Heartworm is a particularly nasty parasite which is spread from host to host by mosquitoes. Dog heartworms are a kind of filaris (a long thin worm), but the condition can affect cats, ferrets, coyotes, wolves, sea lions and even humans. This worm was named as such due to its habit in the final stage of its reproductive cycle to live in the heart of its host.
Heartworm preventative measures include the drugs Ivermectin, Milbemycin, and Selamectin, and in all cases it is much better to use heart worm protection for dogs rather than cure the infestation once established. Once your pet contracts heart worms the treatments used take a long time and also may involve surgery. Treating existing infections is very expensive, even if you carry insurance, so you'll want to stay ahead of the chance of heartworm happening to your pet's health by using some form of heartworm protection.
You need to be careful when choosing which heartworm prevention medication that you give your dog, given the seriousness of a heartworm infestation. The heartworm prevention medicine put on the market in 2001 called Proheart Six only required that your pet be injected two times within the year. At first, the heartworm prevention was believed to be a safe & easy way to administer care and prevention to pets, however the FDA of the USA intervened when reports of more than 5,000 pets became ill as a reaction from the medicine. Of the 5,000 reported there were 415 fatalities.