Mad Cow: Symptoms Emerge Years Later

Researchers Say Incubation Period for Mad Cow Disease May Be Longer Than Thought

Medically Reviewed by Louise Chang, MD on June 23, 2006
From the WebMD Archives

June 23, 2006 -- Symptoms of mad cow disease (bovine spongiform encephalopathy, BSE) may emerge more than 50 years after infection in humans, according to a new study.

Researchers say the findings show that the size of a potential mad cow disease epidemic may be much bigger than previously thought.

John Collinge of University College London and colleagues studied the only other known BSE disease outbreak in Papua New Guinea and found those infected in the initial outbreak in the 1950s were still developing the disease 50 years later.

Researchers say large segments of the U.K. population have been exposed to BSE prions by eating infected meat. So far about 160 cases of the human variant of mad cow disease (variant Creutzfeldt-Jakob disease, vCJD) have been identified in the U.K., with cases also reported in other countries. Prions are unconventional proteins that are behind mad cow disease, vCJD, and other types of degenerative diseases.

Recent estimates on the eventual size of a BSE outbreak are based on current numbers of vCJD patients. But researchers say determining the incubation period for the disease is critical to predicting the true extent of an epidemic and has been unknown until now.

Mad Cow May Wait to Emerge

In the study, published in The Lancet, researchers studied the only example of a human prion disease epidemic, a disease called kuru. Kuru is caused by cannibalism and reached epidemic proportions in the parts of Papua New Guinea where the consumption of dead relatives -- as a mark of respect and mourning -- occurred through the 1950s.

Between 1957 and 2004, the total number of kuru cases was more than 2,700. The average time before symptoms emerged was 12 years but was more than 50 years in some cases.

The last year of birth recorded for a patient with the disease was 1959, and researchers assumed that transmission of the disease by cannibalism stopped when the practice ceased by 1960.

However, they identified 11 people in the region who were diagnosed with new symptoms of kuru from 1996 to 2004, which meant that incubation periods for the disease ranged from 34 to 56 years and may have been even longer.

Genetic analysis showed that people recently diagnosed with kuru had a particular gene variation that is associated with extended periods of incubation and resistance to the disease.

They say the results suggest that the incubation time for kuru and other BSE diseases, including mad cow disease and variant Creutzfeldt-Jakob disease, may be much longer than previously thought.

As a result, Collinge says current predictions of the size of a human BSE epidemic may be substantially underestimated.

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SOURCES: Collinge, J. The Lancet, June 24, 2006; vol 367: pp 2068-2074. News release, The Lancet.
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