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Why Mosquitoes Cannot Transmit HIV
By L. Hannah Gould
PhD Student, Department of Epidemiology and Public Health
Despite years of HIV/AIDS education worldwide, studies continually find that many people, including health care professionals, believe that HIV can be transmitted by mosquitoes and other blood sucking insects [1-3]. A random telephone study in St. Petersburg found that 55.8 percent of the population thought that mosquitoes could transmit HIV. Years of research have not documented a single case of HIV due to entomologic transmission but in an age of media hysteria over vector-borne diseases such as West Nile virus, it is not surprising that people inevitably wonder why a mosquito (or any insect vector) can carry one deadly disease, such as malaria, but not another. In order effectively to dispel these misconceptions, it is important to understand the science behind why mosquitoes cannot transmit HIV. The reasons are best addressed by looking at the process a pathogen must go through to be transmitted from person to person by a mosquito.
Even if a mosquito were to bite an HIV-infected person and then immediately bite someone else, HIV would not be transmitted. The most apt analogy to explain this is that a mosquito is not a "flying syringe." Unlike in a syringe, the input and output receptacles are separated; the proboscis of a mosquito is composed of six stylets, or feeding channels, with completely different functions - some stylets are used to ingest blood from the host, while others are used to inject the host with anticoagulants and other salivary compounds to keep the blood flowing and the biting unnoticed. The contents of these tubes do not mix, so even a mosquito that had previously fed on an HIV-positive individual would not introduce that blood into a second person. Additionally, the amount of circulating virus in an HIV-positive individual is so low that it is unlikely there would be sufficient virus for subsequent transmission. In fact, the chances of transmission occurring even if blood were directly exchanged in this scenario have been estimated to be less than one in ten million.
Because HIV is a fragile virus, it cannot survive within a mosquito, so even if the insect feeds on an HIV-positive individual, any virus in the blood meal will be destroyed before it can be passed on. Upon ingestion of a blood meal, the mosquito's immune system and digestive tract immediately begin to digest blood components, and HIV cannot survive this harsh milieu of digestive enzymes in the mosquito gut. Thus, unlike a malaria parasite, which is able to survive the mosquito's immune system, HIV will be destroyed.
For a mosquito-borne disease to be transmitted to another host, the pathogen must replicate within the vector and travel to the mosquito's salivary glands, where it is present in saliva at the next feeding. This process is extremely complex, with several other roadblocks that must be overcome, including passage through the gut wall and migration to and invasion of the salivary glands. Simply put, even if HIV were to survive the mosquito's gut, the virus would be unable to reach the salivary glands.
Furthermore, some vector-borne diseases, including typhus and Chagas disease, are transmitted because the human host scratches the skin at the moment of feeding and kills the insect, infecting himself with pathogen that is in the crushed remains of the dead insect. However, because HIV exists at such low blood concentration, even if a person accidentally scratched a mosquito carrying HIV-infected blood, it is extremely unlikely that this would result in HIV transmission.
Sources:
1. Amirkhanian, Y.A., J.A. Kelly, and D.D. Issayev, AIDS knowledge, attitudes, and behaviour in Russia: results of a population-based, random-digit telephone survey in St Petersburg. Int J STD AIDS, 2001. 12(1): p. 50-7.
2. Bockarie, M.J. and R. Paru, Can mosquitoes transmit AIDS? P N G Med J, 1996. 39(3): p. 205-7.
3. Booth, W., AIDS and insects. Science, 1987. 237(4813): p. 355-6.
4. Dobe, M., Awareness on AIDS among health care professionals. Indian J Public Health, 1995. 39(3): p. 105-8.
5. Webb, P.A., et al., Potential for insect transmission of HIV: experimental exposure of Cimex hemipterus and Toxorhynchites amboinensis to human immunodeficiency virus. J Infect Dis, 1989. 160(6): p. 970-7.
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