A photograph of a mosquito with a July 7 story about AIDS transmission should have been credited to Dr. Leonard Munstermann of the University of Notre Dame. (Published 8/4/87)

Public health officials are convinced that mosquitoes cannot transmit AIDS, but the issue continues to surface in scare headlines and radio call-in shows.

Now, a report of a government-funded study that found that mosquitoes fed on AIDS-infected blood could harbor the virus for as long as three days has prompted a new round of questions.

On Wednesday, a workshop sponsored by the congressional Office of Technology Assessment will examine the evidence about the ability -- or inability -- of mosquitoes, bedbugs, ticks and cockroaches to harbor and pass on human immunodeficiency virus (HIV), the cause of AIDS.

The latest study, conducted by Dr. Jai Nayar of the Medical Entomology Laboratory of Vero Beach, Fla., under contract to the National Cancer Institute, reported that the AIDS virus can live for several days in the stomachs of mosquitoes that ingested blood with very high concentrations of the virus.

"But it stays there," said Dr. Gary Noble, assistant director for science of the Centers for Disease Control (CDC). "It's not in the bloodstream {of the mosquito}, not in the saliva, and it does not duplicate like the malaria {parasite} does. When the mosquito bites the next person, there's no HIV available to transmit."

Nor does the epidemiological evidence support the view that AIDS is spread by insects. If mosquitoes could transmit the disease, public health officials would expect to find many unexplained cases of infection among children ages 10 to 14. These children are thought to be too old to have been infected in the womb and too young to have been infected sexually, and yet they commonly receive mosquito bites. But according to the Public Health Service, the 26 AIDS cases reported in youngsters from 10 to 14 can all be traced to a previously known risk of AIDS. Nineteen of the infected children are hemophiliac, according to PHS spokeswoman Ellen Casselberry; five received infected transfusions, and two had had homosexual contact with an AIDS carrier.

Scientists know that arthropods -- a group that includes insects, ticks and other invertebrates -- can transmit disease by two routes: biological and mechanical.

The biological route, said Dr. Thomas Monath, director of CDC's Division of Vector-Borne Viral Diseases, requires that the virus reproduce itself in the insect and eventually reach the salivary glands. This step is important because blood-sucking insects secrete saliva before they bite. The saliva contains an anti-clotting ingredient that allows the insect to suck blood without interference from the body's natural defenses.

Biological transmission accounts for "the vast majority" of cases of insect-borne infectious disease, said Monath. Among the conditions passed on in this way are parasitic diseases, such as malaria, and viral diseases, such as yellow fever, dengue fever and viral encephalitis. "It requires a delay of several days to a week or more between the ingestion of infected blood and the virus reaching a site {in the insect} from which it can be transmitted," he said.

The second route of transmission is mechanical. A small amount of infected blood -- 0.00001 teaspoon -- would remain in the insect's proboscis, or stinger, potentially infecting the next person bitten.

But mechanical transmission in humans is to date only hypothetical. According to Monath, mechanical involvement is suspected in certain cases of hepatitis B and, occasionally, Rift Valley fever, a viral disease in Africa that is more commonly transmitted by direct contact with infected domestic animals. Mechanical arthropod-borne disease transmission, he said, has been clearly demonstrated only in animals. Two of these mechanically transmitted diseases -- equine infectious anemia in horses and bovine leukosis in cows -- are caused, as AIDS is, by a retrovirus.

Mechanical transmission of AIDS is considered highly unlikely because the AIDS virus is difficult to transmit and it takes a lot of virus to cause infection. CDC's Monath, for example, estimated that it would take 2,800 mosquito bites to infect an individual with enough virus to pose a threat.

"And that's not just any old mosquito bites," he said. Every one of them would have to occur after an interrupted feeding on an AIDS carrier. The insect would then have to complete its feed on another person. Mosquitoes don't eat for four days or more after a complete feeding.

"Anything's possible," Monath said, "but this scenario, repeated 2,800 times, is so beyond the pale of reasonable possibility as to be ridiculous."

Still, the presence in the animal world of two mechanically transmitted retroviral diseases makes some specialists worry whether AIDS could be transmitted the same way. "If {equine infectious anemia} is transmitted from horse to horse by flies, why not the human AIDS disease?" asked Dr. Ricardo Veronesi of Brazil's Sao Paulo University School of Medicine in an interview with the Reuters news agency.

