Exploring Tick Borne Pathogens Circulating Mongolia Through Collection of Ticks

Abstract

Introduction: Mongolia is a country known for its rich nomadic and pastoral culture, with populations of people who work in environments that are densely populated with ticks and TBP animal reservoirs. TBPs typically undergo transstadial transmission, but transovarial transmission may also occur. Transovarial transmission events have been demonstrated in laboratory settings, but few studies have evaluated transovarial transmission of TBPs in field settings within the host-vector ecosystem. Tick borne pathogens of most concern in Mongolia are Rickettsia spp., Anaplasma spp., Borrelia spp., Babesia spp., Ehrlichia spp., and tick-borne encephalitis. In this study, specific aims were: 1) To determine the prevalence of tick borne pathogens, particularly Rickettsia spp., Anaplasma spp., and Ehrlichia spp. among various tick species at different developmental life stages; and 2) To understand the role of animal reservoirs and vertical transmission of TBPs among feeding ticks at different life stages using larval and nymph ticks collected in the wild from small mammal reservoirs, as well as eggs laid by engorged wild-caught adult female ticks and reared in the laboratory.

Methods: In this cross-sectional study, ticks in their larvae and nymph life stages were collected off of captured rodents across seven soums (districts) in three aimags (provinces) situated in the Northern region of Mongolia from June 20th to July 23rd, 2016. Engorged adult ticks were collected from livestock located in three soums within three aimags from May 6th to 22nd, 2016. Tick eggs were collected from engorged ticks from May 9th to June 1st, 2016. Ticks were tested by PCR to detect the presence of Rickettsia spp., Anaplasma spp., and Ehrlichia spp.

Results: There were 546 (88%) larval and 72 (12%) nymphal Dermacentor spp. ticks collected. There were 588 (95%) of 618 ticks allocated into 42 larvae and 18 nymph pools (60 pools total). All tick pools were PCR-positive for Rickettsia spp. and no tick pools were PCR-positive for Anaplasma/Ehrlichia spp. minimum infection rate (MIR) for R. raoultii ranged from 6.7% to 28.6%. Of the 60 tick pools, 50 (522 ticks total) were matched with rodent rickettsial infection history status. There were 31 (62%) tick pools or 362 (69%) of individual ticks found on rodents with no history of rickettsial infection. The majority of ticks discovered to have no association with rodents with rickettsial infection history were larvae (352/362 individual ticks). There were 38 adult fed female ticks collected. All adult fed ticks were PCR-positive for Rickettsia spp. and 2 (5%) were PCR-positive for Anaplasma/Ehrlichia spp. There were 33 ticks that laid eggs. PCR testing of eggs showed a 91% (30/33) positivity for Rickettsia spp. and one pool of eggs was PCR-positive for Anaplasma/Ehrlichia spp. All sequenced Rickettsia spp. products were identified to be R. raoultii and all sequenced Anaplasma/Ehrlichia spp. were An. ovis.

Conclusions: This study identified transovarial transmission of Rickettsia spp. and Anaplasma spp. among D. nuttalli ticks. This study also found a low association between rodents with history of Rickettsia spp. infection and infection status of biting ticks. Additional study is needed to further assess the proportion of transovarial transmission found in nature. Specifically, testing of individual tick eggs and larvae should be conducted. A better understanding of the ecology of TBPs in nature can provide public health and human and veterinary medicine with a greater awareness of the burden of TBPs in Mongolia.