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(1) Background: Gamasid mites are a large group of arthropods, and some of them are of medical importance. Besides directly biting humans and causing dermatitis, some gamasid mites are the vector of rickettsialpox and potential vector of hemorrhagic fever with renal syndrome (HFRS). The Three Parallel Rivers Area of China is one of the hotspots of biodiversity research in the world, with complicated topographic landforms, different types of vegetation, special elevation gradients and high biodiversity. (2) Methods: Species richness ( S ): the Shannon-Wiener diversity index ( H ), Simpson dominance index ( D ) and Pielou evenness index ( E ) were used to analyze the basic community structure. The β diversity (Cody index) was used to reflect the diversity difference between any two adjacent elevation gradients. The method based on Preston's lognormal model for species abundance distribution was used to estimate the total number of gamasid mite species. (3) Results: A total of 3830 small mammal hosts captured from the nine survey sites were identified as 44 species, 27 genera and nine families in five orders. Apodemus chevrieri , Eothenomys miletus and A. draco were the dominant host species with a total constituent ratio C r = 52.037%. From the body surface of the hosts, 26,048 gamasid mites were collected and identified as 10 families, 21 genera and 82 species (excluding 847 unidentified specimens) with high species richness ( S = 82) and diversity ( H = 2.33). The three dominant mite species were Dipolaelaps anourosorecis , Laelaps nuttalli and L. echidninus , with a total C r = 64.46% (16,791/26,048). There are significant differences in the species composition, species diversity and dominant species of gamasid mites on different hosts. The species diversity of the mite community fluctuated greatly in different elevation gradients. The highest peaks of species richness and β diversity appeared at altitudes of 3000-3500 m ( S = 42) and 1500-2000 m ( β = 17.5), respectively. The species abundance distribution of the mites was successfully fitted by Preston's lognormal model with S^(R)=19e-[0.22(R-0)]2 ( α = 0.22, R 2 = 0.9879). Based on fitting the theoretical curve by Preston's model, the total number of gamasid mite species was estimated to be 153 species. (4) Conclusions: Gamasid mites on small mammals are abundant with complex community structures and high species diversity in the Three Parallel Rivers Area of China. There is an apparent community heterogeneity of the mites on different hosts and in different environments.

(1) Background: As a species of gamasid mite, the tropical rat mite ( Ornithonyssus bacoti ) is a common ectoparasite on rodents and some other small mammals. Besides stinging humans to cause dermatitis, O. bacoti can be a vector of rickettsia pox and a potential vector of hemorrhagic fever with renal syndrome (HFRS). (2) Objective: The present study was conducted to understand the host selection of O. bacoti on different animal hosts and the distribution in different environmental gradients in Yunnan Province of Southwest China. (3) Methods: The original data came from the investigations in 39 counties of Yunnan, between 1990 and 2015. The animal hosts, rodents and some other small mammals were mainly trapped with mouse traps. The O. bacoti mites on the body surface of animal hosts were collected and identified in a conventional way. The constituent ratio ( Cr ), prevalence ( P M ), mean abundance ( MA ) and mean intensity ( MI ) were used to reflect infestations of animal hosts with O. bacoti mites. The patchiness index and Taylor's power law were used to measure the spatial distribution pattern of O. bacoti mites on their hosts. (4) Results: A total of 4121 tropical rat mites ( O. bacoti ) were identified from 15 species and 14,739 individuals of hosts, and 99.20% of them were found on rodents. More than half of O. bacoti mites (51.78%) were identified from the Asian house rat ( Rattus tanezumi ), and 40.09% of the mites from the Norway rat ( R. norvegicus ) ( p < 0.05). The infestations of R. tanezumi ( P M = 7.61%, MA = 0.40 and MI = 5.31) and R. norvegicus ( P M = 10.98, MA = 1.14 and MI = 10.39) with O. bacoti mites were significantly higher than those of other host species ( p < 0.05). The infestations of two dominant rat hosts ( R. tanezumi and R. norvegicus ) with O. bacoti mites varied in different environmental gradients (latitudes, longitudes, altitudes, landscapes and habitats) and on different sexes and ages of the hosts. The prevalence of juvenile R. norvegicus rats with O. bacoti mites ( P M = 12.90%) was significantly higher than that of adult rats ( P M = 9.62%) ( p < 0.05). The prevalence ( P M = 38.46%) and mean abundance ( MA = 2.28 mites/host) of R. tanezumi rats with O. bacoti mites in the high latitude were higher than those in the low latitudes ( p < 0.05). The majority of the total collected 4121 O. bacoti mites was found in the flatland landscape (91.28%) and indoor habitat (73.48%) ( p < 0.05). The P M (10.66%) and MA (0.49 mites/host) of R. tanezumi rats with O. bacoti mites were significantly higher in the indoor habitat than in the outdoor habitat ( p < 0.05). The tropical rat mites showed an aggregated distribution pattern on their first dominant host, R. tanezumi . Conclusion: The tropical rat mite ( O. bacoti ) is a widely distributed species of gamasid mite in Yunnan Province, Southwest China, and its dominant hosts are two synanthropic species of rats, R. tanezumi and R. norvegicus . It is mainly distributed in the flatland landscape and indoor habitat. It has some host-specificity, with a preference to rodents, especially R. tanezumi and R. norvegicus . The O. bacoti mites are of aggregated distribution on R. tanezumi rats.