The biology and epidemiology of the filarial parasites affecting dogs and cats are summarized in Table 5.1. The most important and geographically widespread filarial disease in the dog and cat is caused by D. immitis and is known as heartworm disease (HWD). D. repens is also frequently observed in pet animals and represents a more common zoonotic risk than D. immitis. Dipetalonema reconditum is another filarial nematode commonly found in dogs, but having minimal clinical significance.