My dog was diagnosed with adenocarcinoma almost two years ago. He's doing okay but has been going down hill the past few weeks. He will probably have to be euthanized within a month or two due to a decreased inability to have a bowel movement.

He takes some sort of steroid daily and we keep him on a high protein diet.

Our local animal clinic has recommended he get a single shot worth of 6 months of heartworm prevention medicine. He used to get the chewable preventative heartworm medicine up until a year ago. Since then he has not had any.

Is a heartworm preventative necessary?

Here is the client information sheet for the drug ProHeart 6:


1 Answer 1


When considering treatments for elderly animals, I usually go through a risk assessment to pick the best option.

  • What are the risks/hazards to each action I can take?
  • What is the likelihood of each risk/hazard?

For this situation, you can either give the heartworm preventative or not give the heartworm preventative.

Administering the Preventative

If you are considering give your dog the heartworm preventative, you should first test your dog to see if he is already infected. The Federal Drug Administration states:

If a heartworm-positive dog is not tested before starting a preventive, the dog will remain infected with adult heartworms until it gets sick enough to show symptoms. Also, giving a heartworm preventive to a dog that has an adult heartworm infection may be harmful or deadly. If microfilariae [heartworm larvae] are in the dog’s bloodstream, the preventive may cause the microfilariae to suddenly die, triggering a shock-like reaction and possibly death in some dogs.

Even if your dog does not have heartworms, the administration of a preventative can result in side effects. The 6 month heartworm shot is probably ProHeart 6. Their client information sheet gives the following as possible side effects:

  • Allergic reaction – The most common side effects of ProHeart 6 are allergic symptoms, including swelling of the face, itching, hives and/or inflamed skin. Allergic reactions have been reported when ProHeart 6 and vaccines have been given at the same time. Some allergic reactions can be severe, such as difficulty breathing or collapse.
  • Vomiting and/or diarrhea (Either with or without blood)
  • Seizures
  • Change in your dog’s appetite or activity level

These reactions are rare, but when they do happen they typically happen immediately after the shot is administered. For a dog who's health is already compromised they can potentially shorten his life, or decrease his quality of life significantly.

Withholding the Preventative

If you do not administer the heartworm preventative, the risk is that your dog may develop heartworms. VCA Animal Hospitals states:

It usually takes several years before dogs show clinical signs of infection. ...Unfortunately, by the time clinical signs are seen, the disease is usually well advanced.

Heartworms cause disease by clogging the heart and major blood vessels leading from the heart. They also interfere with the valve action in the heart. By clogging the main blood vessel, the blood supply to other organs of the body is reduced, particularly blood flow to the lungs, liver and kidneys,causing these organs to malfunction.

The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been in the dog and the degree of damage that has been sustained by the heart, lungs, liver and kidneys.

The most obvious clinical signs of heartworm disease are a soft, dry cough, shortness of breath, weakness, nervousness, listlessness and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint or become disoriented. Your veterinarian may notice abnormal lung and heart sounds when listening to the chest with a stethoscope. 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.

Heartworm disease is not a fast disease, and if your dog is in poor health already, he may not live long enough to be affected by the worms.

  • Regarding this statement in your answer "For a dog who's health is already compromised they can potentially shorten his life, or decrease his quality of life significantly."... Is this documented in the drug paperwork?
    – Samee87
    Commented May 27, 2015 at 16:39
  • 1
    @Samee87 no, that's my interpretation/experience. If a dog who's expected lifespan is measured in months has a serious health event (such as seizures, or difficulty breathing, or the dog already has bowel problems and experiences further GI disturbances like vomiting/diarrhea) then that is going to have a major affect on remaining lifespan/QoL. The dog probably does not have the resiliency to heal after a major event.
    – Zaralynda
    Commented May 27, 2015 at 16:46
  • Thank you. I believe you are right. I also found this online... proheart6.com/PDF/clientinformationsheet.pdf
    – Samee87
    Commented May 27, 2015 at 16:47

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