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Wondering About Mosquitos

EDITOR: Manuel F. Lluberas
Vector Control Systems Manager

You may be wondering why is it that mosquitoes have not been implicated in the spread of far more deadly parasites than malaria or viruses like dengue fever. If you think of HIV as one of the many other potentially dangerous illnesses that mosquitoes may transmit, you are not alone. A look at the anatomy of a mosquito's mouthparts may provide some clues.

Before delving further into our subject, let's get some facts. The pathogens transmitted by mosquitoes into unsuspecting hosts are not transmitted through blood. A mosquito might have ingested infected blood, but this blood flows in one direction throughout the mosquito's body. Think of a mosquito as always sucking blood through one of the two straws inside its proboscis. The straw used for sucking blood is used only for this purpose while the other tube is used only for injecting her saliva into the host. This saliva contains special proteins that act as an anti-coagulant, preventing the host's blood from coagulating before she has ingested it or prematurely in the mosquito's mouth. The blood meal is carried from the proboscis to the intestinal tract and, once all possible nutrients have been extracted is eventually excreted as waste. In short, this blood meal never returns to the proboscis. The few blood cells that might remain on the proboscis after it is removed from the host following the bite are wiped off by the labium, one of many structures protecting the proboscis, as the mosquito retracts her mouthparts from the host.

All the pathogens and parasites transmitted by mosquitoes are transported and injected into her host through the mosquito's saliva, which runs in the other tube inside the proboscis and never comes in contact with the blood meal.

However, why West Nile encephalitis and not HIV, when both are viruses? According to entomologists with the federal Centers for Disease Control and Prevention's epidemiology division in Fort Collins, Colorado, when West Nile is ingested with a blood meal from an infected host, it can leak from the mid-gut of the vector, a Culex mosquito, to her salivary glands. As it turns out, the virus infects gut cells and some of them burst, allowing the virus to cross the mid-gut membrane. Once the virus gets across the mid-gut barrier, there is nothing to stop it from traveling through the mosquito's body cavity and infecting almost every tissue, including the salivary glands. HIV, on the other hand, does not infect the mid-gut cells or cross this barrier and is eventually excreted. Even if it manages to set in the gut, HIV is not adapted to infect those surrounding cells and replicate. HIV is only able to replicate in human T-4 lymphocytes, which are not found in mosquitoes. In other words, HIV lacks the genetic code to take over a mosquito cell, and is adapted only to certain primate cells.