures. Exposure to a very strong base is also effective. But all of these things will make any cut of meat inedible. Thus, normal cooking procedures are not going to inactivate the agent. Q: So when a patient says, "I ate beef last night. Do I need to be worried?" how do you respond? Dr Lurie: You need to find out exactly what the patient ate so that you can educate him or her about the riskiest types of beef. However, in general, I would assure the patient that there is very little need to worry. The risk at the public health level is different from the risk for an individual. As much as I may be personally concerned about the overall risk of vCJD occurring in the United States, and as critical as I am of the public health job being done, I still think the risk to individual Americans posed by BSE so far is small. Q: Are cattle parts used in any other products that could pose a risk? Dr Lurie: One that bears mentioning is dietary supplements. The supplement industry is hardly regulated at all in the United States. It is theoretically possible to take a piece of cow brain, crush it up, put it into a dietary supplement, and import it into this country with very little chance of its being detected. There is no requirement that such supplements be clearly labeled as to their contents or the country of origin of their ingredients. The supplements that pose the greatest risk are those that contain ground-up organs--so-called glandular supplements. I would stress to patients that it is foolish to take a dietary supplement that contains something like bovine brain or testis. The FDA has said it will ban parts of cattle older than 30 months from use in dietary supplements; however, since there is no evidence that beef products are medically effective for anything, there is no reason not to ban them entirely. Q: Aren't animal materials also used in the manufacture of some standard medications and biologics--such as various kinds of gel caps and vaccines? Is there any danger from these? Dr Lurie: In the early 1990s, the FDA banned the use in vaccines of bovine material from countries with BSE. However, the manufacturers ignored the ruling and the FDA failed to enforce it. In 2000, several companies admitted that they had used fetal calf serum and other bovine materials from British cattle in the preparation of some vaccines. There was great concern that millions of doses of many different vaccines might be dangerous. However, the consensus of those who studied the matter was that, because of the processing that vaccines undergo, the risk to patients was very, very small. So far, no one is known to have contracted vCJD from a vaccine. The risk cannot be conclusively eliminated, because the incubation for the disease is 10 years on average. Still, it is small enough that no patient should forego a necessary vaccination as a result. The gelatin used in medical products such as gel caps and depo injections has also been somewhat of an issue, because gelatin is made from cow hooves and hide. Much of the gelatin in this country is imported from Germany, which has had a modest BSE epidemic. Although the risk from imported gelatin is most likely minimal, because of the extensive processing it undergoes during manufacture, it would be safer to require that only domestic gelatin be used in medical products. Q: What advice would you give patients who hunt and eat the game they kill? Dr Lurie: A type of transmissible spongiform encephalopathy-- chronic wasting disease--is seen in deer and elk. Chronic wasting disease appears to be much more infectious than BSE. However, there has never been a case of vCJD attributed to consumption of a deer or elk with chronic wasting disease. Even so, we suggest that anyone who hunts his or her own meat either have someone else dress it, or else acquire the necessary expertise to ensure that he or she will not contaminate the meat with fragments of brain or spinal cord. Q: Has there been any evidence that vCJD can be transmitted from human to human, for example, through blood transfusion? Dr Lurie: There is now a case of vCJD in Britain which very likely resulted from transmission by blood transfusion. Even before this case, the United States placed restrictions on blood donation by people who had resided for a length of time in Britain (3 months or longer, between 1980 and 1996) or in Europe (5 years or longer, between 1980 and the present). The risk of contracting vCJD from a blood transfusion is very, very low. Physicians should reassure patients that fear of vCJD is no reason not to receive a transmission that is medically necessary. Q: Although it seems clear that the chances of a physician in this country seeing a patient with vCJD are minimal, it is always best to be prepared. What are the symptoms that suggest vCJD? Dr Lurie: Psychiatric manifestations, such as depression, are very often the presenting symptom in patients with vCJD. This helps distinguish vCJD from ordinary CJD, in which gait difficulties, jerky movements, and rapidly progressing dementia are more commonly seen early on. Also, vCJD tends to develop in patients who are much younger (average age, 29 years) than those in whom CJD is seen (average age, 65 years), and it has a slower progression (median duration of illness, 14 months) than CJD (median duration of illness, 4.5 months). Q: What should you do if you suspect that a patient has vCJD? Dr Lurie: The most important thing, as always, is to exclude other potential causes of the condition that you are evaluating--by the usual means. Get a good history of travel and nutritional habits and a medical, surgical, and transfusion history from the patient and his or her family. Pay particular attention to patients with a history of neurosurgery; cadaveric dura mater grafts have been a source of numerous cases of CJD, approaching in number the well-known epidemic of CJD caused by human growth hormone. At this point, vCJD can only be diagnosed by identification of the characteristic spongiform brain changes in a biopsy specimen. A specialized laboratory at Case Western Reserve University in Cleveland accepts specimens for diagnosis and study.

 
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