Diversity of rickettsiae in domestic, synanthropic, and sylvatic mammals and their ectoparasites in a spotted fever-epidemic region at the western US-Mexico border

Over one hundred cases of human rickettsiosis, many fatal, are reported annually across the US-Mexico transboundary region, representing a likely undercount. Although cases are often attributed to Rickettsia rickettsii, the agent of Rocky Mountain spotted fever, multiple other Rickettsia pathogens are present in North America. We conducted multiple-host surveillance of domestic, synanthropic, and sylvatic mammals and their ectoparasites to investigate the ecology of Rickettsia species in this region. A total of 499 mammals, including 83 dogs, 23 wild carnivores, five lagomorphs, and 388 rodents were sampled, and 413 fleas and 447 ticks belonging to 15 and 4 species, respectively, were collected during 2017 and 2018. We detected Rickettsia spp. DNA in one blood sample of coyote (Canis latrans), 11 ear tissues of rodents (10.6%), and 79 ectoparasites (9.5%). Of the 64 Rickettsia-positive fleas, 54 were Echidnophaga gallinacea and 10 were Pulex simulans, while of the 15 ticks, 11 were Rhipicephalus sanguineus s.l. and four Ixodes pacificus. The DNA sequence alignment of gltA and ompB regions revealed one and ten genetic variants of Rickettsia spp., respectively. These variants were clustered in clades of zoonotic species (R. felis, R. massiliae, R. parkeri, R. rickettsii, and R. typhi) and organisms of unknown pathogenic significance (R. asembonensis and Candidatus Rickettsia tarasevichiae). The finding of a coyote infected with R. rickettsii and the multiple zoonotic SFG rickettsial agents in the study area suggest that: 1) wild canids could serve as an amplifying host for RMSF, an alternate host for Rh. sanguineus s.l. ticks, and a means to spread infection and ticks over large areas; and 2) at least some of the human rickettsiosis cases attributed to R. rickettsii could be caused by other Rickettsia species. This study strongly supports the importance of multiple-host and vector eco-epidemiological studies and the One Health approach to better understand disease in a RMSF-epidemic region.