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Why Mosquitoes Cannot Transmit AIDS by Wayne J. Crans, Associate Research Professor in Entomology Rutgers Cooperative Extension Fact Sheet # FS736 Media releases concerning the possibility of mosquitoes transmitting AIDS (Acquired Immune Deficiency Syndrome) were common when the disease was first recognized, and the subject is still addressed by tabloids that seek captivating headlines to increase their circulation. The topic was initiated by reports from a small community in southern Florida where preliminary evidence suggested that mosquitoes may have been responsible for the higher on average incidence of AIDS in the local population. The media was quick to publicize claims that mosquitoes were involved in AIDS transmission despite findings of scientific surveys of the National Centers for Disease Control (CDC) that clearly demonstrated that mosquito transmission of AIDS in that community appeared highly unlikely. Nevertheless, media releases perpetuated the concept that mosquitoes transmitted AIDS, and many people still feel that mosquitoes may be responsible for transmission of this infection from one individual to another. There are three theoretical mechanisms which would allow blood-sucking insects such as mosquitoes to transmit HIV.
1. In the first mechanism, a mosquito would initiate the cycle by feeding on an HIV positive carrier and ingest virus particles with the blood meal. For the virus to be passed on, it would have to survive inside the mosquito, preferably increase in numbers, and then migrate to the mosquito's salivary glands. The infected mosquito would then seek its second blood meal from an uninfected host and transfer the HIV from its salivary glands during the course of the bite. This is the mechanism used by most mosquito-borne parasites, including malaria, yellow fever, dengue, and the encephalitis viruses.
2. In the second mechanism, a mosquito would initiate the cycle by beginning to feed on an HIV carrier and be interrupted after it had successfully drawn blood. Instead of resuming the partial blood meal on its original host, the mosquito would select an AIDS-free person to complete the meal. As it penetrated the skin of the new host, the mosquito would transfer virus particles that were adhering to the mouthparts from the previous meal. This mechanism is not common in mosquito-borne infections, but equine infectious anemia is transmitted to horses by biting flies in this manner. 3. The third theoretical mechanism also involves a mosquito that is interrupted while feeding on an HIV carrier and resumes the partial blood meal on a different individual. In this scenario, however, the AIDS-free host squashes the mosquito as it attempts to feed and smears HIV contaminated blood into the wound. In theory, any of the mosquito-borne viruses could be transmitted in this manner providing the host circulated sufficient virus particles to initiate re-infection by contamination. Each of these mechanisms has been investigated with a variety of blood sucking insects and the results clearly show that mosquitoes cannot transmit AIDS. News reports on the findings, however, have been confusing, and media interpretation of the results has not been clear. The average person is still not convinced that mosquitoes are not involved in the transmission of a disease that appears in the blood, is passed from person to person and can be contracted by persons that share hypodermic needles. Here are just some of the reasons why the studies showed that mosquitoes cannot transmit AIDS: Mosquitoes Digest the Virus that Causes AIDS When a mosquito transmits a disease agent from one person to another, the infectious agent must remain alive inside the mosquito until transfer is completed. If the mosquito digests the parasite, the transmission cycle is terminated and the parasite cannot be passed on to the next host. Successful mosquito-borne parasites have a number of interesting ways to avoid being treated as food. Some are refractory to the digestive enzymes inside the mosquito's stomach; most bore their way out of the stomach as quickly as possible to avoid the powerful digestive enzymes that would quickly eliminate their existence. Malaria parasites survive inside mosquitoes for 9-12 days and actually go through a series of necessary life stages during that period. Encephalitis virus particles survive for 10-25 days inside a mosquito and replicate enormously during the incubation period. Studies with HIV clearly show that the virus responsible for the AIDS infection is regarded as food to the mosquito and is digested along with the blood meal. As a result, mosquitoes that ingest HIV-infected blood digest that blood within 1-2 days and completely destroy any virus particles that could potentially produce a new infection. Since the virus does not survive to reproduce and invade the salivary glands, the mechanism that most mosquito-borne parasites use to get from one host to the next is not possible with HIV. Mosquitoes Do Not Ingest Enough HIV Particles to Transmit AIDS by Contamination Insect-borne disease agents that have the ability to be transferred from one individual to the next via contaminated mouthparts must circulate at very high levels in the bloodstream of their host. Transfer by mouthpart contamination