What is bovine spongiform encephalopathy(BSE), or “mad cow disease,” and how is itthought to infect cattle?
Q: What is bovine spongiform encephalopathy(BSE), or "mad cow disease," and how is itthought to infect cattle?Dr Lurie: BSE is a progressive neurologic disorder of cattlethat is currently understood to be an infectious disease--but not in the customary sense. It does not appearto be caused by a virus, a parasite, a bacterium, or a fungus.Instead, a misfolded protein, or prion, is believedto be the infectious particle that is transmitted betweenanimals, or from an animal to a person. These misfoldedproteins occur in high concentration in the diseasedareas of the brain of animals with BSE and are highly resistantto heat, chemicals, irradiation, and other methodsof sterilization.Q: What's the chief modeof transmission?Dr Lurie: It is not thought to be transmitted horizontally--that is, from one cow to another through a mechanismsuch as sneezing. The chief mode of transmission appearsto be ingestion of those parts of an infected animal thatcontain prions. The epidemic of BSE in Great Britain thatpeaked in the late 1980s has been linked to changes in theway cattle feed was produced in that country in the early1980s. One of these changes led to an increase in the useof cattle parts in cattle feed.One theory of how that epidemic started is that partsof sheep infected with scrapie were included in cattle feed.Scrapie is another type of transmissible spongiform encephalopathythat for hundreds of years has been presentat relatively low levels in the British sheep population. Alternatively,there might have been a spontaneous mutationamong cattle. Either way, infected cattle were slaughtered,ground up, and included in the feed of a large number ofother cattle. Some of these animals became infected andwere then ground up and "recycled" yet again. This ongoingcycle of cannibalism eventually caused an epidemic ofsome 183,000 cases of BSE in the British cattle herd.Q: So humans contract the disease in the sameway, through consumption of infected meat?Dr Lurie: When the British BSE epidemic first receivedpublic attention in the mid-1980s, scientists and publichealth officials worried whether the disease might crossthe species barrier and become a problem for humans.Because it typically takes several years after infectionfor the symptoms and detectable brain changes of thedisease to appear, there was no way to know at first.Then, in 1996, the first case of what is now called variantCreutzfeldt-Jakob disease (vCJD)--the human counterpartof BSE--was detected. As of December 1, 2003, atotal of 153 cases of vCJD had been reported worldwide.The great majority of human cases have occurred inBritain. But there have been a modest number in France,and others have occurred in places to which people wholived for extensive periods in Britain have subsequentlymoved (including Florida, where vCJD in a young womanwas attributed to her prior residence in Britain). It is assumedthat all contracted the illness by eating parts of infectedcattle.Q: What has been done to prevent the spreadof BSE to American cattle?Dr Lurie: The major barriers against BSE were erectedin 2 phases in this country. In 1989, a ban was placed onthe import of ruminants and ruminant products (such asfeed made from cattle parts) from any country that hadreported a case of BSE. (Ruminants are animals withmulti-chambered stomachs, which include goats, sheep,cows, and so forth.) Then, in 1997, a ban was instituted onthe feeding of ruminants to other ruminants. This "feedban" is probably as important as any safeguard we have inthis country--probably more important than any testingregimen.Q: The fact that BSE was diagnosed in an Americancow in December seems to indicate that thesemeasures have not been sufficient to ensure the safetyof American beef. So, where are the loopholes?Dr Lurie: The first problem is that the ban on foreignanimals and feed products is far from airtight. Even if youban animals from each country in which a case of BSEcrops up, infected animals may have been imported beforethe detection of BSE. An example is the Americancow in which BSE was diagnosed; it was brought in fromCanada before the disease was detected there. Also, realquestions exist about the extent to which foreign feedproducts can be excluded when the rate of inspection atour borders is very, very low--probably under 1%.Q: So, if it's still possible that infected foreigncattle are being imported into this country,can the ban on feeding ruminants to other ruminantsat least prevent BSE from spreading?Dr Lurie: It is very helpful, but there have been worrisomeexceptions to the ban. For example, you could feed cowsto pigs and chickens. And then, any feed that chickensdid not totally consume--so-called chicken litter--was allowedin turn to be fed to cattle.Another permitted