1.b. relationship between physical factors and the prevalence of disease which can change over time

global patterns of climate + relief and their effect on disease

temp and precipitation= drivers of vector borne diseases and epidemics. endemic in tropics= malaria, dengue fever, yellow fever and sleeping sickness- depend on warm humid conditions. disease influenced by climate= seasonal patterns- partly due to temp determining rates of vector development and behaviour and viral replication. precipitation= aquatic habitats= ponds/stagnant water which allow insects/disease vectors to flourish and complete their life cycles.

relief= altitude causes abrupt changes in climate/ disease habitats. Ethiopia- malaria is concentrated in humid lowlands but absent in cooler highlands. many diseases=water borne. in developing world millions rely on water from wells and surface supplies contaminated by sewage. bacteria responsible for cholera etc thrive in these conditions. unprotected and stagnant drinking water supplies also provid habitats for disease vectors= copepod vectors that transmit parasitial disease guinea worm to humans in west africa

physical factors and disease vectors

dengue fever= widespread in tropics. annually infects 400 mil people, responsible fo 20,000 deaths. climate controls its epidemiology and life cycle of aede mosquitoes that transmit to humans. mosquitoes thrive in warm humid- favours the outbreak of dengue. south pacific sustained temps more than 32^o and humidity levels above 95% trigger waves of epidemics. these conditions occur in summer months but short term weather changes and exceptional rainfall events can also lead to outbreaks of the disease

seasonal variation in disease outbreaks

health risks associated with disease often have stron seasonal bias. temperate regions in northerm hemisphere epidemics of influenza peak in winter months. although reason for this is seasonality is unknown i is known that transmission of the flu virus is most often in winter. tropics/sub tropics vector borne diseases transmitted by mosquitoes, flies. ticks, fleas and worms often reach a peak during the rainy season. eg diarrhoeal disease in south asia surges in the pre monsoon march to april and the end of it sept to dec periods where fly populations are highest. sand flies which trasmit protozoan are most abundant in rainy season. bilharzia caused by a trematode flatworm hosted by freshwater snail kills estimate 200,000 a year. infection rates follow the life cycle of snails- closely linked to seasonal precipitation and temps in range of 10-30^o

climate change provides conditions for emerging infectious diseases

climate change= increase in temp, rainfall and humidity has stimulated transmission of vector borne diseases and extended their geographical range. warmer and wetter conditions have favoured the growth and spread of mosquitoes, carrying tropical and sub tropical diseases such as west nile virus, malaria and dengue fever.

west nile fever

first identified in uganda in 1937 is transmitted by culex mosquitoes. birds= main host for virus. today= spread globally. prevalent throughout africa, in the americas its range extends from venezuela to canada