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Cracking the Carriers’ Courses

On May 13, 1857, Dr. Ronald Ross was born into an age when many suspected that putrid air was the culprit that killed hundreds of thousands of people and who called this killer disease malaria from the Italian mal’aria [“bad air”]. Toward the end of the 19th century, however, some doctors and scientists believed mosquitoes might be the culprits carrying microscopic parasites that had just been discovered in human blood samples. Since approximately half the world’s population was at risk of malaria and many hundreds of thousands died from severe cases each year, Ross wanted to help with the research. But as a doctor in the Indian Medical Service, Ross faced several huge problems: which mosquitoes? which parasites?

After examining many species, Ross identified the Anopheles mosquito as the malaria carrier after finding the same parasite living in its stomach as he had found in the blood of malaria victims. He set out to discover and detail the life cycle of this parasite, a single-cell organism, Plasmodium.

Anopheles drilling [left] and beginning to pump blood [right]
Anopheles drilling [left] and beginning to pump blood [right]

Ross illustrated how the mosquito was the secondary host of the parasite that transferred to and from the primary host through the mosquito’s salivary glands. He showed how a victim of Anopheles – from bird to gorilla or human – was the primary host who became the liver-stage breeding ground from which the infection migrated to the blood cells where it continued to circulate, perhaps killing the host, but also available to the enterprising Anopheles to sample in the night.

That’s one insect + one animal for the parasite to have a full and productive life. Clever? This means Plasmodium ensures it continues to exist if one of its hosts dies. Since a mosquito can infect other hosts and other hosts can infect mosquitoes, the microscopic parasite has covered all its bases.

Knowing that mosquitoes lay their rafts of eggs in still water, Ross demonstrated that mosquito populations decreased when stagnant pools or ditches of water were drained. He advocated the nighttime use of mosquito netting over beds to block the female mosquito. For his successful research on malaria and methods of combating it, Ronald Ross received the Nobel Prize.

Here, in two short animations from the Howard Hughes Medical Institute, is how the malaria parasite uses its two unwitting hosts:

The Mosquito Host

The Human Host

Today, 20 Anopheles species are active around the world. Building on Ross’s work, scientists employing newer technologies have discovered that several Plasmodium parasite species are exceedingly clever and, once inside either host, can wait, deploy, hide, and camouflage themselves from an animal’s immune system. Scientists continue trying to perfect a vaccine that can outperform Plasmodium and defeat malaria disease.

B Bondar / Real World Content Advantage