Author: Mosquito Squad of Leominster

On June 15 the State of Massachusetts started tracking the arboviruses West Nile Virus (WNV) and Eastern Equine Encephalitis (EEE) across the state for the season. Arbovirus is a term to describe a group of viruses that are transmitted by mosquitoes, ticks or other arthropods. The goal is to provide an estimate for human risk from EEE & WNV. Based on the surveillance information actions are taken to inform the public, reduce human risk and implement planned programs as needed.

The process takes a three pronged approach that can sometimes cause confusion. It is not just a matter of reporting human cases or testing mosquitoes. The process for tracking arbovirus is complex and a great system for keeping the public aware of the severity of the risk. Here is how it works according to the Massachusetts’s Department of Health (MDPH) website:


MDPH uses long term traps in areas of high-risk, trapping egg-bearing mosquitoes during arbovirus season. The mosquitoes are sorted by species and tested in groups for EEE and WNV. The groups are called “pools”. Results are generally available within 24 hours. If EEE or WNV is discovered in an area, additional trap sites may be added.

By testing the species of mosquitoes that are involved in spreading disease, the MDPH can provide the public with an estimated risk to humans. In addition to testing for WNV and EEE, MDPH uses national and regional data and up-to-date scientific literature to assess the risk of emerging arboviruses in Massachusetts. They also test the appropriate mosquito pools for these new-to-the-area viruses.

The presence of EEE, WNV or an emerging virus in the mosquito population does not mean there has been a human or animal case. It just means the virus is present in the mosquitoes in that area and that steps to reduce your risk for exposure should be taken.


When WNV first emerged in the United States it caused high mortality rates in the bird population, so testing of deceased birds was a great surveillance tool. The practice of testing individual dead birds has been discontinued because so many birds survive as carriers of WNV and EEE making the practice inaccurate. Certain bird species are highly susceptible to EEE so testing of highly suspect bird specimens for EEE and/or WNV is done, but only as-needed.


When horses or other domestic animals have severe neurological disease that is suspected to be caused by EEE or WNV, Hinton State Laboratory Institute (HSLI) tests specimens to determine if infection is indeed present. During the 9 days it takes to get the complete test result, animals that test positive in initial screening will be reported as a preliminary result to its veterinarian, local boards of health where the animal lives and where the animal was exposed, as well as the local mosquito control project if it exists. These results may be used to inform clinical decisions and plan public health and mosquito control activities. The animal will not be considered a confirmed case until the full testing is complete. Specimens from other sources such as zoos, horse stables and wild animals are also tested as appropriate as well.

Animals can be immunized against WNV and EEE, which is the best form of prevention for animals. Check with your veterinarian to get more information.


Humans presenting with encephalitis and meningoencephalitis should be reported and specimens provided to HSLI for WNV and EEE testing. An initial screening will result in a preliminary finding, but the confirmatory results will take 3-7 days. Due to an increase in commercial laboratory testing, MDPH does not receive as many samples, making the state’s reported numbers lower than they actually are. Commercial labs cannot complete the final confirmatory testing, only HSLI can. All suspected cases should go to MDPH to be tested at HSLI so that reporting can be as accurate as possible.

Local medical personnel, local boards of health, and local mosquito control will be notified if a sample comes up positive. The goal here being to keep the public informed so proper steps can be taken to ramp up mosquito control efforts. To track results of the state’s surveillance program check out the Massachusetts Arbovirus Daily Update page.

To lower your risk for infection all season long, eliminate mosquitoes in your yard with help from Mosquito Squad of Leominster. Our traditional barrier spray eliminates mosquitoes on contact and continues to work for up to 21 days. By lowering the populations of mosquitoes in your yard by 85-90% we can help keep you and your family safe from WNV and EEE this season. Contact us to sign up today!