Emergence of Dirofilaria immitis in humid coastal zones: Epidemiological predictors and molecular characterization

Canine heartworm, a parasitic zoonosis caused by Dirofilaria immitis, is expanding geographically. This study investigated the prevalence, associated risk factors, and molecular characteristics of D. immitis in dogs from Ilhéus, Bahia, a coastal tourist region in northeastern Brazil previously considered non-endemic. In a cross-sectional study with 481 domestic dogs, blood samples were tested for adult D. immitis antigens (SNAP® 4Dx® Plus) and microfilariae (modified Knott's test). Risk factors were analyzed using logistic regression and spatial analysis, including the association with the Normalized Difference Vegetation Index (NDVI). Positive samples underwent polymerase chain reaction (PCR), sequencing, and phylogenetic analysis targeting the cox1 gene. The overall prevalence of D. immitis, defined as positive in at least one test, was 19.6% (84/428; 16.0%-23.7%). Seroprevalence was 16.0% (77/481; 12.7%-19.3%), while microfilaremia was 14.3% (61/428; 11.2%-17.9%). Most infected dogs were asymptomatic (71.4%). Larger dogs, those kept outdoors, and those exposed to environmental risk factors (e.g., lack of doghouse or standing water) showed significantly higher odds of seropositivity. The presence of dense vegetation (NDVI median = 0.51) and a rural environment were also associated with increased risk. Spatial analysis revealed a cluster of cases in the southern area of the municipality. Phylogenetic analysis indicated high genetic similarity between local isolates and those from other regions worldwide. These findings confirm the circulation of D. immitis in Ilhéus and highlight the role of environmental and management factors in transmission. The high proportion of asymptomatic carriers reinforces the need for regional surveillance and prevention strategies.