8  Environmental and Climatic Factors and Their Role in Transmission

Climate is one of the most important determinants of malaria transmission. Temperature, rainfall, and humidity all influence mosquito survival, breeding, and the development of the malaria parasite within the mosquito.

• Temperature: Optimal temperatures for malaria transmission range between 20°C and 30°C (68°F to 86°F). Higher temperatures shorten the extrinsic incubation period, allowing mosquitoes to become infectious more quickly. However, extremely high temperatures (above 40°C) can reduce mosquito survival, limiting transmission.
• Rainfall: Rainfall creates breeding sites for Anopheles mosquitoes by forming pools of stagnant water, which are ideal for laying eggs. However, excessive rainfall can wash away breeding sites, reducing mosquito populations.
• Humidity: High humidity levels extend mosquito lifespan by reducing the rate of desiccation. Mosquitoes are more active and feed more frequently in humid conditions, increasing the risk of malaria transmission.

8.1 Breeding Habitats and Environmental Conditions

Anopheles mosquitoes breed in a variety of aquatic environments, and the availability and quality of these breeding sites play a crucial role in malaria transmission.

• Natural Breeding Sites: These include swamps, marshes, ponds, and slow-moving streams. These sites are abundant in rural and forested areas, making such regions more prone to malaria.

• Man-Made Breeding Sites: Urbanization and human activities have led to the creation of artificial breeding sites, such as irrigation channels, rice fields, and water storage containers. These sites can support large mosquito populations even in densely populated areas.

8.2 Seasonality of Malaria Transmission

Malaria transmission is often seasonal, particularly in regions with distinct wet and dry seasons. The transmission typically peaks during or after the rainy season when breeding sites are most abundant, and mosquito populations are at their highest.

• Endemic Regions: In stable endemic regions, malaria transmission occurs year-round, but there are still seasonal peaks.

• Epidemic-Prone Regions: In areas where malaria is not endemic or is close to elimination, transmission can occur in short, intense outbreaks during favorable climatic conditions, leading to outbreaks.