A swarm of trouble: Mosquito season is new-disease season

Consistent outbreaks of West Nile virus indicate that it's here to stay. Experts caution that other mosquito-borne diseases lurk just offshore.

By Susan J. Landers — Posted May 21, 2007

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Ready the repellants. Warn patients to slip on long-sleeved shirts and consider staying inside during prime biting time. Mosquito season has arrived in some parts of the country, and protection is in order.

The tiny insects are not just a nuisance. They cause more human harm than any other organism, according to the American Mosquito Control Assn., a nonprofit scientific and educational association in Mount Laurel, N.J. At least a million people die from mosquito-borne diseases every year.

Although malaria, which takes the largest toll, is no longer a major health problem in this country, complacency is not an option, experts say. West Nile virus, transmitted by mosquitoes and carried by wild birds, can serve as a model for how other mosquito-borne diseases can reach this country and spread. It took only five years for the virus to travel from coast to coast, and now it's here to stay.

"Since 1999, we've had epidemics every single year," said Lyle Petersen, MD, MPH, director of the Centers for Disease Control and Prevention's division in Fort Collins, Colo., on vector-borne viral and bacterial diseases. New vector-borne diseases generally follow a pattern. They land in a hospitable environment, cause an epidemic and then disappear for a while, Dr. Petersen said. But not West Nile virus. It has become endemic in the entire U.S.

The first human cases for 2007 were reported in Mississippi in early April.

Why this illness has taken hold in the U.S. is a mystery, he said. But it may have to do with the fact that there are so many potential carriers -- 60 mosquito species have been found infected with West Nile virus.

Defeating those mosquitoes is critical, and that's where physicians can play a big role, Dr. Petersen said. "It is critical that they continue to see, diagnose and report patients." Data from physicians can spur local mosquito-control efforts.

Last season, Chicago and Sacramento, Calif., deflected what surveillance data predicted would be a very large outbreak of West Nile by instituting proper mosquito control efforts early. This was possible because physicians had reported cases and sounded an alert.

Mistaken identity

When it arrived on the East Coast in 1999, the new virus, an import from Africa, West Asia and the Middle East, took physicians by surprise. Many mistook it for St. Louis encephalitis, another mosquito-borne disease that has long been seen on American soil.

The public health community quickly learned that an introduced pathogen can take off and spread disease like a match sparks a wildfire.

Another lesson from the experience is that a network of suspicious physicians can serve as a terrific early warning system. By acting on the inkling that a patient's fever could be more than just a case of summertime flu, a new disease can be identified quickly, and its spread halted, said Roger Nasci, PhD, chief of the CDC's Arboviruses Diseases Branch in Fort Collins, Colo.

A number of zoonotic diseases, such as SARS and monkeypox, have made their national debuts in recent years, but they have not, fortunately, been able to establish a presence, thanks to alert physicians, quick public health actions and some luck.

But West Nile virus is entrenched in this country as well as throughout the Western Hemisphere from central Canada to Argentina and has had a significant health impact in the temperate areas of North America, Dr. Nasci said.

The bright side is that West Nile, though serious for many, is not as serious as are some other mosquito-borne diseases.

That's not to say that contracting West Nile virus is a walk in the park -- though that's one way to encounter an infected mosquito. Dr. Petersen, who not only studies disease-bearing mosquitoes but has had firsthand experience, believes he met an infected mosquito in 2003 as he and his then-10-year-old daughter walked to the mailbox. They hadn't applied repellant even though it was dusk. "I thought, 'What could be the risk in one minute?' " he recalled. But the minute stretched into several as he stopped to chat with a neighbor.

Three days later, he, his daughter and the neighbor all fell ill within about an hour of each other.

His was supposedly a mild case of West Nile fever, said Dr. Petersen, who was then 48 and a marathon runner. But it took months to fully recover. "I could barely walk up the stairs. It was the last time I called West Nile fever a mild illness."

"We've learned that after you've had this horrible acute illness, there is a long period where you have chronic fatigue. That lasted in me for about two months." The experience also earned him the moniker "West Nile Lyle."

But the disease can be much worse, notes Joe Conlon, technical adviser to the American Mosquito Control Assn. "One of the great untold stories is the amount of neuroinvasive disease caused by the virus," he said. Nearly 10,000 cases of encephalitis or meningitis have resulted from the virus since reporting began in 1999, and about 1,000 people have died.

If that's not enough reason to break out repellants, here's another: Eastern equine encephalitis, also carried by mosquitoes, kills nearly one-third of humans infected.

As its name implies, EEE occurs in the eastern half of the United States and also can infect horses, where it takes an even greater toll. It has a counterpart in the west, western equine encephalitis. An average of five EEE cases are reported each year, and most occur in Florida, Georgia, Massachusetts and New Jersey. Transmission is most common in and around freshwater hardwood swamps.

But as few mosquito species carry that disease, large outbreaks are considered unlikely.

There is no human vaccine or specific treatments for EEE. Control measures for the mosquitoes are difficult because of laws that protect the wetlands where these mosquitoes breed, according to the CDC.

