Malaria, mosquitoes and the legacy of Ronald Ross
Posted by ajcann on January 28, 2008
Malaria is a vector-borne infectious disease caused by protozoan parasites of the genus Plasmodium. It is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Each year, it infects approximately 515 million people and kills between one and three million people, the majority young children in Sub-Saharan Africa.
Malaria is thought to have infected humans for over 50,000 years, and may have been a human pathogen for the entire history of our species. Close relatives of the human malaria parasite are common in chimpanzees. References to the unique recurring fever of malaria are found throughout recorded history, the earliest from China in 2700 BC. The term malaria originates from the Medieval Italian: mala aria meaning “bad air”, and the disease was also formerly called ague or marsh fever due to its association with swamps, the home of the mosquitos which transmit the parasite.
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In 1880, a French doctor Charles Laveran was the first to observe parasites inside the red blood cells of people suffering from malaria. This was the the first time that protozoan was identified as causing a disease. In 1908, Carlos Finlay, a Cuban doctor treating patients with yellow fever in Cuba, first suggested that mosquitoes were transmitting the disease to and from humans. However, Sir Ronald Ross, working in India, finally proved that malaria is transmitted by mosquitoes in 1898. He did this by showing that certain mosquito species transmit malaria to birds and by isolating malaria parasites from the salivary glands of mosquitoes that had fed on infected birds:
For the last two years I have been endeavouring to cultivate the parasite of malaria in the mosquito. The method adopted has been to feed mosquitos, bred in bottles from the larva, on patients having crescents in the blood, and then to examine their tissues for parasites similar to the haemamoeba in man. The study is a difficult one, as there is no a priori indication of what the derived parasite will be like precisely, nor in what particular species of insect the experiment will be successful, while the investigation requires a thorough knowledge of the minute anatomy of the mosquito. Hitherto the species employed have been mostly brindled and grey varieties of the insect; but though I have been able to find no fewer than six new parasite of the mosquito, namely a nematode, a fungus, a gregarine, a sarcosporidium, a coccidium, and certain swarm spores in the stomach, besides one or two doubtfully parasitic forms, I have not yet succeeded in tracing any parasite to the ingestion of malarial blood, nor in observing special protozoa in the evacuations due to such digestion.
Apart from combating malaria, what else do Ross’s experiments teach us?
How about the value of persistence? Ross records that before the reported successful experiment, work in the preceding two years involving the examination of approximately a thousand mosquitoes had failed to reveal any parasites. It also shows the benefit of sharing data before publication so as to put forward possibly conflicting interpretations of the results. Today, many a journal editor may have rejected such a speculative, uncontrolled and unreplicated study as Ross’s original paper. And if they had, we might still be waiting to discover the infectious agent responsible for malaria.
Malaria, mosquitoes and the legacy of Ronald Ross.
Bull World Health Organ. 2007 85: 894-896
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