requires sufficient infectious particles to initiate a new infection. The exact number of infectious particles varies from one disease to the next. HIV circulates at very low levels in the blood--well below the levels of any of the known mosquito-borne diseases. Infected individuals rarely circulate more that 10 units of HIV, and 70 to 80% of HIV-infected persons have undetectable levels of virus particles in their blood. Calculations with mosquitoes and HIV show that a mosquito that is interrupted while feeding on an HIV carrier circulating 1000 units of HIV has a 1:10 million probability of injecting a single unit of HIV to an AIDS-free recipient. In laymen's terms, an AIDS-free individual would have to be bitten by 10 million mosquitoes that had begun feeding on an AIDS carrier to receive a single unit of HIV from contaminated mosquito mouthparts. Using the same calculations, crushing a fully engorged mosquito containing AIDS positive blood would still not begin to approach the levels needed to initiate infection. In short, mechanical transmission of AIDS by HIV-contaminated mosquitoes appears to be well beyond the limits of probability. Therefore, none of the theoretical mechanisms cited earlier appear to be possible for mosquito transmission of HIV. Mosquitoes Are Not Flying Hypodermic Needles Many people think of mosquitoes as tiny, flying hypodermic syringes, and if hypodermic needles can successfully transmit HIV from one individual to another then mosquitoes ought to be able to do the same. We have already seen that HIV-infected individuals do not circulate enough virus particles to result in infection by contamination. However, even if HIV-positive individuals did circulate high levels of virus, mosquitoes could not transmit the virus by the methods that are employed in used syringes. Most people have heard that mosquitoes regurgitate saliva before they feed, but are unaware that the food canal and salivary canal are separate passageways in the mosquito. The mosquito's feeding apparatus is an extremely complicated structure that is totally unlike the crude single-bore syringe. Unlike a syringe, the mosquito delivers salivary fluid through one passage and draws blood up another. As a result, the food canal is not flushed out like a used needle, and blood flow is always unidirectional. The mechanics involved in mosquito feeding are totally unlike the mechanisms employed by the drug user's needles. In short, mosquitoes are not flying hypodermic needles and a mosquito that disgorges saliva into your body is not flushing out the remnants of its last blood meal.
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Ever had a cold and no-one wants to go near you in case they catch it? Well, like a cold, HIV is a virus, but you can't become infected with it in the same way. HIV is the virus that causes AIDS.
In the past decade, there has been a lot of publicity and education work undertaken to teach people the ways in which they might be at risk of HIV transmission. In spite of this, some of the messages have been mixed and occasionally contradictory, and there is still some confusion about what is safe and what is not. This section will clear up some of the myths.
Many people are concerned about whether certain situations could result in someone becoming infected. Worries about the possibilities of infection lead to people with HIV being very isolated and sometimes resulted in prejudice and paranoia. HIV is a virus that is found in blood and other body fluids such as semen and vaginal fluids. It can't live for long outside the body, so to be infected with HIV you need to allow some body fluid from an infected person to get inside your body.
The most common ways that people become infected with HIV are:
- by having sexual intercourse with an infected partner
- by injecting drugs using a needle or syringe which has already been used by someone who is infected.
HIV can be passed on in both these ways because the virus is present in the sexual fluids and blood of infected people. If infected blood or sexual fluid gets into your body, then you can become infected.
You can get HIV from . . . Vaginal sex
HIV is found in the sexual fluids of an infected person. For a man, this means in the fluids which come out of the penis before and during sex. For a woman, it means HIV is in the fluids produced by the vagina before and during sex to help make intercourse easier.
If a man with HIV has vaginal intercourse without a condom, infected fluid can pass into the woman's blood stream through a tiny cut or sore inside her body. Such a cut or sore wouldn't always be visible, and could be so small that the woman wouldn't know about it.
If a woman with HIV has sexual intercourse without a condom, HIV could get into the man's blood through a sore patch on his penis or by getting into the tube that runs down the penis.
If there is any contact with blood during sex, this increases the risk of infection. For example, there may be blood in the vagina if intercourse occurs during a woman's period.
Oral sex
Oral sex with an infected partner does carry some risk of infection. If a person sucks on the penis of an infected man, for example, infected fluid could get into the mouth. The virus could then get into the blood if you have bleeding gums or tiny sores or ulcers somewhere in the mouth.
