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Malaria-parasite's life-cycle discovered

 mosquitoes are vectors for malaria

At the end of the 19th century, malaria was believed to be contracted through inhalation of dirty water. Several biologists, Manson, Koch, KingANCHOR and LavernANCHOR, separately developed the theory that malaria may be caused by mosquito bites, but there was little evidence to support this idea.

According to Manson’s theory the mosquito bite drew blood and the parasite was absorbed into the mosquito. The mosquito then laid eggs and died, leaving the parasite to escape from the mosquito into the marsh water and infect humans through digestion.ANCHOR Manson also summised that the parent cells in the blood gave rise to small motile filaments, which were visible in the blood of infected people, and that these infected mosquitoes through their bites.

An opposing theory from Italian biologist Bignami stated that the parasites infected larval mosquitoes in the water, and that the mosquito bites infected man. Studying the blood of people suffering from malarial fever in India, Ronald Ross investigated the motile filaments in the blood, and felt that Manson’s theory was correct.

He began to systematically dissect mosquitoes and study them in detail under a microscope. He was able to trace the parasite to the stomach of the mosquito, where it appeared to vanish. This was the first time a protozoan parasite had ever been known to infect more than one species in its life cycle. After years of work Ross was no closer to tracing the form of the parasite in the mosquito and began to think that Manson may have been wrong about the route of infection. He had dismissed the theories of Bignami whose central premise was that the mosquitoes brought the parasite from the marshes to people, while Ross felt that the connection between marshes and malaria could be explained simply because mosquitoes breed in stagnant water.

In November 1897 Ross heard that an American, MacCallum, had researched the avian form of malaria in the crow. Malaria in birds has many similarities to malaria in humans, and Manson and Ross had seen great advantages in working with these models, but had failed to get mosquitoes to bite them. MacCallum found that there were two types of parasite taken into the stomach cavity of insects – male and female. One form was motile like sperm and the other was motionless like eggs.ANCHOR This explained why the parasites disappeared from the stomach of the mosquitoes. The male and female parasites underwent meiosis, forming a new type of motile parasite which burrowed into the stomach lining to grow.

Having difficulties finding humans to work with, Ross moved his research to birds, studying crows, sparrows, pigeons, weaver birds and larks. Ross continued to conduct a long series of experiments on birds to confirm the findings that the malarial parasite infected hosts of two different species during its life cycle. These experiments gave the final proof that the developmental stage of the malaria parasites takes place in mosquitoes.

Continuing his studies, in June 1898 Ross found that when the young parasites in the mosquito stomach grew to maturity, they ruptured, releasing thread-like bodies into the insect, where they were scattered around the circulation. A few weeks later he found that the thread-like bodies could enter cells, and that they seemed to be accumulating at a point in the thorax, as if they were heading towards a destination. Eventually Ross realised that the parasite was accumulating in the salivary gland of the mosquito, containing the anti-coagulant that it injects before feeding.

Following these findings Ross demonstrated that the route of infection is through the bite of a mosquito through experiments on 4 sparrows and a weaver bird, whose blood contained many parasites following bites from infected mosquitoes. The account of these findings was presented to the British Medical Association in July 1898.ANCHOR

Ronald Ross was awarded the Nobel Prize in physiology or medicine for his discoveries in 1902.


References

  1. King, "Insects and disease, mosquitoes and malaria.", Popular Sci. Monthly, Sep. (1883)
  2. Laveran, Traité des Fièvres Palustres, Paris, 1884, p. 457.
  3. Manson, "On the nature and significance of the crescentic and flagellated bodies in malarial blood.", Brit. Med. J., Dec. 8 (1894).
  4. MacCallum, "On the flagellated form of the malaria parasite.", Lancet, Nov. 1 (1897). Also J. Exptl. Med., 3 (1898).
  5. Manson, "The mosquito and the malaria parasite.", Brit. Med. J., Sep. 24 (1898). (Read at the Edinburgh Meeting of the British Medical Association at the

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