ROLE OF CLIMATE,
Rainfall, streamflow, temperature and relative humidity have been identified as the key meteorological variables that influence malaria transmission. The large-scale climate phenomenon El Nino and drought and flood conditions were also implicated in malaria transmission previously.
We undertook an analysis of the relationships of malaria incidence with hydro-meteorological variables such as rainfall, stream flow, temperature and relative humidity and analyzed the seasonality, trends, spatial distribution and statistical relationships between malaria epidemics and rainfall, and stream flow anomalies. These are described below
The regional character of the malaria distribution is brought out in relation to topography by considering the four climatically homogenous regions – Eastern, Western, Northern and Southern Plains. The Eastern and Western regions are further subdivided into the Eastern and Western coasts and hills. This classification makes sense from the point of view of the monsoonal wind directions (North-Easterly and Westerly) which lead to enhancement of rainfall on the Eastern and Western hill slopes during the respective monsoon seasons.
we also undertook preliminary analysis of climate change impacts using our annual island-wide data for malaria morbidity and mortality that goes up to 1911. We also constructed annual time series for rainfall, temperature; minimum, maximum and mean and relative humidity; day and night time. An examination of the relationship between rainfall, temperature and malaria incidence shows that higher morbidity prevailed during periods of lower temperature. The relationship with rainfall was not as clear cut.
In general, the rainfall and stream flow during years of high malaria incidence may be characterized as higher flows during wet seasons and lower flows during the dry seasons. Extended dry periods leading up to September have led to increased malaria risk.
Menik Ganga (river) which is often a locus for malaria outbreaks in the Uva province in January 2006 when the river has its seasonal high flow. During dry seasons, the stream flow declines leaving pools in the riverbed favorable for mosquito breeding.
There is a deficit in stream flow during the October to December season preceding an epidemic year. Such a deficit was also found in the records of the Gin Ganga River in the South-West. Further work shall examine such diagnostic for other rivers. Such a deficit if confirmed is a potential precursor of malaria risk. Our work is ongoing and we shall seek to understand the island-wide variation of stream flow and its relationship to epidemicity and report on it (Zubair et al., 2005e).
Environmental factors are also important contributors to malaria control. For instance, larviciding of stagnant pools with diesel and construction of buffer dams to prevent pooling were undertaken in the first half of the century as control measures. People without acquired immunity who were from non-malarious regions were resettled in malarious regions during large land settlement projects in malarious areas from 1950’s such as the Gal Oya scheme in the South-East to the Mahaweli Development scheme in the 1980’s. Of late, environmental control has re-emerged as a means of vector control. In all this however, the epochal change in climate may also be implicated in changes in the intensity and distribution of malaria.
The high preferences of major malaria vector and secondary vectors breeding habitats of malaria vectors in all RMO regions in a larval survey was carried out in 2016. It indicates that the highest density of Anopheles culcifacies was found in river bed pools.Anopheles culcifacies breeding contributes mainly in sand pools, rock pools and connected pools.
Temporary water collections like hoof prints, tire prints and wells, rainwater collections were the other breeding places. The main breeding sites of secondary malaria vector Anopheles subpictus were temporary water collections like hoof prints, tire prints and abandoned brick and burrow pits. Main breeding sites of Anopheles varuna were river and stream margins while Anopheles annularis was abundant in tanks.
(Source- Anti Malaria Campaign, Ministry of Health, Nutrition and Indigenous Medicine, Sri Lanka AMC, Annual Report, 2016 – Page No.30-31)