The mosquito then passes the infection to other animals they bite. Outdoor dogs are more likely to be infected with heartworms than those that stay inside. However, mosquitoes can slip through cracks around doors and in screens and every dog and cat may be at risk, whether indoors or out. There are differences in the heartworm disease in cats and dogs. Dogs may have as many as 100 worms that can live in the heart and vessels for up to seven years or as long as the dog can live with the worm burden.
Cats usually have only two or three worms and the worms live only up to two years.
The general signs of heartwormscoughing, difficulty breathing, lethargy, and weight lossare common to cats and dogs, but the course of the disease is very different. In untreated dogs, heartworms will increasingly compromise the circulatory system over time and will eventually lead to death. In cats the signs of heartworms can be vague and can mimic other feline diseases, making diagnosis difficult. Many cats are able to clear the infection before the worms reach the heart. Other cats can appear healthy and then suddenly die from heartworms.