Heartworm disease is a debilitating and often fatal disease caused by a parasite, Dirofilaria immitis, which invades the dog's circulatory system causing damage to vital organs such as the heart, lungs, liver, kidneys, and blood vessels. Heartworm is primarily a disease of dogs but occasionally infections are seen in cats, ferrets and other mammals.

The heartworm's life cycle starts with transmission of the infective larvae from a mosquito to a dog via a bite. It takes approximately six months for a larvae to grow to an adult in the dog's heart. The adult heartworms breed and create the juvenile form, the microfilaria, which are seen in the blood.

Clinical signs of a heartworm infection occur when the disease is well advanced and considerable damage to the dog's body has probably already occurred. Common symptoms are a chronic cough, exercise intolerance, weight loss, episodes of fainting or collapse, and shortness of breath. If left untreated, the dog will die of heart failure.

Since this disease is so serious, the treatment is so difficult, and the symptoms are not seen until late in the disease, the best method of control is with heartworm preventative medication. Every spring, the dog should have a blood test, performed by a veterinarian to be sure that he is not already infected with heartworm. Once a negative heartworm test is obtained, the dog should be started on either a monthly preventative (Interceptor). In central Illinois, it is best to give the preventative from March or April thru December. An alternative we recommend is to keep the dog on preventative year round.

If a dog becomes infected with heartworms, he can be treated successfully, but the treatment is prolonged, expensive, and can involve some side effects.

1. The dog is treated with an arsenic compound (Thiacetarsamide) that kills the adults in the heart. This is a very critical time in the therapy since the dying worms can cause obstructions and because the drug is very toxic. Therefore, we hospitalize the dog for 3 days during this treatment.
2. After the dog goes home, it is very critical in the overall treatment that you strictly confine and carefully observe your dog for the next 4 weeks. Strict confinement means cage or kennel rest, leash walking only, no exercise or play. If you are to observe any of the following symptoms call your vet immediately: coughing, vomiting, decreased appetite, difficulty breathing, decreased activity.
3. Four weeks after the first treatment, the dog is treated with another drug (Ivermectin) that kills the microfilaria in the bloodstream.
4. One week later, the dog gets tested for microfilaria, and if it is negative, he is started on preventative medication.
5. A heartworm test for adults can be run 4 to 6 months later or the next spring.

All Parts of the therapy are necessary for a successful treatment!

Dr. Mary Welle is a Veterinarian at All Creature's Animal Hospital in Urbana, Illinois.

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