Unveiling filariid infections in dogs living in the Western Amazon, Brazil.

Dirofilaria immitis and Dirofilaria repens, zoonotic vector-borne helminths of the family Onchocercidae, primarily cause cardiopulmonary and subcutaneous dirofilariosis in dogs, respectively. In the Western Amazon region of Brazil, information on the prevalence of Dirofilaria spp. remains limited, despite environmental conditions such as deforestation, high humidity, and a growing population of stray dogs and cats that may favor vector expansion and parasite transmission. However, the underuse of molecular tools may contribute to both low detection and misidentification of filariid species. Thus, this study investigated the prevalence of filariids in dogs from Rio Branco, Acre, Western Amazon, using a multi-testing diagnostic approach, including a mean of microscopic and molecular tools. A total of 444 owned dogs were screened for microfilariae (mff) using modified Knott's and Woo tests. A total of 59 (13.29%) dogs were positive for mff at the microscopic examination, with 47 and 23 dogs positive by the modified Knott's test and the Woo test, respectively. All microfilaremia-positive samples were subsequently analysed by duplex qPCR for the simultaneous detection of D. immitis and D. repens. Samples negative by qPCR were further examined by conventional PCR targeting the 12S rRNA gene, followed by Sanger sequencing. Molecular analyses confirmed Acanthocheilonema reconditum in 44.10% of microfilaremic dogs and D. immitis in 8.50%. These findings demonstrate the circulation of filarial species with zoonotic potential in the Western Amazon and underscore the need for strengthened epidemiological surveillance. Improved monitoring may support early detection and the implementation of preventive strategies in areas previously considered non-endemic.