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:
- 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
- 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
- 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
- A study testing one U.S. Gulf Coast population of colonized Ae. taeniorhynchus found it was not able to transmit Zika virus8
- 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
- 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
- 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.
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1 Centers for Disease Control and Prevention (2018) Zika virus. Accessed March 23, 2018: https://www.cdc.gov/zika/index.html
2 Cohen J (2017) Where has all the Zika gone? Science 357:631-632.
3 Centers for Disease Control and Prevention (2018) Zika Cases in the US. Accessed April 9, 2018: https://www.cdc.gov/zika/reporting/2017-case-counts.html
4 Epelboin Y, Talaga S, Epelboin L, Dusfour I (2017) Zika virus: An updated review of competent or naturally infected mosquitoes. PLoS Neglected Tropical Diseases 11:e0005933.
5 Hahn MB, Eisen RJ, Eisen L, Boegler KA, Moore CG, McAllister J, Savage HM, Mutebi JP (2016) Reported distribution of Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus in the United States, 1995-2016 (Diptera: Culicidae). Journal of Medical Entomology 53:1169–1175.
6 O’Donnell KL, Bixby MA, Morin KJ, Bradley DS, Vaughan JA (2017) Potential of a Northern population of Aedes vexans (Diptera: Culicidae) to transmit Zika virus. Journal of Medical Entomology 54: 1354-1359.
7 Gendernalik A, Weger-Lucarelli J, Garcia Luna SM, Fauver JR, Ruckert C, Murrieta RA, Bergren N, Samaras D, Nguyen C, Kading RC, Ebel GD (2017) American Aedes vexans mosquitoes are competent vectors of Zika virus. American Journal of Tropical Medicine and Hygiene 96: 1338-1340.
8 Hart CE, Roundy CM, Azar SR, Huang JH, Yun R, Reynolds E, Leal G, Nava MR, Vela J, Stark PM, Debboun M, Rossi S, Vasilakis N, Thangamani S, Weaver SC (2017) Zika virus vector competency of mosquitoes, Gulf Coast, United States. Emerging Infectious Diseases 23: 559-560.
9 Aliota MT, Peinado SA, Osorio JE, Bartholomay LC (2016) Culex pipiens and Aedes triseriatus mosquito susceptibility to Zika virus. Emerging Infectious Diseases 22: 1857-1859.
10 Rosilainy SF, Campos SS, Ribiero PS, Raphael LMS, Bonaldo MC, Lourenco-de-Oliveira R (2017) Culex quinquefasciatus from areas with the highest incidence of microcephaly associated with Zika virus infections in the Northeast Region of Brazil are refractory to the virus. Memorias Do Instituto Oswaldo Cruz 112: 577-579.
11 Guo XX, Li CX, Deng YQ, Xing D, Liu QM, Wu Q, Sun AJ, Dong YD, Cao WC, Quin CF, Ahao TY (2016) Culex pipiens quinquefasciatus: A potential vector to transmit Zika virus. Emerging Microbes and Infections 5:e102.