Why Can't Mosquitoes Transmit HIV?

Currently, I am reading a fabulous book by Bill Bryson entitled A Short History of Nearly Everything. It covers a wide range of scientific

Mosquitoes are carriers for several infamous viruses, most notably malaria and dengue fever. In fact, mosquitoes, through mosquito borne diseases, kill more people per year than any other animal. Luckily for humans, the HIV virus is not carried or spread by mosquitoes. Several reasons account for the inability of mosquitoes to transmit HIV.

1. Mosquitoes' Blood-Sucking Mechanism

As Professor Wayne Crans of Rutgers University so nicely puts it "mosquitoes are not flying hypodermic needles." The "snout" of a mosquito, the part that looks like a needle, is actually composed of six mouthparts. Four of these are used to pierce the skin of the person or animal that the mosquito is biting. The other two parts are composed of two tubes. One of the tubes sends saliva into the host and the other sends blood up to the mosquito. This two tube system is one reason why mosquitoes are unable to transmit HIV. Only saliva is injected into humans when a mosquito bites and thus HIV positive blood that a mosquito may have previously ingested is never transmitted to other humans.

2. The HIV virus gets digested in the mosquito's gut

Unlike mosquito borne diseases, HIV is unable to replicate within the mosquito's gut and therefore is broken down. In humans, HIV binds to T cells and begins replicating. No T cells exist inside the mosquito's gut and so the virus has no way of replicating or migrating to the mosquito's salivary glands. HIV particles are therefore digested by the mosquito alongside the actual blood meal. During the digestion process, the HIV particles are "completely destroyed."

3. HIV circulates at low levels in human blood

In order for mosquito-borne diseases to be spread from person to person, the associated virus needs to circulate within the host's blood at sufficient levels. HIV circulates in human blood at a far lower level than would be necessary to create a new infection. If a mosquito were to inject HIV positive blood into a human (which, as evidenced by reasons 1 and 2, is not possible), then it would take a whopping ten million mosquito bites to transmit one unit of HIV. By comparison, people who are HIV positive generally carry no more than ten units of HIV. Accidentally swallowing a mosquito or squashing one cannot lead to HIV infection either. In these situations the mosquito once again carries an insufficient amount of HIV positive blood to cause a new infection.

Sources:

Scientific American. "If a used needle can transmit HIV, why can't a mosquito?" Scientific American. June 4, 2001.

Crans, W. "Why Mosquitoes Cannot Transmit AIDS." Rutgers University. April 2006.

Yong, E. "Here's What Happens Inside You When a Mosquito Bites." National Geographic. August 6, 2013.

Department of Epidemiology, School of Public Health, University of Alabama. Can we get AIDS from mosquito bites? The Journal of the Louisiana State Medical Society 151, 429-33 (1999).