Geography

Canine heartworm disease occurs all over the world. In the United States, it was once limited to the south and southeast regions. However, it is spreading and is now found in most regions of the United States and Canada, particularly where mosquitoes are prevalent (or the State Bird, as in Minnesota).

Contagion

The disease is not spread dog to dog. An intermediate host, the mosquito, is required for transmission. Therefore, spread of the disease coincides with the mosquito season: the number of dogs infected and the length of the mosquito season are directly correlated with the incidence of heartworm disease in any given area.

It takes a number of years before dogs show outward signs of heartworm infection. Consequently, the disease is diagnosed mostly in 4 to 8 year-old dogs. It is seldom diagnosed in dogs under 1 year of age because the young worms take up to 7 months to mature once they've infected a dog.

Heartworm Life Cycle

Adult heartworms live in the heart and pulmonary arteries of infected dogs. They have been found in other areas of the body, but this is unusual. They survive up to 5 years and, during this time, the female produces millions of young. The microfilaria live in the bloodstream, mainly in the small blood vessels. Although they cannot complete their entire life cycle in the dog and are therefore not infective, they do cause problems. As many as 30 species of mosquitoes can transmit heartworms as follows:

1. The female mosquito bites an infected dog and ingests the microfilariae.

2. The microfilariae develop further for 10 to 30 days inside the mosquito, then enter the mouthparts. At this stage, they are called infective larvae because they will grow to adulthood when they enter a dog.

3. The mosquito bites another dog where the fur is thinnest (having long fur does not protect a dog from getting heartworms).

4. The infective larvae enter the dog's bloodstream and move to the heart and adjacent vessels.

5. The larvae grow to maturity in 2 to 3 months and start reproducing, thereby completing the full life cycle.

Effects from Adult Worms

Adult worms cause disease by a) interfering with valve action in the heart and b) clogging the heart and major blood vessels. This reduces the blood supply to other organs - particularly the lungs, liver, and kidneys - leading to malfunction.

Most dogs infected with heartworms do not show any signs of disease for as long as 2 years. Unfortunately, by the time signs are recognizable, the disease is well advanced. Specific signs depend on several factors: the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver, and kidneys.

The most obvious signs are: a soft, dry, chronic cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. These signs are most noticeable following exercise, when some dogs may even faint.

Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds. In advanced cases, congestive heart failure may be apparent and the abdomen and legs will swell from fluid accumulation. There may also be evidence of weight loss, poor condition, and anemia.

Severely infected dogs may die suddenly during exercise or excitement.

Effects from Microfilariae

Microfilariae (young heartworms) circulate throughout the body but remain primarily in the small blood vessels. Because they are as wide as the small vessels, they may block blood flow to the body cells and deprive them of nutrients and oxygen. The lungs and liver are most affected.

Destruction of lung tissue leads to coughing. Cirrhosis of the liver causes jaundice, anemia, and general weakness. The kidneys may also be affected, allowing poisons to accumulate in the body.

Treatment

There is some risk involved in treating dogs with heartworms, although fatalities are rare. In the past, the drug used to treat heartworms contained arsenic, so toxic effects and reactions occurred somewhat frequently. Today's drugs treat 95% of dogs with heartworms effectively without these side effects.

Treatment involves:

1. Killing the Adult Worms: Two injections are given to kill adult heartworms in the heart and adjacent vessels. Complete rest is essential after treatment. The adult worms die within a few days and start to decompose. As they break up, they are carried to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This is a dangerous period in which it is absolutely essential that the dog be kept quiet and not be allowed to exercise for one month following treatment. A cough is noticeable for 7-8 weeks after treatment in many heavily-infected dogs.

   Owners need to watch for any significant reaction to the initial treatment, although such reactions are rare. For example, if your dog exhibits loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, and/or depression, notify your vet immediately. Antibiotics, cage rest, and supportive care - such as intravenous fluids - may be needed.

2. Killing the Microfilaria: Approximately one month following treatment to kill the adults, the dog is returned to the hospital for administration of a drug that kills the microfilariae. Your dog needs to stay in the hospital for the day. 7-10 days after this treatment, your dog will be tested to determine if microfilariae are still present. If they have been all killed, the treatment is done. If there are still some present in the blood, the second treatment will need to be repeated.

   In some cases, the heartworm infection is "occult," meaning that no microfilariae were present. In this case, a follow-up treatment at one month is not needed.

Treating Severe Heart Disease

In dogs with severe heartworm disease, it may be necessary to treat them with antibiotics, special diets, diuretics to remove fluid accumulations, and drugs to improve heart function prior to treatment for the heartworms. In addition, these dogs may need lifetime treatment for a failing heart, even after the heartworms have been killed. This includes the use of diuretics, heart drugs, aspirin, and special low-salt, low-protein diets.

After Treatment

Dog owners are usually pleasantly surprised at the change in their dog following treatment for heartworms, especially if the dog had been showing signs of heartworm disease. The dog has a renewed vigor and vitality, improved appetite, and weight gain.

Prevention

Even after a dog has been successfully treated for heartworms, they can be re-infected Therefore, it is essential to begin and maintain a heartworm prevention program all year long.

There are three drugs that can be administered monthly to prevent heartworm infection. Two products, HeartGard and Interceptor, are chewable tablets. Revolution is a topical product. One of these should be started immediately after heartworm treatment has been completed, or when a new puppy has reached 8 weeks of age. All three are effective, very safe, and similar in cost.

Since no preventative is 100% effective, your dog should also be tested for heartworms once per year in order to catch the disease before severe damage can occur.