Altitudinal variation and bio-climatic variables influencing the potential distribution of Culicoides orientalis Macfie, 1932, suspected vector of Bluetongue virus across the North Eastern Himalayan belt of Sikkim

- Culicoides orientalis was first reported from Sikkim in 1963, with no further study.
- In 2001, C. orientalis was suspected a vector from Tamil Nadu, where Bluetongue disease was reported.
- Forty locations of Sikkim were studied for two years and potential distribution sites of C. orientalis were generated based on nineteen bioclimatic variables and altitudinal variation.
- Nine bioclimatic variables were involved in regulating the species distribution pattern.
- The combined effect of altitude with bioclimatic variables showed the highest potential occurrence site of C. orientalis in the southern part of Sikkim.

Culicoides orientalis was first recorded from Sikkim, in the year 1963, but no evidence based disease outbreak were available. In the last 50 years, 260 Bluetongue disease outbreaks caused by Culicoides species have been evidenced from India. Moreover, in recent years with increase of average temperature worldwide and increase in longevity of arthropod vectors like Culicoides along with a geographical range shift to new suitable warmer regions has increased the potentiality of vector borne disease outbreak throughout the world. The Himalayan range of Sikkim in India is a biodiversity hotspot and is extremely sensitive to such global climate changes. An attempt has been made to evaluate the altitude, climate and environmental data on selected study sites of Sikkim for a period of two years (2014-2015) for discerning potential distribution of C.orientalis in this region. The altitude, temperature, precipitation and potential distribution range maps of C. orientalis showed the areas of highest species abundance within the altitudinal range of 550-1830 m, with some species extending its range up to 3750 m, with average precipitation of 2010-2590 mm and mean temperature of 11-18 °C. The Maximum Entropy Modelling (MaxEnt) and the Jackknife test of the MaxEnt model further revealed that the major contributing factors governing C. orientalis distribution are annual precipitation (78.8%), followed by precipitation of driest quarter (8.3%) and mean temperature of the warmest quarter (3.3%). Accuracy of the study was evaluated by the area under the curve (AUC = 0.860). The Biplot on F1-F2 axes (N = 16, α = 0.05) in the PCA showed the linear depiction of all the variables considered in our study, major contributors were annual precipitation, precipitation of driest quarter and mean temperature of warmest quarter being the primary factors governing species distribution, as analogous to results of the MaxEnt model. This study would help in developing strategies for monitoring and managing surveillance programmes to control the chances of disease spread to livestock animals and also validate the predictions on the eventual spread of the vector midges to higher altitudes of Sikkim.

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