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

## TL;DR

This study uses CT scans to detect early signs of heartworm disease in cats by measuring airway changes that appear before blood vessel changes.

## Contribution

The study introduces CT-based bronchial-to-vertebral body ratios as a novel diagnostic tool for early-stage feline heartworm disease.

## Key findings

- Infected cats showed 42-47% higher bronchial-to-vertebral body ratios in specific lung lobes.
- Pulmonary artery measurements did not differ significantly between infected and healthy cats.
- CT ratios demonstrated excellent reliability for consistent diagnosis.

## Abstract

Feline heartworm disease, caused by the parasite Dirofilaria immitis, is a serious condition that often goes undetected in cats. Unlike dogs, which typically develop infections with adult worms in the heart, cats more frequently experience a larval form of the disease known as Heartworm-Associated Respiratory Disease (HARD), which primarily affects the lungs. Because affected cats often show vague signs such as coughing or breathing difficulty, diagnosing can be challenging. In this study, we used advanced imaging with computed tomography (CT) to examine the lungs of naturally infected cats. By comparing CT measurements from infected cats with those from healthy controls, we found that infected cats showed clear widening of the airways (bronchi), even when the pulmonary arteries appeared normal. To ensure reliable comparisons, lung measurements were standardized using the cat’s spine as a stable reference point. These findings show that specific CT-based lung measurements can help veterinarians detect early-stage heartworm infection in cats more accurately. This approach may improve diagnosis in areas where the parasite is common and support earlier treatment and prevention, ultimately benefiting feline health and welfare.

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.