1.8 Malaria — An Example of Biological Method of Study

Short Introduction Malaria has killed more people than any other disease. It is a classic example of a biological problem and shows how such problems can be solved using the biological method. This subtopic covers the symptoms of malaria, how its cause was discovered (by Laveran), and how its spread was determined (by King and Ronald Ross). It also introduces the concept of a vector and briefly discusses dengue fever.

Main Idea

Malaria is a fatal disease that has affected humans since ancient times. Through the step-by-step application of the biological method — making observations, formulating hypotheses, drawing deductions, and performing experiments — scientists discovered that malaria is caused by a parasite called Plasmodium and is spread by female mosquitoes. The mosquito is called a vector because it carries the parasite from one organism to another.

Core Concept

Symptoms of Malaria The patient of malaria feels very chill and cold. His temperature rises above the normal value of 98.6°F. The patient suffers from headache and has a feeling of nausea. After some time, the person begins to sweat and feels better. The whole series of events is repeated after every 24, 48, or 72 hours, depending on the species of Plasmodium. 1. Cause of Malaria By adopting the steps of the biological method, it was proved that malaria is caused by Plasmodium. Recognition of the problem: Malaria was a problem since ancient times, but its cause was not known. Observations: In the 19th century, many different causes of malaria were being suggested. By that time, there were four major observations about malaria: a. Malaria and marshy areas have some relation. b. Quinine is an effective drug for treating malaria. c. Drinking the water of marshes does not cause malaria. d. Plasmodium is seen in the blood of a malarial patient. Hypothesis: Based on these observations and other information, the following hypothesis was formulated by a French physician Laveran in 1882: "Plasmodium is the cause of malaria." generated by DeepSeek to PDF

Deduction: Although a hypothesis is a tentative idea, to draw deductions it is accepted to be true. 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." Experiments: This deduction was tested through an experiment. Blood of 100 patients was examined under a microscope. For the purpose of having a control group, the blood of 100 healthy persons was also examined under a microscope. Results: The results showed that almost all malarial patients had Plasmodium in their blood. Only 7 out of 100 healthy persons had Plasmodium in their blood. The other 93 healthy persons were without any trace of Plasmodium. In the 7 healthy persons with Plasmodium in their blood, Plasmodium was in the incubation period. The incubation period is the time between the entry of the parasite into the host and the appearance of the symptoms of the disease. After a few days, those 7 healthy persons became ill with malaria. The results were quite convincing to prove the hypothesis that "Plasmodium is the cause of malaria." Reporting the results: Results of these experiments were announced worldwide, which helped to control malaria. 2. Spread of Malaria The biological method helped to find that mosquitoes spread malaria. Recognition of the problem: Malaria is a fatal disease since ancient times. After the confirmation that malaria is caused by Plasmodium, it was important to find how Plasmodium gets into the blood of man. This disease was more common in areas near stagnant water ponds where mosquitoes breed. It was found that: a. Malaria is associated with marshes. b. Drinking water of marshes does not cause malaria. From these points, it can be concluded that Plasmodium was not present in the marshy water. So Plasmodium must be carried by something that comes to marshy water. The problem in this study was to find that agent. Observations: An American scientist A. F. A. King listed 20 observations in 1883 about the spread of malaria. Some important observations were: a. People who slept outdoors were more likely to get malaria than those who slept indoors. b. People who slept under fine nets were less likely to get malaria than those who did not use such nets. c. People who slept near smoky fires usually did not get malaria. Hypothesis: On the basis of these observations, King suggested a hypothesis: "Mosquitoes transmit Plasmodium so are involved in the spread of malaria." Deductions: Following deductions were made considering the hypothesis true: Deduction I: "Plasmodium should be present in mosquito." Deduction II: "A mosquito can get Plasmodium by biting a malarial patient." Experiments: In order to test the above deductions, many experiments were performed. Experiments of Ronald Ross: Ross, a British army physician working in India, performed an important experiment in 1897. He allowed a female Anopheles mosquito to bite a malarial patient. He killed the mosquito some days later and found Plasmodium multiplying in the mosquito's stomach. Next, Ross used sparrows in his experiments. He allowed female Culex mosquitoes to bite sparrows suffering from malaria. He then allowed these mosquitoes to bite healthy sparrows. After a few days, these sparrows became ill with malaria. In the end, the hypothesis was tested by direct experimentation on human beings. An Italian biologist allowed an Anopheles mosquito to bite a malarial patient. The mosquito was kept for a few days and then allowed to bite a healthy man. The person later became ill with malaria. Results: All these experiments confirmed that mosquitoes transmit Plasmodium and spread malaria. Vector generated by DeepSeek to PDF

