Intraspecific sensilla dimorphism in Ascoschoengastia indica (Prostigmata, Trombiculidae).

Ascoschoengastia indica is one of the dominant chigger species in Southeast Asia and a potential carrier of scrub typhus, due in part to its cosmopolitan nature. This study explored the possible biological significance of the observed dimorphism in the shape of its scutum sensilla. Sensilla are specialized structures that are generally adapted to perform specific functions related to sensory capabilities, so their shape and sizes are expected to vary between taxa. We describe morphological variation of the sensilla of A. indica in Thailand. The sensilla had either a round or an ovoid, club-shaped form, which was not dependent on the particularly locality or host. Ignoring the precise function of the sensilla and their morphological variation, our study attempted to answer the following single question: Do the distinct forms of the sensilla indicate possible heterogeneity of the A. indica species? The two forms, named S1 and S2, were compared by genetic and morphometric techniques. The genetic analysis was based on the COI sequences, while the morphometric comparison used the scutum, an organ shown to be of taxonomic value for chigger mites. Neither morphometric nor genetic data revealed any evidence of a speciation process underlying the morphological variation in sensillum types.

Detection of Orientia tsutsugamushi in Novel Trombiculid Mite Species in Northern Tamil Nadu, India: Use of Targeting the Multicopy traD Gene.

Detection of Orientia tsutsugamushi DNA in a trombiculid mite chigger species suggests that it might be a potential vector of scrub typhus in an endemic area. Over a period of 20 mo, 85 rats were trapped, 57 had chiggers that were identified by standard morphometric techniques. The chigger pools were assessed by performing PCR assays targeting fragments of the single-copy genes 56 kDa type-specific antigen gene (TSA56) by nested PCR and the 47 kDa (htrA) quantitative real-time PCR (qPCR). The novel traD SYBR green assay that detects a multicopy gene was also performed. In total, 27 chigger pools were positive by traD qPCR, of which only 7 were positive by 47 kDa qPCR and in 3 of these, 56 kDa gene was amplified by nested PCR. Orientia tsutsugamushi-specific DNA was detected in Ascoschoengastia spp., Schoengastiella ligula, Leptotrombidium rajasthanense, Leptotrombidium deliense, and Leptotrombidium jayawickremei chigger pools. Therefore, they could be potential vectors of scrub typhus in Southern India. The three 56 kDa sequences belonged to TA716 genotype and Kato genogroup. Further studies are needed to confirm these chigger species as scrub typhus vectors in Northern Tamil Nadu.

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.