Study Examines How 'Mad-Cow' Might Jump Species

October 22, 2001 Reuters by Merritt McKinney

NEW YORK (Reuters Health) - A new report provides a glimpse of how brain-wasting diseases move from one species to another, as probably happened with ``mad cow'' disease in the UK.

The findings call into question the wisdom of giving livestock feed that contains the byproducts of other animals, one of the study's authors told Reuters Health.

Mad cow disease, or bovine spongiform encephalopathy (BSE (news - web sites)), is one of several fatal brain-wasting diseases thought to be caused by the build-up of abnormal brain proteins called prions.

In 1996, a new form of Creutzfeldt-Jakob disease (news - web sites) (CJD), a human brain-wasting disease, was reported in young adults in the UK. There is strong evidence that this new type of CJD arose when people ate meat from cows infected with BSE.

Such leaps from species to species are not thought to happen very often, but little is understood about the process.

To see what happens when one species is infected with a prion disease that normally occurs in another species, investigators at Rocky Mountain Laboratories in Hamilton, Montana, injected mice with a hamster version of the prion disease scrapie. This group of mice never developed symptoms of scrapie, and standard laboratory testing did not detect the disease-causing agent in the mice.

But when the researchers extracted material from the mice's brains and injected it into hamsters, all of the hamsters developed scrapie, according to the report in the November issue of the Journal of Virology.

The researchers are not certain that their findings will apply to other prion diseases, but they conclude that current testing methods may not be sensitive enough to detect prion diseases.

Even though the mice originally infected with hamster scrapie did not get sick, other mice were not as lucky. When scientists took hamster scrapie agents from the brains of the first group of mice, who hadn't gotten sick, and transferred the material to other mice, they became sick.

``The scrapie seemed to have learned how to deal with this new species, and it worked much better,'' according to Dr. Bruce Chesebro, one of the study's authors. ``It replicated faster in additional rounds of mice and even became more lethal to them,'' he said in a press release.

But the way scrapie developed in the mice was not consistent. The time it took each mouse to get sick varied, and the disease did not affect the same part of the brain in each animal.

In an interview with Reuters Health, Dr. Richard Race, another of the study's authors, said that said it may be a good idea to halt the practice of using animal feed containing the byproducts of other animals.

Farmers no longer give cows feed that contains byproducts of other animals, but such feed is still given to poultry, pigs and other livestock, he said.

It may be time to rethink this practice, Race explained, since the study showed that animals that are not sick may be able to pass on disease to other animals.

The research shows that when a prion disease moves from one species to another, the process is a very slow one that is not always easy to detect, Race added.

SOURCE: Journal of Virology 2001;75:10106-10112.

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