Definition of Malaria
Malaria is a parasitic disease caused by an obligate intracellular protozoan of the genus Plasmodium. The disease is systemic, acute and sometimes severe.
It is usually characterized by shivering, chills alternating with fever, headache and nausea and sometimes vomiting. After an interval free of fever, the cycle is repeated either daily or every third day depending on the species of the malaria parasite.
Malaria is the one of the leading causes of morbidity and mortality in Kenya, accounting for 15% of all out-patient attendance in the country's health facilities admissions (DHIS 2015). However there has been a steady decline of malaria cases in the last five years (DHIS 2015). In Kenya the distribution of malaria is not uniform, due to geographical differences in altitude, rainfall and humidity. These geographical factors influence the transmission patterns as they determine the vector densities and intensity of biting. The higher the ambient temperature, the shorter the sporogonic cycle of the parasite in the mosquito, hence the shorter the duration of the gonotrophic cycle.
All four species of human Plasmodium: P. falciparum, P. malariae, P. ovale and P. vivax occur in Kenya. P. falciparum which causes the severest form of the disease accounts for 98 percent of all malaria infections.
The principal vectors of malaria parasites in Kenya are members of the Anopheles gambiae complex and An. funestus. The species of An. gambiae complex found in Kenya are An. gambiae s.s., An. arabiensis, which are usually predominant during and after the rains and An. merus, which is mainly restricted to the coastal strip. An. funestus exist in low densities throughout the year.
They are among the most efficient vectors in the world as they feed predominantly on humans. In Kenya these malaria vectors are susceptible to all pyrethroid insecticides, although a decrease in susceptibility to permethrin was reported from an area of insecticide treated bed nets in western Kenya round, 1994).
Several studies carried out on malaria vectors in Kenya have shown that they are indoor feeders, preferring mainly human blood, apart from An. arabiensis which prefer animals and some feed out door. They also rest indoors after feeding.
The biting activities for most Anophelines occur between 20.00 and 06.00 hours, with a peak biting rate from midnight to 04.00 hours when most people are in bed, and as such they are susceptible to insecticide treated bed nets. Based on these known biological characteristics, the Kenyan Government has promoted use of insecticide treated bed nets as the main malaria control intervention measure.