Risco de exposição a carrapatos Amblyomma sculptum e Amblyomma dubitatum e à bactéria Rickettsia bellii em área urbana indene para Febre Maculosa Brasileira

Abstract

Diseases caused/transmitted by ticks presuppose physical contact and trophic exchange between the host, vector, and pathogen. In this context, we can establish three levels/sequential stages of relationship among these beings and that are essential for the epidemiology of diseases associated with ticks: 1- hazardous area, 2- exposure and 3- parasitic interaction. The hazardous area refers to the occurrence, within a certain area, of the tick and in this, when applicable, of the pathogenic microorganism. The second level, exposure, refers to physical contact with the tick and that will occur in a hazardous area. The third level, parasitic interaction, refers, after physical contact, to the host’s attractiveness to the tick and its parasitic capacity towards this specific host. Facing the unlikelihood of controlling/eliminating ticks from a hazardous area and/or impossibility to avoid the parasitic interaction for disease prevention, it is necessary to lessen the risk of host exposure to ticks. To this end, it is essential to know the bioecology, especially host-seeking behavior of tick species that are associated with host disease in a hazardous area. Two species of ticks in Brazil, Amblyomma dubitatum and Amblyomma sculptum, occur in environmental sympatry when maintained by capybaras. The number of these parasites is particularly high in areas under anthropic impact in southeastern Brazil and, in some places, they are associated with Brazilian Spotted Fever (BSF), a disease with extremely high lethality caused by the bacterium Rickettsia rickettsii. Although the tick nymph A. sculptum is considered the main vector of the pathogen, the proportions between the two species and the behavior of each stage of the ticks influence the risk of human exposure to ticks and the bacteria. In addition, Rickettsia bellii, a non-pathogenic bacterium and that frequently infects A. dubitatum interferes with the transmission of the lethal Rickettsia by mechanisms that are still unclear. Therefore, this work aimed to contribute with information for protocols that reduce the risk of human exposure to ticks at areas hazardous for the transmission of BSF or other diseases associated with these two tick species. Among others, the density, behavior, and proportional relationship between these two tick species in the urban area of an BSF-free municipality and related to the risk of human exposure to ticks were evaluated. For this purpose, ticks were collected in ten locations with capybaras and bordering water bodies in the three seasons of the year characterized by peaks of activity of the parasitic stages of the A. sculptum tick. The collections of ticks in each location were made exhaustively in a standardized collection unit of 400 m2 and the coordinated application of four techniques. These techniques also allowed the evaluation of the host-seeking behavior of ticks according to the species and stage. Finally, the tick infection rate with R. bellii was correlated to the relative proportion of A. sculptum/A. dubitatum at each location. During the study period, 52,953 ticks were collected exclusively from two species, A. sculptum and A. dubitatum. Of these ticks, 83.4% were A. sculptum (1,523 adults, 10,545 nymphs and 32,104 larvae) and 16.6% A. dubitatum (464 adults, 2,153 nymphs and 6,164 larvae). Although the largest number of ticks collected was of the A. sculptum species, the prevalence of species varied among the ten study sites. During the study period, an average annual density of 4.4 ticks/m2 was found, varying between 0.03 and 31.8 ticks/m2 according to the location, season, species, and stage of ticks. As for the host-seeking behavior of ticks, ambush predominated in larvae and hunter in nymphs and mainly in adults, for both tick species. However, it was noted that the ambush host-seeking occurred in a significantly greater proportion of larvae and nymphs of A. sculptum in relation to A. dubitatum ticks and the latter remained more restricted to the ground. Adults and nymphs of A. sculptum were significantly more attracted to humans in relation to the same stages of A. dubitatum. Conversely, larvae of A. dubitatum were significantly more attracted to humans than those of A. sculptum. Taken together, these observations indicate that a human being in movement along A. sculptum and A. dubitatum hazardous area will have an almost exclusive exposure to A. sculptum larvae and/or nymphs, particularly in autumn and winter. When in a stationary position, humans are more attractive to adults and especially nymphs of A. sculptum, but also to larvae of A. dubitatum. This unexpected observation needs additional studies within the epidemiology of tick-borne diseases. Finally, it was observed that the higher the proportion of A. sculptum in the tick community, the lower the infection rate of A. dubitatum by R. bellii and that indicates a refractory effect of A. sculptum on the diffusion of the bacteria. Based on these observations, preventive measures to avoid exposure to the two species of ticks in hazardous areas are proposed.