Heartworm Disease

The American Heartworm Society website has excellent and detailed information on this debilitating and often fatal disease in dogs. Our discussion here will focus on the dog's disease prevention, and treatment. Cats can get heartworm, but do not suffer fatalities like the dogs. We will provide prevention and treatment to dogs until research deems it necessary and prudent to do so for cats in our area.
The protocols used here are tailored to the individual dog's disease state, lifestyle and personality. This is not a one size fits all program. Prevention is offered after appropriate testing and monitoring and can be in the form of monthly pills, liquid, or a twice a year injection. Treatment can be a short, fairly inexpensive process, or it can be a long, step by step expensive series of medications. It is always best to prevent this disease in the first place.
There is no state that is immune to this disease nor is there a time of year when it
is safe to not be on prevention medication.

Wild canids like the coyote also carry the disease and serve as a permanent reservoir of infection for our dogs.


Heartworms are thread-like worms that can be 6 to 10 inches long. They live inside your dog's heart near or into the arteries that feed the lungs. Adult worms will reproduce and lay thousands of microscopic baby worms that circulate throughout the dog's body. Mosquitos feed on your dog's blood and suck up these infant worms and the worms go through a developmental stage inside the mosquito. The next time the mosquito feeds on your dog or your neighbors dog or a visitor's dog, the new teenage form of heartworm is passed back into the tissue where it matures some more. Six months later the young adult has migrated to the heart where it starts reproducing and the process repeats. This cycle occurs over and over through the life of your dog and you would never know he was infected until one of two things occurs. He is blood tested on a routine pre-prevention exam, or he shows signs of heart and or lung disease and is then blood tested. Adult heartworms have been known to survive inside a dog's heart for 7 years before they die on their own. Your dog does not spit them up, poop them out, or dissolve them on his own. The adult worms will live a long, productive life inside your dog's heart wreaking havoc on his heart and lungs.


Early symptoms can be as slight as a mild cough or as terrifying as sudden death. That's the problem with this disease. It will cause weight loss, lack of appetite, distended abdomen (pot-belly), passing out episodes, dull coat, lack of energy, lung disease, coughing, loss of alertness, and depression as the disease goes undetected and untreated.

Most heartworm infections go undetected since there are usually no symptoms early on in the disease process. Several factors can cause symptoms to be hidden (occult) which are typical in many infections. If there are only a few adult worms in the heart and they are not clogging the flow of blood to the lung, or the dog may be very sedentary and you don't notice lethargy or exercise intolerance. Your dog may have been in heartworm territory in another state while on vacation with you and became infected too.


With sudden death as a symptom...preventing this disease in the first place is paramount! It is critical, however, to test for the presence of the adult worms before starting a prevention program. Not all veterinarians use the same protocol for prevention. Our protocol is based on logical conclusions given what we know about this parasite: The testing at the correct time is critical to keeping your dog protected. Knowing that it takes 6 months from the time an infected mosquito bites your dog until an adult worm can be detected means that dogs less than 6-7 months old do not need to be tested before starting prevention. All dogs older than 6-7 months must first be tested before starting prevention. Since the test only detects the presence of the adult worm in the heart there will be another test 6 months after the first initial test to confirm that there were no immature worms that grew up into adults during that timeframe. The prevention medication only kills the infant form of the parasite and it will not kill the parasite once it is 4 months old. Testing must be a part of the prevention program or you can miss infections and allow them to do damage to your dog.

There are at least 3 good reasons to repeat testing every 2 years:

  1. a dog can vomit up the prevention medication without you knowing it

  2. you can miss a month of prevention or give the wrong dose due to weight changes of your dog

  3. the manufacturer of the prevention drug may make an error in dosing

These are all reasons why we adhere to a strict protocol that requires testing of dogs over 6 months old and retesting 6 months later and then retesting every 2 years or as needed if doses are missed.

Monthly prevention can be a single ingredient to kill only the infant heartworm, or it can be a combination of that drug and two others that kill intestinal parasites too. We also have an injectable drug that is only given every 6 months, but it only kills the infant form of heartworm. You can decide which method works best for your dog and your lifestyle. whatever you choose, your dog should be on preventive all year long.


Although heartworm disease can be fatal, and treatment can be risky, the condition is nearly always curable. Treatment requires careful medical management and strict confinement and rest at home. After the positive test you will be counseled on where to go from there. Some owners decide not to treat due to a variety of reasons like, the dog is suffering from another more serious illness, the dog will not likely survive the treatment due to the severity of disease, or treatment will put the family in financial hardship. Once treatment is authorized, the process to evaluate further can begin. Blood tests to assess risk and possibly X-rays of the chest to evaluate lung and or heart damage are advised at this time. Your dog must be able to handle the arsenic-based drug used to kill the adult worms. He must also deal with the dying worms and be able to dissolve them in his bloodstream during the recovery period.
If we detect existing disease or compromise of any other organ system, then we will likely treat that disease as much as
possible before starting the arsenic compound.

Veterinarians also differ here in the treatment plan. There is no way to know how many worms there are or how they are situated in your dog's heart, so our philosophy is to treat all patients as if they are at high risk. That entails more of a slow kill of the worms
with 3 injections rather than the faster kill done with 2 injections.

Some dogs are pre-treated with steroids, antibiotics, and heart drugs for weeks before the injections start. The dog will also be started on the first monthly dose of prevention during this time. We will watch your dog for 8 hours during the initial exposure to the prevention since some dogs will experience an allergic-type reaction to the dying infant form of the parasite
and need emergency medical treatments.

Immiticide is the name of the arsenic-based drug that is used to kill the adult worms. It is given by injection deep into the back muscle. An anti-inflammatory drug can be given at this time too and for the next few days for the swelling and pain that usually results. It seems to be a bit like how we feel after we get a tetanus shot. Be prepared for your dog to be a bit grumpy and sore.

It is critical for your dog to rest and avoid vigorous activity for 30 days after each of the 3 injections. This is the time when the dog's body is dealing with disposing of the dead and dying worms. If a large chunk of dead worm dislodges from the heart lining and is pumped out into the lungs it will act like a blood clot. A clot to the lung will cause pain, shortness of breath, and lack of circulation to the lungs leading to loss of oxygen and possible death. We will test your dog's blood 6 months later to make sure every
last worm died during our treatment.


  1. Pretreat with steroid, antibiotic, and prevention medication at positive test outcome.

  2. Determine best time to start Immiticide injection.

  3. STRICT rest for 30 days after first injection

  4. Give 2nd injection 30 days from the first and decide if the 3rd injection is given

  5. either 24 hours later or 30 days later.

  6. Continue STRICT rest for 30 days after each injection.

  7. Maintain monthly prevention and supportive medication.

  8. Retest 6 months after last treatment shot


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