Malaria, dengue and chikungunya
Malaria is caused by Plasmodium, spread by the female Anopheles mosquito; dengue and chikungunya are spread by Aedes mosquitoes.
Plasmodium and its two hosts
Malaria is caused by Plasmodium, a tiny protozoan. Different species cause different types: P. vivax, P. malariae and P. falciparum. (Your textbook prints 'P. malaria'; the correct species name is P. malariae.) Malignant malaria, caused by P. falciparum, is the most serious and can be fatal. Plasmodium needs two hosts, a human and a female Anopheles mosquito, to complete its life cycle. The mosquito is also the vector (the transmitting agent).
Life cycle of Plasmodium
- 1An infected female Anopheles bites a person and injects sporozoites (the infectious form) into the blood.
- 2Sporozoites reach the liver, where the parasites multiply asexually inside liver cells.
- 3The parasites are released into the blood and attack red blood cells, multiplying asexually inside them.
- 4★The RBCs burst and release a toxin, haemozoin, which causes chills and high fever recurring every 3 to 4 days.
- 5Some parasites form gametocytes (sexual stages) in the RBCs.
- 6A female Anopheles bites the infected person and takes up gametocytes. Fertilisation and development take place in the mosquito's gut.
- 7The parasites multiply to form sporozoites, which are stored in the mosquito's salivary glands, ready for the next bite.
★ key step
Draw it: life cycle of Plasmodium
- back to Sporozoites enter human blood
Label the human part (liver and RBCs) and the mosquito part (gut and salivary glands) clearly.
Malaria, dengue and chikungunya compared (extra: your textbook gives only the Aedes vector for dengue and chikungunya)
| Feature | Malaria | Dengue | Chikungunya |
|---|---|---|---|
| Pathogen | Plasmodium (protozoan) | Dengue virus | Chikungunya virus |
| Vector | Female Anopheles | Aedes mosquito | Aedes mosquito |
| Main signs | Chills and high fever every 3 to 4 days | Sudden high fever, severe headache, pain behind the eyes, muscle and joint pain, rash; platelet count may fall | Fever with severe joint pain that can last weeks or months; rash |
| Treatment | Anti-malarial drugs | No specific antiviral drug; rest, fluids and medical care | No specific antiviral drug; rest, fluids and medical care |
Controlling mosquito-borne diseases
- Do not let water stagnate near homes; clean and dry water coolers, tyres, flower-pot trays and other containers regularly.
- Use mosquito nets and put wire mesh on doors and windows.
- Introduce fish such as Gambusia into ponds; they feed on mosquito larvae.
- Spray insecticides in ditches, drainage areas and swamps.
- Raise public awareness. Extra, not in your textbook: Aedes bites mainly in the daytime and breeds in clean stored water.
Every monsoon, Indian cities run campaigns to empty coolers and pots and fog drains, because dengue and chikungunya cases rise as Aedes breeds in stored water. Checking your own home once a week is the simplest protection.
2 marks: “What is haemozoin? What symptoms does it cause?” A toxic substance released when Plasmodium-infected RBCs rupture. It causes chills and high fever recurring every three to four days.
3 marks: “Why is the female Anopheles mosquito called both a host and a vector of Plasmodium?” Fertilisation and development of sporozoites take place inside it, so it is a host. It carries sporozoites in its salivary glands and injects them into humans, so it is the vector.
5 marks: “Describe the life cycle of Plasmodium with a labelled diagram.” Sporozoites injected by female Anopheles; liver cells; RBCs, rupture, haemozoin, fever; gametocytes; taken up by mosquito; fertilisation in gut; sporozoites to salivary glands. Labelled cycle diagram.
Plasmodium's tour: Saliva, Liver, RBC, Gametes, Gut, Saliva. Malaria = Anopheles; dengue and chikungunya = Aedes.