We know Malaria is a common disease in many countries. We will go through the history of this disease to know how biology solved the biological problem concerning the cause and transmission of malaria.
Historical Background of Malaria
In ancient times (more than 2000 years ago), physicians were familiar with malaria. They described it as a disease of chills and fevers with recurring attacks. They also observed that the disease was more common among the people living in low, marshes areas.
It was thought that the stagnant water of marshes poisoned the air and as a result of breathing in this “bad air” people got malaria. This belief led to the name of this disease. The Italian words “mala” means bad and “aria” means air. For further classification of the observation, some volunteers drank stagnant water from the marshes. They did not develop malaria.
Major Observation in 17th Century
In 17th century when the New World (America) was discovered, many plants from America were sent back to Europe to be used as medicines. The bark of a tree known as, quian-quina was very suitable for curing fevers. It was so beneficial that soon it was impossible to carry enough bark to Europe.
Some dishonest merchants began to substitute the bark of another tree, cinchona which closely resembled quina-quina. This dishonesty proved much valuable for mankind. Cinchona bark was found to be excellent for treating Malaria. Now, we know the reason: cinchona bark contains quinine that is effective in treating the disease.
At that time, physicians treated malaria with cinchona without understanding the cause of malaria. Two hundred years later, it was found that some diseases are caused by tiny living organisms. After this discovery, it also became a belief that malaria, too, might be caused by some micro-organism. In 1878, a French Army Physician Laveran began to search for the cause of malaria. He took a small amount of blood from a malaria patient and examined it under microscope. He noticed some tiny living creatures. His discovery was not believed by other scientists.
Two years later, another physicians saw the same creatures in the blood of another malaria patient. Three years after the second discovery, the same creatures were observed for third time. The organism was given a name Plasmodium.
In the last part of 19 century, many different causes of malaria were being suggested. By this time, there were four major observations about this disease.
- Malaria and marshy areas have some relation.
- Quinine is an effective drug for treating malaria.
- Drinking the water of marshes does not cause malaria.
- Plasmodium is seen in the blood of malaria patient.
We know that scientists use whatever information and observation he has and makes one or more hypothesis. The hypothesis made in this case was;
Scientists does not know whether his hypothesis is true or not, but he accepts it may be true and makes deductions. One of the deductions from the above hypothesis was;
“If plasmodium is the cause of Malaria, then all persons ill with malaria should have plasmodium in their blood.”
The next step was to test the deduction through experiments which were designed as;
“Blood of 100 malaria patients was examined under microscope. For the purpose of having a control group, the blood of 100 healthy persons was also examined under microscope.”
The result of experiments showed that almost all patients had plasmodium in their blood while 07 out of 100 healthy persons also had plasmodium in their blood (now we know that Plasmodium in the body of healthy people was in incubation period i.e. the period between the entry of parasite in the host and the appearance of symptoms). The results were quite convincing and proved that the hypothesis “Plasmodium is the cause of Malaria” is true.
Next biological problem was to learn about “How Plasmodium gets into the blood of man”. Biologists were having following observations:
- It is associated with marshes.
- Drinking water of marshes does not cause disease.
Result of Observations
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.
- Individuals who slept near a smoky fire usually did not get malaria.
Suggestions of A.F.A King about Malaria
On the basis of above 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.”
Improvements in Observations/Results about Malaria
In order to test the above deductions, Ronald Ross: a British army physician working in India in 1880’s; performed important experiments. He allowed a female Anopheles mosquito to bite a malaria patient. He killed the mosquito some days later and found Plasmodium multiplying in 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 malaria.
But scientists avoid using human beings for experiments when results can be more 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.
In the end, hypothesis were tested by direct experimentation on human beings. In 1898, Italian biologists allowed an Anopheles mosquito to bite a malarial patient. The mosquito was kept for a few days and then it was allowed to bite a healthy man. This person later becomes ill with malaria. In this way, it was conformed that mosquitoes transmit Plasmodium and spread malaria.