Malaria, an example of biological method: two problems, solved step by step

How biologists solved malaria

Malaria is common in many countries, including Pakistan; your book notes it has killed more people than any other disease. Physicians knew it over 2000 years ago, and quinine was the only remedy from the 17th to the 20th century.

Problem 1: what causes malaria? Late 19th-century observations: malaria relates to marshy areas; quinine treats it; drinking marsh water does not cause it; Plasmodium was seen in patients' blood. In 1878 the French army physician Laveran found and named it.

Hypothesis: "Plasmodium is the cause of malaria." Deduction: if so, all malarial patients should have Plasmodium in their blood. Experiment: blood of 100 patients (experimental group) and 100 healthy persons (control group). Most patients had Plasmodium.

Problem 2: how does Plasmodium get into human blood? In 1883 the physician A. F. A. King listed 20 observations. People sleeping outdoors got malaria more; fine mosquito nets lowered the chances; people near a smoky fire usually did not get it.

King's hypothesis: mosquitoes transmit Plasmodium. Deduction: if so, Plasmodium should be present in mosquitoes. Ronald Ross, a British army physician in India, found Plasmodium multiplying in a female Anopheles that had bitten a patient.

Ross would not risk a human test, so he used sparrows. Female Culex bit a sick sparrow; Plasmodium moved to their salivary glands; they then infected healthy sparrows. In 1898 Italian biologists confirmed it in humans with Anopheles.

Two full runs of the method: Plasmodium causes malaria (Laveran, 1878), and mosquitoes transmit it (King's hypothesis, Ross's experiments, confirmed in humans in 1898).

Key terms

Plasmodium

The microorganism in the blood that causes malaria.

Anopheles

The female mosquito that passes malaria to humans.

Culex

The female mosquito Ross used to pass malaria between sparrows.

Quinine

The only remedy for malaria from the 17th to the 20th century.

Short questions with model answers

1. Q1. Exercise A Q9: people who slept near a smoky fire had less chance of malaria. Why?

   • Recall the answer to Problem 2.
   • Rule out options about the fire killing Plasmodium.
   • Keep the option about mosquitoes.

   (c) Mosquitoes cannot tolerate smoke and are repelled.

2. Q2. In Problem 1, which group was experimental and which was control?

   • Experimental group: 100 malarial patients.
   • Control group: 100 healthy persons.

   Patients were the experimental group; healthy persons were the control group.

Common mistakes

• ✗ Saying Culex carries human malaria.

  ✓ Fig. 1.6: Culex in sparrows, Anopheles in humans.

• ✗ Crediting Ross with discovering Plasmodium.

  ✓ Laveran saw Plasmodium (1878); King made the mosquito hypothesis (1883); Ross tested it.

• ✗ Writing that drinking marsh water causes malaria.

  ✓ The book's observation is the opposite: it does not.

MCQs

1. 1. Who first saw Plasmodium in a malarial patient's blood, in 1878?

   1. (a) Ronald Ross
   2. (b) A. F. A. King
   3. (c) Laveran
   4. (d) Italian biologists
   Show answer

   (c) A French army physician (p. 18 box).

2. 2. People who slept near a smoky fire had less chance of malaria because: (Exercise A Q9)

   1. (a) Smoke kills Plasmodium in their blood
   2. (b) Mosquitoes cannot tolerate smoke and are repelled
   3. (c) Fire kills Plasmodium in the air
   4. (d) Smoke kills Plasmodium in mosquitoes
   Show answer

   (b) Mosquitoes carry Plasmodium; smoke keeps them away.

3. 3. Ross used which mosquito to pass malaria between sparrows?

   1. (a) Female Culex
   2. (b) Female Anopheles
   3. (c) Male Culex
   4. (d) Male Anopheles
   Show answer

   (a) Fig. 1.6 and p. 19.

4. 4. The deduction from "Plasmodium is the cause of malaria" was:

   1. (a) Mosquitoes transmit Plasmodium
   2. (b) Quinine treats malaria
   3. (c) Marsh water causes malaria
   4. (d) All malarial patients should have Plasmodium in their blood
   Show answer

   (d) The if-then result tested on 100 patients (p. 18).

5. 5. Mosquito transmission was confirmed in humans in:

   1. (a) 1878
   2. (b) 1883
   3. (c) 1898
   4. (d) 2003
   Show answer

   (c) By Italian biologists, using Anopheles (p. 20).

Quick revision

• Problem 1: observations led to the hypothesis that Plasmodium causes malaria, proved on 100 patients against 100 healthy people.
• Problem 2: King's observations led to the mosquito hypothesis; Ross tested it with sparrows and Culex.
• In 1898 Italian biologists confirmed that Anopheles passes malaria to humans.