practice was the feeding of "platewaste" to cattle. Plate waste is food and even packaging that is left over at the end of a meal. These leftoverswere collected from large institutional eating places,treated in various ways, and pressed into pellets for useas cattle feed. These could easily have contained leftoverhamburger.The FDA has indicated that it will eliminate the foregoing2 exceptions to the ruminant feed ban. However, aneven more serious concern than these obvious loopholesis lack of proper enforcement of the current feed ban.Historically, the ban has not been carefully enforced. Ittook the FDA many years to finally inspect all the registeredfeed mills for the first time. Yet some have not beenreinspected in years--and there are thousands of feedmills that are not registered.Q: Can you tell me about some of the thingsthat are being done to minimize the risk oftransmission to humans?Dr Lurie: For years there have been regulations to preventanimals with a high possibility of infection, such as thosewith neurologic disease, from entering the human foodsupply. Since January, the United States Department ofAgriculture (USDA) has also excluded all cows that are unableto ambulate ("downer" cows); these are thought to beat higher risk for BSE than healthy animals. The Canadiancow in which BSE was diagnosed last year was reportedto have been a downer cow. However, because downer cattleare 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, butits impact is limited.Q: Wouldn't testing of cattle be a simpler and moreeffective way to keep infected animals out of thefood supply?Dr Lurie: I think one has to have modest expectationsabout what testing can accomplish. There is no blood orcerebrospinal fluid test, either for animals or humans.The only way to diagnose BSE is by brain biopsy, whichin cows is done at slaughter. Also, evidence suggests thatwhile animals (and humans) are infected at a fairly youngage, symptoms of the disease and detectable brain changesdevelop only as they get older. Thus, younger animalswill test negative for the prion even if they have alreadybeen infected. Worldwide, only a small number of the cattleknown to have been infected have been younger than30 months.Last year, about 20,000 cows were tested. This representsa tiny fraction of the 40 million or so cattle that areslaughtered in this country each year.Q: Are there parts of a cow that are more risky toeat than others?Dr Lurie: Fortunately, the infectious prions are primarilyconfined to neurologic tissue. Thus, to the extent that youcan 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 forhumans, of skull, brain, trigeminal ganglia, eyes, vertebralcolumn, spinal cord, and dorsal root ganglia of cattle aged30 months or older, as well as the tonsils and small intestinesof all cattle.It may also be wise to avoid cuts of meat that havebone on them, such as T-bone steak, because they aremore likely to contain neurologic tissue. Even ribs couldbe risky, because ribs attach to the vertebrae. However,most authorities feel that consumption of a cut of beef thatis pure muscle meat represents minimal risk--if any.Although infectious material has recently been detectedeven in skeletal muscle, this was only with a highly sensitiveassay.Q: What aboutground meat?Dr Lurie: This can be somewhat worrisome, because ofthe use of a process called "advanced meat recovery."When an animal is slaughtered, the meat is trimmed fromthe bones with a circular saw. Still, little scraps adhere tothe bone. Advanced meat recovery is the process bywhich these bones are either crushed in powerful machinesor exposed to belts that shave off the last scrapsof muscle. The material that results can be called "meat,"according to US regulations, and can then be processedand used in hamburger, pizza toppings, or hot dogs.However, a number of tests have demonstrated thatsmall amounts of neurologic tissue do get into the advancedmeat recovery end product. The government hasnow banned advanced meat recovery for animals olderthan 30 months. We think it should just be banned entirely.More fundamental, the new regulations do not actuallyban the inclusion of spinal cord in advanced meat recoveryproduct; they only prevent any such product frombeing 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 wellthe meat is cooked?Dr Lurie: No, that's irrelevant. You can inactivate the infectiousagent 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 toinactivate 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 ateso that you can educate him or her about the riskiesttypes of beef. However, in general, I would assure the patientthat there is very little need to worry. The risk at thepublic health level is different from the risk for an individual.As much as I may be personally concerned about theoverall risk of vCJD occurring in the United States, and