Mosquito-borne diseases

Mosquito-borne diseases have killed millions of people over the years. In 2015 alone, the World Health Organization estimated that 480,000 people died of malaria, which is caused by a parasite transmitted by mosquitoes. Here in New England we had outbreaks of malaria and yellow fever in colonial times, right up to the 1940s. In New England, there are currently only two major illnesses spread to people by mosquitoes: EEE and West Nile virus. However our local ecology can still support mosquito-borne viruses and parasites from the tropics, such as Zika. These diseases are transmitted by a limited number of mosquito species, and these species have very specific habits and habitats. There are 51 species of mosquitoes in Massachusetts. Some thrive when it’s hot and dry, some when it’s cool and wet, and others have their own requirements of “just right.”

Eastern Equine Encephalitis (EEE): Although rare, “Triple E” is the deadliest New England mosquito-borne virus. According to the CDC, there is an average of 8 human cases in the US each year of which 33% will die, and many survivors will have brain damage. There is no vaccine to prevent EEE and the only treatment is to provide supportive care until the infection passes. EEE is typically spread only between birds by Culiseta mosquitoes, which rarely bite mammals. Horses and humans can get EEE from mosquitoes that bite both birds and mammals (like Aedes, Coquillettidia, or Culex mosquitoes) but infected mammals do not have a high enough level of virus in their blood to pass the virus back to another mosquito.
According to the Massachusetts Dept. of Public Health (DPH), there have been 20 human cases of EEE in Massachusetts in the last 10 years. Birds do not usually die from EEE infection, so monitoring for EEE is currently limited to trapping mosquitoes, and evaluating infections of humans and horses.

West Nile Virus (WNV): The first documented case of West Nile in the United States was in 1999, but it is quickly making its way across the US. According to the CDC most people with West Nile do not show any symptoms, but 1% develop neurological illness similar to EEE, and 10% of those die. There is no specific treatment or vaccine. Like EEE, West Nile circulates between mosquitoes and birds, and only infects mammals by chance. According to DPH, the virus is most often carried by Culex mosquitoes, which feed primarily on birds, but are more likely to feed on humans and other mammals than the Culiseta mosquitoes. During the first years of the WNV outbreak, blue jays and crows often died from the infection, and DPH monitored dead birds to look for outbreaks. But in just 17 years, North American birds have developed a tolerance for WNV, and now DPH limits their surveillance to trapping mosquitoes and tracking human cases.

Weather conditions favorable for development of elevated WNV risk include hot, generally dry weather with rain occurring as downpours rather than light precipitation.

Zika Virus: So far, the only cases of Zika fever in Massachusetts are from travelers returning from infected areas, but scientists are watching for signs that this virus could develop a local transmission cycle. According to the CDC, most people recover quickly from the mild symptoms of Zika, so most outbreaks may have gone undetected. Though observed in tropical Africa, Southeast Asia, and the Pacific Islands, until 2007 it has only been noticed incidentally while researchers were studying Zika’s more virulent cousins, dengue fever and yellow fever. In 2015, a widespread outbreak of Zika virus in Brazil was associated with a severe birth defect, microcephaly. In the past year, CDC has developed tests to identify Zika fever. Zika appears to be transmissible from person to person through sexual contact, not just from mother to fetus or through transfusions.

In February 2016, the World Health Organization declared Zika virus to be a world health emergency. According to the CDC Arbovirus Disease Branch, as of June 2016, 60 countries report continuing mosquito-borne transmission of Zika. Many other countries and states are reporting traveler-associated cases, including 27 cases in Massachusetts.

There are signs that Zika virus transmission could spread to Massachusetts. Zika is typically spread by Aedes mosquitoes, which are active during the day, not just at dawn and dusk. Aedes mosquitoes bite mammals, particularly humans. Unlike EEE, it appears that a mosquito that bites an infected person can then pass on Zika to other people – in other words, Zika’s natural host is human beings, not birds. The good news is that right now, Aedes mosquitoes are not normally found quite as far North as Massachusetts.

By planning ahead, we can encourage the most effective mosquito-borne illness control strategies that cause little or no environmental damage. Reducing mosquito breeding areas, protecting our bodies, and using pesticides are all methods in our toolbox.


How to reduce mosquito habitats

How to protect yourself from mosquito-borne diseases

Using pesticides to prevent mosquito-borne diseases

What will global warming bring?