Kenya has four malaria epidemiological zones. This stratification has been historical and is related to experiences in relation to malaria burden occurrence in the country. It is envisaged that when new mapping exercises are undertaken there could be a revision of the stratification. These zones are:
a) Endemic zones
Areas where malaria is present have altitudes ranging from 0 to 1300 meters, these areas are around lake victoria in western Kenya and in the coastal regions. Rainfall, temperature and humidity are the determinants of the perennial transmission of malaria. The vector life cycle is usually short with high survival rate due to the suitable climatic conditions. Transmission is intense throughout the year with annual entomological inoculation rates between 30 and 100.
b) Seasonal transmission zones
These epidemiological zones are found in the arid and semi-arid areas of northern and south-eastern parts of the country. They experience short periods of intense malaria transmission during the rainy seasons. Temperatures are usually high and water pools created during the rainy season provide the malaria vectors breeding sites. Extreme climatic conditions like El Niño and southern oscillation lead to flooding in these areas thus epidemic outbreaks with high morbidity rates due to low immune status of the population.
c) Epidemic zones
Malaria transmission in the western highlands of Kenya is seasonal, with considerable year-to-year variation. The epidemic phenomenon is experienced when climatic conditions favour sustainability of minimum temperatures of around 1800C. This increase in minimum temperatures during the long rains period favours and sustains vector breeding resulting in increased malaria transmission. The whole population is vulnerable and case fatality rates during an epidemic can be up to ten times greater than what is experienced in regions where malaria occurs regularly.
d) Low risk zones
These zones cover the central highlands of Kenya including Nairobi. The temperatures are usually low to allow completion of the sporogonic cycle of the malaria parasite in the vector. However, with increasing temperatures and changes in the hydrological cycle, there is likelihood of increased vector breeding areas and new areas getting vulnerable.