Feline heartworm disease: Solving the puzzle - Veterinary Medicine
Medicine Center
DVM Veterinary Medicine Featuring Information from:


Feline heartworm disease: Solving the puzzle
Does that cat you suspect of having asthma really have a heartworm infection? In cats, heartworm disease manifests quite differently than in dogs and has an altered infective cycle. These researchers are seeking to create a definitive model for feline heartworm disease to improve our overall understanding.


Group 1 cats served as controls. As such, they received no chemo-intervention that would affect the subsequent development of the heartworms.

Group 2 cats were treated with ivermectin (Ivomec—Merial) orally at a dose of 150 g/kg every two weeks beginning on study Day 84. We hypothesized that treatment with ivermectin beginning on Day 84 would target the early arriving L5 larvae in the lungs. We also hypothesized that continued administration of ivermectin to cats in this study group would mimic the natural death of developing heartworms in naturally infected cats.

HARD: A proposed name for lesions and disease associated with the death of immature heartworms in the lungs of cats
Group 3 cats were given selamectin (Revolution—Pfizer Animal Health) topically at the label dose of 6 mg/kg monthly beginning 28 days after experimental infection and continuing monthly until the termination of the study. The purpose of the monthly treatment with selamectin was to prevent development of heartworms beyond the L4 stage and eliminate them before their arrival in the lungs. For study purposes, cats in this group would correlate to uninfected control cats and would corroborate the importance of monthly heartworm prevention.

For the study, the cats were group-housed, were not stressed by handling, and were sedated for radiographic examination and other procedures required for data collection.

Data collection procedures

To evaluate the efficacies of treatments given to cats in Group 2 and Group 3 and to confirm the consequences of infection in all groups, numerous procedures were conducted on all cats throughout the study period of 240 days after infection. In addition to daily clinical observations, we collected blood samples for complete blood counts, serum chemistry profiles, and heartworm antigen and feline heartworm antibody detection. We also performed thoracic radiographic examinations and bronchoalveolar lavages at intervals to assess pulmonary pathologic changes and the development of vascular and airway disease. All cats were examined at necropsy about 252 days after experimental infection. All hearts and lungs were removed for gross and histologic evaluation.

Lung lesions (arteries, arterioles, capillaries, alveoli, bronchioles, bronchi, and tracheas) were assigned a lesion score of 0 to 3 with increasing severity. These methods were similar to those used in a previous study.8 Lesion scoring was performed under the guidance of an anatomic pathologist who was blinded to treatment group assignments. Multiple group lesion scores were analyzed by using the nonparametric Kruskal-Wallis test of variance, with specific differences detected by using Levene's test of error variances (p ≤0.05).

Study results

Some cats in Group 1 and Group 2 developed intermittent signs of heartworm infection (lethargy, depression, dyspnea) beginning three months after infection.* Intermittent signs in Group 1 cats continued throughout the second half of the study and were present in some cats up to and at the time of necropsy. This suggested that clinical responses were the result of the death of immature as well as adult heartworms and comparable to what is observed during natural infections. One cat in Group 2 developed acute dyspnea and died 120 days after infection. That cat had radiographic lesions similar to those of other cats in Group 2. Cats in Group 3 remained clinically normal.

Figures 1A & 1B & 2A & 2B
Lesions typical of what was observed in the lungs of cats from the three treatment groups are presented in Figures 1-4. Lesion scores from lungs recovered from cats in each of the treatment groups were demonstrably different. Lung arterial lesions were present in Group 1 and 2 cats but were most severe in the Group 1 cats. However, lesions in the alveoli, bronchioles, and bronchi of Group 2 cats were equally as severe as in Group 1 cats. Alveoli, bronchi, and bronchioles from cats treated with selamectin (Group 3) had significantly lower lesion scores than in cats in Groups 1 and 2 and were generally normal in their appearance.


Click here