Heartworm Disease in Dogs

A dog peeking above the frame of the image

Heartworm Prevention is Key

If you don’t give your dog heartworm preventative, now is the time to start! Heartworms are becoming more and more common near Mason and Fulton county. There are several different types of heartworm preventative medications available; topicals, chewable pills/tablets, and injections. In the long run, it is much healthier for your dog and much cheaper for you, to provide monthly heartworm preventative instead of having to treat for heartworms later.

Heartworm preventatives are products designed to periodically kill larval heartworms in the dog. Most products are monthly, oral medications but there are also injectables available now that last for 6 months (ProHeart). It’s very important for owners to be sure to give oral or apply topical heartworm preventatives once a month, all year round. Owners who stop giving heartworm preventative medications in the winter usually stop giving it too early or they start giving it too late in the spring. All it takes is one bite for your dog to become infected!


Heartworms can measure up to 14 inches long! They survive in the pulmonary arteries of an infected dog. Mosquitoes transmit this infection by biting an infected dog, then biting a heartworm-free dog. The pulmonary artery is close to the heart and a large accumulation of worms causes the heart to work extra hard to pump blood through blocked arteries, causing inflammation. Microfilariae (baby worms) spend several months maturing under the skin of a dog but they are still able to be killed with heartworm preventatives, like Heartgard Plus or Trifexis. However, when the heartworms mature, these medications will no longer be affective.


Heartworm tests are designed to look for microfilariae or adult proteins, so immature worms or too few of them can escape detection. It takes approximately 5-7 months from the time of exposure to get a valid heartworm test reading. During a worm’s maturity cycle, the dog is asymptomatic (not showing symptoms) so owners don’t realize the infection until it’s too late. Eventually, exercise and anxiety puts extra stress on the heart, then symptoms appear:

  • coughing
  • shortness of breath
  • tires easily
  • fluid accumulation in abdomen or chest
  • nose bleeds
  • sudden death

There are several diagnostic methods for detecting heartworms in a dog’s body. The most common is antigen testing, which detects adult female worm antigen (protein) circulating in the blood stream. Male worms are undetectable because they don’t produce enough antigen to be read by the test. All that is needed is a small blood sample from the dog. This type of testing can detect even a single worm! Radiographs can also be useful when diagnosing heartworms. They can show the abnormal enlargement of the heart and problems with the pulmonary arteries. The American Heartworm Society recommends yearly testing so infection can be detected early and treatment can be administered quickly.

Treatment Can be Dangerous

Treatment is not simple and can be dangerous to the dog. There are 4 categories of heartworm risk which are based on the size and age of the dog, number of worms present, other health issues, severity of heartworm disease and degree of exercise restriction during recovery. These range from Class 1 through Class 4 with Class 4 being the most severe. Patients receive an intramuscular injection into the lower back with an arsenic-based product, which kills the adult worms. Oral heartworm preventatives are then started to kill microfilariae. Anti-inflammatory doses of steroids can be prescribed to control inflammation. In advanced stages, too many worms dying at once can cause circulatory shock which is why one treatment dose is given, followed by two more doses one month later.


Strict confinement is needed for 4-6 weeks after the last treatment. This means no running, jumping, swimming, playing, etc. The reason for this is to prevent a pulmonary embolism (clot) from forming; this can be fatal to the dog. Symptoms to look for include: coughing, fever, and nose bleeds. Ultimately, it’s healthier and more economical to give your dog some form of heartworm preventative, so you won’t have to put him through the treatment and recovery process.

If you would like more information on heartworms, treatment, or would like to schedule an appointment to have your dog tested, please contact us at 309-543-2091.

posted in:  Pet Safety