A virus that had never been seen in the Western Hemisphere until it killed seven people last fall in New York has re-emerged, and researchers from the School of Public Health (SPH) are working with Massachusetts officials to guard against an outbreak here.
Last August, birds began dying in New York City, and tests identified West Nile encephalitis virus as the culprit. Officials estimated that 5,000 birds succumbed. Following the bird deaths, nearly 70 people suffered brain infections and seven of them died, according to reports from the Centers for Disease Control and Prevention (CDC).
West Nile encephalitis infection can cause the brain to swell but rarely leads to death. Many people carry the virus with mild if any symptoms, but people with severe reactions may suffer convulsions, fever, and paralysis. There are no specific treatments for the disease.
Two months ago, a red-tailed hawk tested positive for the virus and confirmed fears that the mosquito-borne disease had survived the winter. Although the virus has yet to be found in Massachusetts, state agencies have turned to SPH researchers for guidance on how to track the disease as well other mosquito-borne illnesses such as Eastern Equine encephalitis, which has wreaked havoc in Massachusetts in the past.
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 SPHs Department of Immunology and Infectious Diseases. Nicholas Komar, who received his doctoral degree from the SPH in 1997, now works on mosquito-borne viruses for the CDC. All three studied under Andrew Spielman, professor of tropical public health, and Richard Pollack, instructor, who run the laboratory of public health entomology.
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 dont know exactly how many species of mosquitoes are involved,” Pollack said. “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.”
Culex pipiens is well adapted to conditions found in urban areas. “The species is fairly cosmopolitan,” Pollack said. “You will find this mosquito virtually anyplace you look, except perhaps Antarctica.”
Unlike the relatively urban mosquitoes suspected of transmitting West Nile, a much more rural species, Culiseta melanura, carries the Eastern Equine encephalitis virus. Although rare, the disease kills about half of the people who are severely affected, according to the CDC. The virus was first isolated in the United States in 1933 and infected 35 people in Massachusetts in 1938, killing more than half of them.
“This state was really where Eastern Equine encephalitis made its impact,” Spielman said. “It was a searing experience.” Spielman later found Culiseta melanura on the SPH campus in the early 1960s, placing the swamp mosquito in an urban campus setting. He published his findings in Science.
Since the 1938 outbreak, fewer than 40 cases of Eastern Equine encephalitis have occurred in Massachusetts, but the state Department of Public Health has developed a surveillance system for the virus. The Department is now building onto that system to also monitor West Nile. It will add eight mosquito surveillance sites to its regular 18 traps statewide and initiate a nontoxic bacterial larvacide program that will help kill the mosquitoes before they mature.
The word “equine” in Eastern Equine encephalitis suggests that horses actively transmit the virus, but, like humans, horses are dead-end hosts; they dont infect others. Birds, however, form an odd relationship with mosquitoes, and viruses are the direct beneficiaries. In fact, when birds die en masse for no obvious reason, such as they did in New York last year, arboviruses can easily rank among suspected causes.
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 to another bird or other animal in their saliva when they bite again. Certain birds such as crows, starlings, and robins appear to serve as hospitable hosts for arboviruses, Spielman said. These birds tend to collect in large flocks that create available populations on which the mosquitoes may feed and in which viruses may amplify.
Spielman and Pollack are now trying to identify the reservoirs for arboviruses. Their research represents a shift in the field, Spielman said, because past investigations focused on the organisms responsible for transmission and not on those essential for maintenance of the viruses in nature.
“The manner in which these viruses perpetuate in nature is something that really interests us,” Pollack said. “Some types of birds will only transiently maintain the infection, and 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. Why is that important? It takes a certain concentration of virus to infect each kind of mosquito, and the virus must be present at such dense concentrations in the blood for sufficient time to ensure that other mosquitoes will have a chance to become infected.”
What you can do
1. Get rid of standing water. Drain buckets, trashcans, birdbaths, roof gutters, tire swings, and other containers that may serve as breeding sites.
2. Wear long-sleeved clothing, pants, and other protective 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.
Sources:, Gregory Ebel, Chris Mores, Richard Pollack, and the Massachusetts Department of Public Health.