Once-Silent Diseases Are Sending a Deadly Message : Health: The Four Corners hantavirus is the latest onslaught. Urban sprawl, unexpected events increase risk.
The unexpected epidemic of a mysterious flu-like disease that struck the Southwest this summer was no surprise to virologists. For years they have warned that humanity’s increasing encroachment on nature will eventually unleash on the United States and other developed nations exotic diseases previously confined to poverty-ridden Third World countries.
Even the culprit in the so-called Four Corners illness, a hantavirus carried by deer mice and spread in their droppings, is far from unknown to scientists. For decades it has been lurking silently in this country--and not so silently in other parts of the world.
A close cousin of the virus had previously been found in virtually every state in the Union and in many U.S. cities, including San Francisco and Los Angeles. A few scientists believe that the cousin has caused disease in the cities and that it may be linked to the increased incidence of high blood pressure and kidney disease among inner-city residents.
“This is the kind of thing that can happen anywhere,” said virologist Stephen Morse of Rockefeller University in New York City. “It happens when ecological and demographic changes allow viruses buried in the environment to gain access to people and cause disease. We need to be working on rodent control and working to warn people about the potential for epidemics.”
The new outbreak first drew attention on Memorial Day weekend when young people in the Four Corners area--where Utah, Colorado, Arizona and New Mexico converge--started dying. They developed headaches, muscle aches, fever and coughing, followed by a sharp decline when their lungs filled with fluid, suffocating them.
The Four Corners illness subsequently appeared outside its initial locus, popping up in Texas, Nevada, California and, most recently, Oregon and Louisiana. So far, it has left 33 dead, and authorities believe there may be many other victims whose deaths have passed unnoticed.
Public health authorities quickly linked the epidemic to a new strain of hantavirus. Harmless to the deer mice, the hantavirus spreads to humans when rodent droppings dry out and fine particles become airborne.
“This is a widespread group of viruses and we still have a lot to learn about them,” said Dr. James LeDuc, an epidemiologist at the World Health Organization in Geneva. “It wouldn’t surprise me at all to see cases throughout summer and into fall.”
Hantaviruses represent one of a group of potential threats to human health that scientists call “emerging viruses.” These infectious agents lurk all around us in the environment waiting for some unusual event--like the heavy rainfall in the past two years that triggered a proliferation of rodents in the Southwest--to catapult them into an unexpected, and frequently deadly, encounter with humans.
Such events can also include the spread of cities into rural areas, bringing humans into unusually close contact with previously isolated animals. The urban sprawl of cities in the Northeast brought suburbanites near deer that carried the deer tick, carrier of Lyme disease, now the second most common insect-borne disease in the United States.
The penetration of roads and towns deep into primeval forests in South America and Africa triggered the release of deadly Ebola and Lassa fever viruses.
HIV, which causes AIDS, is the prototypal emerging virus. It may have been around for decades, lost in the clutter of infections that afflicted humans, particularly in the bacterial caldron of the tropics. It came into the collective consciousness only when medical science conquered the myriad diseases that had masked it, globalization of travel allowed its spread from country to country, and the relaxation of sexual mores combined with new medical technologies such as transplants and blood transfusions to enhance its spread from person to person.
In the last three years, a number of major reports, including one by the National Academy of Science’s Institute of Medicine, have concluded that humans are at significant risk from a number of viruses that now exist primarily in animals or insects. And residents of industrialized countries, including the United States, are at increased risk of diseases that are major problems only in developing countries.
Among these potential problems are such threats as cholera, tuberculosis and malaria, which are undergoing marked resurgence as a result of urban crowding and poor sanitation. Rift Valley fever, once primarily a disease of sheep and cattle, is infecting humans in southern Africa. Anisakiasis is increasing in incidence as people eat more raw fish. Purpuric fever first appeared in Brazil and has since spread to Australia and other areas.
And near the top of all the lists of threats that virologists have compiled are hantaviruses. These little-known but deadly viruses have caused severe health problems elsewhere in the world, and many scientists believe it is only a matter of time until they do so in the United States. A few believe that that time has already come.
Hantaviruses were first recognized during the Korean War when thousands of U.S. and U.N. troops developed a previously unrecognized disease characterized by severe damage to the kidneys. Several hundred victims died. The best virologists in the United States were recruited to find the cause of this so-called hemorrhagic fever, said Dr. Theodore Tsai, an epidemiologist at the National Institutes of Health. But they failed.
In 1976, virologist Ho Wang Lee of Korea University in Seoul finally isolated the causative virus, which he called Hantaan virus after the Hantaan River, from the common striped field mouse.
