Understanding Heartworm Disease

24 Aug

By Dr. Casey Hill
Hutto Veterinary Clinic

Lucy & Buddy

Heartworm disease is endemic in Texas. That means there is an ever-present population of the parasite and all of our pet dogs are susceptible to infection. The adult heartworm infests a dog’s heart and the large vessels leading to the heart. Heartworm infection can cause serious problems, including heart failure and sudden death.

Life Cycle

            Dogs become infected with heartworm disease when they are bitten by mosquitoes carrying a larval stage of the parasite. The larvae enter the dog’s blood stream during the mosquito bite and once inside the dog, mature through several other larval stages. Eventually, about two to three months later, the parasite matures into the adult worm which lives in and near the heart. The adult worm takes an additional three months to produce more offspring. These first larval stages (microfilaria) circulate in the blood and are eventually picked up by new mosquitoes as they bite the infected dog.


            During your dog’s annual checkup, your veterinarian can perform a heartworm test. This test should be done annually regardless of whether your pet is on heartworm preventive. Though unlikely, it is possible for a pet to become infected with heartworm disease while on preventive, and it is important for you and your veterinarian to know that this has happened. The most common heartworm test done by veterinarians is an ‘antigen’ test which tests for the presence of mature adult worms. A less sensitive test is to look for microfilaria (the first larval stage) in a blood sample. Neither of these tests will be consistently positive in an infected dog until about six to seven months after infection because of the nature of the lifecycle of the Heartworm.


            The best way to keep your dog safe from heartworms is to use a monthly preventive. The medication in heartworm preventive actually kills the immature stages of the parasite.  The way it works is that after an infective mosquito bites your pet, the immature heartworm larvae are exposed to the heartworm medication circulating through your dog’s system. They then die before they can mature into the adult parasite that is responsible for all of the problems associated with heartworm disease. It is important for dogs in Texas, where heartworm disease is endemic, to receive heartworm preventive medication from their owners every month for life.


            If your dog should become infected with heartworms, she will start off having no symptoms at all. If your pet is being tested regularly for heartworm disease, this is likely the stage he will be in when the infection is discovered. It usually isn’t until more adult heartworms develop that we see problems. The first noticeable sign in an infected dog is usually a cough. Later in the disease we might see an infected dog become weak or lethargic and sometimes these animals will develop a pot belly as fluid accumulates because the blood is not able to flow freely through the heart. Complications of heartworm disease include thrombus, or clot, formation which can cut off circulation to part of the lung or to a limb. This can result in sudden death depending on the location of the clot.


Heartworm Treatment

            Treatment of an adult heartworm infection is costly, painful and dangerous to your pet. It involves deep intramuscular injection of a medication that kills adult heartworms, not just the larval stages which preventive medication kills. It requires pain medication as the injection leave the muscle at the site very tender. Most importantly, a pet undergoing heartworm therapy will have to be on strict cage rest with no exercise for one month after each treatment (usually resulting in two consecutive months of cage rest).

The reason for this is that as the adult heartworms die, they can dislodge and travel to the lungs and form an embolism which cuts off blood flow to that section of lung. This can result in sudden death. With restricted activity we can reduce the blood flow through the heart and reduce the chances of this happening.

Alternative therapy for heartworm disease is to give the pet monthly preventive and kill the larval stage of the parasite. This won’t get rid of the adults in the dog’s heart, but it will keep more larvae from maturing to the adult stage. The adults that are living in the heart of the dog will eventually die and the infection will come to an end.

The danger of this approach is that your pet will remain infected with heartworms during this entire time, which usually takes several years. Also, during this time, your pet is acting as a reservoir for mosquitoes to pick up heartworm disease and spread it to other dogs. But, in older dogs, or ones where strict cage rest cannot be accomplished, this course is sometimes preferable.

Heartworm Disease and Cats

            Cats are also susceptible to heartworm disease. The parasite is adapted to the dog as its primary host, so it is far less likely to establish an adult infection in a cat than in a dog, even if that cat is bitten by many infected mosquitoes. Unlike the dog, a cat with heartworm disease will only harbor one or two adult worms.

Also unlike the dog, a cat with one worm is at a significant risk of sudden death and this is often the fate that befalls a cat who is unlucky enough to contract heartworm disease.

Despite the low chance of infection, the severity of the disease in a cat is so great that the American Heartworm Society, as well as many veterinarians, recommend monthly preventive in cats as well. Even indoor cats are potentially exposed to the occasional mosquito and should be considered at risk.

Dr. Hill joined Dr. John Holmstrom about a year ago at the Hutto Veterinary Clinic on Front Street, and she welcomes new clients and questions from pet-owners.


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