The Carlisle Mosquito Online

Friday, August 28, 2009

EEE and WNV share link to mosquitoes, birds

On July 22, Massachusetts Department of Public Health Chief Medical Officer Dr. Al DeMaria described the history and life cycle of Eastern Equine Encephalitis (EEE) and West Nile Virus (WNV) during a presentation on vector-borne diseases present in Massachusetts.

EEE was first recognized in Massachusetts in the 1930s, when over 100 horses died of brain fever. Outbreaks had occurred earlier in the 1800s, but the pathogen was only identified in 1933. EEE initially kept a low profile as an infectious disease because humans were heavily harvesting cedar trees for roofing shingles, almost entirely eliminating the habitat of mosquitoes that spread EEE. However, with the advent of asphalt roofing the cedar trees have since returned – as well as the mosquitoes that carry EEE. Recent surveillance efforts have documented that there is an outbreak of the virus every seven to 19 years, with an average outbreak occurring every 13 years.

According to the CDC, EEE has a 33% mortality rate which makes it one of the most deadly mosquito-borne diseases in the United States. EEE, however, is contained to a very specific environment – hardwood, freshwater swamps like those found in Massachusetts. The virus occurs naturally in bird populations which are asymptomatic. It cycles from bird to bird by bird-biting mosquitoes. Bridge vectors, mosquitoes that bite both birds and mammals, are required for the virus to jump from avian populations to mammalian ones, like horses and humans. An individual infected with EEE may have flu-like symptoms that vary in their severity.

WNV shares some similarities with EEE but it has a distinctly different ecology. WNV primarily cycles between birds and bird-biting mosquitoes but birds infected with WNV often become sick and die due to the virus. Like EEE, bridge vectors are required for the virus to jump from its usual avian-based populations to mammalian ones. Unlike EEE, WNV is not tied to a specific ecosystem and that fact is demonstrated by its presence in many states across the country. In fact, the mosquitoes most involved with the transmission of WNV prefer dirty water for breeding sites, enabling even urban-based populations of mosquitoes to be potential sites for WNV.

Incidences of WNV are also more difficult to track. Approximately 80% of infected individuals are asymptomatic or present only very mild signs of sickness while 20% of those infected demonstrate flu-like symptoms that may last anywhere from a few days to a few weeks.“ ∆

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