Vectors of Disease (Cambridge (CIE) O Level Biology): Revision Note

The Mosquito as a Vector of Disease

Transmission of malaria

• Malaria is caused by one of four species of the protoctist Plasmodium

• These protoctists are transmitted to humans by an insect vector:

  • Female Anopheles mosquitoes feed on human blood to obtain the protein they need to develop their eggs

  • If the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen with the blood as they feed

  • When feeding on the next human, Plasmodium pass from the mosquito to the new human’s blood

  

  Dunpharlain, CC BY-SA 4.0, via Wikimedia Commons

  Infected mosquitos can transmit malaria when they bite human skin

• Malaria may also be transmitted during blood transfusion and when unsterilised needles are re-used

• Plasmodium can also pass from mother to child across the placenta

  • Malaria may also be transmitted during blood transfusion and when unsterilised needles are re-used

  • Plasmodium can also pass from mother to child across the placenta

The life cycle of the malaria parasite diagram

The malaria parasite is transmitted to humans by mosquito vectors

Control of malaria

• One of the main methods for reducing malaria is to control the spread of the Anopheles mosquito which carries the pathogen

• This can be achieved by

  • Spraying living areas with insecticides, such as DDT

  • Spreading oil over the surfaces of water bodies that the mosquitoes breed in such as ponds and irrigation or drainage ditches (the mosquitoes lay their eggs in water but the larvae breathe air at the water surface – an oil layer makes this impossible and kills the larvae)

  • Draining marshes and other unnecessary bodies of water

  • Ensuring ponds and irrigation or drainage ditches are stocked with fish that feed on mosquito larvae

  • Spraying these water bodies with a preparation containing the bacterium Bacillus thuringiensis, which kills mosquito larvae but is not toxic to other organisms

• Unfortunately, mosquitoes can lay eggs in very small puddles and pools of water and therefore it is practically impossible to control all breeding sites using the methods listed above

Prevention of malaria

• Drugs (e.g. chloroquine, mefloquine) are taken before, during and after a visit to a location where malaria is prevalent

  • However, the use of these drugs has resulted in drug-resistant strains of Plasmodium or the drugs are expensive and can have unpleasant side effects

• One of the best ways to prevent malaria is to avoid being bitten in the first place

• People in malarial zones should sleep under bed nets (which can also be soaked periodically in insecticide to increase effectiveness) and should try to avoid exposing their skin at dusk when mosquitoes are most active