The Mosquito Connection

Understanding avian malaria begins in a mosquito-filled room

By Mari Kimura

Mari shows off one of her mosquito buckets.

Mari Kimura/CLO

As I walk into the mosquito rearing room in Cornell's Entomology department, I inhale damp, warm air and vividly remember from my childhood the sensation of walking into the reptile house at the zoo. Dozens of mesh-covered plastic buckets filled with mosquitoes and flies are stacked from floor to ceiling; scattered between the buckets are Tupperware pans of murky water containing hundreds of wiggling larvae. The buzzing of the insects is interrupted by sporadic hissing as a humidifier in the ceiling releases bursts of water vapor. Years earlier, I had envisioned myself conducting my graduate research hiking through lush alpine meadows under clear blue skies, binoculars in hand, studying birds. How did I end up tending mosquitoes in this hot, windowless room?

Actually, my studies of birds--House Finches--led me here, as I tried to find answers about a common but poorly understood avian disease. The disease is avian malaria, infamous for wiping out native Hawaiian birds that had no resistance to the disease. However, many birds in North America carry malarial parasites in their blood with no apparent ill effects--including one set of House Finches that I sampled in California that had a whopping 95 percent infection rate. Avian malaria parasites rely on both birds and mosquitoes to complete their life cycle. I realized that if I wanted to understand this bird disease I needed to learn more about mosquitoes.

House Finches throughout North America carry a particular strain of avian malaria that infects many species of birds. House Finches, native to the West, were once popular in the pet trade, but when the sale of these birds became illegal, pet shop owners in the East released their finches. House Finches are now one of the most common birds in towns and cities. Parasites that already existed in the eastern bird community were transmitted by mosquitoes to these introduced House Finches and persist in them to this day.

My research to date, however, has revealed something of a mystery: House Finches in the West are infected by an additional strain not found in eastern House Finches. Why? Could it be that there are geographic differences in the ability of mosquitoes to transmit malaria in birds, as has been shown with malaria in humans?

A female Culex pipiens pipiens mosquito. Kent Loeffler

Back in the mosquito room, I peer into my experimental buckets. I am rearing two mosquito species: Culex pipiens and Culex quinquefasciatus. Both species are known to transmit avian malaria. Only female mosquitoes seek blood meals, because they need the protein to produce eggs. Male mosquitoes mostly feed on nectar. This is why most people never get to see a male mosquito. The males have furry-looking antennae, which are more developed than the female antennae because the males seek females by detecting the frequency of their wingbeats. The other distinctive feature of the males is that their mouthparts appear to be split in three at the tip, resembling permanently puckered lips.

Culex mosquitoes lay their eggs in clusters called rafts, which are only a few millimeters long and look like tiny, black, fuzzy bananas. Each raft contains about 50 eggs. I scoop up the floating rafts with a makeshift tool--a bit of mesh taped onto a wooden Q-tip stick, like chasing cornflakes around a bowl with a spoon. I place the rafts in a plastic pan with food for the larvae: liver powder and other tasty treats. The larvae hatch out after a day or two, and about a week later, the larvae become pupae. Mosquito pupae are lively, bouncing little blobs, and when I blow gently on the surface of the water they all shoot down to the bottom of the pan. Two days after pupation the young adults emerge. Although there is some variation, the males generally emerge 12 to 24 hours before the females. When they are mature, they will mate, and the females will be ready to have a blood meal and start the cycle over again.

My goal is to rear mosquitoes in the lab, have them feed on bird blood that contains malarial parasites, and let the parasites develop in the mosquitoes. Then I'll extract DNA from the mosquito salivary glands to see if I can recover parasite DNA using a method called PCR that can produce billions of copies of DNA from even a single DNA fragment. If this works, I will sample the mosquito species that like to feed on birds in Ithaca, New York, to determine which ones are responsible for transmitting malarial parasites.

Research has a way of taking us to unexpected places, and this warm, dark room in the entomology lab is no exception.

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu