Abstract
BackgroundScrub typhus is a lethal infectious disease vectored by larval trombiculid mites (i.e. chiggers) infected with Orientia tsutsugamushi (OT) and recent decades have witnessed an emergence of scrub typhus in several countries. Identification of chigger species and their vertebrate hosts is fundamental for the assessment of human risks to scrub typhus under environmental changes, but intensive and extensive survey of chiggers and their hosts is still lacking in Taiwan.MethodsChiggers were collected from shrews and rodents in nine counties of Taiwan and were assayed for OT infections with nested polymerase chain reaction (PCR). PCR products were further sequenced to reveal probable OT strains. Rodents were assessed for OT exposure by immunofluorescent antibody assay. Lastly, incidence rate of scrub typhus in each county was associated with loads and prevalence of chigger infestations, seropositivity rate in rodents, and OT positivity rate in chiggers.ResultsRattus losea was the most abundant (48.7 % of 1,285 individuals) and widespread (occurred in nine counties) small mammal species and hosted the majority of chiggers (76.4 % of 128,520 chiggers). Leptotrombidium deliense was the most common (64.9 % of all identified chiggers) and widespread (occurred in seven counties) chigger species but was replaced by Leptotrombidium pallidum or Leptotrombidium scutellare during the cold seasons in two counties (Matsu and Kinmen) where winter temperatures were lower than other study sites. Seropositivity rate for OT exposure in 876 assayed rodents was 43.0 % and OT positivity rate in 347 pools of chiggers was 55.9 %, with 15 OT strains identified in the 107 successfully sequenced samples. Incidence rate of scrub typhus was positively correlated with chigger loads, prevalence of chigger infestations, seropositivity rate but not OT positivity rate in chiggers.ConclusionsOur study reveals R. losea as the primary host for chiggers and there exists a geographical and seasonal variation in chigger species in Taiwan. It also emphasizes the importance of recognition of chigger vectors and their vertebrate hosts for a better prediction of human risks to scrub typhus under rapid environmental changes.
Highlights
Scrub typhus is a lethal infectious disease vectored by larval trombiculid mites infected with Orientia tsutsugamushi (OT) and recent decades have witnessed an emergence of scrub typhus in several countries
We have found that R. losea hosted the majority of chiggers (76.4 %) and OT positivity rate in these chiggers was higher than those collected from most other host species
With R. losea as a surrogate, we found that incidence rate of scrub typhus was positively correlated with chigger loads, prevalence of chigger infestations and seropositivity rate but not OT positivity rate in chiggers
Summary
Scrub typhus is a lethal infectious disease vectored by larval trombiculid mites (i.e. chiggers) infected with Orientia tsutsugamushi (OT) and recent decades have witnessed an emergence of scrub typhus in several countries. Scrub typhus is a potentially severe febrile disease transmitted by trombiculid mites infected with rickettsia Orientia tsutsugamushi (OT). Identification of chigger species is fundamental for the evaluation of human risks because only a subset of chigger species, (mostly the genus Leptotrombidium [12]) are responsible for transmitting scrub typhus. An implicit assumption for such investigation is that chiggers at different locations respond to environments identically, which might not be true when the primary vector species vary geographically. This can be exemplified, as mentioned above, by the difference in seasonal occurrence of L. akamushi versus L. pallidum or L. scutellare, which is more abundant at higher and lower temperatures, respectively [11]
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