Experimental infection of capybaras Hydrochoerus hydrochaeris by Rickettsia rickettsii and evaluation of the transmission of the infection to ticks Amblyomma cajennense
Introduction
The bacterium Rickettsia rickettsii is the etiological agent of Rocky Mountain spotted fever (RMSF), an acute febrile, tick-borne disease that has been reported in Canada, United States, Mexico, Costa Rica, Panama, Colombia, Brazil, and Argentina (Dumler and Walker, 2005, Paddock et al., 2008). There has been a clear reemergence of RMSF in Brazil. For example, from 1988 to 1997, there were 25 laboratory-confirmed cases distributed among six municipalities in the state of São Paulo. During the subsequent decade, from 1998 to 2007, there were 255 confirmed cases distributed among 54 municipalities of the state (official data from the São Paulo State Health Secretary at http://www.cve.saude.sp.gov.br). Indeed, this increase is partially attributed to a much more efficient surveillance, especially after the disease became nationally notifiable in 2001 (Silva and Galvão, 2004). However, it is possible that ecological factors have played a major role in this disease reemergence.
Studies in the United States demonstrated that R. rickettsii is transmitted transovarially and transestadially in several tick species, however infection rates by R. rickettsii in ticks are very low (usually <1%) under natural conditions, mainly because R. rickettsii is pathogenic for ticks (Burgdorfer, 1988, Niebylski et al., 1999). Thus, the role of amplifier hosts in the ecology of RMSF is crucial. Amplifier hosts are vertebrate animals that develop rickettsemia for some days, when new uninfected ticks become infected and start new lineages of infected ticks within the tick population (Burgdorfer, 1988, Dumler and Walker, 2005). In the United States, several small rodent species have been implicated to act as amplifier hosts of R. rickettsii, as for example, Microtus pennsylvanicus in the eastern part of the country (Burgdorfer et al., 1966, McDade and Newhouse, 1986). In Brazil, the opossum Didelphis aurita is the only vertebrate that has been shown to be a competent amplifier host of R. rickettsii for Amblyomma cajennense ticks (Horta et al., 2009).
The tick A. cajennense is the main vector of R. rickettsii in Brazil, where horses and capybaras (Hydrochoerus hydrochaeris) are among the most important primary hosts for all parasitic stages of the tick (Vieira et al., 2004, Guedes et al., 2005). In the state of São Paulo, capybaras were considered threatened of extinction during the 1950s. During subsequent decades, capybara populations increased in many areas, and nowadays it is considered a major cause of crop damage in the state (Verdade and Ferraz, 2006), besides various complains of high environmental burdens by A. cajennense due to increased capybara populations. Since capybaras have been suspected to be amplifier hosts for R. rickettsii since the 1940s (Travassos and Vallejo, 1942), the increasing capybara population in the state of São Paulo has also been suspected to be related to the reemergence of RMSF. Based on these statements, the present study performed experimental infection of capybaras by R. rickettsii, and evaluated the role of these animals as amplifier hosts for A. cajennense ticks, under laboratory conditions.
Section snippets
Capybaras
Eight capybaras, 3 months old, provided by a commercial breeding were kept in individual boxes (3 m × 3 m) for the experiment in Mogi Guaçu, state of São Paulo. Capybaras were fed daily with fresh green forage and commercial pellets for rodents, and water ad libitum. Before starting the experiments, capybaras were clinically healthy and their paired serum samples (14 days interval) were shown to contain no reactive antibodies to R. rickettsii, Rickettsia parkeri, Rickettsia felis and Rickettsia
Results
During the study, capybaras of the control groups (CG1 and CG2) remained uninfected because no Rickettsia DNA or R. rickettsii-reactive antibodies were detected in their blood. In contrast, rickettsial infection was evident in both G1 (infected through infestation with R. rickettsii-infected ticks) and G2 (inoculated intraperitoneally with R. rickettsii) capybaras, since they all seroconverted (Fig. 1) and their blood caused rickettsial infection in inoculated guinea pigs (Table 1, Table 2). In
Discussion
The present study showed that capybaras are susceptible to the infection by R. rickettsii. In contrast to other mammal species (humans, dogs, guinea pigs, and domestic rabbits), in which R. rickettsii usually induces high fever and even death (McDade and Newhouse, 1986), capybaras showed no signs of fever. Decades ago, Travassos and Vallejo (1942) reported that R. rickettsii-inoculated capybaras developed no fever during the rickettsemic period of the study. On the other hand, our results
Acknowledgements
We thank the Centro de Controle de Zoonoses (CCZ) of São Paulo Municipality for providing conjugate anti-capybara IgG and Usina Colombo S/A for donating the capybaras. This work was supported by FAPESP (Grants 06/50650-8 to M.C.H. and 06/50918-0 to M.B.L.) and SUCEN (Grant 01/05 to C.E.S.).
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