Serologic survey for antibodies to Rickettsia sibirica in Inner Mongolia, People's Republic of China

Molecular Characteristics of Rickettsia in Ticks Collected along the Southern Border of Mongolia

Tick-borne infections are a significant threat to public health, particularly in regions where individuals frequently enter tick habitats. Roughly 26% of the population in Mongolia practice nomadic pastoralism and are considered at high risk of exposure to ticks and the diseases they carry. This study tested ticks from Mongolia’s southern border for Rickettsia spp. to better understand the epidemiology of tick-borne diseases in the region. Dermacentor nuttalli and Hyalomma asiaticum ticks (n = 4022) were pooled and tested for Rickettsia spp. by real-time PCR. Melt-curve analyses and Sanger sequencing were used to identify Rickettsia species. Approximately 64% of the 786 tick pools tested positive for Rickettsia bacteria. Melt curve analyses identified four different Rickettsia species circulating in these tick pools. Amplicon sequencing of the ompA gene identified Rickettsia spp. that closely resembled R. raoultii and R. sibirica. Dermacentor nuttalli ticks from Govi-Altai had the highest maximum likelihood estimation infection rate 48.4% (95% CI: 41.7–56.5%), while Hyalommaasiaticum collected in Omnogovi had a rate of 7.6% (95% CI: 6.2–9.2%). The high detection of Rickettsia suggests a substantial risk of infection in southern Mongolia. Further studies are necessary to investigate the clinical burden of tick-borne diseases in Mongolia.

Evidence for transovarial transmission of tick-borne rickettsiae circulating in Northern Mongolia

Transstadial transmission of tick-borne rickettsiae has been well documented. Few studies, however, have evaluated the role of transovarial transmission of tick-borne rickettsiae, particularly in nature within the host-vector ecosystem. This cross-sectional study aimed to understand the role of transovarial transmission of tick-borne rickettsiae among feeding ticks at different life stages. Tick eggs laid by engorged wild-caught adult female ticks were pooled and tested for Rickettsia spp. and Anaplasma/Ehrlichia spp. using molecular techniques, while adult fed ticks were tested individually. Additionally, larval and nymphal ticks were collected in the wild from small mammals, pooled and tested for Rickettsia spp. and Anaplasma/Ehrlichia spp. There were 38 fed adult and 618 larvae/nymphs (60 pools total) Dermacentor spp. ticks collected from livestock and rodents. All individual adult ticks and tick pools were positive for Rickettsia spp. While none of the larvae/nymphs were positive for Anaplasma/Ehrlichia spp., two adult fed ticks were positive. Rickettsia spp. DNA was detected in 91% (30/33) of the pooled eggs tested, and one pool of eggs tested positive for Anaplasma/Ehrlichia spp. Sequencing data revealed Rickettsia spp. shared ≥99% identity with R. raoultii ompA. Anaplasma/Ehrlichia spp. shared ≥89% identity with A. ovis 16S ribosomal RNA. This study identified potential transovarial transmission of Rickettsia spp. and Anaplasma spp. among D. nuttalli ticks. Additional studies are needed to further assess the proportion of transovarial transmission occurring in nature to better understand the burden and disease ecology of tick-borne rickettsiae in Mongolia.

In this study, we evaluate the probability or likelihood that tick-borne rickettsiae might be transmitted vertically from wild engorged adult female ticks collected throughout the Northern region of Mongolia during the summer of 2016. While significant effort has been directed to study tick-borne rickettsiae, this public health challenge is complicated by the limited knowledge and understanding of tick and tick-borne rickettsiae ecology within Mongolia. Tick-borne rickettsiae of concern to humans and animals in this region of the world are Rickettsia spp., Anaplasma spp., and Ehrlichia spp. Using molecular techniques, we detected rickettsiae among all Dermacentor spp. tick life stages and demonstrated potential vertical transmission of Rickettsia spp., and Anaplasma spp. among wild engorged adult female Dermacentor nuttalli ticks. We believe our findings provide important information regarding the ecology of key rickettsiae associated with tick-borne disease in Mongolia.

Estimated seroprevalence of Anaplasma spp. and spotted fever group Rickettsia exposure among herders and livestock in Mongolia

BACKGROUND

To better understand the epidemiology of tick-borne disease in Mongolia, a comprehensive seroprevalence study was conducted investigating exposure to Anaplasma spp. and spotted fever group (SFG) Rickettsia spp. in nomadic herders and their livestock across three provinces from 2014 to 2015.

METHODS

Blood was collected from 397 herders and 2370 livestock, including sheep, goats, cattle, horses and camels. Antibodies against Anaplasma spp. and SFG Rickettsia were determined by indirect immunofluorescence using commercially available slides coated with Anaplasma phagocytophilum and Rickettsia rickettsii antigens. Logistic regression was used to determine if the odds of previous exposure differed by gender, location, and species, with or without adjustment for age. To examine the association between seroprevalence and environmental variables we used ArcGIS to circumscribe the five major clusters where human and animal data were collected.

