Our findings indicate that there is spatial expansion of CL transmission in Sri Lanka involving hitherto non-endemic regions in the wet zone of the country. The districts of Anuradhapura, and Polonnaruwa in the North Central province, and Hambantota in the Southern province of Sri Lanka were the initial hotspots of CL [24]. More recently (2014–2016), the district of Kurunegala in the North Western province has become a major hotspot for CL [24]. The current focus of infection in the district of Gampaha may have originated from the adjacent Kurunegala district or from a more distant focus due to patient migration. However, the majority of our study population had minimal travel to areas outside the district. Clustering of cases within the MOH area with multiple cases occurring within the same household is in support of established transmission within the locality with the possibility of peri-domestic or domestic transmission as reported in the Southern province [32].
Phlebotomus argentipes has been identified as a potential vector of CL in Sri Lanka [14–16]. The presence of P. argentipes in the Mirigama MOH area suggests that the same species may be responsible for transmission in the Western province as well. Further, the highest sand fly density in HC observed from Wewaldeniya PHI area which was detected to be the area with the highest number of cases confirmed that the availability of the vector is a contributing factor for disease occurrence. However, according to the World Health Organization guidelines for leishmaniasis vector incrimination [33, 34], further studies on the presence of parasites within female sand flies, and evidence of luxuriant growth in the anterior midgut and on the stomodeal valve of the sand flies and experimental transmission using animal models are required for the incrimination of the exact vector species.
Similar to past reports, CL was found to affect a broad age range (2–77 years) and both genders with a slight adult male preponderance [35]. However, the recent disease trends of increased female and older adult involvement documented by Siriwardane et al., was noted [35]. Most of those affected were housewives, unskilled workers and school children who engaged in outdoor activities such as home gardening (housewives, unskilled workers and pensioners) and sports activities (school children). Even among the population categorized as indoor-based skilled workers (six), many reported involvements in agricultural activities after duty hours (afternoons and evenings) which coincided with the preferred biting time of the vector (dusk to dawn) [30].
The ecology of the current focus of CL is somewhat distinct from the previous endemic foci, which were located in the dry and intermediate climatic zones with abundant scrub forest cover. The current focus is in the wet zone of the country with an abundance of agricultural fields but minimal forest cover. However, wooded areas were observed on residual hills in the vicinity of most infected households. Galgamuwa et. al., reported that the highest incidence of CL was in lowlands < 100 m above sea level [18). In contrast over half the study participants (57.53%) in Mirigama resided at altitudes > 100 m above sea level, on residual rocky hills. The presence of isolated rocks and stone barriers were also noted on most (17/32) of the residences located at lower elevations (< 100 m above sea level).
During the day, sand-flies rest in dark, sheltered and humid but dry surfaces [30, 36, 37] as they require protection from sunlight, wind and rain [38, 39]. Moist microenvironments rich in humus are preferred for egg laying. Populations of sand-flies tend.to be localized as they do not disperse far from their breeding sites [30, 40]. Thus the existence of breeding and resting places in very close proximity to human settlements facilitates the transmission of leishmaniasis [41]. The cracks and crevices in house walls, stone-barriers and rocky terrains in and around most patient residences perhaps provide suitable resting places while the wooded areas with rotting leaf litter may provide the humus rich microenvironment required for egg-laying. Poor housing conditions were reported as significant risk factors of CL in southern Sri Lanka [7, 42]. The houses of 64.38% in the current study were either of poor quality or poorly maintained and one third of houses screened were positive for P. argentipes indicating the possibility of domestic transmission. The significance of pets and livestock animals in the peri-domestic area is not known as zoonotic reservoir hosts are unknown.
The current report was based only on case analysis; thus the risk potential of ecological characteristics observed cannot be analyzed statistically for their significance. The study was limited to the Mirigama MOH area which reported the highest number of cases within the district of Gampaha. Thus the current observations may provide only a superficial glimpse of the CL situation in the Gampaha district and may not be reflective of the entire area affected. Vector studies require further in-depth investigations as to their breeding, resting and, feeding behaviors, insecticide susceptibility patterns and the vectorial capacity of the sand fly populations.
Although CL lesions may heal spontaneously, treatment is recommended to minimize scarring and the spread of disease. Most patients were managed with a course of intra-lesion SSG as per guidelines formulated by the Sri Lanka College of Dermatologist (SLCD 2012-14) while for more complicated lesions SSG combined with cryotherapy was administered.
The mean DLQI of 8.137 (range 0–23) suggests that CL does have a moderate negative impact on the QOL of the current study population. This is in contrast to the mean DLQI of 3.18 (range 0–30) reported by Refai et. al. [43] among civilian patients in Anuradhapura, but is somewhat comparable to the mean DLQI of 6.02 (males) reported by Ranawaka et. al. [44]. In agreement with Ranawake et. al., the most affected domains were emotions (feelings of shame and embarrassment) and shopping and household duties [44].
Early diagnosis and complete treatment, integrated vector management combined with disease surveillance and social mobilization are some of the recommended control strategies for leishmaniases [33, 45]. Although disease surveillance is in place, deficiencies were noted as 15% (11/73) of cases were not notified. An integrated vector management strategy is yet to be implemented in CL endemic areas of Sri Lanka. At this point of time, a multi-pronged approach is urgently required to control the spread of CL which is threatening the entire country. Ensuring the availability of treatment even in non-endemic areas, implementation of vector control measures and creating awareness among medical professional as well as the community is important to combat the spread of infection.