Study On The Distribution Law Of Important Chiggers Such As Chiggers In The Ground In Southwest China

Chigger mites(trombiculid mites,tsutsugamushi mites)are the exclusive vector of scrub typhus(tsutsugamushi disease).The mites Leptotrombidium deliense and L.scutellare are the two major vector species of scrub typhus in China.Southwest China covers five provincial administrative divisions,Yunnan,Guizhou,Sichuan,Chongqing and Tibet,and it is an important epidemic area of scrub typhus.Based on the field investigations in Southwest China between 2001 and 2019,the ecological distribution and host selection of L.deliense and some other five chigger species were studied.The six involved chigger species are L.deliense,L.scutellare,L.sinicum,L.densipunctatum,Helenicula simena and Walchia ewingi.Based on a consecutive 12months'investigation at Jingha village in southern Yunnan from April 2016 to March2017,the seasonal fluctuations of L.deliense and L.scutellare were illustrated.1.Distribution and related ecology of six major species of chiggers in the whole Southwest China1.1 Collection and identification of animal hosts and chiggersA total of 19,375 small mammal hosts were captured from the 91 investigated counties of Southwest China between 2001 and 2019.Of the 19,375 captured hosts,19,300 were identified as 84 species,41 genera and 12 families under 6 orders.A total of 170,945 chiggers were collected from 19,375 animal hosts.Of 170,945 chiggers collected,166,925 of them were identified as 308 species in 26 genera under 3subfamilies of 2 families(Trombiculidae and Leeuwenhoekiidae).Of the 308identified chigger species in the 91 investigated counties,six species of them were the most dominant,accounting for 33.91%(56,610/166,925)of the total chiggers.The six major species of chiggers were L.scutellare(10.55%,17,617/166,925),L.sinicum(6.25%,10,438/166,925),L.deliense(5.08%,8,479/166,925),L.densipunctatum(4.18%,6,982/166,925),H.simena(4.15%,6,927/166,925)and W.ewingi(3.69%,6,167/166,925)respectively.Of the six major species of chiggers,L.deliense is the first major vector of scrub typhus in China,followed by L.scutellare(the second major vector).The abundant L.scutellare mites in Southwest China suggest that there may be two major vectors of scrub typhus coexisted in southwest China,L.deliense and L.scutellare,not just one major vector(L.deliense)as the traditional view suggested.1.2 Geographical distribution of six major species of chiggersLeptotrombidium deliense mites were collected from only 14 of the 91investigated counties,and 12 counties were in Yunnan province.And 45.87%of L.deliense mites were concentrated in Jinghong county of Xishuangbanna prefecture in southern Yunnan,and 22.17%of the mites were found in Rongjiang county of southeast Guizhou.The constituent ratio of L.deliense collected from Jinghong and Rongjiang was as high as 68.04%.The results suggest that L.deliense might be the dominant species and the main vector in some parts of the southwest,but not the whole region.The results did not support the historical view that L.deliense was the exclusive dominant species of chiggers in the vast southern region(including most of Southwest China)and the main vector of scrub typhus.Leptotrombidium scutellare mites were collected from 19 of the 91 investigated counties.And 72.31%of L.scutellare mites were collected from Dali county in southwest Yunnan.The results suggest that L.scutellare,which was widely believed to be mainly distributed in northern China,might be the dominant mite species and vector species in some parts of Southwest China.A large number of L.sinicum and H.simena mites were also identified from Dali county in Yunnan province,and their constituent ratios were95.39%and 75.16%respectively.Leptotrombidium densipunctatum and W.ewingi seemed to be evenly distributed in Southwest China,which suggests that the ecological environment of Southwest China is suitable for the two chigger species.1.3 Horizontal and vertical distribution of six major species of chiggersIn the horizontal distribution of different latitudes,L.deliense was mainly distributed in the low latitudes of 21-23°N,and its prevalence and mean abundance decreased with the increase of latitudes.The mite L.deliense was not found on 3,250small mammals in north of 28°N.The results indicate that latitudes have a great effect on L.deliense,and the mite prefers lower latitudes to higher ones.The rest of five chigger species(L.scutellare,L.sinicum,L.densipunctatum,H.simena and W.ewingi),however,were mainly concentrated in middle and high latitudes,and they were very rare or even absent in the northern latitudes of 29°N.At longitude levels,W.ewingi mite was mainly distributed in the longitude of 97-99°E,and the other 5species of chiggers were distributed in the longitude of 97-101°E.However,the number of six chigger species did not show an obvious variation with the change of longitudes,suggesting that the longitude may have no obvious effect on the infestation of the 6 major chigger species.In the vertical distribution of different altitudes,80.80%of L.deliense mites came from low altitudes below 1000m,and the prevalence of the mite decreased with the increase of altitude,suggesting that the mites prefer to be in low altitudes.However,74.18%of L.scutellare mites were collected from high altitude areas(2000-2500m),and the prevalence and mean abundance of L.scutellare increased with the increase of altitudes,suggesting that altitudes have a significant effect on L.scutellare.The rest four chigger species,L.sinicum,L.densipunctatum,H.simena and W.ewingi,were also mainly collected in middle and high altitudes,and the prevalence and mean abundance of H.simena increased with the increase of altitudes.1.4 Geographical landscape and habitat distribution of six major species of chiggersIn the distribution of different geographical landscapes,L.deliense tended to be distributed in flatland landscapes with higher infestation indexes than in mountainous landscapes(P<0.05).The individuals of the rest five chigger species(L.scutellare,L.sinicum,L.densipunctatum,H.simena and W.ewingi)in mountainous landscapes were much higher than those in flatland landscapes with higher prevalence and mean abundance in mountainous landscapes(P<0.05).In terms of habitat selection,many individuals of six chigger species were collected from outdoor habitats.Except for L.densipunctatum,the prevalence