The same is true if infected sexual fluids from a woman get into the mouth of her partner.
But infection from oral sex alone seems to be
very rare, and there are things you can do to protect yourself.
For more information go to our
Oral Sex page.
Anal sex
If a couple have anal intercourse the risk of infection is greater than with vaginal intercourse. The lining of the anus is more delicate than the lining of the vagina, so it's more likely to be damaged during intercourse, and any contact with blood during sex increases the risk of infection.
Injecting drugs
There is a good likelihood of becoming infected with HIV if you share injecting equipment with someone who has the virus. The virus can be passed by sharing needles, syringes, spoons, filters and water. Disinfecting equipment between use can reduce the chance of transmission, but doesn't eliminate it.
For more information, go to our
Injecting Drug Use page.
Blood transfusions
Some people have been infected through a transfusion of infected blood. These days, in most countries all the blood used for transfusions is now tested for HIV. In those countries where the blood has been tested, HIV infection through blood transfusions is now extremely rare.
Blood products
Blood products, such as those used by people with Haemophilia, are now heat-treated to make them safe.
Mother to child transmission
An infected pregnant woman can pass the virus on to her unborn baby either before or during birth. HIV can also be passed on during breastfeeding.
For more information, go to our pages about
Pregnancy and
Mother to child transmission of HIV.
If a woman knows that she is infected with HIV, there are drugs that she can take to greatly reduce the chances of her child becoming infected.
Infection in the health-care setting
Some health-care workers have become infected with HIV by being stuck with needles containing HIV-infected blood. A very few have become infected by HIV-infected blood getting into the health-care worker's bloodstream through an open cut or splashes into a mucous membrane (e.g. eyes or the inside of the nose).
There have only been a few documented instances of patients becoming infected by a health-care worker.
We have more information about
healthcare workers and HIV infection.
Tattoos / piercings
Anything which allows another person's body fluids to get inside your body is risky. If the equipment is not sterile, having a tattoo done could carry a very small risk. In the UK, there are hygiene regulations governing people who do tattoos and piercings, and all instruments used should be sterile. If you are thinking of having a tattoo or piercing, ask staff at the shop what procedures they take to avoid infection.
You can't get AIDS from. . . Kissing
At the moment, scientific opinion is pretty clear that you cannot become infected with HIV through kissing.
To become infected with HIV you must get a sufficient quantity of HIV into the bloodstream. Saliva does contain HIV, but the virus is only present in very small quantities and as such, cannot cause HIV infection.
Unless both partners have large open sores in their mouths, or severely bleeding gums, there is no transmission risk from mouth-to-mouth kissing.
Sneezing, coughing, sharing glasses/cups, etc
HIV is unable to reproduce outside its living host, except under very extreme laboratory conditions. HIV does not survive well in the open air, and this makes the possibility of this type of environmental transmission remote. In practice no environmental transmission has been seen.
This means that HIV cannot be transmitted through spitting, sneezing, sharing glasses or musical instruments.
You also can't be infected in swimming pools, showers or by sharing washing machines or toilet seats.
Insects
Studies conducted by many researchers have shown no evidence of HIV transmission through insects, even in areas where there are many cases of AIDS and large populations of insects such as mosquitoes. Lack of such outbreaks, despite considerable efforts to detect them, supports the conclusion that HIV is not transmitted by insects.
Also, HIV only lives for a short time and does not reproduce in an insect. So, even if the virus enters a mosquito or another sucking or biting insect, the insect does not become infected and therefore cannot transmit HIV to the next human it feeds on or bites.
Injecting with sterile needles
Drug use with sterile works will not transmit HIV either, as long as clean works are used every time - this means needle, syringe and spoon, water and filters. There are still many other risks associated with injecting drug use. And, if you are on drugs, even alcohol, this may cloud your judgement and make you more likely to become involved in risky sexual behaviour - it's harder to make the effort to use a condom when you're off your head.
Protected sex
If an unbroken latex condom is used, there is no risk of HIV transmission. There are myths saying that 'some very small viruses can pass through latex' - this is
not true.
Anal sex is not necessarily a risk if unbroken condoms are used and there is no blood-to-blood contact. You can't 'create' HIV by having anal sex.
Further information can be found on our
Condoms page.