To the Editor: Fleaborne disease is highly prevalent in Laos, mainly attributed to murine typhus (Rickettsia typhi infection), transmitted by Xenopsylla cheopis fleas, but data on other fleaborne diseases are limited (1). We screened blood and cerebrospinal fluid (CSF) from participants in 2 large prospective studies in Laos for Rickettsia spp. using a genus-specific 17-kDa-based Rickettsia real-time quantitative PCR assay, and positive results were confirmed by DNA sequencing (2,3). In samples from >2,500 patients (2,540 blood and 1,112 CSF), we detected 3 cases of sequence-confirmed R. felis infections.

A 50-year-old man, an official in Vientiane City, was admitted to a hospital with fever and headache in October, 2008. HIV infection and cryptococcal meningitis were diagnosed. Treatment with intravenous amphotericin B, then oral fluconazole, was successful; antiretroviral treatment was initiated 1 month after diagnosis. Among a panel of diagnostic PCRs, the CSF sample specimen tested positive for genus-specific 17-kDa-Rickettsia quantitative PCR, but was negative for Orentia tsutsugamushi and R. typhi. DNA sequencing of 434 bp of the 17-kDa gene (Macrogen, Seoul, South Korea) revealed a 100% homology to the R. felis URRWXCal2 strain (Table).

R. felis positivity in CSF is rare; 4 have been reported (3). The combined findings of R. felis infection and severe immunodeficiency in this patient led to a reevaluation of the 2 reported R. felis infections in Laos (2). Before this study, R. felis DNA or culture had not been handled in our facility. The interval between processing positive samples, dedicated separate areas for samples before and after PCR, and the low positivity rate make DNA contamination highly unlikely.

A 39-year-old housewife from Luang Namtha in northern Laos had a history of diabetes mellitus, which had been treated with glibenclamide. On arrival at the hospital in November, 2008, she had fever, headache, myalgia, and an eschar. She was empirically treated with doxycycline (Table). An eschar biopsy specimen was PCR-positive for Rickettsia spp. and O. tsutsugamushi; PCR of buffy coat detected O. tsutsugamushi DNA only (2). Molecular characterization included 17-kDa�?" and sca4- gene sequencing, which both revealed amplicons of 100% identity to the R. felis-URRWXCal2 strain. Serologic evidence for O. tsutsugamushi infection (scrub typhus) included a 4-fold rise in IgM and IgG titers, and IgM and IgG titers against typhus group rickettsiae, spotted fever group rickettsiae, and R. felis (isolate B377 in XTC-2 cells, Australian Rickettsial Reference Laboratory) were negative in admission and convalescent-phase samples (6-day interval) (Table).

A 13-year-old boy from Salavan, in southern Laos, had fever, headache, and nonspecific symptoms in July, 2009. P. falciparum malaria and dengue were diagnosed, both confirmed by PCR (Table). PCR results for the buffy coat specimen were positive for the 17-kDa gene; subsequent sequencing confirmed R. felis with 100% identity to the URRWXCal2 strain. The fever resolved after treatment with antimalarial drugs and ceftriaxone; neither would be expected to be efficacious for R. felis infection.

These data suggest that R. felis occurs in Laos, and is possibly emerging, but whether it results in clinical disease or commonly causes subclinical infection is unknown. The screened cohorts of consecutively enrolled patients with febrile illnesses across 3 diverse geographic regions are representative of etiologic agents of fever across Laos. PCR has previously been used for detection of R. felis and resulted in the discovery of a new R. felis-like organism in fleas in Kenya, Candidatus Rickettsia asemboensis (4). Reports from Southeast Asia suggest that R. felis is not a common cause of febrile illness (1,2), which contrasts with findings in Kenya, where R. felis was found in �%^7% of febrile patients (4,5), and also in �%^3% of afebrile patients (5).

The high R. felis carriage rate in fleas found in Laos (77% overall; 53% in Ctenocephalides felis felis, 89% in C. f. orientis) contrasts strongly with the apparent low incidence of R. felis human infections (6). Among febrile hospitalized patients in Vientiane, 1 case of R. felis infection was serologically diagnosed by using species-specific cross-absorption (1). Seroprevalence studies in the region could elucidate exposure to this pathogen and unmask subclinical infections missed in fever etiology studies.

The 3 patients from Laos described herein had comorbidities associated with variable degrees of immunodeficiency (HIV infection and malaria with cellular and humoral deficiencies, diabetes with functional neutrophil/macrophage impairment) (7,8). R. felis infections have not been associated with immunosuppression, but few investigations of this possible association have been published. Of the 3 patients, the woman and the boy had other vectorborne infections: scrub typhus, transmitted by Leptotrombidium mites, and P. falciparum malaria and dengue, transmitted by Anopheles and Aedes mosquitoes, respectively. Recent reports described R. felis within a great diversity of vectors, including mites in South Korea and Anopheles and Aedes mosquitoes in Africa (9,10).

More work is needed on the role of non-flea vectors in transmission of R. felis and the consequences this may have in terms of mixed infections of diverse vectorborne pathogens. The rare detection of R. felis in patients, combined with high flea carriage rates, unusual signs and symptoms linked to immunodeficiencies or multiple infections, and reports from Africa describing R. felis in asymptomatic patients, underscore the need for further investigations into the organism�?(tm)s natural history and its uncertain role as a pathogen.