Human exposure to Dirofilaria immitis following a canine heartworm disease elimination program in Linosa island (Sicily, Italy)

The recent reports of Dirofilaria immitis causing canine heartworm disease (HWD) in highly endemic foci of Italy suggested the increasing risk for dogs and humans. In southernmost areas, such as Linosa island (Sicily), previous studies showed D. immitis infection in 58.9 % of dogs and a parasitic exposure in 7.9 % of islanders. These findings prompted a targeted elimination program, which led to HWD elimination by 2022. Hence, the present study aimed to assessantibody response and kinetics among Linosa residents one year after the implementation of canine HWD elimination program. In 2023, blood samples were collected from 89 residents and 39 dogs. Participants completed a questionnaire with demographic and clinical data, including symptoms and relevant risk factors. Human sera were tested for D. immitis exposure using an in-house ELISA. Dog samples were analyzed using Knott test and a commercial ELISA rapid test (SNAP 4DX Plus, IDEXX). Additionally, previously infected dogs underwent echocardiographic evaluation. Genomic DNA was extracted from all human and dog samples and tested by PCR using Dirofilaria spp. and Wolbachia-specific protocols. Serological analyses revealed that 25.8 % of the volunteers were seropositive for D. immitis, indicating an increase of seropositivity compared to 2020 (8 %). None of the human and dog samples tested positive for Dirofilaria spp. or Wolbachia by PCR. Despite the successful elimination of D. immitis in dogs, the increased seroprevalence in humans indicates that the level of human exposure remains stable, even in the absence of canine infection. However, new seropositive cases may reflect exposures that had not yet seroconverted at the time of initial testing in 2020, or exposures that occurred outside the island. Long-term studies on antibody persistence and vector surveillance are essential to improve elimination efforts.