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General Information

Canine heartworm disease is a very serious problem in our area. It is caused by the heartworm Dirofilaria immitis.  Severe infestations of the adult worms result in coughing, labored breathing, heart failure, and death. Heartworms are spread from an infected dog by mosquitoes. Since our area is heavily infested with mosquitoes, heartworm disease is very common among dogs not on a heartworm preventative.

Life Cycle

Adult heartworms live in the right side of the dog’s heart and in nearby blood vessels. Adult females produce microscopic immature heartworms called microfilariae. These microfilariae are in the bloodstream of an infected dog. When a mosquito bites a dog with heartworms, the mosquito ingests microfilariae. After the microfilariae mature in the mosquito, they are transmitted to a non-infected dog when the mosquito feeds again. Six to seven months after transmission, the microfilariae mature in to adult heartworms which then produce microfilariae.


Heartworm disease can be prevented with medication. There are several drugs available to protect dogs from contracting heartworms. All of them are given monthly and should be administered during the mosquito season, generally from March or April until October or November. It is not necessary to give heartworm preventatives when there are no mosquitoes in the environment. At the Molitor Pet and Bird Clinic, we recommend Revolution, Tri-Heart Plus, Heartgard Plus or Sentinel. Please discuss these options with Dr. Molitor at any of your appointments. It is extremely important the dog has a blood test before beginning heartworm preventative. Some of the preventatives may cause an anaphylactic reaction in dogs that have heartworms and your dog could die from this reaction.


Dogs with heartworms can be treated. There are two phases to the treatment: 1. destruction of the adult heartworms, and 2. elimination of the microfilariae from the bloodstream. The first phase of the treatment may be dangerous so dogs are closely monitored for any signs of toxic reactions to the drug used. Following this treatment the dog must be kept very quiet for 4-6 weeks to avoid complications. At the end of this the 4-6 weeks, the dog is returned to the hospital for phase 2 of the treatment. The dog is then given 2 heartworm tests at monthly intervals to determine the success of treatment. If both tests are negative, the dog is started on a heartworm preventative.

Our Recommendations 

Dogs over 4 months of age should be checked for heartworms yearly (in the spring) and if negative should be kept on a preventative during the mosquito season. Dogs under 4 months of age do not need to be tested for heartworms, but should be started on a preventative at 3 months of age. 
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