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2. Solving a Biological Problem eLearn.Punjab

Next biological problem was to learn about “How Plasmodium gets into the blood of man”. Biologists

were having following observations;

• Malaria is associated with marshes.

• Drinking water of marshes does not cause malaria.

From these observations, it can be concluded that Plasmodium was not in the marsh water. But it

must be carried by something that comes to marsh water. In 1883, a physician A. F. A. King, listed

20 observations. Some important observations of King were:

• People who slept outdoors were more likely to get malaria than those who slept indoors;

• People who slept under fine nets were less likely to get malaria than those who did not use such

nets; and

• Individuals who slept near a smoky fire usually did not get malaria.

On the basis of these observations King suggested a hypothesis:

“Mosquitoes transmit Plasmodium and so are involved in the spread of malaria.”

Following deductions were made considering the hypothesis as true i.e. If mosquitoes are involved

in the spread of malaria then;

“Plasmodium should be present in mosquitoes.”

“A mosquito can get Plasmodium by biting a malarial patient.”

In order to test the above deductions, Ronald Ross: a British army physician working in India in

In fact quinine was the 1880’s; performed important experiments. He allowed a female
only effective remedy for Anopheles mosquito to bite a malarial patient. He killed the
malaria from the 17th to mosquito some days later and found Plasmodium multiplying in
the 20th century. mosquito’s stomach.
The next logical experiment was to allow an infected mosquito

(having Plasmodium) to bite a healthy person. If hypothesis was

true, the healthy person would have got malaria. But scientists avoid using human beings for

experiments when results can be so serious. Ross used sparrows and redesigned his experiments.

He allowed a female Culex mosquito to bite on the sparrows suffering from malaria. Some of the

mosquitoes were killed and studied at various times. Ross found that Plasmodium multiplied in

the wall of mosquito’s stomach and then moved into mosquito’s salivary glands. He kept some

mosquitoes alive and allowed them to bite healthy sparrows. Ross found that the saliva of the

infected mosquito contained Plasmodia (plural of Plasmodium) and these entered the sparrow’s

blood. When he examined the blood of these previously healthy sparrows, he found many Plasmodia

in it.

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