Seroprevalence of Rickettsia rickettsii and Rickettsia amblyommii in horses in three municipalities in the state of Pará, Brazil

Abstract: Brazilian Spotted Fever (BSF) is caused by Rickettsia rickettsii and is considered to be the most important zoonotic tick-borne disease in Brazil. The disease seems to be reemerging and cases have been documented in several states, but very little is known about the occurrence in the Amazon basin. There are no cases of BSF reported in this area but medical care and diagnostic tools are not always available, therefor it is possible that cases are occurring without being diagnosed. Because of the severity of the BSF pathology, this disease must be considered to be of great public health importance. Rickettsia rickettsii is the most pathogenic rickettsia species and because the agent more frequently occurs in horses than in humans, horses may be used as sentinel animals, even though there are no descriptions of clinical signs or symptoms in horses seropositive to R. rickettsii. Monitoring rickettsial seroprevalence in horses is therefore a valuable action for increasing the possibility to prevent Brazilian Spotted Fever in humans. To provide epidemiological input on BSF, the seroprevalence of Rickettsia rickettsii and Rickettsia amblyommii was determined among horses in three municipalities (Óbidos, Brasil Novo, Santarém) in the state of Pará, northern Brazil, through serological testing by indirect immunofluorescence assay (IFA). Three groups of horses were included in the study, namely; farm horses, urban horses and sport horses. The results were used to evaluate whether there were differences between the three groups studied regarding the prevalence of seropositivity and to assess whether urban horses and horses used in sport are a risk of contamination for animals raised on farms, which represent the vast majority of the horse population in the state of Pará. Furthermore, the aim was also to detect possible associations between seroprevalence and determinants like gender, age, breed category and area. By systematic cluster sampling, sera from a total of 436 horses were collected. Immunofluorescence assay was performed using vero cells infected with R. rickettsii (strain Taiaçu), and R. amblyommii (strain Ac37). Titers equal to and greater than 64 were considered positive. Sera from the positive samples were diluted in two-fold increments until reaching endpoint-titration. Of the 436 sera tested, 37 (8.5%) reacted to R. rickettsii and 85 (19.5%) to R. amblyommii. The R. rickettsii titers ranged from 64 to 512 and the R. amblyommii titers ranged from 64 to 16384. In Brasil Novo the seroprevalence was significantly higher than in Santarém. The reason for this is not known. None of the determinants sex, age, breed category or area had any significant association with an increased seroprevalence. Nor did we see endpoint antibody titers that were higher than what can be expected in a non-endemic area.