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Heartworm disease is a serious and potentially fatal disease in dogs. It is caused by a ten-inch long parasite called Dirofilaria immitus. These worms live in the heart and adjacent blood vessels of infected dogs.

How are heartworms transmitted?

The disease is transmitted when mosquitos carry microscopic baby heartworms from one dog to the next. It takes six months after your pet is bitten by an infected mosquito for him to have adult worms in his heart.

My dog stays mostly inside and has a thick hair coat. Can he still become infected with heartworms?

Yes! A thick hair coat will provide your pet with some protection, but it is no guarantee because the hairs on the face and ears are thin enough that mosquitos can easily reach the pet’s body to feed.

How common is this disease?

Heartworm disease occurs in 66-100% of dogs not on medication exposed to mosquitos and who live in endemic regions. There are many different subspecies of mosquitos, each with their own individual habitat (environmental) preference. Some mosquitos live almost completely indoors. In Texas, ALL dogs (even those that spend all their time indoors) need to be on heartworm preventive.

How is Heartworm disease diagnosed?

A small amount of blood is taken from your dog. There are two different tests that can be performed. A blood smear is examined under the microscope for the presence of microfilaria (immature heartworms). A negative blood smear does not rule out heartworm infection. An antigen test is performed to detect the presence of protein molecules from adult worms in the heart.

How accurate is the antigen test?

False positive results are extremely rare. False negative tests are also rare but can occur if the numbers of worms are too small in number, if they are all males (the antigen tested for is in the sex organs of the female worms), if the antigen level is low when tested (the circulating antigen levels will vary during the day) and if the pet is tested too soon after infection. It takes from four to six months from the time the mosquito bites until the adult worms are in the heart and the dog will test positive.

Why does my pet need to be tested before he can be given the medication?

It is important to determine if your pet is infected with heartworms because some heartworm medications are dangerous to give to dogs with heartworms. Dogs who have heartworms should not be put on heartworm preventive unless you have talked to a veterinarian first.

Should your pet test positive for heartworms, we strongly recommend that he be given treatment to kill the adult worms and then put on preventive medication.

Based upon the results of the heartworm test, your veterinarian will discuss with you which heartworm medication is the best and safest for your pet to be on.

How often does my pet need to be on medication and for how long?

Dogs need to be on heartworm preventive on the same day each month YEAR-ROUND. Unlike vaccinations, which are boostered periodically, heartworm prevention works only as long as you continue to give it. Should your pet be off of the medication, your dog will be at risk for getting heartworms. Prevention is safer and less costly than treatment. Do not stop giving preventive in the winter months. Dogs need to be put on preventive for their entire lives.

What if I can’t remember to give the medication on time?

It is very important that your pet receive the medication on the same day each month. If you are a few days off, it is okay to give the medication at that time. If it has been more than a few weeks, we recommend calling and talking with your veterinarian prior to restarting the medication.

We have text message medication reminders that we can send you when it is time for your pet’s monthly heartworm prevention. Click here for more information and to sign up.

Is it really that important for my pet to be given this medication?

Most owners would agree that in order for their pets to have a healthy and happy life, that the following things are essential: food, water, shelter, love and good health. One important step towards maintaining good health is preventing potentially fatal diseases such as heartworm disease. We, as veterinarians, feel that heartworm prevention is ESSENTIAL and NECESSARY for your pet to enjoy a good, long, healthy life!

Once my pet is on heartworm prevention, does he need to be tested again?

We recommend yearly heartworm checks so that if a dog is found to be infected, we can eliminate the heartworms before they cause damage to the heart and other internal organs. Annual testing also verifies that the medication is effective. Heartworm disease, like cancer, is much more treatable if caught early.

If your pet has previously been off of the medication, we recommend another heartworm test in six months to make sure that your pet was not infected prior to starting the medication.

What are the signs of heartworm disease?

The most common clinical sign of heartworm disease usually is no sign at all until the disease is well advanced. Later signs may include chronic coughing, tiring easily, unwillingness to exercise, difficulty breathing, lethargy and/or an enlarging abdomen. The purpose of an annual heartworm test is to discover the presence of heartworms and treat them BEFORE your pet shows clinical signs. By the time your pet shows signs of heartworm disease, damage to the heart and lungs has occurred.

I don’t want to test my dog for heartworms because I wouldn’t treat him anyway.

Some clients may be apprehensive/afraid to find out that their pet has heartworms. Some clients don’t want to feel guilty about the fact that they let their pet get heartworms or that they don’t want to go to the expense of treating their pet for heartworms. While we know that you love your pet and we can certainly understand these feelings, they do NOT allow your pet to live a long and healthy life!

Ignoring a potential problem DOES NOT MAKE IT GO AWAY. The longer you wait to treat your pet or put your pet on heartworm preventive, THE WORSE THE DISEASE WILL GET.

  • What would increase your feelings of guilt?
  • Not preventing a deadly disease?
  • Letting your pet’s health get worse?
  • Not treating your pet?

Please talk to your veterinarian about any concerns that you may have. What is important is your pet’s health! Even if you don’t want to treat your pet, you can still help your pet by putting him on appropriate heartworm prevention. We simply need to do the blood test first.

“But heartworm prevention is so expensive”.

Actually, with the different types of heartworm medications now available, owners have a wide choice of medications to choose from, many of them quite inexpensive. This is especially true on the two products that we carry and strongly recommend: Sentinel & Trifexis. Our prices are lower than what you’ll find online. Click here for more information.

What we ask you is to weigh the cost of medication (inexpensive) against the cost of treatment (expensive) and the cost to the pet’s health (priceless). Heartworm disease can rob your pet of a good, high quality life; and, it can also rob you of valuable time with your pet because of a shortened life span.

FACT: Mosquitos bite dogs!

FACT: Mosquitos carry heartworms!


We can’t prevent old age and we can’t prevent cancer, but, working together we can prevent heartworm disease.

Please do not hesitate to talk with your veterinarian about any questions that you may have regarding heartworm disease, treatment and prevention.

For even more information, see this pdf from the American Heartworm Society on current canine guidelines for prevention, diagnosis and management of heartworm disease in dogs.