In Part 2, we suggested that mosquitoes are not possibly vectors of the new coronavirus. Let us explain why.
There are many species of mosquitoes. In the Philippines, the number can reach 330. Not all mosquitoes are blood-feeders. Other species suck plant sap while there are also nectar feeders. There are also mosquitoes which have their larval stages feeding on the wrigglers of other mosquito species as well as those that specialize on inhabiting the water contents of pitcher plants. For those that feed on blood, only the females do; the males feed on plant juices. Thus, only the females of blood-feeding species are potential vectors of blood-borne pathogens.
The blood meal is necessary for the development of the eggs inside a female mosquito’s body. For this reason, the mosquito usually feeds by sucking blood from several hosts. Natural selection has produced viruses that are favored by this biological requirement of female mosquitoes to move from host to host. During the process, mosquitoes may plainly acquire a pathogen in or on its mouthparts or body or they may become infected as intermediate hosts. Either way they could function as carriers or vectors.
Being a vector means transmitting disease-causing pathogens from one animal, including human, host to another. The transfer may be through mechanical or biological means. Mechanical transfer occurs when a mosquito transfers a virus from a bat to another in the blood on its proboscis. A similar event happens when a cockroach or house fly acquires bacteria when feeding on faeces, and then physically transfers some bacteria from its mouthparts, legs, or body to human food, the bacteria later causes gastroenteric diseases like typhoid fever. The pathogen is transported passively from one host to another, without them multiplying inside the vector. Generally, for mechanical transfers, the insect is just one of several means to spread a pathogen.
In the case of biological transfer, there is a closely knit association among the insect vector, the virulent pathogen, and the susceptible host. All three are needed for the transfer to occur and the disease to develop. The pathogen multiplies within the insect vector, and oftentimes, there is specificity between the vector and the pathogen. Such close association can only evolve through long-term processes governed by natural selection. For viruses that multiply in an arthropod vector and a vertebrate host, they are termed arboviruses (from arthropod-borne viruses). Arboviruses belong to several virus families and hence, the grouping is artificial i.e., it is not based on the evolutionary relationships of the viruses.
For a mosquito to become infected, the first necessary step is to bite an infected animal, such as a human. While it is known that mosquitoes can transmit a number of viruses like dengue, chikungunya, Zika, and Japanese encephalitis viruses, they cannot transmit many other viruses, such as HIV and Ebola, either due to low concentrations of the virus circulating in the blood, or mosquitoes do not become infected at all. Therefore, a mosquito carrying virus is more complicated than a contaminated syringe used on several persons or pipette from one tube to another. When a mosquito pierces through a host skin and sucks up blood that contains a virus, the virus quickly ends up in its gut. From there, the virus needs to infect the cells lining the gut so that it may pass through to infect the rest of the body (head, thorax and abdomen) of the mother mosquito, mainly through the haemolymph and spreading even to the legs and wings. The virus then has to infect the salivary glands before being passed on by the mosquito when it next bites the next victim. This process can take from a few days to weeks.
Aside from the time factor needed for the incubation or the process to be completed, the virus also needs to escape from the gut, to be incorporated into the haemolymph and through the body of the insect, and then into the salivary glands. Each step in this process can be an impenetrable obstacle or barrier for the virus. This may be quite easy for viruses that have adapted to this process. However, for other viruses, they will just be digested or excreted. The viruses causing dengue, zika, and Japanese encephalitis belong to the virus family Flaviviridae while that of chikungunya belong to Togaviridae. They have an entirely different protein envelope structure compared to the coronaviruses (family Coronaviridae). In terms of size, even if SARS-CoV-2 is relatively larger than the dengue virus, it may still pass through the internal tube of a mosquito’s proboscis (~22 micrometers). However, the basic differences in physical structure, and in their longevity and behavior inside a potential carrier may imply differences in their admissibility through the mosquito’s feeding apparatus.
So far as known, coronaviruses have not evolved to be transmitted by mosquitoes.
– Jun Lit (Ireneo L. Lit, Jr.)