Efficacy of a combination of fipronil and permethrin in reducing the risk of Dirofilaria immitis transmission to dogs in field conditions

Dirofilaria immitis is a life-threatening mosquito-borne parasite affecting dogs. Prevention mainly relies on macrocyclic lactones (MLs), but D. immitis resistance to MLs and lack of compliance may occur. Multimodal strategies combining chemoprophylaxis and vector control with insecticide and repellent products are often recommended. This study evaluated the efficacy of Frontline Tri-Act® (IVP), a topical combination of fipronil and permethrin, in preventing the transmission of D. immitis by repelling mosquitoes in naturally exposed dogs. This study was conducted in northern Greece, in an area highly endemic for D. immitis and with seasonal (spring to fall) mosquito season. Dogs were housed in outdoor kennels to allow natural vector exposure throughout the whole study. Twenty-four healthy, heartworm-naive, Beagle dogs were randomly allocated 1:1 to an IVP or an untreated control group. Phase 1 was conducted during the mosquito season, when dogs from the IVP group were treated monthly for seven consecutive months. Phase 2 was conducted after the mosquito season, when all dogs were left untreated and diagnosed for D. immitis infection via SNAP® Leish 4Dx and Knott’s test at scheduled intervals. Mosquito traps were also used to verify D. immitis infection rates, mosquito species composition and presence or absence in relation to Phase 1 and Phase 2. At the end of the study, 7 in 12 (58.3%) untreated control dogs were heartworm-positive, while the 12 IVP treated dogs remained negative, indicating 100% preventive efficacy (p < 0.001). Several hundred mosquitoes were trapped during Phase 1 and pooled for D. immitis Polymerase Chain Reaction (PCR) detection, 8 pools in 30 (27%) were positive. No treatment-related adverse events were reported. Frontline Tri-Act® provided 100% protection against D. immitis transmission under natural mosquito challenge. These findings support its inclusion in multimodal strategies for heartworm prevention in endemic areas.