Editor’s Note: This is the second in a two-part series about Mad Cow Disease. This article examines the theories of how the disease transferred into the human population.

The frenzy over Mad Cow Disease in recent weeks has brought an old subject back into the spotlight.

Although scientists have studied the disease for years, they still are not certain how bovine spongiform encephalopathy, or Mad Cow Disease, developed in humans.

Prion diseases, or transmissible spongiform encephalopathies [TSEs], exist in a variety of animals, said Nolan Hartwig, ISU Extension veterinarian specializing in beef, dairy, sheep and veterinary public health. Scrapie, a TSE that occurs in sheep, has been present in Europe for more than 200 years and is also present in the United States. A TSE also occurs in mink, and there is a disease called Chronic Wasting Disease in elk and deer.

Hartwig said there are several TSEs in humans also. A disease called Kuru developed in New Guinea native tribes who practiced ritualistic cannibalism. When a person died, family members would eat parts of the brain. This practice led to the development of a TSE. Other TSE diseases in humans are Gertsmann-Straussler Syndrome and Creutzfeldt-Jakob Disease, which usually appears in older adults.

The human version of BSE, first diagnosed in 1986, is known as New Variant Creutzfeldt-Jakob Disease, or vCJD. The disease appears in younger people and appears to be nearly identical to BSE in cows, Hartwig said.

BSE is mainly passed to other animals through feed, said Harley Moon, professor at the ISU Veterinary Medical Research Institute. He said there is no evidence that it can be passed by contact – it must be ingested or injected.

“They are not, in the ordinary sense of the word, contagious,” he said.

Hartwig said it is not known how BSE developed in cows, but the working theory is that sheep byproducts infected with scrapie were fed to cows in England, and the scrapie mutated to become infectious to cattle.

“At this point, we have to say that’s an unknown,” he said. “But we still have to use the working hypothesis so that we err on the side of safety.”

Hartwig said the current theory is that BSE somehow transferred to the human population, possibly from infected beef. There are currently about 90 people either dead or dying from BSE. Besides England, cases have been diagnosed in France, Spain, Italy, Belgium and Germany, according to CNN.com.

Janice Miller, veterinary medical officer with the National Animal Disease Center of the USDA Agricultural Research Service, said it is difficult to know exactly how many cases of BSE exist because of its long incubation period – probably four to five years.

“Most [infected] animals are four to five years old,” she said, “and they’re assuming that the [contaminated meat and] bone meal was ingested when they were young calves.”

However, some infected animals are 10 years old, so she said it is unclear how long the incubation period is. Some TSEs, such as Kuru, can have incubation periods of several decades, she said.

Hartwig said the disease probably spread to the continent from animal byproducts exported from England.

Although Moon said it is unlikely that BSE will be imported into the United States, he said there is a possibility that the disease could appear in the United States as it did in England. However, if that occurred, he said, the United States has extensive safeguards to ensure BSE does not enter the cattle population.

These safeguards include a ban on the importation of cattle and the importation of animal byproducts. Also, regulations are in place that prevent feeding byproducts from ruminants such as cows, sheep and deer, to other ruminants.

“If [BSE] were to emerge, we’ve put in place the things we need to do to detect it, [probably] reasonably early,” he said.

In addition to importation bans, the United States has an aggressive surveillance program, Hartwig said. Brain tissue from animals showing signs of a nervous system disease are sent to the United States Department of Agriculture’s National Veterinary Services Laboratory [NVSL] in Ames, where the animals are examined for BSE.

Miller said the brain samples are stained with an antibody designed to detect the BSE protein. Currently, no animals have ever been diagnosed with BSE in the United States, she said.

Some people have criticized the surveillance program, which began in the early 1990s, for not being proactive enough and screening animals that are not showing signs of the disease, Miller said. However, she said such a program would most likely be futile in the United States.

“Europe’s finding a lot of those animals that aren’t showing clinical signs,” she said, “but then they also find those animals that do show signs.”

So far, Miller said the NVSL has looked at about 12,000 brain specimens and found no sign of BSE. The brain specimens are taken from dead animals and sent to the NVSL, where they use a special test for the prion protein that causes BSE.

“That’s been a source of confusion for a lot of people, because they don’t know that all the tests are [not] being done on live animals,” she said.

Miller said it is possible to do tests on living subjects, or patients. Doctors could do a brain biopsy, which would extract tissue from the brain of a live animal. However, the procedure is very costly, and it is not done on animals because of its expense.

Nancy Degner, vice president of consumer marketing for the Iowa Beef Industry Council, said the surveillance program in the United States would catch any outbreak of BSE early.

“With all that has been put in place by the industry, it should be identified and contained, and that’s it – it’s over,” she said.

Degner said the American beef industry is concerned by the threat from BSE, but they are confident the regulatory system will prevent any cases of BSE in the United States.

“The cattle industry is extremely concerned that we don’t want it here,” she said, “but we’re also confident that there’s a lot of safeguards in place, both regulatory and in the industry, that we should not have a problem.”

She said the United States has been fortunate in having time to prepare for the disease and put effective safeguards in place.

“We have the luxury of looking back at what happened in England and making sure that won’t happen again here,” she said.

Degner said the Iowa Beef Industry Council is trying to educate the public about BSE. She said they have been communicating with newspapers around the state and food safety professionals to keep them informed of any developments with BSE.

Although beef prices dipped when a cattle herd in Texas was quarantined after eating feed made from ruminant byproducts, she said the markets have recovered, and there has been no major economic impact since then.

“Anytime there’s negative information of such a scary nature, I think it damages the industry,” she said. “Whether people are choosing to not eat beef, we have not seen that.”

Hartwig said beef is safe to eat.

“Do I eat beef? You bet,” he said. “And would I eat beef in the UK or Ireland? Yes.”