Serological evidence of human infection with rickettsial strain Bar29 in Catalonia, northeastern Spain

Abstract

Mediterranean spotted fever (MSF) is an endemic disease in Catalonia, northeastern Spain. The causative agent is Rickettsia conorii, which is transmitted by the dog tick Rhipicephalus sanguineus in the Mediterranean area [1, 2]. However, it is strongly suspected that another spotted fever group (SFG) rickettsial strain could be implicated in some cases of MSF diagnosed in Catalonia [3, 4]. In 1990, Mtu1 and Mtu5 were isolated from Rhipicephalus turanicus ticks collected in the south of France. Both were new serotypes related to the SFG rickettsiae [5]. Mtu1 was named Rickettsia massiliae and proposed as a new SFG Rickettsia species [6]. Strains related to R. massiliae have been identified throughout the world [7]. In Catalonia, a new SFG rickettsial strain (Bar29) was isolated by our group in 1996 from R. sanguineus; it is identical to the Mtu5 strain and closely related to R. massiliae [3]. R. massiliae Bar29 belongs to the R. massiliae group of SFG that includes R. massiliae, Rickettsia aeschlimannii, Rickettsia montanensis and Rickettsia rhipicephali [7]. A previous study we conducted using sera from Catalonian patients suggested R. massiliae Bar29 could be implicated in cases of MSF diagnosed in our region [4]. Currently, R. massiliae and R. aeschlimannii are recognized as human pathogens [7, 8]. The aim of the study presented here was to determine the seroprevalence of R. massiliae Bar29 in a representative population of Catalonia and to identify possible risk factors for infection. The study was performed in a predominantly urban area near the coast in northeastern Spain. Between September 1993 and January 1994, 211 subjects who had received attention at Sabadell’s Hospital were chosen to participate in a study searching for antibodies to R. conorii and Borrelia burgdorferi. The sample size had been estimated taking into account overall prevalences of 11 and 3% of antibodies to R. conorii and B. burgdorferi, respectively (given an alpha error of 0.05). The sample included adults undergoing minor surgery and children who visited the Pediatric Emergency Service. Taking into account the actual population, the study population was stratified by age and residential area. After informed consent was obtained, a serum sample was collected. Age, sex, demographic area, occupation, contact with dogs, contact with wild animals, contact with farm animals, and history of MSF were surveyed. One milliliter of blood was collected in tubes containing heparin. The sample of heparinized blood was sedimented and the supernatant (0.5 ml) was collected and stored at −80°C. Human sera were evaluated using an indirect immunofluorescence assay (IFA) for IgG antibodies [9]. Antigen was obtained from the R. massiliae Bar29 strain previously isolated from ticks and cultured in our laboratory [3]. Titers ≥1/40 were considered positive. Statistical analysis was performed using the chi-square test, Fisher's Eur J Clin Microbiol Infect Dis (2006) 25:541–543 DOI 10.1007/s10096-006-0176-1