But in the early 1980s, Lee isolated a different strain of the virus in Norwegian rats in Seoul. “That happens to be one of the most common rats of the world, spread around the globe by humans,” said Dr. James Childs of the U.S. Centers for Disease Control and Prevention in Atlanta. “Everything changed. People became very concerned that, in addition to shipping us cars, the Koreans might be shipping us a virus.”
The new strain was called Seoul virus and the class became known as hantaviruses. Soon, researchers showed that another hantavirus, Puumula virus, was responsible for a widespread disease in Scandinavia and the Soviet Union called nephropathia epidemica, characterized by severe kidney damage.
Virologists have identified eight strains of hantaviruses. Nobel laureate D. Carleton Gajdusek of NIH even isolated one strain from a small rodent called the meadow vole on his estate, Prospect Hill, outside Washington, D.C. The hantavirus responsible for the Four Corners illness will likely be the ninth strain found. The Louisiana death was caused by a different strain of hantavirus, which would be the 10th.
The hantaviruses are a significant public health problem around the world. Puumula is spreading into Western Europe. Many outbreaks occur in the spring when people open their summer cabins and clean them out, exposing themselves to airborne particles from rodent droppings.
The Seoul virus is the second-most important viral infection in China, behind the hepatitis B virus. It is also a public health problem throughout Southeast Asia.
Most people thought the United States was free of hantaviruses, but that has proved a false hope. Tsai and his colleagues reported in 1985 that they had found significant numbers of rats infected with hantaviruses, particularly the Seoul virus, “in every large city we looked at”--including Los Angeles and San Francisco. “We found them not only in port cities on the coasts, but inland as well, in cities like Cincinnati and Columbus.”
More recently, Dr. Bruno Chomel, an epidemiologist at UC Davis, has found hantaviruses in deer mice in the Plumas National Forest in Butte County.
Despite the high prevalence of the virus in cities, Tsai found no evidence of the disease in humans.
Researchers knew that the virus could easily spread to humans, however. There have been many reports of scientists and laboratory technicians in Japan and Europe who have been infected after working with laboratory animals contaminated by the virus.
As a result of his studies in Baltimore in collaboration with Childs, the World Health Organization’s LeDuc believes that exposure to virus-infected rats in the inner city may be one reason blacks have a higher rate of hypertension and kidney damage than whites.
LeDuc, Childs and Dr. Greg Glass of Johns Hopkins found three people hospitalized at the university with an active hantavirus infection. Two went on to develop hypertension and kidney disease (which can cause hypertension).
Dr. Pierre Rollin, now at the CDC, also found three patients in France with active hantavirus infections. Two developed hypertension and kidney disease.
“That’s pretty convincing,” said Dr. James Holland, a virus expert at UC San Diego.
Meanwhile, teams of scientists at the CDC and elsewhere are trying to grow the Four Corners hantavirus in the laboratory so they can develop a definitive test for it. If the test now in use shows the presence of hantavirus in one of the Four Corners patients, that means it is almost certainly there, said Dr. Steven Ostroff of the CDC. But a negative test result can often mean that the test is not sensitive enough.
Researchers are also eager to see how the new virus’ genes differ from those of the other viruses. Only a very small change may have been necessary for the virus to begin attacking the lungs rather than the kidneys, they speculate.
Meanwhile, researchers around the world are keeping their eyes open for new outbreaks of hantaviruses or other threats caused by ecological disturbances. As Rockefeller’s Morse said: “We never know which virus will suddenly be propelled by chance and circumstances fortunate to it--and unfortunate to us--to disease status.”
The Deer Mouse
The deer mouse is the most common and widespread of the white-footed mice native primarily to wooded areas of western North America. It has been found to carry the hantavirus, which is transmitted to humans when they breathe the dried excrement of deer mice after it has turned to dust and dispersed into the air.
Appearance: Grayish to reddish brown above and white below. The tail is distinctly bicolored and short-haired.
Habitat: Forests, woodlands and brush.
Range: Canada and virtually all of the United States except the Gulf Coast. Two cases reported last week, in Louisiana and East Texas, are outside the normal range of the deer mouse and may have been caused by another rodent.
Symptoms of the disease: The first stages of the Four Corners illness include coughing, headache, muscle ache and conjunctivitis (reddening of the eyes). Death occurs when the lungs fill with fluid, suffocating the victim.
Tips for avoiding exposure to virus: Avoid rodents in general. Any mouse excrement found in campsites or homes should not be swept up or vacuumed. Instead, contaminated areas and dead animals should be soaked in disinfectant; handlers should wear rubber gloves. Dead animals should be double-bagged in plastic and buried.
Sources: Staff reports; “Western Forests,” by Steven Whitney, Alfred A. Knopf, 1985.
