On Sept. 19, the City of Waltham posted on their news page that State Health Officials have  found a mosquito infected with West Nile virus in Waltham. The mosquito was found in the Northeast Quadrant of Waltham and tested positive on Sept. 11.

According to the Massachusetts Department of Public Health, mosquitos have been testing positive for WNV and eastern equine encephalitis since July, but the first human case of WNV and animal case of EEE occurred on Aug. 6. The first human case of EEE this season was reported on Aug. 16. The MDPH releases WNV and EEE risk maps based on seasonal testing from June to October. The tracker shows the number of positive mosquito samples as well as both human and animal cases for WNV and EEE. Data on mosquitoes that tested positive for WNV or EEE include the collection date, county, town and mosquito species. For information on patients who test positive for WNV or EEE, onset date, county of exposure, age, gender/sex of the patient and clinical presentation are included in the dashboard.  

As of Sept. 27, there are a total of 13 human cases of WNV and four human cases of EEE in Massachusetts. Currently, there are no WNV or EEE cases in Waltham. According to the MDPH there were six human cases of WNV in 2023. Comparatively, there were 12 human cases of EEE statewide in 2019 with six deaths, and five human cases with one death in 2020. There were no human cases of EEE in Massachusetts in 2021, 2022 or 2023.

Each town is assigned a risk level for WNV and EEE. The scale for WNV risk level goes from low to moderate to high to critical. For EEE, a five point scale is used: remote, low, moderate, high and critical. Waltham’s risk level for WNV is moderate, meaning that infection with WNV is likely or has already occurred. Waltham’s EEE risk level is remote, meaning that EEE is not usually found in the area.     

According to Brian Farless, superintendent of the East Middlesex Mosquito Control Project and the Suffolk County Mosquito Control Project, in a Sept. 27 email to The Justice, each mosquito control district in the state submits samples and data to the MDPH, who then tests the mosquitoes for EEE and WNV. As the MDPH receives data, they update the WNV and EEE risk maps, daily case data and notify towns.

A full-time entomologist on staff for the EMMCP runs the mosquito surveillance program. The program uses three kinds of mosquito traps, says Farless. One trap targets mammal biting mosquitoes, and this type catches the species that spread EEE. The second trap targets the mosquito species that is the main transmitter for West Nile virus. The third trap is an Aedes oviposit trap and is used to look for the presence of the Aedes albopictus mosquito. Appearing in Massachusetts in 2009, this new species has been gradually moving throughout the East Middlesex district. It is currently not a primary vector for disease but could become a future concern. 

Established in 1945, the EMMCP is a state agency “that conducts science based mosquito control for the good of public health” and practices “Integrated Pest Management (IPM), which means that we focus on source reduction[,] … mosquito larvae management, adult mosquito management, mosquito surveillance[,] … and education,” Farless wrote. The program also aims to fulfill its goals with the least negative impact on people, other animals and the environment. The Project services 26 participating communities, including Waltham. 

The state is experiencing a recent outbreak of WNV and EEE because of the weather, explained Waltham’s Director of Public Health, Michelle Feeley, in a Sept. 27 interview with The Justice. While usually by this time temperatures should have dropped, they have remained high. Once the first frost comes, the Health Department expects cases to drop. 

With climate change and rising temperatures, the incidence of mosquito-borne diseases can be affected. According to National Public Radio, the transmission process –– a mosquito biting an infected animal, living long enough for the virus to develop in them and transmitting it to a human –– depends on temperature since mosquitoes are cold-blooded animals. According to the Central Massachusetts Mosquito Control Project, mosquitoes thrive at 80 degrees Fahrenheit, become lethargic at 60 degrees Fahrenheit and cannot survive below 50 degrees Fahrenheit. However, climate is only one of the factors causing outbreaks, and EEE is hard to study because it is a rare disease. 

For example, people’s proximity to infected birds and mosquitoes matters. For WNV and EEE, the virus is transmitted from infected mosquitoes to humans. Humans cannot spread the disease to each other, nor can mosquitos bite infected humans and become infected. Humans are dead-end hosts because they cannot spread the virus to mosquitoes. The Centers for Disease Control and Prevention states that WNV “is rarely transmitted from person-to-person by blood transfusion, organ transplantation, and mother to baby, during pregnancy, delivery, or breastfeeding.” There are no vaccines to prevent these two diseases, nor medicines to treat them.  

