A leukemia virus commonly found in dairy cattle can be transmitted in cow's milk to at least one other animal species, sheep, and in the test tube to human cells, according to scientists at the University of Pennsylvania.
The scientists, who reported the findings yesterday in the weekly journal Science, noted that it is safe to drink milk, particularly that which has been pasteurized, as is virtually all milk sold in the United States.
"I do feel infants should not be fed raw milk from infected cows," said Dr. Jorge Ferrer, the report's chief author. "People drink raw milk in many countries."
Ferrer said Americans and others who drink pasterurized milk face only a "small possible risk" of contracting leukemia from the bovine virus. The risk is far outweighed, he said, by "milk's enormous nutritional benefits."
"People have been drinking milk from infected cows for many years, yet leukemia remains relatively rare in human populations," he said. Leukemia, or cancer of the blood-forming tissues, accounts for 2.8 percent of human cancers.
Dr. Robert Gallo, National Cancer Institute virologist, agreed with Ferrer.
In examination of dairy farmers, "hundreds of cases of leukemia" and "thousands of human sera" or blood samples, Gallo said, no one has ever found any antibody, or bodily reaction, to bovine virus "or any other proof of human infection."
Ferrer noted that methodology has not been sophisticated enough to rule out the possibility of some human infection, and said more study is needed.
Gallo called the bovine virus "an important model" for the way one or more human viruses might conceivably work in helping to cause leukemia in humans. It has long been known that viruses can cause leukemia in chickens, cats, mice and, since the late 1960s, cattle. But existence of a human leukemia virus has not been proved.
"In many species, including cows, we do find leukemia is due to a virus," Ferrer said. "I believe this increases the chance that in humans it is also caused by a virus."
Evidence of the bovine virus has been found, he said, in at least 20 percent of all cattle examined and 60 percent of all herds. About one infected animal in 20 actually incurs the disease.
The cattle become infected mainly by contact with other infected animals, he said, "and there also is evidence that insects, possibly flies and ticks," may transmit the virus.
Ferrer and colleagues at the University of Pennsylvania School of Veterinary Medicine are working on a leukemia vaccine to protect cattle herds. "On the basis of our recent progress, it is realistic to expect we will have as commercially available vaccine" within three to four years, he predicted.