When a female mosquito pierces the skin with its mouthparts, a small amount of saliva is injected into the wound before drawing blood. The saliva prevents the blood from clotting in the food canal of the mosquito. The word vector means transmitter. Any organism which carries a parasite and transfers it from one organism to another is called a vector. Dengue Fever Dengue fever is caused by a Dengue virus and is transmitted by the mosquito Aedes aegypti, which has zebra-like white and black stripes on its body. A typical case of Dengue haemorrhagic fever is characterized by high-grade fever, bleeding from the nose, blood in urine, and an enlarged liver. There is no specific antiviral drug available for the treatment of patients suffering from Dengue fever. The second attack can be more serious and dangerous. The best prevention is personal protection from mosquito bites and measures to prevent mosquito breeding.

Detailed Explanation

Malaria is a textbook example of how the biological method is applied in real life. The investigation into malaria proceeded in two major phases: first, finding the cause of malaria, and second, finding how it spreads. 1. Finding the Cause: In the 19th century, people had several observations about malaria. They noticed it was common near marshes, quinine (a drug) treated it, drinking marsh water did not cause it, and Plasmodium was found in patients' blood. Laveran formulated the hypothesis that Plasmodium causes malaria. To test this, blood from 100 patients and 100 healthy people was examined. Almost all patients had Plasmodium, and 93 out of 100 healthy people did not. The 7 healthy people who had Plasmodium were in the incubation period — they later developed malaria. This strongly supported the hypothesis. 2. Finding the Spread: Once it was known that Plasmodium caused malaria, the next question was: how does Plasmodium get into human blood? Since malaria was common near marshes but drinking marsh water didn't cause it, something from the marshes must be carrying it. A. F. A. King made observations in 1883, noting that people sleeping outdoors were more likely to get malaria, those using fine nets were less likely, and those near smoky fires usually did not get it. These observations led to the hypothesis that mosquitoes transmit Plasmodium. 3. Testing the Spread Hypothesis: Ronald Ross, in 1897, allowed a female Anopheles mosquito to bite a malarial patient. He found Plasmodium multiplying in the mosquito's stomach. He then used sparrows and Culex mosquitoes to confirm transmission. Finally, an Italian biologist allowed an Anopheles mosquito that had bitten a malarial patient to bite a healthy man, who then became ill. These experiments confirmed that mosquitoes are the vectors of malaria. 4. Dengue Fever — Another Vector-Borne Disease: The text also introduces dengue fever as another disease spread by a mosquito vector. The Aedes aegypti mosquito has zebra-like stripes and transmits the Dengue virus. The text notes that there is no specific antiviral drug for dengue and that the second attack can be more dangerous. Prevention focuses on avoiding bites and stopping mosquito breeding.

