Infectious Disease: Question 3

Worked solution

Part (a): Pathogen and transmission

Malaria is caused by Plasmodium, a protoctist (protozoan); several different species of Plasmodium can infect humans. The parasite is transmitted between people by the bite of a female Anopheles mosquito. As the mosquito takes a blood meal, it injects saliva containing infective stages of the parasite, called sporozoites, directly into the person’s bloodstream.

Part (b): Role of the Anopheles mosquito and its control

The Anopheles mosquito is the vector for malaria. It does not simply carry the parasite on its body, but is essential to the parasite’s life cycle. A mosquito becomes infected when it feeds on the blood of a person already carrying Plasmodium, taking up a stage of the parasite that then develops further inside the mosquito. When this mosquito next feeds on another person, it injects sporozoites in its saliva, transmitting the infection onward.

Because the mosquito is essential to this cycle, controlling it interrupts transmission even without treating every infected person. Two effective approaches are:

• Insecticide-treated bed nets, which physically and chemically stop mosquitoes biting people while they sleep, when Anopheles mosquitoes are most active.
• Removing or treating stagnant water (the mosquito’s breeding sites), for example by draining pools or adding larvicide, which reduces the number of new mosquitoes able to become infected and transmit the parasite.

(Indoor spraying of insecticide is another valid method, working in the same way by reducing mosquito numbers or contact with people.)

Part (c): Biological, social and economic factors

Biological factors: Year-round high rainfall leaves large numbers of pools of standing water, providing abundant breeding sites for Anopheles mosquitoes throughout the year, so mosquito (and therefore parasite) numbers stay high. In addition, both the mosquito and Plasmodium can evolve resistance, to insecticides and to antimalarial drugs respectively, reducing the effectiveness of existing control measures over time.

Social factors: With only one clinic serving a large population, many people cannot easily reach a health worker for diagnosis or treatment, allowing infections to persist and be passed on for longer. Limited local health education may also mean fewer people understand or consistently use preventive measures such as bed nets.

Economic factors: Widespread poverty means many households cannot afford to buy bed nets, insecticides or antimalarial drugs for themselves, and the region as a whole is unlikely to have the funds needed for large-scale mosquito control programmes such as widespread spraying or drainage of breeding sites.

Together, these biological, social and economic factors reinforce each other, which is why malaria remains difficult to control in a region like this.

Final answers

• (a) Plasmodium, a protoctist, transmitted when a female Anopheles mosquito injects sporozoites (in its saliva) into a person’s blood during a bite.
• (b) The mosquito is the vector in which part of the parasite’s life cycle occurs; controlling it (e.g. insecticide-treated bed nets, removing breeding sites) reduces transmission.
• (c) Biological (year-round breeding sites, drug/insecticide resistance), social (poor healthcare access, limited education) and economic (inability to afford prevention, no funds for large-scale control) factors together make malaria difficult to control in this region.