RESULTS

Anaplasma spp. exposure was detected in 37.3% (136/365) of humans and 47.3% (1120/2370) of livestock; SFG Rickettsia exposure was detected in 19.5% (73/374) humans and 20.4% (478/2342) livestock. Compared to the southern province (aimag) of Dornogovi, located in the Gobi Desert, humans were significantly more likely to be exposed to Anaplasma spp. and SFG Rickettsia in the northern provinces of Tov (OR=7.3, 95% CI: 3.5, 15.1; OR=3.3, 95% CI: 1.7, 7.5), and Selenge (OR=6.9, 95% CI: 3.4, 14.0; OR=2.2, 95% CI: 1.1, 4.8).

CONCLUSION

The high seroprevalence of Anaplasma spp. and SFG Rickettsia in humans and livestock suggests that exposure to tick-borne pathogens may be common in herders and livestock in Mongolia, particularly in the more northern regions of the country. Until more is known about these pathogens in Mongolia, physicians and veterinarians in the countryside should consider testing for Anaplasma and SFG Rickettsia infections and treating clinically compatible cases, while public health authorities should expand surveillance efforts for these emerging infections.

Tick-borne rickettsioses in international travellers

BACKGROUND

Tick-borne rickettsioses are of emerging importance in today's travel medicine but have until recently received little attention. We describe the current knowledge of tick-borne rickettsioses as they relate to international travel, their microbiological diagnosis, treatment, possible prevention, and future prospects.

METHODS

Literature-based review and personal observations.

RESULTS

During the last decade, some 400 cases of tick-borne rickettsioses have been reported in international travellers, the vast majority being African tick bite fever caused by Rickettsia africae and Mediterranean spotted fever caused by Rickettsia conorii. Only a minority of infected travellers can recall a preceding tick bite. Most patients present with a mild-to-moderately severe flu-like illness typically accompanied by a cutaneous rash and an inoculation eschar at the site of the tick bite, but potentially life-threatening disease with disseminated vaculitis is occasionally seen. Definite microbiological confirmation of tick-borne rickettsioses by isolation or antigen detection is only available at reference laboratories and diagnosis must in most cases rely on clinical and epidemiological data supported by serology. Doxycycline is the recommended treatment for tick-borne rickettsioses and prevention is based on personal protective measures against tick bites when travelling in endemic areas.

CONCLUSION

Tick-borne rickettsiosis should be suspected in febrile returnees from endemic areas, especially in cases with skin eruptions. Travellers to endemic areas should be encouraged to use personal protective measures against tick bites.

Rickettsia sibirica infection in members of scientific expeditions to northern Asia

The risk of acquiring North Asian tick typhus (infection by Rickettsia sibirica) during travel to regions of Asia where this disease is endemic is unknown. We investigated prospectively 13 paleontologists on expedition to Mongolia. Four paleontologists had acute illness characterised by fever, rash, headache, and lymphadenopathy. All had IgM and IgG antibodies to R sibirica. Paleontologists with no illness and people who went on expeditions in other parts of the world did not have antibodies to R sibirica. Only two of the four infected persons were aware of tick bites. Travellers to regions endemic for R sibirica are at risk of contracting North Asian tick typhus even in the absence of recognised tick-bites.

Evidence for a high prevalence of spotted fever group rickettsial infections in diverse ecologic zones of Inner Mongolia

A 3-year study of spotted fever group rickettsial ecology in Inner Mongolia revealed that nearly half of the human population tested had antibodies to Rickettsia sibirica detected by complement fixation test. Infected persons, ticks and a high proportion of seropositive livestock and wild rodents were found in all five vegetation zones (desert, steppe, forest, forest-grassland and grassland).

Epidemiology of rickettsial diseases

Rickettsial diseases have a diversity of epidemiologic characteristics reflective of the variety of ecologic situations in which the obligate intracellular bacteria are transmitted to humans. For the spotted fever group (SFG) rickettsiae, Rickettsia typhi, R. tsutsugamushi, Coxiella burnetii, and the human ehrlichial agent, humans are a dead-end host who plays no role in the maintenance of the organism in nature. All rickettsioses exist as zoonoses. Moreover, all rickettsiae are found in infected arthopods, which generally serve as the natural hosts and can transmit the infection to the next generation of ticks, mites, chiggers, or fleas. From our anthropocentric viewpoint, Q fever aerosol infection from parturient animals and Brill-Zinsser disease ignited epidemics of louse-borne epidemic typhus are exceptions. However, silent cycles of C. burnetii in ticks and R. prowazekii in the flying squirrel flea may have maintained these agents in transovarial or enzootic cycles for eons before humans and their domestic animals arrived on the scene. Thus, the epidemiology of rickettsial diseases must be recognized as an unfortunate aberration of the rickettsial economy. Several excellent reviews of rickettsial ecology contain a wealth of useful information.