and mean abundance of the other 5 chigger species were higher in outdoor habitats than in indoor habitats(P<0.05).Therefore,it is necessary to make an efficient surveillance and control of L.deliense in flatland landscapes and to make an efficient surveillance and control of L.scutellare in mountainous landscapes.Through the prevention and control of L.deliense and L.scutellare,the risk of spreading Orientia tsutsugamushi from rats to human beings can be greatly reduced.1.5 Host selection of six major chiggersThe six major species of chiggers could parasitize a variety of host species.The parasitism of the six chigger species across different taxonomic categories(species,genus,family and order)of hosts reflected its low host specificity.Although the infestation of chiggers varied greatly in different families and orders of hosts,the majority of chiggers were collected from rodents,which was not only related to the preference of chiggers to rodent hosts,but might be also related to the most abundant individuals of rodents captured in the field investigations.The rat Rattus tanezumi was the dominant host of L.deliense in Southwest China,while Eothenomys miletus was the dominant host of L.scutellare and H.simena.Eothenomys miletus and Tupaia belangeri were the dominant hosts of L.sinicum.Eothenomys miletus and Apodemus chevrieri were the dominant hosts of L.densipunctatum.Rattus tanezumi and Niviventer confucianus were the dominant hosts of W.ewingi.All the six species of chiggers were of aggregated distribution among their animal hosts,and they tended to choose adult hosts.The prevalence and mean abundance of the mites L.scutellare,L.sinicum,and L.densipunctatum were higher in male hosts than in female hosts(P<0.05).The body parameters(body weight and body length)of animal hosts had no significant effect on the infestation of the six chigger species.1.6 Interspecific relationship and host niche of six major chigger speciesThe interspecific relationships of six major species of chiggers on their dominant hosts showed that a relatively high association coefficient(V=0.568)existed between L.scutellare and L.sinicum.The association coefficients among the rest species of chiggers,however,were quite low,suggesting that there may be no obvious interspecific cooperation or competition between different chigger species.The host ranges of L.deliense,L.scutellare,L.sinicum,L.densipunctatum,H.simena and W.ewingi were 34,24,22,31,23 and 23 host species respectively.The niche width index of W.ewingi(B_i'=6.735)was the highest of six chigger species.The differences of niche width indices in the rest five species of chiggers were not prominently different,and the niche width index of L.deliense(B_i'=2.113)was the lowest.The host-niches of six chigger species were wide with low host specificity on the whole.When the examined 84 species of hosts were regarded as host resources,the hieratical clustering analysis on the overlaps of six chigger species showed that H.simena and W.ewingi were clustered together at first,indicating that the two chigger species have a relatively high similarity with a high niche overlap in the selection of animal hosts.The three species,L.scutellare,L.sinicum and L.densipunctatum,were then clustered together,indicating that they are also similar in host selection.The niche overlap between L.deliense and the other 5 species of chiggers was very low and it suggests that the host selection of L.deliense is quite different from the other five species with an obvious niche separation.2.Seasonal fluctuation of L.deliense and L.scutellare in southern YunnanFrom April 2016 to March 2017,a total of 2,434 rodents and some other small mammal hosts were captured from the 12 months'field investigation at Jingha village in southern Yunnan.Of the 2,434 captured hosts,2,424 of them were identified as 15species,10 genera and 5 families under 4 orders.Of all captured hosts,2,053individuals of them(84.35%)were the Southeast Asian house rats,R.brunneusculus,a dominant rodent pest in the investigated site.A total of 99,221 chiggers collected from 2,053 R.brunneusculus rats were identified as comprising 102 species and 12genera in 2 subfamilies.Of the 102 species of chiggers,L.deliense and L.scutellare were the dominant species on the body surface of the rats R.brunneusculus,with constituent ratios of 19.02%(18,867/99,221)and 3.83%(3,798/99,221),respectively.The mite L.deliense occurred throughout the year.The seasonal fluctuation of the vector L.deliense belonged to summer-autumn type with the highest peak in July(the first peak),and a second small peak occurred in October.The individuals of L.scutellare mainly appeared in winter and spring with the peak from January to February,and they dropped to a low level in April.The L.scutellare mites appeared occasionally from April to November,but with very few individuals.A negative correlation(P<0.05)existed between the individuals of L.scutellare and monthly temperatures.In conclusion,L.deliense was not the dominant species of chigger mites in the whole Southwest China,but the dominant chigger species in some areas south of 28°N,especially in the outdoor habitats of hot and humid flatland landscapes with low latitudes and altitudes.The mite L.scutellare,which was widely believed to be mainly distributed in northern China,might be the dominant mite species and vector species in mountainous areas with middle and high latitudes and altitudes of Southwest China.In the middle and high latitudes and high altitudes,the dominant position of L.deliense was gradually replaced by L.scutellare.The mites L.sinicum,L.densipunctatum,H.simena and W.ewingi were also mainly distributed in mountainous areas with middle and high latitudes and altitudes,especially in outdoor habitats of mountainous areas.The six major species of chigger mites could parasitize a variety of host species with a low host specificity.The mite L.deliense occurred throughout the year,and the seasonal fluctuation of the mite belonged to summer-autumn type with two peaks.The mite L.scutellare mainly appeared in winter and spring.The temperature and rainfall were two key factors which influenced the seasonal fluctuation of these two major vector species of chigger mites.The researches on the distribution of six main chiggers in Southwest China is of great significance for monitoring and controlling the vectors of scrub typhus in the region.