Dr. Mark Whitehead, co-director of the Institute for Tropical Medicine in Miami, believes that the environmental conditions that contribute to disease spread in animals might also occur in human settings. The horse anemia, he said, occurs when the animals are crowded together amid large numbers of flying insects. "Given well-documented studies in man," he said, "I assume the same will hold true."

Whitehead said poverty and poor sanitation mimic the conditions in which equine anemia is spread. Squalid conditions may help explain the unusually high rate of AIDS infection in the town of Belle Glade, Fla. In those conditions, Belle Glade residents "get 100 insect bites a day," he said.

Yet CDC investigators in Belle Glade found no correlation between AIDS infection and frequent exposure to mosquitoes, according to Dr. Harold Jaffe, chief of epidemiology at CDC. In addition, he said, people whose blood tests showed previous exposure to viruses known to be carried by mosquitoes were no more likely than others to test positive for HIV infection.

The CDC team also concluded that almost all of the AIDS cases could be explained by sexual contacts or sharing of contaminated needles.

Meanwhile, federal scientists who studied the recent Nayar report concluded that even though the virus can survive in mosquitoes, there is nothing to suggest that AIDS can actually be transmitted through mosquito bites. "The fact that the AIDS virus can passively survive in mosquitoes after the ingestion of large concentrations of virus isn't particularly relevant to a situation in nature," said Noble. And even under these exaggerated conditions, he noted, scientists such as Dr. Robert Gallo of the National Cancer Institute are having trouble finding enough AIDS virus in the mosquitoes to grow in culture.

Other insects have been considered as candidates for transmitting AIDS, but no case of the disease has been positively linked to an insect.

Among the insects under study are bedbugs, which have been shown to harbor the AIDS virus, said Dr. Larry Miike, project director of OTA's health program. "From what I understand from the work being done in Africa, it's not unusual to have 3,000 bedbugs in a hut," he said.

Miike said the bedbug connection "is a possibility, but in terms of its importance in the transmission of disease it ranks very low."

CDC's Monath has investigated AIDS infection of bedbugs and found the virus in the bugs. But he said there is no evidence that the virus replicates in bedbugs or that it is excreted in their feces, which is the primary way bedbugs transmit other diseases.

Like mosquitoes, he said, bedbugs are "like tiny test tubes" in which the AIDS virus survives for a number of days and eventually is broken down and disappears. He said digestive enzymes in the insects might actually hasten this process.

There is also some evidence that insects may be carriers of hepatitis B, a liver disease whose pattern of transmission has been used as a model to predict the course of AIDS. Like the AIDS virus, the hepatitis B virus is passed on through blood exchange and sexual contact.

But there is a critical difference, Monath said: The concentration of virus particles in blood is at least 1 million times greater for hepatitis B than for AIDS.

According to Dr. Baruch S. Blumberg of the Fox Chase Cancer Center in Philadelphia, who discovered the hepatitis B virus, studies done in Africa in the 1970s showed that in areas where hepatitis was rampant, field-captured mosquitoes could be found carrying the hepatitis B virus at a rate of one per 200.

"That's very high," Blumberg said. A similar rate of infection exists among field samples of the Anopheles mosquito, which transmits the malaria parasite "and can cause deadly scourges that can wipe out entire populations," he said.

In later work in Africa and Malaysia, Blumberg's group found that bedbugs also could carry the hepatitis B virus. "We could detect the virus for three or four or even five weeks after a single feeding" on a virus-infected solution, said Blumberg, "and could detect it in the droppings for several weeks after that." The infected droppings are especially worrisome, Blumberg said, because "they become the dust of the bed, which is easily inhaled or rubbed into the sores of individuals sleeping there."

In the debate over insects and AIDS, few answers are definitive.

"Even though there's an extremely minimal chance of the virus's spreading by mosquitoes," said Dr. Michael Samuels, special assistant to the surgeon general for AIDS issues, "AIDS is so final and fatal that it scares people."

In science and medicine, said Dr. Robert E. Windom, assistant U.S. secretary for health, "you can't be sure about anything 100 percent." But when it comes to whether insects can transmit AIDS, he said, "we're as sure as you can be about anything in science. "