Industry may have to take a new look at how prions spread in animals, such as cattle.© Getty Images

One assumption lies at the root of efforts to keep the meat we eat safe from mad cow disease: that tissues beyond an animal's brain, spinal cord and immune system are free of the prions that cause the disease.

A disturbing study now shows that assumption to be false. Researchers have found that if an animal falls ill with another infection, its immune response can carry large numbers of prions to organs throughout its body.

"The rules no longer apply," warns pathologist Adriano Aguzzi at Zurich University Hospital, Switzerland, who led the research.

Prion problems

Mad cow disease, more correctly known as bovine spongiform encephalopathy (BSE), is believed to be caused by rogue proteins called prions. When these prions enter the human food chain, they can cause the equivalent disease in humans, called new-variant Creutzfeldt-Jakob disease (vCJD).

Patients develop rapidly increasing dementia, and die soon after the onset of symptoms. Since vCJD first appeared in Britain in the mid-1990s, only a handful of people have succumbed, but uncertainty about the incubation time of the disease leaves open the possibility that the number of people infected is actually much larger.

The outbreak has caused massive public concern over the effects of beef consumption. To prevent the spread of disease, several countries affected by BSE including Britain, Canada and the United States have implemented regulations that exclude the brain, spinal cord and immune-system tissues, such as the spleen, from the food chain.

It was thought that other body parts were safe to eat. "This is why you can still eat products from susceptible animals in the countries with BSE," says Aguzzi.

Inflamed infection

Aguzzi's research has begun to chip away at that belief. In 2003, he and his colleagues announced that they had detected small amounts of prions in the muscle tissue of people who had died from vCJD, suggesting that animal meat might harbour trace amounts of these disease proteins too.

But nothing prepared Aguzzi and his team for what they found in their most recent study. They took mice with the equivalent of BSE and induced an 'inflammatory response' in them. This is the type of immune response that the body mounts in the face of a wide range of injuries and illnesses, including cuts, the common cold and type I diabetes.

There was an explosion of prions in the animals' pancreases, kidneys and livers, the researchers report in this week's issue of Science¹. The amount of prions in these organs was just as high as is generally found in diseased spleens.

"If the animal has an additional infection in the body, the prions are no longer confined to the areas where they normally are," explains Surachai Supattapone, an expert in infectious diseases at the Dartmouth Medical School in Hanover, New Hampshire.

The researchers believe that the cells involved in the inflammatory response somehow help the prions to replicate, and to spread to the parts of the body being targeted by the immune reaction. 

Pathologist at Zurich University Hospital, Switzerland

• References

  1. Heikenwalder, M. et al. Science Published online: doi:10.1126/science.1106460 (2005).