MONDAY, March 27, 2006 (HealthDay News) -- People may be at risk for contracting the human version of mad cow disease, even if they haven't eaten parts from an infected animal, a new study contends.
Instead, new experiments with mice suggest that human-to-human transmission via blood transfusions, unsterilized surgical instruments or other means could be a relatively easy mode of infection with the deadly disease.
British researchers say variant Creutzfeldt-Jakob disease (vCJD), the human version of the disease, was more easily transmitted in mice producing a human gene than bovine spongiform encephalitis (BSE) or "mad cow" disease, the animal version.
vCJD, which is caused by a distorted protein called a prion, also infected mice with gene variants that had previously been thought to be immune to the illness.
"It means that we should be aware and keep an open eye and set up the best possible measures to avoid human-to-human transmission because we have to count on the fact that there might be more asymptomatic carriers that we know of," said Dr. Corinne Lasmezas, professor of infectology at Scripps Florida in Jupiter. "But it doesn't mean that it will be a huge epidemic."
Lasmezas wrote a "reflection and reaction" paper that accompanied the study. Both appeared Monday in the journal The Lancet Neurology.
The total number of human vCJD cases worldwide now stands at 190. The incubation period is so long, however, that the toll may rise anywhere from the low hundreds to the hundreds of thousands, the study authors stated.
In August 2004, British authorities reported evidence that vCJD might be spread by blood transfusion. The misfolded prions characteristic of vCJD were detected in an elderly person who received a blood transfusion in 1999 from a donor who developed the disease after the donation and died in 2001.
The transfusion recipient, whose age and sex were not disclosed, did not have symptoms of the disease and died of other causes.
Another case of transmission linked to a transfusion involved a victim who developed symptoms of the disease six-and-a-half years after receiving a transfusion. In that case, the donor developed symptoms three years after the donation and died of vCJD.
Because of the potential risk, the United States does not accept blood donations from people who spent at least five months in Europe since 1980 or who spent three months in the United Kingdom between 1980 and 1996.
No human equivalent of mad cow disease has yet been found to have originated in the United States, but three BSE-infected cows have been identified.
For the new study, Edinburgh-based researchers engineered mice to carry one of three different human gene variants, known as MM, MV or VV. Among Caucasian humans, 50 percent of individuals are MV, 40 percent are MM and 10 percent are VV.
All of the human cases of vCJD thus far have been in people who are MM. One person who tested positive for prions but died of other causes was MV.
"As far as clinical cases, being MM seems to be a particular factor of susceptibility," Lasmezas said. It had been thought -- or hoped -- that other gene variations were less susceptible. The MV person, she added, "was the first hint that MV people might be susceptible."
The researchers then inoculated the engineered mice, along with mice which had bovine genes, with either vCJD or BSE.
BSE was transmitted to the bovine line but not to the human lines; vCJD, on the other hand, infected all three human lines.
"This means that as far as we can extrapolate to humans, which is something we always have to be careful with, all genotypes allow infection so all humans could be potentially infected if they have been exposed to a sufficient amount of the infectious agent," Lasmezas said.
There was, however, a gradation of susceptibility. Most mice that were infected were MM, followed by MV, who were infected at about the same rate but developed the infection later. Transmissions were much lower among VV mice, indicating that this variant might be protective.
"If this is true in humans, there may be as many MV people incubating but they might develop the disease later or might never develop the disease, but we have to count on the fact that there might be MV people who are infected and are subclinical [no symptoms] carriers," Lasmezas said.
One positive note: The study mice were infected directly with brain material, meaning the findings may be less relevant and less alarming to humans.
"This was brain-into-brain and that doesn't occur very often," said Dr. Carol K. Petito, professor of pathology at the University of Miami Miller School of Medicine. "It's a mistake to say this never will happen... [but] human-to-human transmission will be less efficient."
More information
Visit the U.S. National Library of Medicine to learn more about Creutzfeldt-Jakob disease.
