Researchers Aid in Effort to Track Mosquito-borne Encephalitis
Last fall, a disease called West Nile encephalitis never before seen in the Western Hemisphere killed seven people in the region of New York City and now seems poised to return. Two months ago, a red-tailed hawk dropped dead from the sky in New York and subsequently tested positive for the virus, confirming fears that the mosquito-borne disease had indeed survived the winter. Although the virus has yet to be found in Massachusetts, state agencies have turned to HSPH researchers for guidance on how to track the disease as well other mosquito-borne illnesses that have provoked many questions about these curious insects.
Perhaps
it is only natural that the agencies looked to HSPH. Chris Mores, state
supervisor for mosquito-borne viruses, and Gregory Ebel, soon to join
the New York State Department of Health, are doctoral students in the
Department of Immunology and Infectious Diseases. Nicholas Komar, who
received his doctoral degree from HSPH in 1997, now works on mosquito-borne
viruses for the Centers for Disease Control and Prevention (CDC). All
three studied under Andrew Spielman, professor of tropical public health,
and Richard Pollack, instructor, who conduct research in the Laboratory
of Public Health Entomology. Last year, the lab was recognized by Howard
Koh, the state commissioner of public health, for its outstanding collaboration
with state labs. HSPH was the only academic institution to be so honored.
When it comes to mosquito-borne viruses, or arboviruses, the HSPH investigators are engaged in tantalizing research. Mosquitoes are found worldwide across all temperature zones and comprise more than 2,000 species. Not all of them carry human pathogens, but researchers suspect that one of the most common, Culex pipiens, transmits the virus that causes West Nile encephalitis.
"We don't know exactly how many species of mosquitoes are involved," said Pollack. "Evidence from the New York experience as well as from outbreaks in Africa, Europe and Asia, point to Culex, and the one that seems most likely involved in the New York region is Culex pipiens."
Unlike the relatively rural mosquitoes suspected of transmitting the Eastern Equine encephalitis virus, Culex pipiens is well-adapted to conditions found in more urban areas, which makes control of West Nile encephalitis potentially very troublesome. "The species is fairly cosmopolitan," said Pollack. "You will find this mosquito virtually anyplace, except perhaps Antarctica."
West Nile encephalitis can cause the brain to swell. People with severe reactions may suffer convulsions, fever, and paralysis, but infection rarely leads to death. Many people carry the virus with mild, if any, symptoms.
Scientists first isolated the virus in Uganda in 1937. It was later found in Africa, the Middle East, and Europe. In 1996, the virus infected thousands in Romania and was later detected in the Czech Republic and Italy. Then, in August 1999, birds began dying in New York City. Tests identified West Nile encephalitis virus as the culprit, fixing the virus on North American shores for the first time in history.
When birds die en masse for no obvious reason, mosquito-borne viruses can rank among suspected causes. This is because certain birds and mosquitoes form an odd relationship from which the virus is the direct beneficiary. Female mosquitoes bite animals for blood mainly to develop eggs. While feeding, they may ingest virus from an infected bird and later may pass infection through their saliva to other animals.
Some birds appear to serve as hospitable hosts for these viruses. They tend to collect in large flocks that create populations on which the mosquitoes may feed and in which viruses may amplify. In contrast, other types of birds may not tolerate infection and soon succumb.
"The manner in which these viruses perpetuate in nature is something that really interests us," said Pollack. "Some types of birds will only transiently maintain the infection, perhaps at a level too low to infect other mosquitoes. Birds of other species, in contrast, will support the development of incredible concentrations of virus in their blood."
Identifying which kinds of birds and mosquitoes are significant in maintaining these human pathogens will ultimately help in the design of highly focused interventions that may reduce risk while remaining economically and environmentally sound, said Pollack.
"The virus has to be maintained by and perpetuate in some kind of animal," said Pollack. "There is still much to be learned as to how the virus is maintained in nature, although it's likely that it is some kind of bird, probably one that doesn't get sick."
Although it appears obvious that birds may be implicated as the reservoirs of West Nile virus, it is too early to rule out other potential hosts, said Pollack.
Following the bird deaths in New York, nearly 70 people suffered brain infections and seven of them died. Pesticides were sprayed, traps were set, and officials were sent door to door in the designated "hot zone" of Queens to collect human blood samples. The virus was also later detected in Connecticut, New Jersey, and Maryland. This year, New York City officials announced they will add fish that eat mosquito larvae to water treatment facilities and use trucks to spray pesticides in neighborhoods where the virus is detected, if necessary. On April 18, the New York City Board of Health said standing pools of water are a public nuisance and authorized sanitation workers to remove them.
No one knows for sure from where the New York City virus came. During the outbreak, officials took the unusual step of sequencing the full genome of the virus and comparing it to isolates from other parts of the world. The New York City genome was strikingly similar to one isolated in Israel in 1998.
The death of the red-tailed hawk in February and the detection of the virus in Connecticut this winter have alerted officials that the virus is still around, and they have wasted little time in reacting. The CDC has allocated more than $2 million to ensure containment of West Nile encephalitis. Last week, the Massachusetts Department of Public Health unveiled its plan to combat West Nile.
The department is adding eight mosquito surveillance sites to its regular
18 traps statewide and initiating a non-toxic bacterial larvacide program
that will help kill the mosquitoes before they mature. They also will
track bird populations.
| What You Can
Do
1. Get rid of standing water. Culex pipiens likes standing water with some organic material in it. Drain buckets, trashcans, bird baths, roof gutters, and other containers that may serve as breeding sites. 2. Wear long-sleeved clothing, pants, and other clothing if you are exposed to mosquitoes. 3. Avoid spending time outdoors at dusk and dawn when mosquitoes are most active. 4. Use repellents that contain "DEET," but take particular care if using such products on children. Formulations with less than 10% DEET are generally recommended for children. 5. Repair holes in screens. 6. Be aware that spraying lawns is relatively ineffective.
--Source Richard Pollack, Chris Mores, Gregory Ebel, and the Massachusetts Department of Public Health |
