What is malaria?
Malaria is a complex disease. Its severity is a function of the interaction between the parasite, the Anopheles mosquito vector, the human host and the environment. The risk of malaria infection is determined by the number of vectors, their survival rate, the incubation rate for both the vector and the parasite and the probability of the vector feeding off a human host.
These parameters are directly influenced by meteorological variables such as rainfall, temperature and humidity that give rise to differences in stability of disease transmission and seasonal variations in disease incidence.
Behavioural traits, genetic variation and immune status in the human population will also influence the degree of exposure and the disease outcome.
The Vector: anopheles mosquito
There are over 3,000 species of mosquito of which approximately 100 are vectors of human disease. Disease is transmitted when the female of the species takes a blood feed in order to provide nourishment for the development of her eggs. The female anopheles mosquito is responsible for transmitting malaria but different species such as aedes and culex mosquitoes transmit other diseases such as yellow fever, dengue and filariasis. Some anopheles mosquitoes may also transmit filariasis.
Anopheles mosquitoes usually bite from dusk to dawn although in some situations they will bite earlier than this. In many localities the principle vectors of malaria are late night biters and the older mosquitoes (more likely to be infected) are often found to be biting between 12am to 4am. Different species of anopheles however, may have different peak biting times, preferences (animals or humans) and different resting habits (indoor or outdoors) and these factors will influence the choice of control methods. Indoor resting is most common in dry or windy areas where safe, outdoor resting sites are scarce.
Anopheles mosquitoes breed in numerous different water habitats from shaded ponds and pools to hoof prints and tyre tracks. They tend to prefer water that is not too polluted but some Anopheles gambiae species have been shown to breed in stagnant drains. Artificial containers such as pots or tanks are usually only suitable breeding sites for aedes vectors. The exception to this is An. stephensi in South West Asia.
The female mosquito lays her eggs on the water and these subsequently develop into larvae and then into pupae. The pupa finally hatches to produce a mosquito. This process can take between 7-16 days but is influenced by humidity and temperature – the higher the temperature and humidity the more rapid the life cycle. Digestion of the blood meal and simultaneous development of the eggs takes about two to three days during which time the mosquito does not usually bite.
Malaria is caused by a parasite known as Plasmodium that is carried by the mosquito. There are four different species of Plasmodium that infect human beings, each with different incubation times:
Plasmodium falciparum 9 – 14 days incubation
Plasmodium vivax 12 – 17 days incubation
Plasmodium ovale 12 –17 days incubation
Plasmodium malariae 18 – 40 days incubation
Plasmodium falciparum is the most dangerous of the malaria parasites. It causes ‘malignant’ or cerebral malaria that can quickly progress to unconsciousness and death.
Untreated or poorly treated infections can cause recurring fevers and are communicable from several months to two years (P.falciparum) and up to fifty years (P.malariae).
The female anopheles will usually only feed once in a night, however if she is disturbed she will continue feeding until she has sufficient blood for the nourishment of her eggs. This may then be from more than one host. Following ingestion of Plasmodium infected blood, the parasite undergoes various stages of reproduction and development within the mosquito. The parasite will then migrate to the salivary glands of the mosquito and once she bites another host, the parasite will be transmitted. As the female mosquito feeds, saliva, containing Plasmodium is injected as an anticoagulant and the host becomes infected. The extrinsic development of the parasite in the mosquito takes between ten to fourteen days.
source : Oxfam