Climate and Malaria

            The three main climate factors that affect malaria are temperature, precipitation, and relative humidity (Pampana, 1969).  Climate predicts, to a large degree, the natural distribution of malaria (Bouma and van der Kaay, 1996).

Temperature

            Temperature affects many parts of the malaria life cycle.  The duration of the extrinsic phase depends on temperature and on the species of the parasite the mosquito is carrying (Pampana, 1969).  The extrinsic cycle normally lasts nine or ten days, but sometimes can be as short as five days (Bradley et al., 1987).  As the temperature decreases, the number of days necessary to complete the extrinsic cycle increases for a given Plasmodium species.  P. vivax and P. falciparum have the shortest extrinsic incubation times and therefore are more common than P. ovale  and P. malariae (Oaks et al., 1991).  The extrinsic phase takes the least amount of time when the temperature is 27°C (Pampana, 1969).  The time required for development of the ookinete, the egg of the parasite, in the midgut of the Anopheline mosquito, decreases as temperature increases from 21°C to 27°C (Patz et al., 1998).  Below 20°C, the life cycle of P.  falciparum is limited.  Malaria transmission in areas colder than 20°C can still occur because Anophelines often live in houses, which tend to be warmer than external temperatures.  Larval development of the mosquito also depends on temperature (Russell et al., 1963).  Higher temperatures increase the number of blood meals taken and the number of times eggs are laid by the mosquitoes (Martens et al., 1995).

            The intersections of the ranges of minimum and maximum temperature for parasite and vector development determine the impact of changes in temperature on malaria transmission.  The minimum temperature for mosquito development is between 8-10°C, the minimum temperatures for parasite development are between 14-19°C with P. vivax surviving at lower temperatures than P. falciparum.  The optimum temperature for mosquitoes is 25-27°C, and the maximum temperature for both vectors and parasites is 40°C (McMichael et al., 1996).  There are some areas where the climate is optimal for malaria and Anopheles mosquitoes are present, but there is no malaria.  This is called “Anophelism without malaria” which can be due to the fact that the Anopheles mosquitoes present do not feed primarily on humans (Bruce-Chwatt, 1985) or because malaria control techniques have eliminated the parasite (Martens *check this).  If any changes, whether environmental or otherwise, were to occur to bring another species to the area that does act as a vector for human malaria, then the potential for outbreaks of malaria is very high since there is no immunity in the human population there.

Precipitation

            Anopheline mosquitoes breed in water habitats, thus requiring just the right amount of precipitation in order for mosquito breeding to occur.  Little is known about the biology of this aquatic phase (Oaks et al., 1991).  However it is known that different Anopheline mosquitoes prefer different types of water bodies in which to breed (Nagpal and Sharma, 1995).  Too much rainfall, or rainfall accompanied by storm conditions can flush away breeding larvae.  Not only the amount and intensity of precipitation, but also the time in the year, whether in the wet or dry season, affects malaria survival (Russell et al., 1963).  Rainfall also affects malaria transmission because it increases relative humidity and modifies temperature, and it also affects where and how much mosquito breeding can take place (Pampana, 1969).

            Some contend that the amount of rainfall may be secondary in its effects on malaria to the number of rainy days or the degree of wetness that exists after a rain event.  The degree of wetness can be calculated by the following equation:  (# of wet days in a month)*(total rainfall) / (# of days in the month).  Malaria has also been found to be dependent on the groundwater level (Russell et al., 1963).

Relative Humidity

            Relative humidity also affects malaria transmission.  Plasmodium parasites are not affected by relative humidity, but the activity and survival of Anopheline mosquitoes are.  If the average monthly relative humidity is below 60%, it is believed that the life of the mosquito is so shortened that there is no malaria transmission (Pampana, 1969). 

Wind

            Wind may play both negative and positive roles in the malaria cycle because very strong winds can decrease biting or ovipositing by mosquitoes, while at the same time extending the length of the flight of the mosquito.  During a monsoon, wind has the potential to change the geographic distribution of mosquitoes (Russell et al., 1963). 

Climate and vector succession

            In addition to changing the amount and rate of transmission of the vectors and parasites that are already in a certain location, changing the climate of an area can allow the introduction of different vectors and parasites that may be more efficient.  Since P. malariae and P. ovale have longer extrinsic cycles, some mosquitoes do not live long enough to transmit them.  However, if environmental conditions change in ways that would increase the survival time of those mosquitoes, then they would be able to transmit other species of malaria that were not present in that area before (Pampana, 1969). 

Epidemics

            Epidemics of malaria are caused by a disturbance of the equilibrium between host, parasite and vector.  Najera et al. (1998) define three different types of epidemics.  Type I epidemics are caused by meteorological conditions, which create temporary epidemics that will eventually revert back to the previous condition.  Type II epidemics are caused by landscape changes or colonization of sparsely populated areas that create a new equilibrium level of endemicity.  And type III epidemics are caused by interruptions in measures that were controlling malaria. 

Meteorologically-created epidemics normally only last one season of transmission.  Many areas experience epidemics caused by meteorological changes that occur in interannual cycles.  These cycles, which have been well illustrated by ENSO (El Niño Southern Oscillation), have been found in many parts of the world to follow the paraquinquennial cycle, which means epidemics happen every 5 to 7 years, however, in some areas the period of the cycle is longer.  Because of the periodicity of cycles caused by meteorological factors, if those variables are monitored, there should be a way to predict epidemics based on the risk factors related to epidemics including:  a sudden increase in the number of non-immunes that are exposed to malaria, a rapid increase in vectorial capacity (increased density of vectors or invasion of a more efficient vector), land-use change, and failure of control efforts (Najera et al., 1998).

            P. vivax and P. falciparum cause different types of epidemics.  P. vivax epidemics occur mainly in areas with only seasonal transmission and show a bimodal peak, the second peak caused by relapses, whereas P. falciparum epidemics grow slowly and then explode causing only one peak of transmission (Najera et al., 1998).