Mosquitoes have become a major public health threat in Massachusetts where some species transmit eastern equine encephalitis (EEE) and the West Nile virus to humans. During 2019, public health authorities announced that 12 Massachuseetts residents had contracted EEE, four of whom died as a result. In response to last year’s high rate of mosquito-borne disease cases in Massachusetts, authorities are aggressively expanding area-wide mosquito control operations in the state, and the governor is pushing for new legislation that will enable mosquito control operations to be initiated and carried out with greater expedience. In Massachusetts, the bloodsucking habits of certain mosquito species only serve to expand the reservoir of disease pathogens in the environment, but a few species are direct disease vectors, including the invasive Asian tiger mosquito, which is rapidly expanding its habitat range in the state.

Historically, two prevalent mosquito species in the northeast, Culex pipiens and C. restuans, were not considered by experts to be important in the spread of the West Nile virus in the region because they feed mainly on bird blood and they rarely bite humans. Because of this, these two species were not targeted for elimination during area-wide mosquito abatement programs. However, it is now understood that C. pipiens and C. restuans may be responsible for around 80 percent of West Nile virus infections in the northeast.

pipiens and C. restuans are more abundant than other mosquito pests in the northeast, and their populations have the highest prevalence of West Nile virus pathogens because they feed mainly on the blood of birds where the pathogens originate. This makes both C. pipiens and C. restuans “amplification vectors,” which means that, due to their habit of feeding on numerous birds every day, they spread West Nile pathogens from infected to uninfected birds, thereby increasing the reservoir of West Nile pathogens. As a result, other mosquito species are more likely to acquire the virus when feeding on bird blood, and many mosquito species feed on the blood of both birds and humans.

Mosquitoes that feed on both bird and human blood are known as “bridge vectors,” and while they transmit disease directly to humans, it is important to note that they most likely  would not have acquired the disease pathogens in the first place had amplification vector species not increased the number of birds carrying carrying the pathogens. After researchers discovered the significance of this distinction, officials in the northeast began targeting C. pippiens, C. restuans and other amplification vector species during area-wide control operations rather than bridge vector species.

Were you aware of the difference between disease-carrying mosquitoes species that serve as bridge vectors and ones that serve as amplification vectors?