Airmont Animal Hospital

200-H Route 59
Suffern, NY 10901


Dr. Ness' article as printed in the Hudson Valley Humane Society publication "PAWssibilities!" (

Heartworm: Don't Let It Break Yours

Heartworm disease is a parasitic disease caused by a worm (Dirofilaria Immitis) which lives in the right side of the heart and the pulmonary arteries of the lungs. This disease which affects dogs, cats and other mammals is potentially fatal if left untreated. Heartworm disease has been reported in all 50 states.

 The Lifecycle

When a mosquito feeds on an animal that has heartworm it becomes infected with microfilaria which are heartworm larvae. The larvae will mature while in the mosquito and in about 2 weeks and become infective larvae. If one of these mosquitoes bites your pet, these infective larva will enter your pet through the bite wound, and burrow into your pets skin and migrate into their tissue. This process takes several weeks. After several months these larva undergo a final molt and become juvenile worms. These worms will enter your pet?s vascular system and move to the heart and lungs where they will mature to adult heartworms. These adult worms then produce more microfilaria which is then transmitted to mosquitoes that feed on your pet. Microfilaria cannot mature into adult worms without first passing through a mosquito.

Signs of Heartworm Disease:

Dogs with 1 or 2 heartworms may not appear sick, however a dog with a large number of worms will. The right side of the heart normally pumps blood to your pet?s lungs with ease. When the right side of the heart or the pulmonary arteries become clogged with worms the heart must work harder to pump blood. This can lead to pulmonary hypertension and heart failure. Since the heart must work harder to pump blood past these worms the heart may thicken or develop arrhythmias which could lead to sudden death.

Some of the signs of heartworms are:

  • Coughing and exercise intolerance
  • Nose bleeds may occur due to abnormal blood clotting in the lungs
  • A form of non-infectious pneumonia, due to the parasite in the lungs
  • Due to chronic immune stimulation may other organs can be damaged i.e.: eye, kidneys and liver
  • Heart Failure and death

In cats, heartworm may mimic many other feline diseases. Cats may start vomiting, gagging, have difficulty breathing, appear lethargic or lose weight. Many cats may be diagnosis with Feline asthma but actually have (HARD) Heartworm Associated Respiratory Disease.



Veterinarians now have very sensitive tests to diagnose heartworm in your pet. Most pets are tested yearly when your pet is presented for its annual exam. Puppies under 5 months of age do not need to be tested for heartworm since they are too young to test positive.

Any cat that has been diagnosed with any form of chronic respiratory disease should also be checked for heartworm disease.


Prior to treatment the heartworm positive dog is ranked into 1 of 4 classes:

  • Class 1: lowest risk, these pets have no signs of disease
  • Class 2: moderately affected, healthy dogs with minimal signs of disease
  • Class 3: severely affected, these dogs are suffering from the disease
  • Class 4: Caval syndrome, these dogs are collapsing and in shock

The first step in treating your pet is clearing the migrating immature worms. If we kill the adult worms first they would only be replaced by these worms when they matured. The migrating larva can be killed by using any monthly heartworm preventative such as Heartgard, Tri-Heart and Interceptor. The American Heartworm Society recommends 1-3 months of a preventive prior to treating the adult worms. Many veterinarians will also start your pet on Doxycycline at the same time to kill Wolbachia which is a rickettsial organism which lives in the heartworm. By killing the Wolbachia first we weaken the adult worm, sterilize the female worms, and minimize secondary problems with the treatment of the adult worms.

Immiticide is the only drug available to kill adult heartworms. Depending on the class of infection 2-3 intramuscular injections in the lower back are administered. Most pets are painful at the injection site and are given pain medications for 3-5 days.

After treatment your pet should refrain from any strenuous activity for about 1 month, and watched closely for coughing, fever or nose bleeds.


As with any illness preventing it is easier than treating it. There are many heartworm preventatives on the market today. We have oral medications that come in flavoured tablets such as Heartgard, Tri-Heart, Interceptor, Sentinel to name a few, these medications are administered once a month. Some dogs may develop diarrhea or vomit these oral meds. For those dogs we recommend topical medication such as Revolution or Advantage Multi.

Remember Heartworm prevention is safe, easy and inexpensive, but treating it can be costly and dangerous for your pet. Heartworm can be completely prevented.