Heartworm disease is caused by infection of dogs (and sometimes cats) with the heartworm; Dirofilaria immitus. The mosquito transmits the worms from pet to pet. When the mosquito bites an infected pet and takes a blood meal, it also ingests the immature form of the heartworm known as a “microfilaria”. Once inside the mosquito, the microfilaria matures into an infective larval stage of the heartworm. When the mosquito feeds on an uninfected pet, the infective larvae are transmitted into the animal’s blood stream. From there, the larvae circulate through the body for up to 6 months as they mature. Then, they ultimately settle down in the heart and arteries that supply the lungs (pulmonary arteries). As the worms develop into adults, they can grow to be 2-3 inches in length. There presence in the heart and arteries obstructs the flow of blood and causes inflammation of the blood vessel walls which leads to their thickening. This causes the heart to work harder to pump the same amount of blood, which eventually leads to heart failure. Occasionally, there may be so many worms in the heart that they can lead to a complete obstruction of the blood vessels and acute heart failure. The life cycle of the heartworm is depicted below:

What are the symptoms of Heartworm disease?

It can take up to 2 years from the time a pet is infected with heartworms to the time symptoms begin to show. Unfortunately, during this time, much of the damage that occurs is irreversible. The symptoms that develop as a result of heartworm infection are directly related to the failure of the heart. They include:

  • Decreased exercise tolerance
  • Coughing
  • Increased thirst
  • Fluid accumulation in the belly and/or chest.

How does our veterinary clinic diagnose heartworm disease?

A simple blood test that can be run in the office can tell us if your pet is infected with heartworms well in advance of any symptoms. If a pet is found to be infected, then x-rays of the chest, ultrasound of the heart and blood tests would be indicated. Our veterinarians recommend that all dogs be tested yearly for the presence of heartworms.

Can Heartworm disease be treated at our veterinary office?

Heartworm disease can be treated with drugs, which will kill the worms in the body. Once they are dead, the body will slowly remove them. This treatment has risks and can even cause the pet to die. While killing the worms is essential to slow progression of the heart failure, it will not usually correct the heart failure. Instead, various drugs must be employed to help decrease the symptoms of heart failure.

Can Heartworm disease be prevented?

There are very safe and effective drugs available that can be given once a month to prevent heartworm infestation. We recommend a once-monthly tablet for this purpose. It has the advantage of protecting dogs from heartworm as well as fleas and intestinal parasites (roundworms, hookworms, and whipworms). Before being placed on this drug, it is necessary to have a blood test to check for the presence of heartworms in the body. If they are present, then they must be treated before Iverhart can be administered to prevent a severe reaction. We recommend that all dogs in our area be placed on heartworm prevention starting as early as 4 weeks of age.

Which pets are at risk of developing heartworm infection?

Heartworm disease has been found in San Bernardino and surrounding counties. Our veterinarians have diagnosed and treated 2 cases in the past few months who have never been out of this area. Pets that are subject to being bitten by mosquitoes are at risk of getting infected. The mountainous areas around us currently appear to have a higher incidence of heartworm infestation. In our area, the risk of cats developing the disease is not very high.