Dogs used by the U.S. military in Vietnam, and presumably exposed to the same conditions as their human handlers, developed almost twice as many tumors of the testicles as did their canine counterparts deployed elsewhere, according to research published yesterday by government scientists.

The military working dogs in Vietnam, mostly male and mostly German shepherds, were exposed to the same harsh working conditions, chemicals, antibiotics and infectious agents as infantrymen.

"Dogs may be a particularly sensitive sentinel for man," said Howard Hayes, lead author of yesterday's report in the Journal of the National Cancer Institute. "{They} could be used to foretell the risks for exposure to the same chemical insults as soldiers in Vietnam."

Hayes and his colleagues, however, note they have no convincing evidence that service in Vietnam led to increased risk of testicular cancer in humans.

The researchers examined records of thousands of military dogs and found those in Vietnam had a significantly higher rate of testicular seminoma, a generally benign tumor in canines and one to which German shepherds are especially prone. In humans, however, testicular seminoma is often malignant and is a leading testicular cancer found in middle-aged and older men.

Studies by the federal Centers for Disease Control have found that some but not all groups of Vietnam veterans had an increased risk of testicular abnormalities, including lower sperm counts and decreased sperm activity. However, most studies have not shown a link between service in Vietnam and testicular cancer deaths in humans.

The scientists identified a number of agents that might have caused the testicular cancer in dogs. In addition to tropical parasites and harsh working conditions, the dogs and their human employers may have been exposed to the herbicide Agent Orange, which contains dioxin and other ingredients linked to cancer in laboratory animals. The dogs also were exposed to pesticides for ticks and were given antibiotics such as tetracycline to ward off infections.

"We're dealing probably with

an admixture of these chemicals," said Hayes, a researcher at the Environmental Epidemiology Branch of the National Institutes of Health in Bethesda. "I don't believe we'll able to be able to tie it to one chemical."