Estimates of Future Human Death Toll From Mad Cow Disease Vary Widely

October 30, 2001 The New York Times by Sandra Blakeslee
The human form of mad cow disease may peak at 100 cases per year in Britain and kill no more than a few thousand people in coming decades, according to new estimates from a highly respected group of epidemiologists in London.

But other leading experts who study the disease say that these numbers are based on overly optimistic assumptions. They say more than 100,000 people may die from the disease in the next several decades. Such is the state of Britain's struggle to predict how many of its people are going to contract a grisly disease that emerged five years ago and has thus far taken the lives of 107 Britons. It is believed that all the victims contracted the disease from eating meat from cattle infected with mad cow disease, a uniformly fatal malady that slowly destroys brain tissue.

Mad cow disease, known formally as bovine spongiform encephalopathy, occurs when a normal cellular protein, called a prion, misfolds and can no longer be broken down by the body. The misfolded, or rogue, proteins accumulate, particularly in the brain, leaving spongy holes and killing healthy tissue.

A related ailment in people, sporadic Creutzfeldt-Jakob disease, has been recognized for decades and kills about 300 Americans a year, usually in old age. The new disease, dubbed variant Creutzfeldt-Jakob disease, typically strikes younger people. No cases of the new variant form have been reported in the U.S.

Though mad cow disease has been unambiguously diagnosed in fewer than 200,000 cows since 1987, experts estimate that the British population consumed at least three-quarters of a million infected cattle between 1980 and 1996, when variant C.J.D. appeared in people. Mad cow disease has not appeared in American herds, though some native deer and elk are infected with a closely related disorder called chronic wasting disease, which is now spreading among captive elk herds here and in Canada..

The new estimates were made by Dr. Jerome Huillard d'Aignaux and his colleagues at the infectious disease epidemiology unit in the London School of Hygiene and Tropical Medicine. They described their work last week in the journal Science.

Dr. Huillard's group used a statistical method called back calculation, which was developed to predict patterns for the spread of H.I.V. and AIDS. The method, which calculates backward from current case numbers to recreate the original conditions that gave rise to the epidemic, uses three criteria for determining the size and scope of the problem.

"You need to know when people were infected, how long it takes people who have been infected to develop the disease and how many people were infected," said Dr. Simon Cousens, an epidemiologist who helped carry out the study. "When you put the three together, you can write equations to tell you how many cases will occur and when they will occur."

If two out of the three criteria can be determined, Dr. Cousens said, the third can be estimated. The question then becomes one of selecting assumptions for building back calculation models.

The groups' first assumption concerned when people were infected. Cattle first showed signs of infection, thought to be spread via animal feed made from rendered cattle and sheep flesh, in the early 1980's. But because the disease was not formally recognized until 1988, when restrictions were placed on animal feeding practices, the researchers assumed that people were exposed to the disease only after 1988.

In 1996, when scientists realized that the disease had spread from cattle to people, further controls were placed on the food supply, Dr. Cousens said. And so the model assumes that there was no further transmission of the disease to humans after 1996.

The second assumption involves possible incubation periods for variant C.J.D. -- the length of time it takes for a person who eats infected beef to show signs of the disease. The biology of variant C.J.D. is very complicated, Dr. Cousens said. Considerations include how much infected meat a person must consume to contract the disease, the genetic profile of patients and the rate at which the infectious agent that causes the disease, called a prion, replicates. A few such variables were placed into different "curves" of possible incubation periods, he said, but the models were kept simple.

The third assumption -- how many people were infected -- can only be guessed, Dr. Cousens said. The researchers plugged low, medium and high numbers into their equations to see what the different models predicted in terms of how many people would eventually get the disease.

Given these assumptions and their variables, all the models suggested that the variant C.J.D. epidemic was very close to its peak, Dr. Cousens said. No model came up with a number exceeding 10,000 deaths and most were far lower, in the range of a few thousand deaths.

The incubation time is a critical factor in determining how many people may die from variant C.J.D., Dr. Cousens said. If the incubation period is short, the epidemic should play itself out in the next few years. If the incubation period is long, say more than 50 years, most people who harbor the infection will die of other diseases like strokes and cancer before showing any signs of the brain wasting disease. Thus even if a large number of Britons now carry the infection without knowing it and the incubation is long, the death rate from variant C.J.D. will remain relatively low, he said.

But Dr. Neil Ferguson, an epidemiologist in another group of highly respected researchers led by Dr. Roy Anderson at Imperial College in London, said the new estimates were "unjustifiably optimistic." His group published estimates a year ago predicting that the number of variant C.J.D. cases might reach 136,000 in coming decades. They have since revised those numbers only very slightly downward in a study of their own, which will be published soon.

There are many problems with the new study, Dr. Ferguson said. The assumption that no one was exposed to B.S.E. before 1988 is extremely naive, he said. In fact, there was much underreporting of disease by farmers and veterinarians who did not understand what was happening to their animals. Thus, the problem is larger than their models assume.

Moreover, the assumption that people are no longer contracting variant C.J.D. from cattle cannot be proved. Recent research shows that cattle and other animals infected with prion diseases can remain asymptomatic for many years and yet still spread the disease. And biological factors that predispose people to the disease or protect them from it are not well understood.

More important is the question of human- to-human transmission of variant C.J.D. via contaminated blood products, donor tissue and surgical or dental instruments, Dr. Ferguson said. If a large number of people are carriers of the disease without their knowledge and their body fluids or tissues carry the infection, it is possible that many people will be exposed to the disease. The risk remains theoretical, Dr. Ferguson said, but it worries many public health experts.

Finally, genetics play a huge role that needs to be included in any modeling of variant C.J.D. projections. It is now assumed that all victims of the disease have the same genetic profile; their normal prion protein has two copies of the same amino acid, methionine, at a single point called codon 129. This so-called met-met trait is shared by 40 percent of the British population. But if this turns out to be false, Dr. Ferguson said, and the entire population is at risk, the worst case numbers would have to be multiplied by a factor of 2.5.

"We won't know which estimates are more likely until more cases are diagnosed," Dr. Ferguson said.

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