Heartworms are parasites that live inside of the heart of mostly dogs but, also cats. In dogs, heartworm infection can cause heart failure and death.

How do dogs get heartworms? 

Heartworms are carried by mosquitoes and can be passed from one dog to another through the bite of an infected mosquito. Heartworms are not passed directly from one dog to another. The mosquito is required for transmission

to occur. However, when a mosquito feeds on an infected dog, that mosquito becomes infected with a larval form of heartworms, called microfilaria. These microfilaria continue to mature inside of the mosquito and when the infected mosquito feeds on another dog, the microfilaria are injected into the dog through the mouthparts of the mosquito, where they continue to mature into adult heartworms.

What happens to dogs infected with heartworms?

As the heartworms develop into adults, they migrate to the heart of the infected dog, where they live and reproduce. Dogs can be infected with numerous heartworms at once, sometimes numbering into the hundreds.

As the worms establish themselves within the heart, they begin to affect the heart, pulmonary arteries and the lungs of the infected dog, eventually leading to heart failure. The damage done to the heart, arteries and lungs of dogs infected with heartworms may be irreversible and can be fatal. A great deal of damage can be done before the worms are even detected.

Caval syndrome and heartworm disease

In dogs which are heavily infected with heartworms, the worms can become so numerous that they fill the entire right side of heart and begin to “back up” into the veins that feed the heart. When this happens, the effect is often disastrous and all too often fatal.

Can dogs infected with heartworms be treated?

Dogs infected with heartworms can be treated, but treatment is risky and the outcome can be unpredictable. The death of all the worms infecting the dog is not guaranteed with treatment and treatment is prolonged, lasting 4-6 months or longer. The recommended method of heartworm treatment involves injecting the infected dog with an arsenic-derived compound which acts to kill the adult heartworms. Needless to say, the treatment itself can be dangerous, though heartworm treatment is safer today than it was in the past. Heartworm treatment is also extremely costly.

Though it is possible (and recommended under certain conditions) for dogs infected with heartworms to clear the infection over time when placed on heartworm preventive medications such as ivermectin, this method can take years for all of the adult worms to die and carries a great deal of risk with it as well.

Preventing heartworm infection in dogs

Heartworms are easily prevented with a variety of different heartworm preventive medicines. Prevention is much safer and much more effective for dogs than trying to treat an existing heartworm infection. It is recommended that any dog which lives in a heartworm endemic area be placed on heartworm preventive medication. Heartworms have been diagnosed in all 50 states, according the American Heartworm Society.

from:http://www.examiner.com/article/pet-health-101-what-are-heartworms-and-how-do-they-affect-dogs