Mosquitoes carrying Eastern Equine Encephalitis virus

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

As detailed in the recent press release from the MDA and MDCH , Eastern Equine Encephalitis (EEE) outbreaks are occurring in southwest Michigan and the virus activity is likely to be high in other areas of the state as well. This is a very serious disease for horses and potentially for humans. At the time of this writing, additional horse cases have been reported and at least one serious human case from Kalamazoo County has been confirmed.

EEE is a mosquito borne disease endemic to Michigan. Like the recently introduced West Nile virus, it is primarily a disease of birds and is amplified via a bird-mosquito cycle. When the virus spills over from birds into other hosts through mosquitoes that feed on both birds and mammals, it can cause acute neurological disease in the “dead end” host such as horses or humans. It can also cause severe disease and death in deer and some domesticated birds. For example, an emu farm in Michigan was hit hard by the disease last year. The virus does not appear to cause serious disease in other livestock, or in dogs or cats. Though the risk of developing serious disease in humans is low, mortality rates are very high (over 30 percent) for those who develop neurological symptoms and survivors often suffer permanent brain damage.

The mosquitoes that contribute most to amplifying the virus in birds breed in bog and hardwood swamp wetlands, while the mosquitoes that are thought to “bridge” the virus between birds and mammals breed in cattail marshes. Therefore, EEE risk is higher in these areas and the disease is almost always rural and localized. Be aware, however, that these vectors are known to fly several miles away from breeding sites when seeking hosts. As most of you know, conditions have been nearly ideal for mosquito populations in general this year and likely contributed to early amplification of the disease and the current outbreaks.

As discussed in the MDA/MDCH article, people in risk areas should take extra precautions to avoid mosquito bites. There are many good repellents available and those, along with avoiding being outdoors during prime mosquito feeding times (dusk/dawn), should be used whenever possible. Horses can be vaccinated against EEE and it’s not too late to do so for this season. Remember – there is no good treatment after the virus is established, the only viable strategy is take preventative measures.

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