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 µg/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.
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.
HARD: A proposed name for lesions and disease associated with the death of immature heartworms in the lungs of cats
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
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).
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.
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
Figures 1A & 1B & 2A & 2B