Infestation and seasonal fluctuation of chigger mites on the Southeast Asian house rat (Rattus brunneusculus) in southern Yunnan Province, China

Chigger mites are the common ectoparasites of rodents and the exclusive vector of scrub typhus. The Southeast Asian house rat (Rattus brunneusculus) is an important reservoir host and infectious source of some zoonoses including scrub typhus. From April 2016 to March 2017, a 12-month consecutive investigation was made at Jingha village in southern Yunnan of China, which is an important focus of scrub typhus. The infestation and seasonal fluctuation of chigger mites on R. brunneusculus were studied based on the investigation. From 2,053 captured R. brunneusculus, a total of 99,221 chiggers were collected and identified as comprising 102 species with very high species diversity. The richness (S), diversity index (H′), evenness (E) and dominance index (D) of the chigger community on the rat varied in different months. Of the 102 chigger species, five main species accounted for 84.81% of the total chiggers (84,147/99,221). The five main chiggers were Walchia (W.) micropelta (32.65%), Ascoschoengastia indica (24.68%), Leptotrombidium (L.) deliense (19.02%), W. (W.) turmalis (4.63%) and L. (L.) scutellare (3.83%). Of the five chigger species, L. (L.) deliense and L. (L.) scutellare are the most important vectors of scrub typhus in China. The five chigger species showed different patterns of seasonal fluctuation. The seasonal fluctuation of L. (L.) deliense belonged to summer-autumn type with the highest peak in July, but L.(L.) scutellare mainly appeared in winter and spring with the peak from January to February. The temperature and rainfall were two key factors which influenced the seasonal fluctuation of chigger mites.