Winter Resort for Viruses

The notion that mosquitoes bite snakes made most scientists laugh. But not the University of Utah's imaginative Microbiologist Louis P. Gebhardt Jr. By following up a hunch, Gebhardt has just climaxed an eight-year effort to trace the life cycle of a virus that causes one form of deadly brain inflammation commonly known as sleeping sickness.

Western equine encephalitis (or "WEE") was so named when it appeared that its chief victims were horses. But among 297 cases of human viral encephalitis in 1965, the U.S. Communicable Disease Center attributed no fewer than 172 to WEE, with four deaths. The outbreak reached epidemic proportions in Colorado, Kansas, Montana, North Dakota, Texas and Wyoming.

Drowsy Victim. Dr. Gebhardt knew that in the summertime, usually beginning in July, at least one species of mosquito carries the WEE virus from infected animal to man or from infected man to man. He also knew that some birds, notably swallows, harbor the virus. But in the Rockies and on the high plains the carrier mosquitoes die off as winter begins and insect-eating birds fly south. Where did the virus spend the winter?

Dr. Gebhardt had good reason to suspect snakes. The Culex tarsalis mosquito, principal carrier of the WEE virus, hatches out in swamps. Early in the spring, when birds are still scarce, the female mosquito lights on the nearest creature for the blood meal she needs before she can lay her eggs. Dr. Gebhardt figured that the victim might be a snake just emerging drowsily from hibernation. Starting in 1961, he hiked miles through swamps and caught plenty of garter, gopher and blue racer snakes, but found virus in only one.

When he decided to test the snakes' blood immediately after capture, Gebhardt got startling results: 37 out of 84 of the relatively small, nonvenomous and supposedly harmless snakes harbored the virus, but only in the early spring and, inexplicably, in the fall. Blood samples taken in the summer have proved negative. In late spring and summer, the mosquitoes feed largely on birds, especially helpless nestlings.

Timely Spraying. By now, researchers working along the same lines have found WEE virus in other cold-blooded animals--such as lizards and frogs. These findings do not mean that reptiles or birds that carry the virus for a while should be exterminated, Dr. Gebhardt emphasizes. What they do show is that mosquito-control spraying should be timed to hit the swamps in spring, when Culex tarsalis is hatching, so that bloodthirsty females of the species get no chance to dine on creatures in which the virus hibernates.

As for evidence that mosquitoes really bite snakes, Dr. Gebhardt has tested the idea in his laboratory, where skeptics have now seen a dozen or more mosquitoes perched on the head of a single snake, eating heartily. Unlike horses or humans, the snakes apparently suffer no discomfort and develop no encephalitis.

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