Mysterious Deaths of Deer May Be Linked to Gnat Virus

In the wilderness of Northern California last fall as many as 8,000 deer simply vanished from mountainous regions.

Then decayed deer carcasses began to be discovered in the remote areas of Mendocino, Trinity, Siskiyou, Modoc and Humboldt counties.

But it often was impossible to pinpoint the causes of the deaths, long after they occurred.

Phillip Jessup, a pathologist for the state Department of Fish and Game, thinks many of the deer died of a mysterious viral disease called bluetongue. It is carried around the state by gnats and possibly also by ticks.

“Bluetongue is no less complicated than AIDS (acquired immune deficiency syndrome). It’s the most important infectious disease of deer in California,” he says.

More Important Than Thought

“We think it’s more important than we previously gave it credit for. It’s implicated in some of the biggest losses we have seen; 6,000 to 8,000 is a pretty big loss. There are other factors, of course, like lack of nutrition and predators. It’s probably a combination of all these.”

The Department of Fish and Game estimates that there are about 750,000 deer “plus or minus 200,000” in California. Sport hunters insist that the numbers are fewer than before the big drought of 1976-77.

The missing deer of 1986 were not the first to vanish. Two years earlier, about the same number disappeared in the western part of Tehama County.

Bluetongue has been in California since the 1930s, but for many years little was known about it. Jessup thinks many deer deaths attributed in the past to other ailments like foot rot probably were bluetongue cases.

Control a Question of Money

Knowledge of viral disease has advanced in the 1980s, but Jessup says that combatting bluetongue will be a question of money.

“We’re at a point where we can do several things,” he says. “We can isolate the virus out of the gnat, and we’re looking at the insect, and ways to destroy it or inhibit reproduction.”

For the current 1987-88 fiscal year, the state has budgeted about $30,000 for research on bluetongue in deer and bighorn sheep. “About $250,000 a year would really allow us to get some progress faster,” Jessup says.

The California Wildlife Federation has been approaching legislators in quest of more state funding for bluetongue research. No bills for it showed up in the Legislature this year. The federation is asking its members for money to lobby the issue.

Only Hoofed Animals Affected

Bluetongue gets its name because it lowers blood oxygen in deer, causing the tongue, gums and mouth lining to turn blue. Autopsies of deer stricken by the ailment also show artery damage and lesions on the heart and feet.

The disease attacks only hoofed animals. Humans are not vulnerable, and Jessup says there is no danger in eating meat of an infected deer.

Bluetongue affects some animals other than deer, though not as seriously. Among them are bighorn sheep, domestic sheep and cattle. Elk catch it, but don’t seem to die from it. In the Rocky Mountain region it has killed antelope. For reasons that are unclear, it has so far not been fatal to antelope in California.

Jessup estimates that bluetongue kills between 10% and 20% of the deer that contract it. Animals are subject to virus-carrying gnat bites almost anywhere in California except the upper elevations of the Sierra. The highest exposure rate for deer is near the Colorado River, where studies show about 60% have had the virus.

Like AIDS and influenza, the bluetongue virus has many different serotypes, or families. About 30 serotypes are found throughout the world, five of them in California.

There are two vaccines, but neither is effective for all serotypes.

Jessup makes it clear that diseases and drought aren’t the only threats to deer.

“A big factor has been the takeover of deer habitat for urban development, especially in the Sierra and Santa Clara County,” he says. “There used to be thousands of deer in the Gilroy and Morgan Hill areas in the south part of Santa Clara County. Now there is more land in row crops and housing. The numbers of deer have declined.”