Abstract:
Dengue is known as the most common arboviral disease in the world. Sri Lanka has been
affected by dengue epidemics for over two decades. In recent years over 50,000 dengue cases per
year have been reported (55,150 in 2016 and 186,101 in 2017). The highest incidence is reported
from Colombo district. Despite extensive vector control strategies carried out so far by health
authorities a significant reduction in dengue prevalence has not been observed. Although
monthly dengue incidences are considered in dengue epidemiological surveys, no proper
mapping techniques are adopted for improved visualization of spatial case distribution. A study
was initiated to examine the spatial distribution of confirmed dengue cases using Geographic
Information System (GIS) in Gangodawila PHI area, Nugegoda MOH area, as one of dengue
high incidence areas in the District of Colombo, Sri Lanka. Monthly dengue incidence was
obtained from MOH office, Nugegoda from January to December 2017. The GIS locations of
each dengue case were obtained and mapped using Arc Map 10.5 software. Inverse Distance
Weighted (IDW) maps were created quarterly throughout the year 2017. Mapping dengue cases
reflected the spatial heterogeneity in dengue case distribution throughout the year in
Gangodawila PHI area. The case distribution from January to March represented a congestion of
dengue cases in North-West and South-West areas covering the Gamsabha Junction (2.19% of
total area). Case distribution from April to June revealed a case shift towards the South
(Delkanda and Pangiriwatta) covering 1.56% of total area. A case shift towards North
(Delkanda) was revealed from July to September covering a 1.73% of total area and a case shift
towards South-East (Gangodawila) in later three months covering a 1.75% area of the total area
was observed. The study revealed a case shifting pattern from dengue high incidence to low
incidence areas throughout the year. Upon dengue prevalence, vector control measures are
intensified only in dengue high transmission areas as a practice. Source reduction, adulticides
and larvicides are routinely administered. Although elimination of adult and larval vectors are
expected by chemical control, insecticide resistance development in vectors may have reduced
the expected vector control. The study postulates that reasons for case shift over time might be
attributed to escape behavioural tendencies to avoid unfavorable conditions and to invade new
niches. The study highlights the importance of adopting effective spacial mapping tools for
better dengue management.
