Heartworm Disease and Prevention

Heartworms can cause a serious and sometimes fatal disease of
the heart, lungs, and other organs. Heartworm disease can be
cured with appropriate therapy. More important, it can be
prevented with heartworm medication. This education sheet will
help you learn more about heartworms.

Q. What causes heartworm disease? A. A nematode (worm) called
Dirofilaria immitis.

Q. What causes the transmission of the disease?

A. Mosquitoes carry the larval stage of the worm. Please see the
diagram on the other side of this page.

Q. What animals are affected?

A. Most commonly dogs; however, cats can occasionally contract
the disease.

Q. What organs are affected? A. Mostly the heart, but the
lungs, kidneys, liver, and the blood vessels to and from the
lungs can also be affected.

Q. What are the symptoms in an animal with heartworm disease?

A. The disease is directly related to the number of worms
residing in the heart, the duration of the infection, and each
animal’s individual response to the disease. Consequently, a dog
with a low worm burden may show no clinical signs. Dogs with
severe heartworm disease show signs of congestive heart failure
(pronounced cough, exercise intolerance, “fainting-like”
episodes.)

Q. What is Linda Mar Veterinary Hospital’s heartworm prevention
protocol?

A. We recommend your dog be placed on heartworm prevention
medication that is given monthly.

Following a physical examination, puppies less than 6 months of
age can be placed on the preventative medication without having
a heartworm blood test. Puppies greater than 6 months of age,
adult dogs that have never been on the preventative medication,
and dogs that have been off the medication for a period of
greater than 6 months require a physical examination and a
heartworm test. A small sample of blood is taken for this test;
if the test is negative the preventative medication should be
immediately started.

If your dog has been off the medication for greater than 6
months, we also recommend repeating the heartworm test 6 months
after restarting the medication.

Q. How often should a heartworm test be done?

A. If your dog is on monthly heartworm preventative all year
round, we now only require a recheck test and examination every
3 years. However, an annual physical examination is strongly
recommended for all dogs. If your dog is not on the preventative
medication, we recommend an annual heartworm test and a physical
examination.

THE HEARTWORM LIFE CYCLE

The transmission cycle of the heartworm begins when a mosquito
bites an infected dog and feeds on blood that contains tiny
immature heartworms. As it feeds, the mosquito takes in the
immature heartworms. During the next two to three weeks, these
tiny worms develop into larvae (the infective stage of
heartworms) within the mosquito. If the infected mosquito bites
another dog or puppy that is not protected with a heartworm
preventative, it can transmit infective larvae to that healthy
animal.

Infective larvae enter the dog’s body through the mosquito bite,
and they continue to develop over the next few months. They
migrate through the dog’s tissues, eventually reaching the heart
and lungs. Once in the dog’s heart, the worms may grow to
between 7 and 11 inches in length and can cause significant
damage to the heart and lungs. If left untreated, heartworm
disease may result in death. The life cycle of the heartworm is
approximately 6 months.

The above is general veterinary information. Do not begin
any course of treatment without consulting your regular
veterinarian. All animals should be examined at least once every
12 months.