To the Editor: Ehrlichia chaffeensis is an obligate intracellular bacterium in the family Anaplasmataceae. It is considered an emerging pathogen in the United States because it is the causative agent of human monocytotropic ehrlichiosis (1), a flu-like illness that can progress to severe multisystem disease and has a 2.7% case-fatality rate (2).

In Central and South America, human cases of ehrlichiosis with compatible serologic evidence have been reported in Venezuela, Brazil, Mexico, and Chile, although the bacterium has not been isolated (3). Recently, molecular evidence of E. chaffeensis infection was reported for a symptomatic 9-year-old child in Venezuela (4). In Argentina, antibodies reactive to E. chaffeensis, or an antigenically related Ehrlichia species, were detected in human serum samples during a serologic survey in Jujuy Province, where fatal cases of febrile illness were reported (5).

Figure

Figure. Agarose gel electrophoresis of PCR products amplified with Ehrlichia chaffeensis (Ec) variable-length PCR target primers. Rc, Rickettsia conorii (negative control). The sources of DNA templates used for amplification are Amblyomma parvum...

During November–December 2006, we collected ticks by dragging the vegetation and by examining mammal hosts, including humans, in semiarid southern Chaco, Argentina, Moreno Department, Province of Santiago del Estero. Ticks, kept in 70% alcohol, were identified as Amblyomma parvum (n = 200), A. tigrinum (n = 26), and A. pseudoconcolor (n = 13). A sample of 70 A. parvum and 1 A. tigrinum ticks collected on domestic ruminants and canids were subjected to PCR and reverse line blot hybridization by using the TBD-RLB membrane (Isogen Life Science, Maarssen, the Netherlands) (6) to look for Anaplasma and Ehrlichia spp. DNA was extracted from individual ticks by using the DNeasy Blood and Tissue kit (QIAGEN Valencia, CA, USA); several negative controls (distilled water) for both DNA extraction and PCRs were run alongside the samples in random order throughout the experiments. Primers Ehr-R (5′-CGGGATCCCCAGTTTGCCGGGACTTYTTCt-3′) (6) and Ehr-Fint (5′-GGCTCAGAACGAACGCTG-3′; Inst. Biotecnologia, Instituto Nacional de Tecnología Agropecuaria, unpub. data) were used to amplify a 500-bp fragment of the 16S gene of Anaplasma/Ehrlichia spp. PCR products were analyzed by reverse line blot hybridization, and 11.3% (95% confidence interval [CI] = 4.9–21.0) showed a positive signal to the specific E. chaffeensis probe: 8 A. parvum ticks collected from a dog (n = 1), a fox (Lycalopex gymnocercus, n = 1), goats (n = 2), and cattle (n = 4). No signals to other probes present in the membrane were recorded (A. phagocytophylum, A. marginale, A. centrale, A. ovis, E. ruminatium, E. sp. Omatjenne, E. canis). Further sequence analysis of 16S fragments confirmed the result, with our sequences showing 99.6% identity with the corresponding fragment of the E. chaffeensis strain Arkansas 16S gene (GenBank accession no. EU826516). To better characterize the positives samples, we then amplified variable-length PCR target (VLPT) of E. chaffeensis (7). PCR products of variable length were detected by conventional gel electrophoresis analysis (Figure). Distilled water and R. conorii DNA were used as negative controls, and E. chaffeensis DNA as the positive control. The finding was confirmed by sequence analysis (GenBank accession nos. EU826517 and EU826518)

In view of these positive results, another set of 108 specimens was tested by E. chaffeensis VLPT PCR: all the ticks collected on humans (80 A. parvum, 1 A. pseudoconcolor, and 4 A. tigrinum), 18 host-seeking A. parvum ticks, and 5 A. parvum ticks collected on armadillos of the genera Tolypeutes and Chaetophractus. E. chaffeensis was detected in A. parvum ticks only: 5 from humans (6.2%; 95% CI 2.1–14.0; Figure, panel A) and 3 fromhost-seeking ticks (16.7%; 95% CI 3.6–41.4). In total, E. chaffeensis was detected in 9.2% (95% CI 5.4–14.6) of tested A. parvum ticks in the study area. Of the 16 positive A. parvum, 5 were infesting humans.

Little is known about E. chaffeensis epidemiology in South America. In Brazil, wild marsh deer (Blastocerus dichotomus) are suspected to be its natural reservoir, but the tick involved in the transmission cycle is not known (8). In North America, E. chaffeensis sp. is maintained principally by the lone-star tick, A. americanum, and the white-tailed deer (Odocoileus virginianus) (2). However, the possibility of transmission by different ticks and infection among other hosts has been reported; specific antibodies to E. chaffeensis were detected in domestic and wild canids and goats (2), and recently experimental infection was demonstrated in cattle (9). We did find E. chaffeensis organisms in ticks collected on both wild and domestic animals, but the possible role of different mammals as reservoir hosts deserves further investigation. Moreover, the finding of polymorphic VLPT gene fragments in our sample indicates the circulation of E. chaffeensis genetic variants in the study area. VLPT repetitive sequences vary among isolates (7); however, it is not known whether genetic variants differ in pathogenicity or are correlated with geographic distribution or host range.

All positive ticks were A. parvum, a common tick of domestic animals that frequently feeds on humans in Argentina and Brazil and is considered a potential vector of zoonoses (10). In our study area, this tick species was by far the most abundant on humans (93.2%), and our results suggest its potential role as a vector of E. chaffeensis.

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