Intermediate Level

The study of malaria demonstrates each step of the biological method. Recognition of the problem: What causes malaria? Observations: Malaria is associated with marshes, quinine treats it, marsh water doesn't cause it, and Plasmodium is in patients' blood. Hypothesis (Laveran, 1882): Plasmodium causes malaria. Deduction: If Plasmodium causes malaria, all patients should have it in their blood. Experiments: Blood from 100 patients and 100 healthy people was examined. Almost all patients had Plasmodium; 93 healthy people did not. The 7 healthy carriers were in the incubation period — the time between parasite entry and symptom appearance. Result: The hypothesis was supported. For the spread, A.F.A. King (1883) made 20 observations (e.g., outdoor sleepers got malaria more, net users got it less, smoky fire users rarely got it). He hypothesized that mosquitoes transmit Plasmodium. Ronald Ross (1897) allowed an Anopheles mosquito to bite a patient and found Plasmodium multiplying in its stomach. He used sparrows and Culex mosquitoes to confirm transmission, and later an Italian biologist confirmed it in humans. A vector is any organism that carries a parasite and transfers it. Dengue is caused by the Dengue virus and transmitted by Aedes aegypti.

Important Points

• 1. Malaria has killed more people than any other disease.
• 2. Symptoms include chill, high fever (above 98.6°F), headache, nausea, and sweating; the cycle repeats
• every 24, 48, or 72 hours depending on the Plasmodium species.
• 3. Laveran (1882) hypothesized that Plasmodium causes malaria.
• 4. Experiments: Blood from 100 patients and 100 healthy persons was examined. Almost all patients had
• Plasmodium; only 7 out of 100 healthy persons had Plasmodium.
• 5. The 7 healthy persons with Plasmodium were in the incubation period.
• 6. Incubation period = time between entry of parasite and appearance of symptoms.
• 7. A.F.A. King (1883) listed 20 observations about the spread of malaria.
• generated by DeepSeek to PDF
• 8. King's observations: outdoor sleepers more likely to get malaria; net users less likely; smoky fire users
• usually did not get malaria.
• 9. King hypothesized that mosquitoes transmit Plasmodium.
• 10. Ronald Ross (1897), a British army physician in India, allowed a female Anopheles mosquito to bite a
• malarial patient and found Plasmodium multiplying in the mosquito's stomach.
• 11. Ross also used sparrows and Culex mosquitoes to confirm transmission.
• 12. An Italian biologist confirmed transmission in humans.
• 13. A vector is any organism that carries a parasite and transfers it from one organism to another.
• 14. Dengue fever is caused by the Dengue virus and transmitted by Aedes aegypti (zebra-like white and black
• stripes).
• 15. Dengue symptoms: high-grade fever, bleeding from nose, blood in urine, enlarged liver.
• 16. There is no specific antiviral drug for dengue; the second attack can be more serious.
• 17. Prevention of dengue: personal protection from mosquito bites and measures to prevent mosquito
• breeding.

Remember

• Malaria symptoms: Chill → high fever (>98.6°F) → headache → nausea → sweating. Cycle repeats every
• 24, 48, or 72 hours.
• Cause: Plasmodium (proved by Laveran, 1882).
• Incubation period: Time between parasite entry and symptom appearance.
• Spread: Mosquitoes transmit Plasmodium (proved by King's observations and Ronald Ross's experiments,
• 1897).
• Vector: An organism that carries a parasite and transfers it.
• Dengue: Dengue virus, transmitted by Aedes aegypti (zebra-striped).
• Dengue symptoms: High fever, nose bleeding, blood in urine, enlarged liver.

Common Mistakes

• 1. Thinking malaria is caused by marshy water: Drinking marsh water does not cause malaria. The disease
• is caused by Plasmodium, which is carried by mosquitoes that breed near marshes.
• 2. Confusing the vector with the cause: The mosquito is the vector (transmitter), while Plasmodium is the
• cause of malaria.
• 3. Forgetting the incubation period: The 7 healthy persons with Plasmodium were in the incubation
• period; they later became ill. This does not contradict the hypothesis.
• 4. Mixing up the mosquito species: Anopheles transmits malaria in humans; Culex transmits malaria in
• birds. Aedes aegypti transmits dengue.
• 5. Thinking there is a specific antiviral drug for dengue: The text states there is no specific antiviral drug
• for dengue; treatment is supportive.
• 6. Thinking the second dengue attack is milder: The second attack can be more serious and dangerous.

