What is Heartworm in Dogs?

Heartworm disease is a problem for dog owners worldwide but more so in warmer areas.  Many dogs are infected with heartworm and the owners are totally unaware until the disease is fairly progressed.   Do note that many herding dog breeds are extra sensitive to some medications used for heartworm.

Cause of Heartworm Disease in Dogs

Heartworm disease is caused by a parasite known as Dirofilaria immitis. This is a worm that is spread by mosquitoes and it is most prevalent in areas with large mosquito populations.

Dirofilaria immitis pass through several life stages, starting when they are sucked up as tiny larvae in the blood by a mosquito. The next time a mosquito bites an animal the larval worms, known as microfilariae enter a new animal and start to grow to a length of 12 inches. When these worms become adults they move to the dogs heart and that is when problems begin.

As the worms grow and build up the dogs heart becomes full of worms causing it to lose energy and will eventually kill the dog.  This is not an overnight problem, it takes months to progress to a life threatening stage.

Drawing by author ©

Symptoms of Heartworm Disease in Dogs

-Coughing, particularly after, or during, exercise
-Lack of energy
-Vomiting blood
-Heavy Breathing
*It must be noted that less active dogs may not have any symptoms.

How is Heartworm Diagnosed?

A diagnosis can be made one of two ways. The most common method of testing for heartworm is by a blood test. The blood test can determine if a dog has microfilariae in the blood and adult worms in the heart. The test is often found to be most effective if done in the early spring.

X-Rays will also show if worms are present in the dog's heart or lungs.

Treatment of Heartworm in Dogs

Once diagnosed there is no guarantee that the treatment will cure the dog, but without it the dog will certainly die.

The veterinarian will want to determine how infected the dog is and if there are other problems that may become issues when treatment is started, such as a risk of heart failure, and liver or kidney failure.

The veterinarian will try to kill the adult worms using twice daily injections, for two days, of an arsenic compound.

The dog must be kept resting, and inactive during treatment. The concern is that the dead worms will circulate and cause other problems. If the dog is allowed to rest its body will absorb the dead worms.
The veterinarian will ask to recheck the dog, usually three weeks after treatment and again the following year.
Treatment for heartworm is both expensive and risky, as such prevention is very important.

Prevention of Heartworms in Dogs

The only way to really prevent heartworm in dogs is to prevent the dog from being bit by mosquitoes, otherwise medications which are said to prevent heartworms are not really doing that; but they are killing the larval heatworms that may be in the bloodstream, and as such are preventing the adult heartworms being a concern. 

There are several products, both oral and topical, for prevention of heartworm. Every dog owner should discuss the level of risk in their area and what are the best prevention methods. Again, the risk of heartowrms is lower in colder areas.   Also remember that some herding breeds, including Border Collies, are sensitive to some medications.

It should be noted that all canines are at risk of  heartworms (in case a person owns an exotic canine such as a Fennec Fox) and cats can get them too.