Researchers Fear Spread of Lethal Dengue Fever to U.S.

When thousands of people in the Caribbean, Central America and Mexico were stricken with high fevers, aching bones and nausea in 1981, American public health officials along the Mexican border rushed into action, hoping to protect the United States from the advancing epidemic of dengue fever.

They dispatched scores of workers into the swamps in the Gulf states to catch mosquitoes to see whether they were carrying the virus responsible for the disease. Although the workers did find some infected mosquitoes, the virus made little headway into this country that year.

But today, U.S. public health officials have more reason than ever to worry about dengue fever, also known as “breakbone fever” for the excruciating pain it produces.

An even more dangerous form of that disease has shown up for the first time in the Western Hemisphere, causing epidemics throughout Latin America, including Mexico.

This concern was heightened by the discovery in the United States of a species of mosquitoes that is responsible for the transmission of this more dangerous form of dengue fever in Asia. So far, these mosquitoes have been found in 12 states, mostly along the Mississippi River and Ohio River valleys and the Gulf states, although there have been no reports of the disease in humans.

Dengue fever is but one of many viral illnesses that occur mostly in developing nations and which U.S. health officials fear could be brought into this country by insects or by travelers. Such diseases often have strange sounding names such as hemorrhagic fever renal syndrome, Lassa fever, Ebola hemorrhagic fever and Marburg viral disease. All of them are more deadly than dengue, although none has established a foothold in the United States.

Dengue fever is widely regarded as the one that poses the most immediate threat to Americans.

Although the milder forms of dengue has been in North and South America for at least 200 years, the most severe form--called dengue hemorrhagic shock syndrome, or DHSS--did not occur in the Western Hemisphere until the early 1980s.

The illness, common in parts of Asia, causes tiny blood vessels to hemorrhage and produces a shock-like syndrome that can be lethal. The species of mosquitoes that is responsible for its transmission is what has been caught in 12 states.

Scientists fear that the appearance of DHSS in this hemisphere could mean that the milder form of dengue could be replaced by DHSS. And they point out that there is no reason why both forms of dengue could not gain footholds here. There is no effective treatment against either form of the virus. Officials say the best tactic is prevention through mosquito control.

DHSS was reported for the first time in the Western Hemisphere in 1981 in Cuba, where 10,000 people contracted it and 158 died. During the same period, an additional 350,000 Cubans became ill with the milder dengue fever.

Since then, both forms of the disease have spread to Puerto Rico and other Caribbean islands and to Central America and Mexico, where the number of cases reported in the last few years has been growing.

Breeding Colony Found

Concern about dengue was heightened even more in 1985 when mosquito control workers in suburban Houston discovered a breeding colony of a species of mosquito never before seen in this country. It was the first report that the so-called “Asian tiger mosquito”--a carrier of dengue fever viruses in Asia--had established itself here.

The significance of this discovery, according to scientists at the Centers for Disease Control’s laboratory at Fort Collins, Colo., is that mosquito control could become more difficult if the imported insect becomes more widespread.

Until now, the dengue carrier here has been Aedes aegypti, a species that breeds well in locations near people because it likes to lay its eggs in receptacles such as rain barrels, tin cans and old automobile tires.

Unlike A . aegypti, the Asian tiger mosquito, known as Aedes albopictus, likes woodlands and other rural settings and consequently is harder to exterminate.

It is the Asian tiger mosquito that has spread to 12 states, according to Dr. Fred Murphy, chief of the division of viral diseases at the CDC. And this, he said, comes at a time when funding for dengue control research is at an “all-time low.”

Campaign Cut Short

The Centers for Disease Control in 1964 launched an ambitious campaign to rid the United States of A . aegypti . Within two years, the program’s budget grew from $3 million to $16.5 million, but it was cut back before the job was completed because of budgetary pressures of the war in Vietnam, according to Dr. Chester Moore, a research entomologist at Fort Collins.

“We have lost ground in the United States since the 1950s and ‘60s,” Moore said.

One result is that the mosquito population in the states along the Gulf of Mexico is now “quite high,” he said. And all it would take to touch off a human outbreak is for the mosquitoes to bite one infected person, such as a traveler or migrant worker from south of the border who already had been bitten by a virus-carrying mosquito before he entered the United States.

U.S. public health authorities are also worried about hemorrhagic fevers caused by viruses other than dengue. Hundreds of thousands of people in the Third World are afflicted by at least half a dozen such diseases. So far, none has occurred in the United States. But new evidence indicates that at least one of these afflictions, called hemorrhagic fever renal syndrome, or HFRS, could gain a foothold here. It is spread to humans by contact with rodents and kills by causing kidney failure.

Serious, Often Lethal

In China, Korea and parts of Siberia, where the disease is considered a serious and often lethal health problem, it is caused by the Hantaan virus.

In the early 1980s, two researchers at the U.S. Army Research Institute of Infectious Diseases set out to determine the extent to which Hantaan-related viruses are distributed around the world. They studied 1,700 rats obtained from 20 cities for evidence of the virus.

Surprisingly, the researchers, Dr. James W. LeDuc and Dr. J. E. Childs, found that the highest prevalence of a virus closely related to the Hantaan virus came from rats found in Baltimore--higher even than in seaport rats in Asia, Africa and South America.

“The cosmopolitan distribution of domestic rats suggests that (hemorrhagic fever renal syndrome) may exist undiagnosed in areas well beyond its traditionally recognized . . . boundaries,” they warned in a recent World Health Organization report.

Childs, who is now at Johns Hopkins School of Medicine in Baltimore, is trying to learn whether Hantaan-related virus, like its cousins in Asia and Europe, is responsible for kidney disease here.