Rickettsia parkeri InfectionRickettsia parkeri is as an emerging agent of tick-borne rickettsiosis in the Americas. More than 60 years after its initial isolation from tick vectors it was recognized as a human pathogen. Dogs have been characterized as sentinels for human rickettsiosis in rural areas in Brazil, e.g., by sharing the same tick species infesting humans and dogs (Labruna et al., 2007). PathogenThe genus Rickettsia is included in the bacterial tribe Rickettsiae, family Rickettsiaceae, and order Rickettsiales. The genus Rickettsia includes many species of bacteria associated with important and severe human or canine disease, including those in the spotted fever group and in the typhus group. The typhus group (TG) is composed of R. prowazekii and R. typhi, which are associated with lice and fleas, respectively; the spotted fever group (SFG), which includes more than 20 valid species, is mostly associated with ticks. Other species, such as R. bellii and R. canadensis, both associated with ticks, have been considered a distinct group sharing antigenic, genetic and ecological characters both TG and SFG rickettsiae (Parola et al., 2005; Roux et al., 1997; Stothard et al., 1994). R. parkeri has first been isolated in 1937 as a distinct rickettsial strain from Gulf Coast ticks (Amblyomma maculatum) by Ralph Robinson Parker showing low-grade fever and periorchitis in the animal experiment (here: male guinea pigs). In 1965, the “maculatum agent” was named R. parkeri and described as a small, rod-shaped bacterium, with an average size of 1.6 x 0.5 µm, found in the nucleus and cytoplasm of infected cells (Lackman et al., 1965). Until 2002 the agent most often was relegated to the group of ‘nonpathogenic’ SFG rickettsiae, found in ticks in the United States, when the first confirmed infection with R. parkeri in a human was identified in Virginia (Paddock et al., 2004). DistributionR. parkeri has been isolated from Gulf Coast ticks (A. maculatum) in Alabama, Georgia, Mississippi, and Texas (Parker et al., 1939; Parker, 1940; Philip and White, 1955; Philip et al., 1978). It further could be detected in Gulf Coast ticks collected again from Georgia, Florida, Kentucky, Mississippi, Oklahoma and South Carolina (Sumner et al., 2007). Apart from the United States, R. parkeri has also been described as an emerging human pathogen in Uruguay (Venzal et al., 2004) and was furthermore detected in ticks and dogs (seroconversion) in Brazil (Silveira et al., 2007; Labruna et al., 2007). As two of the vector ticks, A. maculatum and A. triste, are established in at least 11 other Latin American countries (Guglielmone et al., 2003), the distribution of R. parkeri in the Americas is likely continental (Pacheco et al., 2006). Disease transmissionThe agent has first been isolated in the Gulf Coast tick A. maculatum in 1937 (Parker et al., 1939). Meanwhile it has also been isolated from A. triste in Uruguay (Pacheco et al., 2006) and Brazil (Silveira et al., 2007). Experimental infection of A. cajennense ticks with R. parkeri (Sangioni et al., 2005) and transstadial and transovarial transmission of R. parkeri in A. americanum, the Lone Star tick, could also be proven in the lab (Goddard, 2003), so that the distribution of R. parkeri might even be larger than the documented distribution so far. The agent is transmitted via the infected tick during feeding. DiagnosisSerological assays as the immunofluorescence assay (IFA) are used for the detection of antibodies in man as well as in dogs. In studies, an at least four-fold higher titer compared to the other used Rickettsia antigens is taken indicative for an infection with the respective species or a very closely related genotype. Rickettsia isolation in cell culture can also be used as direct evidence of the pathogen, but is usually not used in routine diagnosis due to high laboratory standard and time needed, e.g. Clinical signsClinical signs in humans have been described with a febrile illness associated with headache, myalgia, a maculopapular rash, and multiple eschars (Paddock et al., 2004). It is assumed that the infection with R. parkeri has often been misdiagnosed as Rocky Mountain Spotted Fever (RMSF). Several compelling lines of evidence suggest that infections with R. parkeri, and probably one or more other tick-borne SFG rickettsiae, are responsible for at least some of the approximately 400-1,100 cases of RMSF reported each year in the United States. These include according to Paddock (2005): (1) cases of eschar-associated spotted fevers for which species-specific confirmatory data are lacking; (2) serologic evidence of subclinical or mild infections with spotted fever group rickettsiae in tick-exposed patients; (3) evidence of mild to severe disease in various rodents inoculated with certain ‘nonpathogenic’ SFG rickettsiae;and (4) the precedent recognition of multiple tick-borne rickettsial diseases in Europe, Africa, and Australia (Parola et al., 2005; Raoult, 2004). Dogs have been shown to be infected with R. parkeri in serosurveys with prevalences between 0.65 and 1.83% in the Monte Negro region in Brazil (rural and urban) (Labruna et al., 2007) and 25.7% in Southern Brazil (Saito et al., 2008). Dogs have especially been described as sentinels for special areas where the transmitting tick species is found to infest dogs as well as humans (rural areas in Monte Negro region, Brazil; Labruna et al., 2007). TreatmentTreatment with antibiotics can control the disease. As with other diseases transmitted by fleas or ticks, ectoparasite control is the basis of prevention. Products which repel and kill ticks and fleas such as those containing permethrins are the appropriate means of choice for dogs. The sentinel position which has been attributed to dogs makes an ectoparasite control even more important. References
|
ServicesLinksLast changed: 7.09.2010
|
Definitions of parasitologic terms