BackgroundCats can be infected byDirofilaria immitis, the causative agent of heartworm disease, characterized by respiratory signs, airway hyperreactivity, remodelling and inflammation. Allergy is a multifactorial pathology, and the role of a number of helminth parasites in the development of allergies in humans and other species has been demonstrated in many studies. The aim of the present study was to verify whether cats seropositive forD. immitispresent hypersensitivity to some environmental allergens.MethodsBlood samples were collected from 120 cats and tested for the presence of specific immunoglobulin G antibodies againstD. immitisand for hypersensitivity to 20 allergens, using commercial allergen test kits.ResultsOf the 120 cats tested, 72 (60.0%) were seropositive for anti-D. immitisIgG and 55 (45.8%) showed clinical signs of heartworm disease of a respiratory nature. The results of testing with the allergen kits showed that 50.8% of cats were seropositive for ≥ 1 allergens, with the most common allergens beingDermatophagoides farinae(25.8%),Dermatophagoides pteronyssinus(20.0%),Malassezia(17.5%) andCtenocephalides felis(14.2%). The prevalence of allergies was significantly higher—by almost threefold—in cats seropositive forD. immitis(68.1% vs. 25%). There were no significant differences between the prevalence of allergic cats and presence/absence of symptoms, and the results confirmed that symptoms were not a decisive factor for the presence of allergies. The risk for developing allergies was 6.3-fold higher in cats seropositive forD. immitisthan in cats that were seronegative, confirming that seropositivity forD. immitisis a risk factor.ConclusionsCats with confirmed heartworm can develop serious respiratory signs, potentially leading to progression to permanent lung injury and predisposing cats to hyperresponsive airway disease. Previous studies have shown that seropositivity forD. immitisandWolbachiais related to the presence of bronchoconstriction and bronchospasm in the affected cat. The results support the suspicion that contact withD. immitismay be a risk factor for the presence of allergies.Graphical abstract