Seasonal dynamics of spinose ear tick Otobius megnini associated with horse otoacariasis in Sri Lanka.

Otobius megnini is a one host, nidicolous soft tick (Family Argasidae) whose larvae and nymphs parasitize the external ear canal of many wild and domestic animals and occasionally humans. The present study was conducted to determine the seasonal dynamics of O. megnini infesting stabled horses in Nuwara Eliya racecourses. Ticks were sampled biweekly for two years from May 2013 to May 2015 from the ear canal of seven thoroughbred male horses. Weather data: temperature, rainfall and humidity for the study period were obtained from the Meteorological Department, Colombo. A total of 23, 287 ticks of O. megnini were collected from the ear canal of horses and all of which were immature stages comprising 41.4% larvae and 59.6% nymphs. Larval counts were high during warmer months of the year with minimum numbers recording September-November. High larval abundance may be due to the high hatching rate of eggs at high temperatures. Larval abundance was significantly affected by maximum temperature, minimum relative humidity and average relative humidity (Multiple regression; r(2)=0.5, F=3.7, p=0.01). Population peaks for nymphs occurred in May, October and January. Nymph counts were low in March and April. Weather conditions did not have any correlation with the abundance of nymphs. Rainfall had no effect on both larvae and nymph counts. Even though control measures have been taken, O. megnini infestation is a serious problem for the stable owners especially because these horses are used in racing and other recreational purposes. Further, there is a high risk of spreading this tick within the country due to the presence of suitable hosts and the adaptability of the tick to survive in changing climatic conditions. Information on the seasonal dynamics of the tick population is important for employing control measures.