Heartworm Disease

What is heartworm disease?

Heartworms are parasites that affect dogs and cats. In their adult form, they are long, white worms that live in the pet's heart and can cause damage to the cardiovascular and respiratory systems.

Heartworm larvae are transmitted by the common mosquito. After a blood meal from an infected animal, the mosquito carries the larva on to its next victim, introducing the larva to the animals' subcutaneous tissues (the area just beneath the skin) as it feeds. Within six months, these larvae mature into adults, migrating into the heart where they attach themselves. If both male and female worms are present, they will begin reproducing.

Heartworm larvae are transmitted from an infected animal to a clean animal by the common mosquito. Within six months, these larvae mature into adults, migrating into the heart where they attach themselves. If both male and female worms are present, they will begin reproducing.

Heartworms can cause permanent damage to the heart, lungs and circulatory system, and may ultimately cause death. Humans and most other mammals are susceptible to heartworms only in very rare instances, and even then the parasite cannot complete its life cycle within these hosts.

It was once believed that cats were not susceptible to heartworm disease, but recent studies have shown that in high-risk areas, cats can become infected if they are not on preventive medication.

Because cats are smaller than dogs, a much smaller amount of heartworms can do the same damage that a larger number of worms would cause in a dog.

What symptoms will I see?

The majority of pets that have heartworm disease are asymptomatic, meaning that they show no signs of disease. Signs of heartworm disease usually do not appear until a year or more after infection and because the signs vary, the disease may easily be mistaken for other problems. Clinical signs of symptomatic dogs and cats include one or more of the following:

• Frequent cough in the absence of respiratory infection
• Exercise intolerance (easily tire with minimal exercise)
• Dyspnea (labored breathing)
• Weakness or fainting
• Weight loss in spite of normal or increased appetite
• Ascites (abdominal swelling from fluid accumulation)

How is it diagnosed?

The technique most frequently used to diagnose heartworm disease is the antigen or occult test, in which a small blood sample is taken. Dogs over six months of age should be tested before starting on a heartworm preventative. They should continue to be tested annually as a part of their annual vaccination visit and exam.

These preventive medications kill heartworms in the infective larval stage. Serious complications and even death may ensue if a heartworm positive dog is started on the preventative.

Why does my dog have to have an annual blood test?

Unfortunately no medication is 100% effective in 100% of pets.

In addition, if your pet experiences vomiting or diarrhea within 24 hours of ingesting the medication, it could pass through their system without being properly digested or absorbed.

And then, there are always the sneaky pets who appear to have swallowed the medication, but have hidden it away only to regurgitate it later.

Really, the bottom line is that all heartworm preventatives are prescription medication. There is the chance of a potentially fatal reaction if a heartworm preventive is given to a heartworm positive dog. For this reason and the others listed above, we have chosen annual heartworm testing for dogs as the requirement for fulfilling the doctor-patient relationship necessary for the dispensing of a prescription medication.

Testing is not required in puppies, as it can take six months from the point of infection for the test to register positive. Annual testing begins with the puppy's first set of adult vaccinations at approximately a year and a half of age.

An annual physical exam is required for cats.

What happens if my pet is positive for heartworms?