A dog can have tens, or even hundreds, of heartworms. Cats normally only have a few, but even one heartworm can be deadly to a cat. Heartworms can grow to be 4 to 6 inches long, and are skinny and white. The larvae go through several stages before they mature into adult worms. One of these stages must be passed inside of a mosquito in order for the maturation process to complete.
Dogs are normally the host population for heartworm infection.
They are the animals in which heartworms live the longest and produce the most larvae. Because there are so few heartworms inside of cats, the chances of them reproducing and generating larvae is extremely unlikely. The spread of heartworm disease occurs like this. A dog that has heartworms that have reproduced will have larvae in its bloodstream for a period of time. If a mosquito bites the dog during this time, the mosquito will pick up some of the larvae.