[Ed Hertel]

Ed Hertel asked the Naked Scientists:

In the 22 Aug show, Dr. Dave explained that mosquitoes cannot transmit HIV because the disease cannot grow and propagate in the insect's stomach like malaria.

If this is true, how is it possible that a used needle is able to transmit the disease so easily?  Wouldn't a dirty mosquito proboscis act in the same way as a dirty needle?

What do you think?

[lyner]

Mosquitos probably have an immune system which actively damages the virus where a needle doesn't.
They have developed a working relationship with the Malaria parasite so they don't kiil it.

[Bored chemist]

Male mosquitos don't bite people- they feed on plants. Female mosquitos only bite people to get blood in order to provide protein and iron for their eggs. They don't do this often so, in most cases, any mosquito that has drawn blood from one person doesn't need to bite a second person.

Enough diseases are spread by mosquitos to show that we are sometimes plain unlucky.

[chris]

Size is mainly the reason that a needle, but not a mosquito, can transmit HIV and other viruses. A mosquito's proboscis (mouthparts) are a fraction of a millimetre across whilst a small needle is over ten times wider. This means that the volume of blood that can be carried within a needle, and hence the infectious viral load, will be far greater than a mosquito's proboscis.

The reason that malaria (a protozoal infection) and certain viruses like dengue fever or Chikungunya can be transmitted by mosquitoes is that these infections have evolved to replicate within the body of the mosquito. This boosts the infectious load, compensating for the tiny number of organisms pulled into the mosquito's body when it first bit. This infectious load is transmitted with the next meal.

Chris

[blakestyger]

If the amount of material injected by the insect is below the infectious viral load what happens to the viruses that prevents them from replicating to become sufficient in number to infect?

Thanks

[chris]

HIV is quite fragile and is not adapted to survive within the environment of the insect's gut. Consequently, even if the source patient had a viral load in the billions (as some occasionally do), the particles would be denatured within the insect's digestive system.

Chris

[blakestyger]

Thank you Chris - it seems that the quality of the viral protein coat varies from species to species.