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
- Vomiting and/or diarrhea (Either with or without blood)
- 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.