People older than 50 and younger than 15 are said to be at greatest risk of developing severe infections. The death of a boy from EEE in Massachusetts last year prompted a flurry of calls to Charles A. Wolff, MD, a family physician in Middleboro, Mass. "People wanted to be put on medications, prophylactically," he said. "My job was to reassure them and tell them to take the proper precautions, including not to be outside when mosquitoes usually bite."

In addition to eastern and western equine encephalitis, La Crosse virus and St. Louis encephalitis virus also are carried by mosquitoes. From a public health standpoint, they may be even more important than West Nile, said Jason Rasgon, PhD, assistant professor in the Dept. of Molecular Microbiology and Immunology at the Johns Hopkins Bloomberg School of Public Health. "But since they've been here for so long, no one pays attention to them anymore."

Cunning adversaries

Mosquitoes do seem to be getting the better of us, Conlon said, and they are formidable foes. There are about 2,500 mosquito species in the world, with nearly 200 residing in the U.S., according to the American Mosquito Control Assn. Most can carry disease; some can carry several.

Still, complacency may have settled in now that West Nile has been in this country for a few years and budgets to control mosquitoes are being cut, Conlon said. The virus "will probably maintain a low level of incidence forever and break out in sporadic epidemics. Unfortunately, we may not have an adequate infrastructure to deal with it if we keep cutting funding."

The ever-shrinking world also makes it necessary to be prepared for the arrival of unexpected vectors carrying new diseases. Mosquito-borne diseases such as Rift Valley fever and chikungunya are only a plane ride away, Conlon said.

Large outbreaks of chikungunya, which is thought to have gotten its name from the local African word for "that which bends up," occurred in India and islands in the Indian Ocean last fall and earlier this year, affecting millions of primarily poor residents. The disease is aptly named, since those afflicted have nausea and severe joint pain that causes a doubling over.

Following the example of West Nile virus, chikungunya also could establish itself in the U.S., or any tropical or temperate area where the mosquito Aedes albopictus is present, say the authors of an article in the Feb. 22 New England Journal of Medicine.

Aedes albopictus, also known as the Asian tiger mosquito, is a common presence in many parts of the nation and is thought to pose a significant public health threat.

Having the right mosquito is often a deciding factor in determining which diseases are likely to establish themselves here, Dr. Nasci said. And another serious disease, Japanese encephalitis, uses a system almost identical to that employed by West Nile virus to set up a new home base. "If [chikungunya and Japanese encephalitis] were established in the right setting, they could follow the West Nile pattern and spread widely," Dr. Nasci said.

Locally acquired malaria also has been reported in the United States, thus demonstrating that this serious disease continues to pose a risk for reintroduction, said the CDC in its Sept. 8, 2006, Morbidity and Mortality Weekly Report.

One 2003 Florida outbreak of eight malaria cases required substantial public health resources. The community notification system sent phone alerts to 300,000 area residents, and additional notices were sent to local hospitals and physicians' offices.

Heeding the threat posed by the multitude of diseases carried by mosquitoes and other vectors, the Institute of Medicine has scheduled a workshop on vector-borne diseases to be held next month in Fort Collins, Colo.

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Lessons from West Nile virus

  • A newly introduced pathogen can spread quickly throughout the nation.
  • No one can predict the severity of the outbreak.
  • A network of vigilant physicians can serve as an early-warning system.

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The buzz on mosquitoes

  • With each bite, a mosquito imbibes 0.001 mL to 0.01 mL of blood.
  • Female mosquitoes need a blood meal to develop eggs. For their own nourishment, adult mosquitoes rely on plant nectars.
  • Mosquitoes serve as a food source for many other creatures.
  • It's hard to say what would happen if mosquitoes were eradicated, but another, possibly worse, species could replace them.
  • Most mosquitoes have a flight range of one to three miles.
  • The smallest mosquitoes weigh about 2.5 mg. The largest, 10 mg.
  • Larger people and those who are active are more attractive to mosquitoes because they emit more carbon dioxide and lactic acid.

Source: American Mosquito Control Assn.

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Disease-carrying mosquitoes

  • Aedes aegypti is found in many states, with the greatest density along the Gulf of Mexico. It is the primary vector for human yellow fever and dengue viruses and a principal vector for chikungunya virus.
  • Aedes albopictus is common in many parts of the country. It can convey a number of diseases, including West Nile virus.
  • Aedes vexans is found throughout the United States with the possible exception of Hawaii. It can carry western equine encephalitis, West Nile virus and eastern equine encephalitis.
  • Anopheles freeborni is found throughout the Western United States. It is a vector for malaria.
  • Culex pipiens is common in urban and suburban settings. It transmits St. Louis encephalitis.
  • Culex restuans is found from central Canada into Mexico and is most common in the Eastern and Central United States. It has been identified as carrying West Nile virus.
  • Culex tarsalis ranges from Mexico to Canada and is relatively rare east of the Mississippi River. It transmits St. Louis and western equine encephalitis viruses.
  • Culiseta melanura is found in freshwater hardwood swamps. It transmits eastern equine encephalitis and West Nile virus.

Sources: Centers for Disease Control and Prevention; various mosquito control agencies

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