Computed Tomography Assessment of the Bronchial Lumen–Vertebral Body and Pulmonary Artery–Vertebral Body Relationships in Cats Naturally Infected with Immature Dirofilaria immitis

Abstract

Feline heartworm disease, caused by Dirofilaria immitis, is often underdiagnosed, particularly during its larval stage, known as Heartworm-Associated Respiratory Disease, (HARD). This study aimed to quantify CT-detectable pulmonary changes in naturally infected cats. Computed tomography (CT) was performed in 38 cats: Group A (n = 30, symptomatic seropositive) and Group B (n = 8, asymptomatic seronegative). Bronchial and pulmonary artery diameters were indexed to the sixth thoracic vertebral body (T6) to calculate bronchial-to-vertebral body (B/VB) and pulmonary artery-to-vertebral body (A/VB) ratios across all lung lobes. Group A cats showed significantly higher B/VB ratios in the left cranial and right middle lobes compared with Group B (p < 0.05), with increases of 42.0% and 47.5%, respectively. Conversely, A/VB ratios did not differ significantly between groups (p > 0.05), indicating the absence of relevant vascular remodelling. Intra-operator and inter-observer reliability were excellent (intraclass correlation coefficients > 0.85). These findings suggest that bronchial dilation in the absence of arterial enlargement represents a key tomographic feature of larval D. immitis infection. Quantitative CT ratios, particularly the B/VB ratio, provide objective imaging markers for the diagnosis of HARD in clinical practice, and may assist in differentiating this condition from other feline respiratory diseases in endemic areas.