attained-the caudal space that finishes in a hook shape (Fig. 2b). Fig. 2 Comparison of typical M. ozzardi microfilaria and the atypical one found in blood samples from human volunteers in Vila Antimary. a Characteristic M. ozzardi microfilaria showing cephalic space and b detail of the tail. c Anterior region of atypical microfilaria with two paired nuclei at the anterior region and d detail of tail and caudal space (scale bars = 7 m) However, 12.5% of the volunteers infected and enrolled in agricultural activities from Vila Antimary presented besides M. ozzardi, an unusual form of microfilaria whose main difference could be noted at the anterior region (Fig. 2c). These forms have two paired nuclei after the cephalic space, followed by a single one, and afterwards begin the proceeding characteristic column of nucleus. Meanwhile, no differences could be noted at the caudal region; that is, they were very similar to the pattern of nucleation with a column of nucleus close to the parasite cuticle that elongate as long as they reach the tail tip (Fig. 2d). Analysis of the material obtained through light microscopy study from the skin snips revealed no infections with O. volvulus (data not shown). Discussion Epidemiological studies conducted by our group in the region started in 2006, and the goal was to show distribution and occurrence of filarial infections in communities along Antimary, Acre, and Purus Rivers. This is the first report of M. ozzardi in this specific region. Up to now, Acre State presented the lowest rates as demonstrated by our results in Mapinguari and Porto Acre, but the prevalence increased along the neighboring of Amazonas State until to reach higher levels at Kamikua Village, an Indian community along Purus River. The suspicious of an unidentified species of microfilaria transmitted by blackflies (Diptera, Simuliidae) in the Amazonia has already been noticed (Shelley 2002). Since the sample with atypical microfilaria was lost, our efforts in the present survey were concentrated in Vila Antimary at Acre River, where that previous detection was reported. A preliminary trial of infected or uninfected material was performed, and the positive material was submitted to an accurate morphometric study where it was possible to detect this unusual microfilaria. This larval stage in the blood material was obtained with the concentration Knott's Method and differed from all microfilarias previously found. Studies concerned with morphological differentiation of M. ozzardi and O. volvulus microfilaria has been previously described (Post et al. 2003). Both filarian species present unsheated microfilaria, and there is a general concern that M. ozzardi microfilarias are smaller than those of O. volvulus, but the ranges frequently overlap when both are found in skin samples which were a reason of misleading diagnosis in the past (Moraes et al. 1983). In the present work, we reported M. ozzardi infections, and only the samples from Kamikua Village, Vila Antimary, and Porto Acre were analyzed through morphometry. The values obtained are in accord with those previously reported for Haitian and Colombian M. ozzardi (Kozek and Raccurt 1983) prepared through Knott's Method but are slightly different from those achieved