Source Pravda.Ru

How 'mossies' spread West Nile, but not HIV

Think of mosquitoes as having two tiny straws inside their proboscis, or "stinger." Through one they spit an anti-clotting saliva to thin your blood, which they sip through the other.

Gross as it is to picture, you're now a step closer to understanding the micro-mechanics of how such a little insect can spread such a potentially lethal virus as West Nile, which has claimed two dozen lives since entering the U.S. in 1999, and now plagues 34 states and counting.

But you may still be left wondering why mosquitoes cannot spread a far more deadly virus – HIV.

For answers to this mystery, WorldNetDaily turned to some of the nation's top entomologists, who study the anatomy of these tiny disease factories for a living. (They're so absorbed with the pesky bugs, they even have a pet name for them – "mossies.")

First things first: As alluded to above, mosquitoes don't transmit viruses, or parasites, into new hosts through blood. They sip tainted blood, yes, through a stylet (one of the straws) inside their proboscis, but the path of blood is one-directional inside the mosquito.

In other words, blood is channeled to the mosquito's belly, and later eliminated as waste. Blood is not returned to the proboscis. (As for the few blood cells remaining on the sharp stylet after a bite, the labium wipes them clean as the bug pulls it out of the skin.)

It's through the mosquito's saliva, rather, that viruses such as West Nile (and in the case of malaria, parasites) are injected into an otherwise healthy host.

And that fluid runs in the opposite direction – from the salivary glands, which are self-contained, through a connecting channel to the other stylet. When a mosquito bites, it injects the saliva, which contains proteases and anticoagulants to aid in penetration and prevent clotting. (The bug's spit causes an allergic reaction in human skin, leaving that maddeningly itchy welt.)

All that said, let's return to the question at hand: Why West Nile, but not HIV, when both are viruses?

When West Nile is ingested with a blood meal from an infected host, it can leak from the gut of a Culex mosquito (the primary vector for the brain-swelling virus) to its salivary glands.

Harry Savage, entomologist with the federal Centers for Disease Control and Prevention's epidemiology division in Fort Collins, Colo., explains how.

"It actually infects those gut cells, and then some of those cells burst," he said. "And then the virus crosses that mid-gut membrane."

"Mosquitoes are kind of funny – they have a gut, and then they have a surrounding body called a hemoseal. It's kind of a fluid-filled cavity," he explained. "So once [the West Nile virus] gets across that mid-gut barrier, then it's free to travel through the fluids of the hemoseal and infect almost every tissue, including the salivary glands."

HIV, on the other hand, "is isolated to the gut and is primarily excreted," Savage said.

And even if it sets in the gut, HIV is not adapted to infect those surrounding cells and replicate – making many thousand times more of itself, like the West Nile virus, entomologists say.

"HIV can only replicate in human T-4 lymphocytes, which are not found in mossies," said an entomologist with the Indiana State Department of Health.

Put another way, HIV doesn't have the genetic code to take over a mosquito cell, and is adapted only to certain primate cells.

"HIV won't replicate," Savage said. "So if you have 20 virus particles coming in on the blood meal, two weeks later you've still got 20 virus particles."

But if a mosquito takes up 20 West Nile virus particles, 14 days later, "you've probably got at least 10,000 virus particles," he added.

By Paul Sperry© 2002