Ecoepidemiology of Rickettsia parkeri in the Paraná Delta, Argentina

Abstract

Background: In South America, several cases of human rickettsiosis caused by Rickettsia parkeri were documented in Uruguay, southern Brazil and the Paraná River delta of Argentina. There, the main tick vector is Amblyomma triste. Adults of A. triste seek blood meals from large mammals (including humans), whereas immature stages feed on small rodents.Methods & Materials: With the aim of shedding light on the ecology of this emerging disease, we conducted field studies at sites of the Paraná River delta, which consisted of systematic collection of ticks and blood samples from rodents (Fig. 2) and cattle, and also questing ticks from the vegetation. Sampling sessions were carried out monthly during 2011 and 2012 at 16 points that differed in their exposure to cattle and vegetation type (natural or implanted forest).Results: Prevalence of infection in adult questing ticks was high (20.4%). Interestingly, the distribution of R. parkeri infection intensity observed in A. triste ticks was distinctly bimodal, with approximately 60% of the infected ticks presenting high rickettsial loads (Fig. 3). Questing ticks were more frequently found in natural grasslands than in implanted forests, and prevalence of infection were greater in those from grasslands (26%) than in forested areas (8.3%). The dominant rodent species were Akodon azarae and Oxymycterus rufus. In both, the seroprevalence to R. parkeri was greater in those captured in grasslands than in implanted forests. The presence of cattle had a significant positive effect on the burdens of ticks on rodents and the abundance of questing ticks in the vegetation. Most cattle (90%) were seropositive, and the seasonality of the titres of antibodies against R. parkeri matched that of the tick infestation on cattle.Conclusion: The risk of human exposure to R. parkeri infected ticks in the Paraná River delta is high. Our results suggest that the silvopastoral activities that are on the rise in the region affect the dynamics of infection of R. parkeri. Cattle appear to favour the occurrence of the pathogen, whereas forestation seems to reduce it.