April is Heartworm Awareness Month. Is your pet heartworm free? Dogs are considered the definitive host for heartworms (Dirofilaria immitis). However, heartworms may infect more than 30 species of animals (e.g., coyotes, foxes, wolves and other wild canids, domestic cats and wild felids, ferrets, sea lions, etc.) Feline heartworms are more deadly and difficult to treat.