How heartworms infect pets, and how they can be treated

Heartworms in this location are best treated with a medication called Immiticide. This drug is very effective at killing the adult worms. The biggest concern is, as the worms die, there is the potential for them to lodge in the lung’s vessels, obstructing them. This can result in labored breathing, fevers and in the worst cases death. Strict exercise restriction is recommended for the month following therapy to reduce the likelihood of this happening.

An alternate therapy is the “slow kill” method. This is where ivermectin is given and the adult worms are slowly killed over time (months to years). While this method is much less expensive than Immiticide, I do not recommend it. The dog is chronically infected, allowing for the inflammation to persist, which can permanently damage the heart and lungs.

The lungs are the primary location for infections; however, worms can end up in other vessels. Almost 20 years ago, Dr. Bianucci had a patient in which the worms had lodged in the arteries supplying blood to the dog’s back paws. The blood supply was obstructed, causing the tissue to die. This dog had learned to shift all of its weight forward and could walk with just his front two legs. Dr. Bianucci was able to amputate the dead rear paws and this dog was, fortunately, able to continue having a good quality of life.

The heart is another location where the adult worms may lodge themselves. When this occurs they are living in the large veins that carry blood to the right side of the heart. With all these worms in this location most dogs will develop symptoms of right heart failure, with the most common being accumulation of fluid in the abdominal cavity. These dogs are often anemic as a result of the worms in the heart and large veins damaging red blood cells as they pass through.

This type of infection occurs when dogs have a large number of worms and often have been continually infected over a long period of time. At this stage, medications to kill the worms cannot be given. The only treatment is to surgically extract the worms from the heart.

The dog is placed under general anesthesia and an incision is made into the jugular vein of the neck. With the aid of fluoroscopy (moving radiographs) a device is passed into the jugular vein down to the heart and the worms are extracted one to two at time. It is impossible to get them all, as most dogs also have worms in the lung vessels, but just removing some will allow the anemia and heart failure to resolve. Once recovered from surgery they then have to be treated medically to kill the remaining worms.

Of course, these patients are not the ideal candidates to be going under anesthesia. In my experience about half survive worm retrieval and half do not.

Cats are less commonly infected and the worms only live about one to two years. However, they can develop similar symptoms as dogs. The frustrating problem is they do not tolerate the Immiticide. Controlling their symptoms until the worms die is the only therapy.

Like most illnesses, prevention is the best medicine. There are many options available for prevention, from every six-months injections, monthly oral pills to topical drops. It is much less expensive (both financially and more important emotionally) to prevent heartworm disease than to treat once infected.

Dr. Henri Bianucci and Dr. Perry Jameson are with Veterinary Specialty Care LLC. Send questions to petdocs@postandcourier.com.