Rickettsia felis Infection in Febrile Patients, Western Kenya, 2007–2010

To determine previous exposure and incidence of rickettsial infections in western Kenya during 2007–2010, we conducted hospital-based surveillance. Antibodies against rickettsiae were detected in 57.4% of previously collected serum samples. In a 2008–2010 prospective study, Rickettsia felis DNA was 2.2× more likely to be detected in febrile than in afebrile persons.

R ickettsioses are a major human health problem in many parts of the world, including sub-Saharan Africa (1,2). Awareness of rickettsiae as causes of public health problems has been increasing; several novel or emerging diseases caused by these pathogens have been recognized. In Kenya, recent reports have documented human infections with Rickettsia conorii (3,4) and R. felis (5) and tick infection with R. africae (6). Our objectives were to assess previous human exposure to rickettsiae and to determine the incidence of rickettsial infections among febrile and afebrile persons in western Kenya.

The Study
The study was conducted among patients visiting the Lwak Mission Hospital, a rural health care facility in western Kenya in the Asembo area, Rarieda District, in western Kenya (Figure 1). Lwak Mission Hospital serves as the fi eld clinic for population-based infectious disease surveillance conducted by the Kenya Medical Research Institute and the US Centers for Disease Control and Prevention as described (7). The study was conducted with ethical approval from these institutions (protocol nos. 932 and 4566, respectively). To assess previous exposure to rickettsiae, we examined a randomly selected subset of 357 serum specimens collected January 2007 through October 2008 from patients participating in population-based infectious disease surveillance. Samples were screened at a dilution of 1:128 for IgG against spotted fever group (SFG) and typhus group (TG) rickettsiae by using an indirect fl uorescence antibody assay (Fuller Laboratories, Fullerton, CA, USA).
Blood specimens were collected from the fi rst 2 outpatients ≈5 years of age and the fi rst 2 outpatients <5 years of age seen each day for acute febrile illness (recorded axillary temperature >38.0°C without an obvious cause, defi ned as cough, diffi culty breathing, chest pain, signs of meningitis, or bloody diarrhea) from November 2008 through February 2010. A positive malaria smear was not an exclusion criterion. During the same period, blood specimens were also collected from controls: a group of outpatients who did not have febrile, respiratory, or diarrheal illness during the preceding 2 weeks and asymptomatic persons who accompanied patients to the clinic.
To detect rickettsial DNA, we performed 3 quantitative PCRs: a genus-specifi c assay selective for a 74-bp segment of the citrate synthase (gltA) gene, a group-specifi c assay that detects a 128-bp segment of the outer membrane protein (ompB) gene for tick-borne rickettsiae, and a speciesspecifi c assay that detects a 129-bp segment of the ompB gene for R. felis (5,8). To identify which Rickettsia sp. was present in the positive specimens, we PCR amplifi ed and sequenced segments of 4 rickettsial genes-17-kDa, ompB, and 2 R. felis plasmid genes (pRF and pRFδ)-by using primers and procedures as described (5).
A total of 699 febrile patients who sought care at Lwak Mission Hospital from November 2008 through February 2010 and 236 afebrile persons enrolled during this same period were tested for rickettsiae (Table 1). Overall, 50 (7.2%, 95% CI 5.4%-9.3%) of the febrile patients and 8 (3.4%, 95% CI 1.5%-6.6%) of the afebrile persons had positive rickettsiae results according to the genus-specifi c gltA assay. Univariate logistic regression indicated that febrile patients were more likely than afebrile persons to have positive PCR results (odds ratio 2.20, 95% CI 1.03-4.70, p = 0.04). According to the ompB assay, all specimens tested were negative for tick-borne rickettsiae. BLAST searches (www.ncbi.nlm.nih.gov/blast/Blast.cgi) for homologous sequences determined that the segments amplifi ed from the 3 genes had 100% nt homology with R. felis URRXWCal2 (Table 2).
In addition to fever, the most common clinical manifestations among patients with positive PCR results for rickettttsiae were headaches (100%), chills (93.8%), muscle aches (68.8%), and joint pains (68.8%). Rash was reported for 4.4% of rickettsiae-positive patients. Among febrile patients, no statistically signifi cant associations were found between specifi c signs or symptoms and positive PCR results for rickettsiae (p>0.05). Samples from all febrile patients were Giemsa stained and examined; malaria parasites were detected in 79.2% and 73.4% of samples from patients who had PCR-positive and PCRnegative results for rickettsiae, respectively.

Conclusions
The 2007-2008 serosurvey found prevalence of IgG against rickettsiae to be high. Other countries in Africa have reported similar (28%-58%) seroprevalence (9,10). The fi nding that prevalence of IgG to SFG rickettsiae increased with age can, in part, be explained by cumulative exposure to the pathogen and lifelong persistence of IgG. The high number of patients seropositive for SFG and TG rickettsiae may be attributed to cross-reactivity between SFG and TG rickettsial antigens (11), although results from studies in animal models show that cross-reactivity between SFG and TG rickettsiae is not consistent (12). Znazen et al. (13) speculate that antibodies against R. felis may be the major cause of cross-reactions to TG-rickettsiae-specifi c and SFG-rickettsiae-specifi c antigens.
The identifi cation of R. felis DNA sequences in febrile patients confi rms the previous fi nding of this pathogen among febrile patients in Kenya (5