Introduction:Mansonella ozzardi is a widely distributed fi laria worm in the Amazon region. This study aimed to determine the prevalence of M. ozzardi infection in riverine communities of Lábrea municipality, Amazonas State, Brazil. Methods: A diagnostic blood fi ltration method in a polycarbonate membrane was used. Results: M. ozzardi was found in 50.3% of the sample, with the highest prevalence in farmers/fi shermen (69.4%; χ 2 = -19.14, p<0.001). The prevalence was higher in longerterm residents (≥11 years; 60.2%). Conclusions: M. ozzardi infection rates are high near the Purus River, much greater than those previously reported based on diagnosis using thick blood smears.Keywords: Mansonelliasis. Mansonella ozzardi. Brazilian Amazon.Mansonella ozzardi is a widely distributed fi laria worm that parasitizes humans in Amazonas State, Brazil, with high prevalences detected in residents of some regions of the Solimões, Negro, and Purus Rivers (1) (2) (3) (4) (5) . Simuliidae (blackfl ies) are the only vector of M. ozzardi in Brazil (6) (7) .An endemic region for M. ozzardi in Brazil is in the municipality of Lábrea, in the Purus River region, Amazonas, where it has been detected since the fi rst survey in the 1950s (1) . Subsequent studies conducted in the 1970s and 1980s confi rmed the wide distribution of M. ozzardi in this region (8) (9) . More recently, the prevalence of the infection had reportedly increased in riverine communities located near the Purus and Ituxi Rivers (2) (3) (4) .In the Purus River region, only Adami et al. (10) utilized the concentration method by Knott to diagnose mansonelliasis. Therefore, the present study aimed to determine the prevalence of M. ozzardi infection in riverine communities of the Lábrea municipality using a diagnostic blood fi ltration method in a polycarbonate membrane, which is more sensitive than thick blood smears (11) .This study was conducted in the riverside communities of the Purus River (Cassianã, Bacural, Jucuri, Buraco, Santa Rosa, Jurucuá and Samaúma), located approximately 200km from the municipality of Lábrea communities (S: 07º15' 34" and W: 64º47'59"), State of Amazonas, Brazil (Figure 1). The sample consisted of volunteer men, women, and children aged 5-60 years selected using convenience sampling. Data and blood samples were collected after signing the free and informed consent form by the individual or the legal guardian. Of the estimated 350 inhabitants in the included communities, 171 (48.6%) inhabitants were present at the time of the study and agreed to participate.To estimate the prevalence of Mansonella ozzardi, blood was filtrated in a polycarbonate membrane (12) . First, 1mL venous blood was collected from each individual, diluted in 10mL 0.9% saline, and fi ltered in a polycarbonate membrane (Nucleopore Corporation, Pleasanton, CA, USA) with a 3µm pore diameter. Then, the membranes were removed from the fi lters and mounted on microscope slides, fi xed in methanol, stained with a Panoptic kit ® , and examined under an optical micros...