heartworm 
noun
A parasitic nematode worm which infests the hearts of dogs and other animals.
- Dirofilaria immitis, class Phasmida.
- Cats can tolerate a few adult heartworms for several months, but when the heartworms grow in size or when the number multiplies, clinical signs begin to appear.
- Some illnesses that are common among house pets - such as distemper, canine parvovirus, and heartworms - can't be transmitted to humans.