Why Can't All Mosquitoes Transmit Viruses?

Stephanie L. Richards, PhD, Medical Entomologist

Mosquitoes are known to transmit a variety of viruses (e.g., chikungunya, dengue, West Nile, Zika) and other pathogens (e.g., protozoans that cause malaria) that can cause human disease. However, not all species of mosquitoes (there are >2,700 species) can become infected with and transmit all pathogens. Even individuals within a population of mosquitoes show variability in infection and transmission rates.

Both female and male mosquitoes feed on plant nectar; however, only female mosquitoes blood feed.

A mosquito is considered to be a vector if she can become infected with AND transmit a pathogen (such as a virus) during blood feeding. Many mosquitoes can become infected with a pathogen, but not all of them are able to also transmit the pathogen to another host (such as a human).

What happens during a mosquito blood feeding?

A female mosquito is ready to take her first blood meal (from a host such as a human or other animal) within 3–4 days after she first emerges as a flying adult from the aquatic pupal stage. The blood is digested, eggs are developed and she can lay the eggs about 4–5 days after she takes the blood meal. After she lays her eggs, she is ready to blood feed again. The female mosquito can take several blood meals and lay several batches of eggs during her lifetime of approximately 3–4 weeks (depending on environmental conditions).

When a female mosquito inserts her mouthparts into the skin of a host to find a blood vessel, she expels a bit of her saliva. The saliva numbs the victim and also acts as an anticoagulant to keep the blood flowing into her gut. If the mosquito is infected with a virus, her saliva may contain virus particles that are also expelled into the blood stream of the host while she is blood feeding. The female mosquito only takes from 0.001 to 0.01 milliliter of your blood during a blood meal, but the repercussions can be enormous if she has transmitted a pathogen to you during that brief feeding.

Mosquitoes have immune systems

Mosquitoes have immune systems that fight virus infections. If the virus survives the initial defenses in the mosquito gut, the virus must surpass several additional barriers to infection before transmission via saliva can occur. The virus must infect and escape the mosquito’s gut cells in order to replicate in the mosquito’s body (called the hemocoel). Virus particles must also infect and escape salivary gland cells to be transmitted to another host during the next blood feeding.

Are mosquitoes affected by virus infections?

In some cases, a mosquito that is infected with a virus experiences lower fitness (e.g., decreased egg laying, decreased survival). When the number of virus particles in a mosquito is high, the virus particles can even start damaging cells in the salivary glands, sometimes disrupting transmission. The degree of fitness costs vary between mosquito-virus systems.

Other factors influencing virus transmission

Environmental conditions, such as temperature, may impact vector competence (the mosquito’s ability to become infected with and transmit a pathogen). In general, warmer temperatures increase the rate of virus replication, hence increasing the chances that a competent vector could transmit the virus to another host. However, warmer temperatures may also decrease mosquito survival rates.

The virus dose ingested by a mosquito during blood feeding can also impact her vector competence. If a mosquito ingests a low virus dose during blood feeding, it takes a longer time for the virus to replicate to the level required for transmission in saliva. Conversely, if a higher virus dose is ingested, virus replication facilitates the virus transmission rate.

A mosquito’s age at the time of blood feeding affects vector competence. The mosquito immune response (along with other differences) may vary in “young” versus “old” mosquitoes. The life span of an adult mosquito is approximately 3–4 weeks (depending on environmental conditions).

These, along with many other factors, may interact with one another in unpredictable ways. Mosquito-virus interactions are complex and may vary from population to population. Since there is variability in mosquitoes’ abilities to transmit pathogens, it is important to have surveillance-based mosquito control programs that regularly evaluate abundance of mosquito species of public health concern in their regions. Targeted mosquito control using a variety of techniques is important to eliminate the most dangerous mosquitoes, and not all mosquitoes are a public health risk.

 

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