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Risk Map Will Help Predict Encephalitis Outbreaks

By:
Cindy Spence

Source:
Jonathan Day jfda@gnv.ifas.ufl.edu, (561) 778-7200

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VERO BEACH—The risk of a widespread St. Louis encephalitis epidemic in Florida this year is almost nonexistent, says a medical entomologist with the University of Florida’s Institute of Food and Agricultural Sciences.

New York City also will likely be spared a repeat of last summer’s epidemic of West Nile virus, a close relative of St. Louis encephalitis, predicts Jonathan Day, a mosquito researcher at UF’s Florida Medical Entomology Laboratory in Vero Beach.

Weather conditions are expected to keep both mosquito-borne diseases in check along the East Coast, Day said. His predictions are based partly on an encephalitis risk map he is developing as part of a project funded by NASA.

Day is combining satellite surveillance data from NASA, weather information from the National Oceanic and Atmospheric Administration (NOAA) and a map of North America to develop a tool that could be made available nationwide via a Web site.

The map would change as various indicators for encephalitis show up or disappear across the country. The hope is that a risk map could provide valuable time for officials to educate residents about the risk of encephalitis and prevent an outbreak.

Mosquitoes get the encephalitis virus from infected birds and pass it on to humans. Encephalitis starts as a flu-like illness but can progress to a fatal inflammation of the brain. Most people never exhibit symptoms but some develop muscle weakness, tremors, confusion and paralysis that can lead to nerve damage and death.

“During most years, encephalitis is not a problem,” Day said. “But in regions where it cycles in wild birds, encephalitis epidemics can erupt explosively; they can happen very quickly, and there can be thousands of human cases.

“South Florida is always a potential St. Louis encephalitis hot spot, but you also see it in Illinois, Louisiana, Texas and Colorado; it can show up virtually any place.”

With the help of the risk map, Day hopes to be able to predict when conditions are ripe for another outbreak of West Nile, a form of encephalitis never seen in the New World before last year. Data on surface temperatures, rainfall, standing water and vegetation cover will be incorporated into the map.

“In modeling West Nile virus, the question is where will West Nile end up in North America?” Day said. “Will it move out of New York City with dispersing infected birds or will it remain in the NYC metropolitan area?”

West Nile epidemics differ from those of St. Louis encephalitis, Day said. The mosquito that transmits West Nile is an urban mosquito that flourishes under dry conditions. The drier the conditions the better the mosquito fares because it breeds underground in sewer catchbasins that are flushed out during heavy rains.

Predictive models from NOAA indicate the East Coast will experience normal rainfall and temperatures during June, July and August, the critical period for West Nile transmission. The Midwest, however, from St. Louis and Chicago down the Mississippi River basin, is predicted to be extremely dry and hot.

Day says if West Nile has been able to move out of New York City — in a dispersing or migrating bird, for instance — then the risk of West Nile will be high in the Midwest.

The current West Nile risk map should be completed by late June. In coming years, Day hopes to have maps posted by January and updated monthly through April, after which updates would become weekly through the end of mosquito season.

Day and his colleagues begin tracking St. Louis encephalitis in Florida each January. A series of events has to occur for an epidemic to take place. The first event, a freeze, never materialized in South Florida this winter. The second indicator, a wet spring, also never materialized.

Freezes kill understory vegetation, opening habitat to ground-feeding birds and prompting a robust breeding season. A wet spring allows mosquitoes to flourish. A wet spring on the heels of a winter freeze brings hordes of mosquitoes together with an abundance of birds, conditions that are ripe for encephalitis epidemics. During epidemic years, the virus usually becomes evident in sentinel chickens by June.

“Since we don’t start to get human cases until late July, we have about eight weeks to institute control methods and get information out to people about how to protect themselves,” Day said. “The idea of a risk map is to provide even more early warning.

“In New York last year, they had no buffer at all. They had a full-blown epidemic before they even knew what was happening.”

The most recent epidemic of St. Louis encephalitis in Florida was in 1990, with 28 counties reporting outbreaks, 226 clinical infections, about 20,000 sub-clinical infections and 11 deaths.

Day said even a break in the state’s drought, with prolonged wet weather, would not prompt a St. Louis encephalitis outbreak.

“If there’s no rain, there are no mosquitoes, and if there are no mosquitoes there is no epidemic,” Day said. “And if the rains began again now, the mosquitoes that transmit the St. Louis encephalitis virus would never catch up.

“That’s why St. Louis encephalitis epidemics are so rare. Mosquitoes, birds and rainfall have to be in perfect synchrony, starting in January, for an epidemic to occur,” Day said.

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