Putting Mad Cow Disease in perspective

A cow in central California was recently discovered to have a rare degenerative neurological disease. Tests showed that the cow had a “sporadic” form of bovine spongiform encephalopathy (BSE). The discovery caused concern after the initial, unconfirmed tests were reported because there is a “classical” form of BSE, which has killed over 200 people worldwide since 1996. No human deaths have ever been linked to the sporadic form of BSE.

Both forms of BSE are also called “Mad Cow Disease” by the general public, since cows that contract the disease lose normal neural functioning.

Sporadic BSE occurs seemingly at random, possibly by genetic mutation, but does not transmit between animals.

“It’s a single animal and they [the sporadic cases] happen in different places around the world in random events,” said James Cullor, who is a professor in the department of population health and reproduction in the UC Davis School of Veterinary Medicine.

Classical BSE is unique in that it is not transmitted by viruses or bacteria, but rather by proteins called “prions.”

“The normal form of the protein in the brain loses its structural integrity and begins to take on a different shape. This leads to changes in the brain and causes lesions,” said Terry Lehenbauer, the director of the UC Davis Veterinary Medicine Teaching and Research Center, which is located in Tulare, Calif.

Veterinarians have concluded that prions are transmitted between cows when cows eat cow brains or cow spinal cord tissue — a type of cannibalistic feeding method which was banned in the United Kingdom in 1996 and in the U.S. in 1997.

When BSE crosses over to the human species, it becomes known as “variant Creutzfeldt-Jakob disease” (vCJD). There is also a sporadic form in humans (sCJD), which typically only occurs in older people and is often mistaken for Alzheimer’s disease.

“[Sporadic CJD] occurs spontaneously in about one out of 1 million people around the world,” said John Maas, an extension veterinarian in the UC Davis School of Veterinary Medicine. On average, 37 people in California contract the sporadic form of CJD every year. Two reported cases received widespread media attention recently in Marin County.

During the prior international human epidemic of vCJD starting in 1996, BSE was typically transmitted to humans by the consumption of beef sausage or processed meats that included cow brain or cow spinal cord material. Those who ate unprocessed cow flesh or drank milk were never at risk since BSE cannot be transmitted through muscle or milk.

Most of the human deaths caused by vCJD have occurred in the UK, where the public health problem was first discovered in 1996, with other deaths occurring elsewhere in Europe, Saudi Arabia, Japan, Canada and the U.S.

Of the three U.S. deaths, none were likely due to the consumption of U.S. beef. All three had most likely contracted the disease while living overseas.

Strict regulatory controls have been instituted in the U.S. and other countries since the discovery of the variant form of CJD in 1996. The short average lifespan of U.S. beef cattle is another reason that BSE is not spread in the U.S.

“Most cattle consumed in the U.S. are less than two years of age and the prions in cattle with BSE take at least 30 months to develop,” Maas said.

Because of regulatory controls and the fact that BSE has never entered the U.S. food supply, Cullor stresses that there is less than one chance in one billion of getting the disease here.

“One part per billion is 1 inch on the circumference of the earth, so if you parachuted down and landed on the equator, what’s your chance of landing on that particular inch of the equator?” Cullor asked.

BRIAN RILEY can be reached at science@theaggie.org.

EDITOR’S NOTE: The print edition of this story said that Creutzfeldt-Jakob was discovered in 1996. This article has been updated to say that it is a variant that was discovered in 1996. The Aggie regrets the error.