Clients sometimes ask about the potential spread of a mosquito-borne illness in our area. To address these questions we decided to put together a list of the definition of important words that help explain the nitty gritty of disease transmission.
Mosquitoes are VECTORS. This is when an organism can transmit a disease or parasite from one animal or plant to another.
Vector competence is the ability of mosquitoes to acquire, maintain, and transmit microbial agents. When a mosquito penetrates the skin, it draws a blood meal which can contain the virus. Although there may be a lot of the virus inside the mosquito, the mosquito itself is not ‘infected’, it is a carrier. If a mosquito bites you, even for just one or two seconds, that is long enough for it to secrete saliva, and that has instantaneously transmitted the virus to you, even if the mosquito does not get its blood meal. The mosquito needs a blood meal for it to become infected, a carrier if you will, but can infect another person without getting a blood meal.
Vectoral Capacity is the clue of how important a particular species would be in the real world for disease transmission based on how common the species is, feeding preferences and vector competence. Certain things can impact vectoral capacity. For example, female mosquitoes need a blood meal to be able to lay their eggs. If a female becomes full with a blood meal, for the next week she will rest, digest that meal, and then lay her eggs. On the other hand, if she didn’t get a blood meal, she will keep probing until she does because she needs to protein in order to lay her eggs. The probing female may still transmit a virus repeatedly because of constant probing – this is an example of vectoral capacity.
Midgut Infection Barrier – Uninfected (No receptors for virus)
If there are no receptors in the midgut or if an insufficient amount of the virus was in ingested in the blood meal, the blood meal will be digested and excreted and the mosquito will now have an uninfected gut with no virus.
Midgut Escape Barrier – Virus cannot get out of the gut. This is a midgut-infected mosquito, but it is not able to transmit the virus by biting.
If receptors are in the midgut and enough of the virus has been ingested, the mosquito will now have an infected gut. This literally occurs within minutes of a blood meal. The virus cannot get out of the midgut and is not able to transmit by biting or probing.
Dissemination – The ability of a virus to escape the midgut to the hemocoel (rest of the body)
Disseminated Infection – A virus has spread throughout a mosquito’s body
Salivary Gland Barrier – Virus is disseminated to the rest of the body, but salivary glands are not infected.
In some cases the virus escapes the midgut to the rest of the body EXCEPT for the Salivary Glands. So although the mosquito has a lot of the virus and its body is infected it still cannot transmit by bite.
Competent Mosquito Vector – Salivary glands are infected, mosquito is ready to transmit by bite.
Once the virus is in the Salivary Glands, the mosquito can transmit the virus by biting and probing.