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hw incidence map


life cycle

Images used courtesy of American Heartworm Society.


Monthly Heartworm Prevention







461 Patriots Road, Rte 2A

Templeton Common

Templeton, Ma 01468 Telephone: 978.939.9348
FAX: 978.939.2048



Heartworm Disease


Heartworm disease is a parasitic infection that occurs in dogs and occasionally in cats, which can be fatal if not treated. The parasite is a worm called Dirofilaria immitis and it makes its home in the animal's heart causing the animal to become very sick.


How does a dog get heartworm?


Heartworm is transmitted to a dog when bitten by an infected mosquito. Mosquitoes become carriers when they bite a dog that has heartworm. Young Dirofilaria immitis worms called microfilaria, circulate through the blood vessels of an infected dog and are drawn up with blood when a mosquito bites.


1The microfilaria mature into larvae, their next growth stage, within the mosquito, and are transmitted to other dogs through the wound produced when the mosquito bites them.


The life cycle of a heartworm


The heartworm larva remains in the tissue surrounding the bite for approximately 2 months where it grows into an immature adult worm.

It then enters the dog's blood stream and in 3 or 4 months reaches the heart and surrounding arteries.

The worms reach final maturity 6 to 7 months from when the dog was first infected and can now produce microfilaria offspring, thus starting the cycle over again.


Symptoms of heartworm


The most common symptoms of heartworm include:


      • Chronic (long-term) coughing
      • Tires easily
      • Unwillingness to exercise
      • Difficulty breathing
      • Lethargy (laziness)


Untreated, heartworm disease can seriously damage the heart and possibly cause death.


How to detect heartworm


The oldest and probably most well known method used by

veterinarians to detect heartworm disease is to draw a sample of blood from the dog, filter and stain it, then look at it under a microscope to see if any microfilaria are present.

We know now, however, that as many as 38% of dogs with heartworm infection don't have microfilaria circulating in their bloodstream.

These dogs have adult worms lodged in their heart, damaging the organ. Dogs with this condition are diagnosed as having “occult” heartworm disease. image


Occult infections are found everywhere heartworm exists, and the majority of infected dogs appear normal. One major reason occult infections can occur is that a monthly heartworm preventative will eliminate the microfilaria in a dog but have no affect on adult worms.

Because no microfilaria are present, these infections can be missed by the traditional screening methods mentioned earlier.


Source: IDEXX Laboratories, Inc.