Once considered confined to the southern US where winters are mild, heartworms now infest dogs and some outdoor cats in all parts of the country.
Many veterinary clinics display a preserved heart infected with heartworm in a jar.
But the effort and the money are well-spent to keep a loved pet from the discomfort and debilitation of the infliction. Like most parasites, the heartworm goes through several life stages before emergence as an adult and needs at least two hosts to complete the cycle. The mosquito serves as the host for the larval stage of the worm, called microfilariae. The mosquito ingests the larva when it bites an infected dog and deposits its cargo in an uninfected dog when seeking another blood meal. The microfilariae burrow into the dog, and for three or four months, undergo several changes to reach adult form.