The status of potential Zika virus vectors in the U.S.

By Stephanie L. Richards, MSEH, Ph.D., Medical Entomologist

By now, most of us are familiar with the Zika virus. It is primarily transmitted to humans during blood-feeding, but can also be sexually transmitted between humans.¹ While human outbreaks have been observed for decades, Zika was thrust onto the world stage in 2015 when widespread symptoms of birth defects, like microcephaly, and other neurological issues were discovered in relation to Zika infection in pregnant women. In recent years, there has been a significant decrease in symptomatic Zika cases (80% of cases are asymptomatic) in former “hot zones”, likely due to “herd immunity”.² According to the Centers for Disease Control and Prevention (CDC), 432 symptomatic disease cases were reported in 2017 in U.S. states, including five cases resulting from local mosquito populations, 419 traveler-imported cases, seven cases from sexual transmission, and one infection accidentally obtained in a laboratory.³

Not all mosquitoes can become infected with and transmit viruses (i.e., display vector competence). Vector capacity for Zika depends, in part, on the virus incubation period, mosquito-to-human ratio, mosquito survival rate and mosquitoes’ inclination to bite humans or other animals. Characteristics of the infecting virus strain, availability and mobility of reservoir hosts, environmental temperature, mosquito immune response and other factors also impact vector competence and virus distribution.

Reports of vector competence for approximately 20 mosquito species for Zika have been published.⁴ Even if a mosquito population has been classified as vector-competent or incompetent, this does not mean that all mosquitoes of that species will be classified similarly, as many factors can influence vector competence and capacity.

See below for a breakdown of Zika virus findings by species:

Aedes aegypti

• Prevalent in subtropical and tropical regions
• Commonly found in southern U.S. states⁵
• Its vector competence and human blood-feeding preference make it the primary vector of Zika virus

Aedes albopictus

• Found in temperate, subtropical and tropical climates
• Widespread in the U.S.⁵
• Is a competent vector of Zika and blood-feeds on a variety of hosts, including humans

Aedes vexans

• Widespread in temperate regions of the U.S.
• Blood-feeds on large mammals, including humans
• Ae. vexans collected from the field in Minnesota were shown to transmit the virus when fed antibiotics and exposed to Zika virus⁶
• Field-collected Ae. vexans from Colorado also showed mosquitoes were capable of Zika virus transmission⁷

Aedes taeniorhynchus

• A study testing one U.S. Gulf Coast population of colonized Ae. taeniorhynchus found it was not able to transmit Zika virus⁸

Aedes triseriatus

• A long-standing lab colony of Ae. triseriatus became infected with Zika virus but was not able to transmit the virus — hence, mosquitoes from this population were not competent vectors⁹

Culex quinquefasciatus

• Suspected as a potential Zika vector because of its abundance and anthropophilic (human-seeking) behavior in Brazil¹⁰
  • However, laboratory reports have shown variation in vector competence between populations
• A study reported Zika in the saliva of laboratory-infected Cx. quinquefasciatus from China, suggesting this species could serve as a vector¹¹
• Another study showed Cx. quinquefasciatus (generation F₂) from Texas unable to transmit Zika⁸

Culex pipiens

• A longstanding Culex pipiens colony originating in Wisconsin was an incompetent Zika vector⁹

More work is needed to evaluate vector competence of mosquito populations that may be infected with Zika virus under a variety of biological and environmental conditions. This information will be especially important in areas where Zika virus is present, but primary vectors (Ae. aegypti and Ae. albopictus) are not prevalent, and to determine the extent to which other potential vectors may maintain Zika virus in nature.

While the incidence of reported symptomatic Zika infections may have recently decreased in some regions, there is still the possibility of outbreaks occurring in naïve populations where Zika virus is introduced and competent vectors are present. We must be vigilant in supporting sustained local mosquito surveillance and control programs to be better prepared for emerging mosquito-borne threats.

For more information about Syngenta solutions for mosquito control, visit www.SyngentaPMP.com/Mosquito.

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