CONICET | Buscador de Institutos y Recursos Humanos

Introduction: The alpha-proteobacterium Rickettsia parkeri was first reported as pathogenic in the United States in 2004. In Argentina, cases of human rickettsiosis caused by R. parkeri were documented in the provinces of Buenos Aires, Entre Rios and Chaco. The cases of R. parkeri infection reported in Argentina were predominantly concentrated around the Paraná River Delta, where the tick implicated in its transmission is Amblyomma triste. Previous studies conducted in the same region reported a prevalence of R. parkeri in A. triste ranging from 8% to 20%. Adults of A. triste can use cattle as hosts. Moreover, it has been reported that cattle are susceptible of infection with several members of SFG rickettsiae. Additionally, it has been demonstrated that R. parkeri¬-infected Amblyomma maculatum ticks are capable of transmitting this SFG rickettsiae to cattle. The Paraná River Delta constitutes a vast human-domestic-wildlife interface where the risk of pathogen transmission across species is substantial. In this region, farming beef cattle is on the rise, gradually displaced from the Pampas by agriculture. As cows suffer frequent A. triste infestations, they are at risk of becoming infected with R. parkeri and could act as amplifier hosts of the disease. Materials and Methods: The study was conducted in fields of an Experimental Station belonging to Instituto Nacional de Tecnología Agropecuaria, Campana (34°9.5′S, 58°51.8′W) in Buenos Aires Province, Argentina. A herd of beef cattle consisting of 21 Aberdeen Angus cows grazing in a mixed system consisting of natural pasture and Salicaceae plantations in INTA Delta were repeatedly bled from the tail vein every five weeks from December 2010 to May 2012. In parallel to blood collection, the left ear of each cow was thoroughly examined in search of ticks. Simultaneously, questing adult ticks were collected from the vegetation by drag-sampling. Presence of R. parkeri antibodies was determined by an indirect immunofluorescence assay using crude antigens derived from R. parkeri strain At24. The presence of rickettsial DNA was assessed in all cows showing an event of seroconversion. For this purpose, both, the sample that seroconverted and the sample previously obtained from the same cow (seronegative) were analyzed. Blood DNA samples were analyzed by SYBR Green quantitative real-time PCR targeting the rickettsial gene gltA using primers CS-5 and CS-6. The integrity of the DNA extracted from the blood was assessed using a real-time PCR that amplifies bovine 18sRNA gene. Results: Ticks (A. triste adults) in the environment were found only from August to February, with a peak in August. The count of female ticks (FF) on the left ear largely reflected the seasonal pattern observed for questing ticks. Both counts were highly correlated (Spearman?s Rho correlation coefficient= 0.813, p= 0.0002). The temporal pattern of seroprevalence for R. parkeri matched that of tick exposure, showing a peak in August. Seroprevalence was positively correlated with total count of questing ticks (Spearman?s Rho coefficient=0.612; p=0.015) and mean count of FF on the left ear (Spearman?s Rho coefficient=0.546; p=0.0353). At the individual level, a generalized linear mixed model with a binary response (seropositivity) using ?cow ID? and ?trapping session? as random effects showed that every female tick attached on the left ear increased the odds of seropositivity in 37.7% (Odds Ratio=1.377). During the entire study, 2 of 21 bovines were never seroreactive to the R. parkeri antigen, one cow presented R. parkeri-reactive antibodies in all the samples and the rest of the herd presented at least one episode of seroconversion, which in most cases was only transient. Serum endpoint titers against R. parkeri antigen varied from 1:64 to 1:512. No presence of rickettsial DNA was detected by gltA real-time PCR in the blood of cows showing events of seroconversion. All bovine blood DNA samples were positive for 18sRNA gene real-time PCR. Discussion: Rickettsia parkeri has previously been reported infecting A. triste ticks in Argentina, Brazil and Uruguay. As it has been shown in the present study, A. triste parasitizes cattle. A large proportion of the ticks at the study site (8-20%) are infected by R. parkeri and the seasonality observed in adult A. triste ticks was from late winter to mid-spring. We found no evidence of rickettsemia by PCR, but the high R. parkeri-infection rate in A. triste in the study area, the common occurrence of this tick parasitizing this herd and the identification of antibodies against R. parkeri antigen in 90% of the animals(19 out of 21) are evidence that infection is taking place. Of the 19 seropositive cows, 8 (42%) presented repeated periods of seropositivity, lasting more than three months, and up to 18 consecutive sampling sessions. The rest of the seropositive animals of the herd presented short periods of seropositivity, which in some cases were recurrent. These intermittent periods of seropositivity could be due to the low titers of R. parkeri-reactive antibodies that are generated in cattle, but we cannot rule out the possibility of new episodes of R. parkeri-infection produced by A. triste ticks feeding on these cows. The time of the year with more animals presenting R. parkeri-reactive antibodies (from late winter to mid-spring) was coincident with the peak in the abundance of adult A. triste ticks, which is evidence of new infections following infestation by ticks. In this respect, an important statistical relationship was observed between cattle seropositivity and the number of female ticks attached to it, which indicated that each additional tick attached on the left ear of a cow incremented in almost 40% the odds of seropositivity for this animal. Our data suggest that A. triste ticks are capable of naturally transmitting R. parkeri to cattle. Nonetheless, the negative PCR results indicate that either the length of the rickettsemic period may not be long, which suggests that cattle might not be very important for the amplification of R. parkeri or the rickettsemic levels in cattle blood may not be high enough to allow real-time PCR detection, suggesting that R. parkeri replication in cattle might not be efficient. Notwithstanding, a feasible role for cattle in the ecology of R. parkeri could be providing a blood meal to a large number of A. triste adult ticks which could increase tick population.