Volume 24, Number 2—February 2018
Rickettsia africae and Novel Rickettsial Strain in Amblyomma spp. Ticks, Nicaragua, 2013
Obligately intracellular Rickettsia spp., typically transmitted by ticks, cause a multitude of mild to severe rickettsial diseases in humans and other animals. Novel Rickettsia species have been identified through molecular techniques (1). Rickettsiae in Central America have primarily been reported in ticks, dogs, and humans, with limited data on tick species and rickettsial prevalence in Nicaragua (1). In an earlier study, 87% of 77 dogs in the Bosawás Biosphere Reserve were seropositive for rickettsiae (2); the ticks in that study were collected from 40 of those dogs.
The Bosawás Reserve in remote northern Nicaragua, part of the second largest tropical rainforest in the Western Hemisphere, is inhabited by 2 rapidly growing populations of indigenous people: the Miskito and the Mayangna. These subsistence-based communities use dogs for hunting in the reserve. Increasing connectivity with outside areas, population growth, and interference of dogs with wildlife pose an increased risk for the emergence of zoonotic rickettsioses. We planned to expand information on zoonotic Rickettsia spp. in Nicaragua by surveying ticks from hunting dogs for diversity, number, and presence of rickettsiae.
We collected ticks in 2013 from villages at similar latitude and longitude measured by using global positioning system (GPS): Arang Dak (14.51583, −84.99944), Amak (14.06542, −85.142233), and Raiti (14.59464, −85.02772) (Table). Arang Dak is the smallest of the 3 villages and closest to the densest part of the rainforest; Raiti is the largest and most developed village of the 3 and is situated on a heavily traveled route through the reserve. We obtained owner consent before physical examination and sampling of ticks from dogs and stored ticks in 70% ethanol. In the laboratory, we identified ticks for sex, life stage, and species by using a key (3) and screened tick DNA for Rickettsia spp. by real-time PCR (4). Rickettsia-positive samples were further tested by conventional PCR targeting the outer membrane protein A gene (ompA) (5). We also amplified the rpmB and 17kDa genes of the rickettsia in the Amblyomma triste ticks we recovered (4). We sequenced each amplicon by using the forward primer at University of California Davis Sequencing (Davis, CA, USA) and compared sequences to those in the GenBank database by using the BLAST algorithm (https://blast.ncbi.nlm.nih.gov).
Of 146 ticks from 40 dogs, 126 (86%) were A. ovale, 12 were A. sculptum, and 7 were A. triste. We detected rickettsial DNA in 24 (16.4%, 95% CI 11.0%–23.7%) of the 146 ticks: 18 A. ovale, 5 A. sculptum, and 1 A. triste. We deposited rickettsial sequences from these ticks into GenBank (accession no. KX530472, KX576685, and KX576686).
By location, the PCR prevalence was 25.5% (95% CI 15.1%–39.3%) in Raiti, 16.0% (95% CI 5.25%–36.9%) in Amak, and 9.09% (95% CI 3.75%–19.4%) in Arang Dak. These differences were statistically significant (p = 0.05 by Fisher exact test). The finding of highest prevalence in the most populated community is consistent with peridomestic animals maintaining the infection, and the rainforest and remote wildlife not being significant sources.
For the 576-bp ompA sequence, all from A. ovale ticks were identical and were 99.6% homologous with sequences from GenBank identified as R. africae. R. africae has not been reported in A. ovale ticks or in North, Central, or South America. R. africae causes a mild rickettsiosis known as African tick-bite fever and was first described in a patient in the Western Hemisphere in 1998 (1). R. africae has been detected in A. variegatum ticks by using PCR and in humans in Guadeloupe by using serology (6) and more recently in A. loculosum ticks from New Caledonia (7). In Brazil, adult A. ovale ticks bite humans most frequently and are present from the borders of Mexico to those of Argentina (8). A. ovale is a common human-biting tick in Central and South America and poses a public health concern.
Sequences of ompA in 2 of 5 PCR-positive A. sculptum matched 99.6% to Candidatus R. amblyommii in GenBank (ompA of the other samples did not amplify, likely because they were relatively weak on real-time PCR). Candidatus R. amblyommii is common among Amblyomma spp. ticks in the New World and was reported in A. sculptum ticks in Brazil (9). Candidatus R. amblyommii has unknown pathogenicity but has been implicated in rickettsiosis cases in humans (9).
The ompA amplicon from A. triste ticks matched Rickettsia sp. ARAGAOI; sequencing of the rpmB and 17kDa genes was unsuccessful. This rickettsial species was originally described in marsupials in Brazil (10). Further monitoring of tick vectors in this remote area is needed to characterize local risk and detect possibly emerging vectorborne disease.
The authors thank members of J.F.’s laboratory for their technical assistance and feedback; and L. Schwartz, J. Liu, J. Koster, F. Diaz-Santos, and U. Coleman for assistance with field logistics. K. Thomas produced the map.
This project was funded by the One Health Institute and the Wildlife Health Center of University of California, Davis.
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