Quick Revision

• Malaria is a fatal disease caused by Plasmodium, as proved by Laveran (1882) through the biological method.
• Symptoms include chill, high fever, headache, nausea, and sweating, repeating every 24, 48, or 72 hours. The disease
• is spread by mosquitoes, as hypothesized by A.F.A. King (1883) based on observations and proved by Ronald Ross
• (1897) through experiments on Anopheles mosquitoes, sparrows, and humans. A vector is an organism that carries a
• parasite and transfers it. Dengue fever is caused by the Dengue virus and transmitted by Aedes aegypti; there is no
• specific antiviral drug, and the second attack can be more serious.

60-Second Revision

• Malaria cause: Plasmodium (Laveran, 1882).
• Malaria spread: Mosquitoes (King, 1883; Ross, 1897).
• Incubation period: Time between parasite entry and symptoms.
• Vector: Organism that carries and transfers a parasite.
• Human malaria vector: Anopheles; Bird malaria vector: Culex.
• Dengue vector: Aedes aegypti (zebra-striped).
• Dengue: No specific antiviral; second attack more dangerous.
• Frequently Asked Questions (FAQs)
• Q1. What are the symptoms of malaria? Answer: The patient of malaria feels very chill and cold. His temperature
• rises above the normal value of 98.6°F. The patient suffers from headache and has a feeling of nausea. After some
• time, the person begins to sweat and feels better. The whole series of events is repeated after every 24, 48, or 72
• hours depending on the species of Plasmodium.
• Q2. Who first hypothesized that Plasmodium causes malaria? Answer: A French physician, Laveran, hypothesized
• in 1882 that Plasmodium is the cause of malaria.
• Q3. Describe the experiment that proved Plasmodium causes malaria. Answer: Blood from 100 malarial patients
• was examined under a microscope. For a control group, blood from 100 healthy persons was also examined. The
• results showed that almost all malarial patients had Plasmodium in their blood. Only 7 out of 100 healthy persons
• had Plasmodium in their blood; these 7 were in the incubation period and later became ill with malaria. The other 93
• healthy persons had no trace of Plasmodium. This proved the hypothesis.
• Q4. What is the incubation period? Answer: The incubation period is the time between the entry of the parasite
• into the host and the appearance of the symptoms of the disease.
• Q5. What observations did A.F.A. King make about the spread of malaria? Answer: A.F.A. King listed 20
• observations in 1883. Some important ones were: a. People who slept outdoors were more likely to get malaria than
• those who slept indoors. b. People who slept under fine nets were less likely to get malaria than those who did not
• use such nets. c. People who slept near smoky fires usually did not get malaria.
• Q6. What was King's hypothesis about the spread of malaria? Answer: King suggested the hypothesis:
• "Mosquitoes transmit Plasmodium so are involved in the spread of malaria."
• generated by DeepSeek to PDF