ascritical as I am of the public health job being done, I stillthink the risk to individual Americans posed by BSE sofar is small.Q: Are cattle parts used in any other products thatcould pose a risk?Dr Lurie: One that bears mentioning is dietary supplements.The supplement industry is hardly regulated at allin the United States. It is theoretically possible to take apiece of cow brain, crush it up, put it into a dietary supplement,and import it into this country with very little chanceof its being detected. There is no requirement that suchsupplements be clearly labeled as to their contents or thecountry of origin of their ingredients.The supplements that pose the greatest risk arethose that contain ground-up organs--so-called glandularsupplements. I would stress to patients that it is foolish totake a dietary supplement that contains something likebovine brain or testis. The FDA has said it will ban partsof cattle older than 30 months from use in dietary supplements;however, since there is no evidence that beef productsare medically effective for anything, there is no reasonnot to ban them entirely.Q: Aren't animal materials also used in themanufacture of some standard medicationsand biologics--such as various kinds of gel caps andvaccines? Is there any danger from these?Dr Lurie: In the early 1990s, the FDA banned the use invaccines of bovine material from countries with BSE.However, the manufacturers ignored the ruling and theFDA failed to enforce it. In 2000, several companies admittedthat they had used fetal calf serum and otherbovine materials from British cattle in the preparation ofsome vaccines. There was great concern that millions ofdoses of many different vaccines might be dangerous. However, the consensus of those who studied the matterwas that, because of the processing that vaccines undergo,the risk to patients was very, very small. So far, no oneis known to have contracted vCJD from a vaccine. Therisk cannot be conclusively eliminated, because the incubationfor the disease is 10 years on average. Still, it issmall enough that no patient should forego a necessaryvaccination as a result.The gelatin used in medical products such as gelcaps and depo injections has also been somewhat of anissue, because gelatin is made from cow hooves and hide.Much of the gelatin in this country is imported fromGermany, which has had a modest BSE epidemic.Although the risk from imported gelatin is most likelyminimal, because of the extensive processing it undergoesduring manufacture, it would be safer to require thatonly domestic gelatin be used in medical products.Q: What advice would you give patients who huntand 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 infectiousthan BSE. However, there has never been a case ofvCJD attributed to consumption of a deer or elk withchronic wasting disease.Even so, we suggest that anyone who hunts his orher own meat either have someone else dress it, or elseacquire the necessary expertise to ensure that he or shewill not contaminate the meat with fragments of brain orspinal cord.Q: Has there been any evidence that vCJD canbe transmitted from human to human, forexample, through blood transfusion?Dr Lurie: There is now a case of vCJD in Britain whichvery likely resulted from transmission by blood transfusion.Even before this case, the United States placed restrictionson blood donation by people who had residedfor a length of time in Britain (3 months or longer, between1980 and 1996) or in Europe (5 years or longer,between 1980 and the present). The risk of contractingvCJD from a blood transfusion is very, very low. Physiciansshould reassure patients that fear of vCJD is noreason not to receive a transmission that is medicallynecessary.Q: Although it seems clear that the chances of aphysician in this country seeing a patient withvCJD 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 withvCJD. This helps distinguish vCJD from ordinary CJD, inwhich gait difficulties, jerky movements, and rapidly progressingdementia are more commonly seen early on.Also, vCJD tends to develop in patients who are muchyounger (average age, 29 years) than those in whom CJDis 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 apatient has vCJD?Dr Lurie: The most important thing, as always, is to excludeother potential causes of the condition that you areevaluating--by the usual means. Get a good history oftravel and nutritional habits and a medical, surgical, andtransfusion 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 sourceof numerous cases of CJD, approaching in number thewell-known epidemic of CJD caused by human growthhormone.At this point, vCJD can only be diagnosed by identificationof the characteristic spongiform brain changes ina biopsy specimen. A specialized laboratory at Case WesternReserve University in Cleveland accepts specimensfor diagnosis and study.