For now, the epizootic that killed a third of the lions in Tanzania's Serengeti National Park is under control, according to the Cornell University veterinarian who pinned the fatal outbreak on canine distemper virus (CDV). But veterinarians and conservation biologists must remain vigilant, said Max J.G. Appel, D.V.M., Ph.D., because the dog disease is on the move, turning up in unrelated species and unexpected places.

"Canine distemper virus was not supposed to be a major problem in Africa because of the sun and heat," said Appel, noting that CDV is spread through the air in respiratory secretions from infected animals, and that the fragile virus usually does not survive more than a few minutes in open air. Once Appel and Cornell veterinary pathologist Brian A. Summers, Ph.D., discovered evidence of CDV infection in lion tissue samples sent from Tanzania, the disease's path was traced: The Serengeti lions are thought to have been infected by spotted hyenas, which share food with lions and which may have been infected by free-roaming domestic dogs around the national park.

Appel, professor of microbiology and immunology in the Cornell College of Veterinary Medicine's Baker Institute for Animal Health, and Summers, an associate professor of veterinary pathology, joined 13 other scientists in reporting the Serengeti lion findings in the journal Nature (Feb. 1, 1996).

An earlier report by Appel and Summers (Veterinary Microbiology, No. 44, 1995) discussed unexpected distemper episodes in captive lions in California and javelinas in Arizona. In 1988, a mutation of CDV allowed the virus to kill 90 percent of the harbor seals in the North Sea. And in 1994, a slightly different morbillivirus (CDV is a morbillivirus, as are the viruses for measles and rinderpest) killed several horses and their trainer in Australia.

Domestic dogs in Tanzania are being vaccinated against CDV, and the approximately 2,000 surviving lions are expected to have immunity for life, but the Serengeti outbreak still has veterinarians worried. "For the Serengeti lions, this was their Black Death," said Douglas F. Antczak, V.M.D., Ph.D., an immunologist and director of the Baker Institute, referring to the 14th-century plague that killed an estimated 25 million people in Europe. "These viruses are capable of change, and they're on the move," Antczak said.

So far, canine distemper virus is not considered a threat to humans, and a theory that CDV was a possible cause of multiple sclerosis has been disproved, Appel said. Pet dogs in the United States are routinely immunized against the disease with a modified live-virus vaccine. However, the live-virus vaccine nearly wiped out the black-footed ferrets of North America when a well-meaning vaccination attempt killed the only known colony of the endangered animals in South Dakota. When another black-footed ferret colony was discovered in Wyoming, enough of those animals were immunized with a killed-virus vaccine from Baker Institute to start rebuilding populations.

The same type of vaccine from Cornell, although not commercially produced in the United States because of limited demand, is protecting red pandas in zoos and other animals in captivity. There is a greater risk of canine distemper transmission among traveling circus animals than for those in stationary zoos and wildlife parks, said Appel, a researcher in distemper virus for nearly 30 years whose laboratory provides CDV vaccine for the big cats in the Ringling Bros. Barnum & Bailey circuses.