• Q7. Describe Ronald Ross's experiments. Answer: Ronald Ross, a British army physician working in India,
• performed an important experiment in 1897. He allowed a female Anopheles mosquito to bite a malarial patient. He
• killed the mosquito some days later and found Plasmodium multiplying in the mosquito's stomach. Next, he used
• sparrows: he allowed female Culex mosquitoes to bite sparrows suffering from malaria, then allowed these
• mosquitoes to bite healthy sparrows. After a few days, these sparrows became ill with malaria. In the end, an Italian
• biologist allowed an Anopheles mosquito to bite a malarial patient, kept it for a few days, and then allowed it to bite
• a healthy man. The person later became ill with malaria. All these experiments confirmed that mosquitoes transmit
• Plasmodium and spread malaria.
• Q8. What is a vector? Give an example. Answer: A vector is any organism which carries a parasite and transfers it
• from one organism to another. The word vector means transmitter. An example is the female Anopheles mosquito,
• which transmits Plasmodium (the malaria parasite) from a sick person to a healthy person.
• Q9. What causes dengue fever and how is it transmitted? Answer: Dengue fever is caused by a Dengue virus and
• is transmitted by the mosquito Aedes aegypti, which has zebra-like white and black stripes on its body.
• Q10. What are the symptoms of dengue haemorrhagic fever? Is there a specific antiviral drug? Answer: A
• typical case of Dengue haemorrhagic fever is characterized by high-grade fever, bleeding from the nose, blood in
• urine, and an enlarged liver. There is no specific antiviral drug available for the treatment of patients suffering from
• Dengue fever. The second attack can be more serious and dangerous. The best prevention is personal protection
• from mosquito bites and measures to prevent mosquito breeding.
• Multiple Choice Questions (MCQs)
• Q1. Malaria is caused by: A. Mosquito B. Stagnant water C. Swamp D. Plasmodium Correct Answer: D Explanation:
• Malaria is caused by the parasite Plasmodium.
• Q2. Who hypothesized in 1882 that Plasmodium causes malaria? A. A.F.A. King B. Ronald Ross C. Laveran D. Aristotle
• Correct Answer: C Explanation: Laveran, a French physician, hypothesized in 1882 that Plasmodium causes malaria.
• Q3. What is the incubation period? A. The time between the entry of the parasite and the appearance of symptoms
• B. The time it takes for a mosquito to bite C. The time it takes for a drug to work D. The time between two mosquito
• bites Correct Answer: A Explanation: The incubation period is the time between the entry of the parasite into the
• host and the appearance of the symptoms.
• Q4. Which scientist listed 20 observations in 1883 about the spread of malaria? A. Laveran B. Ronald Ross C. A.F.A.
• King D. Charles Darwin Correct Answer: C Explanation: A.F.A. King listed 20 observations in 1883 about the spread of
• malaria.
• Q5. According to King's observations, which people were less likely to get malaria? A. Those who slept outdoors B.
• Those who slept under fine nets C. Those who slept near smoky fires D. Both B and C Correct Answer: D Explanation:
• People who slept under fine nets and those who slept near smoky fires were less likely to get malaria.
• Q6. Ronald Ross performed his important experiment in: A. 1882 B. 1883 C. 1897 D. 1900 Correct Answer: C
• Explanation: Ronald Ross performed his important experiment in 1897.
• Q7. Which mosquito transmits malaria in humans? A. Culex B. Aedes aegypti C. Anopheles D. Drosophila Correct
• Answer: C Explanation: The female Anopheles mosquito transmits malaria in humans.
• generated by DeepSeek to PDF

• Q8. What is a vector? A. The cause of a disease B. An organism that carries a parasite and transfers it C. A type of
• drug D. A type of mosquito net Correct Answer: B Explanation: A vector is any organism that carries a parasite and
• transfers it from one organism to another.
• Q9. Dengue fever is transmitted by: A. Anopheles mosquito B. Culex mosquito C. Aedes aegypti mosquito D.
• Plasmodium Correct Answer: C Explanation: Dengue fever is transmitted by the mosquito Aedes aegypti.
• Q10. Which of the following is true about dengue fever? A. There is a specific antiviral drug available B. The first
• attack is always the most dangerous C. There is no specific antiviral drug available D. It is caused by Plasmodium
• Correct Answer: C Explanation: There is no specific antiviral drug available for dengue fever; the second attack can
• be more serious.
• Chapter 1: The Science of Biology is now fully complete (all 8 subtopics covered: 1.1 through 1.8).
• Reply "next" if you would like to begin Chapter 2: Biodiversity, starting with Subtopic 1: 2.1 Definition and
• Introduction to Biodiversity.