For WNV, eight out of 10 people with the disease do not develop any symptoms. Additionally, around one in five people develop febrile illness (fever) from WNV, and symptoms include headaches, body aches, joint pains, vomiting, diarrhea or rash. Most people in this category recover completely, but fatigue can last for weeks or months. According to the CDC, approximately “one in 150 people who are infected develop a severe illness affecting the central nervous system such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord).” The CDC also states that about one out of 10 people who develop a serious illness affecting the central nervous system die.

EEE is rare, but serious. Around 30% of people who develop severe eastern equine encephalitis die. Moreover, many survivors have lasting neurological problems. Most people with EEE do not develop symptoms. Similarly, patients with EEE can either have a febrile illness or a neurological disease. Most people recover completely after one to two weeks if there is no central nervous system involvement. Neurologic disease can include meningitis or encephalitis, and symptoms consist of fever, headache, vomiting, diarrhea, seizures, behavioral changes, drowsiness and coma.

Feeley stated that children, the elderly and people with compromised immune systems are particularly vulnerable populations for these two illnesses.  

Public health officials urge individuals to take preventative measures to avoid mosquito bites and reduce mosquito populations. Wearing long clothes to cover their skin, as well as using insect repellent, can help reduce the chance of being bitten. They also recommend people to avoid being outside during active biting times for mosquitoes which is from dusk to dawn, install and repair screens and drain any standing water where mosquitoes can lay their eggs. 

If bitten by a mosquito, Feeley recommended that people monitor their symptoms, note if they have been in an area at high risk of WNV or EEE and contact their physician with any concerns. 

To lower mosquito bites, the EMMCP implements spraying operations to reduce mosquito populations. Farless wrote that “[m]ore spraying will be considered as disease is found. Spraying stops when mosquito activity drops, which is usually during September of each year. Due to increased EEE risk during 2024, more spraying was conducted, but, as of late September, spraying has concluded for the season.” 

The decision to spray depends on whether there is a high number of mosquitoes in the traps and if disease is found. According to Farless’ email, there are two spraying methods: truck-mounted Ultra Low Volume (ULV) spray and backpack mist blower. The truck travels and sprays areas from sunset until around 11:30 PM to target peak mosquito activity time and avoid pollinators. Sprayed 20 feet into the air and floating up to 300 feet laterally, the ultra-fine mist kills mosquitoes on contact. When the pesticide lands, the product breaks down rapidly, leaving little to no residual, and it becomes no longer effective at controlling mosquitoes. 

The second method consists of an employee on foot with a motorized backpack mist blower spraying the perimeter of an area. This product leaves a residue, and similarly, this activity happens at night to avoid pollinators. 

Each pesticide is authorized by the Massachusetts Pesticide Board, and the board only approves pesticides that have been previously approved by the Environmental Protection Agency.   

When asked about how the program measures success in reducing mosquito populations, Farless responded that “[s]ince mosquitoes are everywhere in very large numbers, … there is no way of knowing the total population of mosquitoes in a city/town at any given time.” However, he stated that their control methods –– source reduction, ditch maintenance, larval control and adult control –– have been proven to shrink mosquito populations. 

One challenge for the EMMCP has been filling seasonal positions. From mid-May to mid-August, an employee usually would walk or bike through cities and towns in the district and place packets of larvicide into catch basins. The main mosquito species for WNV is found in catch basin water. 

Since there are no WNV or EEE cases in Waltham, Waltham’s public health department is mainly monitoring the situation and sharing guidance with citizens through social media. “It’s on our radar but we’re not actively pursuing anything for it … We’re very aware of it, but the most we can do is tell people the guidelines as of right now,” Feeley stated. 

To people who are worried about mosquito-borne illnesses, Feeley encourages residents to reach out and learn how to protect themselves. If Waltham’s risk levels for WNV or EEE rise, stricter policies would be issued. Feeley hopes that as October approaches and subsequently temperatures drop, the likelihood of more positive cases will decrease.