Unwanted invaders: Introduced Species and Vector-Borne Disease
Howard S. Ginsberg, Ecologist
USGS Patuxent Wildlife Research Center
Howard S. Ginsberg earned a BS in biology from SUNY, Stony Brook and a PhD in
entomology from Cornell University. His research at the Patuxent Wildlife Research
Center focuses on the ecology of arthropods that transmit diseases and monitoring
and conservation of invertebrates. He studies ticks that transmit Lyme disease
and mosquitoes that transmit Eastern Equine Encephalitis.
Ecologists generally agree that invasive species are undesirable, but not everyone shares this opinion. Purple loosestrife, for example, is an invasive species that usurps the habitats of several native species, but it is prized by some gardeners and is still sold at garden shops. The honey bee, an alien species that was introduced by the early colonists, is widely considered beneficial and was even nominated for the symbolic title of national insect. However, when it comes to organisms that cause human disease or species that transmit these pathogens, opinion tends to be unanimous: Nobody wants them here.
Unfortunately, introductions of disease-causing and disease-bearing species seem to be increasing. Two factors that figure prominently in this increase are the worldwide trend toward urbanization and the growth and speed of global transportation. Both of these activities are concentrated in the coastal zone.
Just last summer, the West Nile virus (WNV), formerly known in Europe, Africa, and Asia, was responsible for illness in about 60 people and the death of seven in New York City. We don't know exactly how the virus came to this country; perhaps it came in with a captive bird or another host, or it might have arrived in a mosquito. WNV is apparently less virulent than its North American cousin, St. Louis encephalitis (SLE), but it nevertheless can cause serious disease, as we saw in the New York City outbreak. In addition, WNV can have potentially serious effects on North American wildlife species, which have never before been exposed to this virus. Infected birds in Europe and Africa are typically asymptomatic, but several North American species, especially crows, have developed severe illness from WNV infection. The future of WNV in North America remains to be seen. It might well be gone next year. On the other hand, it might behave as it does in Europe and Africa, establishing an endemic zone in warmer climates (perhaps the southeastern United States) and causing occasional outbreaks in northern areas. Only time will tell whether this virus will become established in North America, and what its long-term effects will be.
West Nile virus is an alien pathogen that is transmitted by a cosmopolitan mosquito. However, alien mosquitoes are sometimes introduced as well. One well-known example is the Asian tiger mosquito, Aedes albopictus, which was introduced into the southern United States via used tires that had been imported from Asia. Water that collects in discarded tires serves as habitat for wrigglers, the larval stage of mosquitoes (in nature, larval Ae. albopictus live in water that collects in treeholes at the crotches of tree branches and other small hollows). This mosquito has not been implicated in disease transmission in North America, but it is a competent vector of dengue fever, so health authorities are concerned. Dengue, which was the cause of serious outbreaks in Central and South America, was brought under control nearly a half century ago by controling its primary vector, Aedes aegypti, itself an alien species introduced from Africa. Unfortunately, Ae. aegypti is making a comeback. Larvae of this species can live in just about any cavity that holds water, including clogged gutters, discarded drinking cups, building material, and other debris. Poverty-stricken areas at the outskirts of cities, with large numbers of people living in temporary dwellings, and with no infrastructure to provide sanitation services, can provide ubiquitous larval habitat for this species. Thus, rapid urbanization has fostered a major resurgence of Ae. aegypti, and with it dengue fever in Central and South America where the yearly incidence now runs into the hundreds of thousands of cases.
As if the return of dengue fever were not enough, the resurgence of Ae. aegypti can have another even more serious consequence, involving the movement of yellow fever. Yellow fever virus exists in South American and African forests and is manifested in a sylvan cycle involving monkeys and forest-dwelling mosquitoes. However, if this virus breaks out in an urban setting, it can exist in a transmission cycle that involves humans and Ae. aegypti. This mosquito species was the vector of yellow fever in the Panama Canal Zone when the U.S. Army surgeon Walter Reed first demonstrated the role of mosquitoes as carriers of this disease at the end of the last century. Yellow fever is, in most cases, far more severe than dengue. Public health authorities are hoping that the yellow fever virus stays in the jungle and never breaks out into an urban setting, where the repercussions would be disastrous. However, given the recent explosion of Ae. aegypti populations, it's hard to be optimistic. It is hoped that mosquito management programs responding to the dengue epidemic will prevent the reappearance of urban yellow fever.
On a worldwide basis, the most important parasitic disease is undoubtedly malaria, with hundreds of millions of cases each year in tropical areas. In the early 1900s, this disease was common in North America, but control of the Anopheles mosquito vectors, both by insecticide applications and modification of wetlands to eliminate larval mosquito habitat, has virtually eliminated this disease from the United States. Cases in the United States occur primarily in people who have traveled to or recently arrived from parts of the world where malaria is common. Occasionally, a local Anopheles mosquito picks up the protozoan pathogen from an infected person and transmits it within United States borders, but such cases are rare.
Mosquitoes have been much in the news lately, but they are not the only medically important critters that have appeared in the Western Hemisphere. An African tick, Amblyomma variegatum, appeared in the Caribbean region around 1967. This handsome species (if you can say that about a tick) is brightly colored and adult males have reddish and green markings on a black background. Its behavior is similarly colorful. It is aggressive and seeks hosts with vigor. In Africa, this species carries several pathogens, including the rickettsia that cause heartwater fever in cattle, which has now become a serious problem on Caribbean islands. Characteristic symptoms of this disease include high fever, motor incoordination and other neurological signs, and accumulation of fluids around the heart, hence the name.
Any discussion of vector-borne disease would be incomplete without mentioning plague, a disease that has profoundly altered human history. This flea-transmitted bacterial disease (primarily of rodents) has killed large proportions of the human population on entire continents at various times in the past. The most recent pandemic occurred in China in the late nineteenth century, when the epidemic spread along shipping routes via rats on steamships and was introduced into the western United States in 1899. Plague exists in wild rodents in the southwestern United States, and people who come in contact with the nests of infected rodents still occasionally catch this disease (four cases of plague were reported to the Centers for Disease Control in 1997 and five in 1996). Fortunately, modern sanitation practices, control of urban rodents, and antibiotics have lessened the significance of plague in most parts of the world.
Modern trends of urbanization and rapid global transportation have changed the face of the world. Introductions of alien organisms are now routine. Most of these introductions don't result in obvious impacts on our environment, but others have substantial effects. Long-range transport of diseases and disease vectors has certainly influenced human populations in the past. Today, the greatly accelerated pace of these introductions and the broad environmental modifications resulting from changing trends in human demography can have rapid and potentially unforseen effects on trends in human disease. Many of these problems can be avoided by routinely considering vectorborne disease issues in decisions relating to transportation and urban development and by increasing our vigilance concerning the potential effects of introduced species on human health.