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Heartworm Disease & Prevention

According to the Heartworm Society, the highest infection rates of heartworm disease occur in dogs (not maintained on heartworm preventive) within 150 miles of the Atlantic and Gulf coasts and along the Mississippi River and its major tributaries. Other areas with large mosquitoe populations also have a high rate of infestation.

Heartworm prevention is simple. It involves a blood draw to determine whether the parasite is present and regular dosing with preventive medication.  Radiographs (X-rays) can also detect the presence of adult heartworms in the heart and lungs.

Heartworm infestation is dangerous; untreated dogs die and treated dogs go through weeks of discomfort while the worms are killed and expelled from their bodies.


Parasites go through several life stages before emergence as adults and often need at least two hosts to complete the cycle. In heartworms, a mosquito serves as the intermediate host for the larval stage of the worm, also known as the microfilariae. The mosquito ingests the larva when it bites an infected dog and deposits its cargo in an uninfected dog when seeking another blood meal. The microfilariae burrow into the dog and undergo several changes to reach adult form, then travel to the right side of the heart through a vein and await the opportunity to reproduce. Adult heartworms can reach 12 inches in length and can remain in the dog’s heart for several years.

Dogs can have some microfilariae in their blood and worms in their lungs without manifesting the disease. Once the number of worms exceeds a certain number based on the size and activity level of the dog, however, the adult worms move to the heart and symptoms begin to occur. Very active dogs may experience symptoms with lower numbers of worms than couch-potato dogs.

The time lag between the initial infestation of microfilariae and reproduction by adult worms living in the heart is six-to-seven months in dogs.

Female heartworms bear live young – thousands of them in a day. These young – the microfilariae – circulate in the bloodstream for as long as three years, waiting to hitch a ride in a bloodsucking mosquito. They undergo changes in the mosquito that prepare them to infect a dog, and they transfer back to the original host species the next time the mosquito bites. The process of change in the mosquito takes about 10 days in warm climates, but can take six weeks in colder temperatures.

The worms grow and multiply, infesting the chambers on the right side of the heart and the arteries in the lungs. They can also lodge in the veins of the liver and the veins entering the heart. The first sign of heartworm infestation may not manifest for a year after infection, and even then the soft cough that increases with exercise may be dismissed as unimportant by the owner. But the cough worsens and the dog may actually faint from exertion; he tires easily, is weak and listless, loses weight and condition, and may cough up blood. Breathing becomes more difficult as the disease progresses. The progression is traumatic: the dog’s quality of life diminishes drastically and he can no longer retrieve a Frisbee or take a long walk in the park without respiratory distress. Congestive heart failure ensues, and the once-active, outgoing pet is in grave danger.


If a blood test or the onset of symptoms alert owner and veterinarian to the presence of this devastating parasite, treatment is possible and successful if the disease has not progressed too far.

Treatment involves two basic areas:
The first step is to evaluate the dog and treat any secondary problems of heart failure or liver or kidney insufficiency so that he can withstand the treatment. The next step is to kill the adult worms with an arsenic compound. Veterinarians now have access to a Immiticide, a new compound that has fewer side effects than the previous drug and is safer for dogs with more severe infestations.

The treatment is administered in two doses each day for two days, followed by several weeks of inactivity to give the dog’s system a chance to absorb the dead worms. Exertion can cause the dead worms to dislodge, travel to the lungs, and cause death.

At least three-to-four weeks after the administration of the drug to kill the adult worms, further treatment to kill the microfilariae is needed. The dog is dosed daily for a week, then the blood test is repeated. If microfilariae are still present, the dose can be increased. Follow-up studies should be done in a year.

The bottom line:  Heartworm is a significant disease in dogs and cats.  The treatment involves managing the heart, vascular and systemic disease present as well as eliminating the parasites.  The goal of treatment is to eliminate the worms one way or another so the animal’s body can rebuild itself and return to the best possible post-infection health.  This sounds simple but it can be very complicated depending upon the number of worms present, the patient’s reaction to their presence, the patient’s general state of health, handling the side effects from the medication and the effects on the patient of the dead worms within the circulatory system.

Once the Heartworm is eliminated from your pet, then preventative medication is continued as prescribed by your veterinarian.


Ideally puppies are started on monthly Heartworm preventatives by 8 weeks of age.  Your pet should have a Heartworm blood test at around 7 months of age and then be retested on an annual basis or according to your veterinarian’s recommendations.

Preventive doses come in oral and topical versions and are only available from a veterinarian.. Ivermectin (Heartguard) is given monthly. Selamectin (Revolution(6)) is a new preventive applied topically and recommended for cats. Some of these drugs also kill other parasitic worms, and Revolution also acts against fleas, ticks, and mites.

We recommend using a year-round heartworm prevention program to guard against heartworm disease.  As long as they are given every month, they are very effective in preventing Heartworm infection and subsequent development of Heartworm Disease.  If your pet has already had his yearly check-up and did not receive his yearly supply of heartworm prevention, please call us to schedule a heartworm check and pick up your heartworm prevention. If he’s due for yearly vaccination, be sure to include a heartworm check and preventative medication in the visit.

For more information on Heartworm check out HeartGard

HEARTGARD®PLUS (ivermectin/pyrantel)

What it does…
Heartgard® Plus is a beef-flavored chewable administered to dogs once a month to prevent heartworm disease and to treat and control roundworm and hookworm infections.
Deals when purchased through Care Animal Hospital…
If you purchase a 12 month supply of Heartgard® Plus from our facility, you are eligible for a $5.00 mail-in rebate from the manufacturer.
Price Matching…
When charges are calculated to include the above rebate, our prices are very comparable to most outside sources.  However, we will price match most competitors, such as Pet Meds and Drs. Foster and Smith.  We’ll even verify the prices for you online!
Manufacturer Guarantee…
If you purchase this product from your veterinarian, use it appropriately for 12 consecutive months, and your dog contracts heartworm disease, Merial (the makers of this product) will cover the cost of your dog’s heartworm treatment.  Also, if your dog becomes infected with roundworms or hookworms during this time of protection, Merial will cover the cost of intestinal parasite treatment.

REVOLUTION® (selamectin)

What it does…
Revolution® is a topical medication used monthly on cats for protection against fleas and heartworm disease and for the treatment and control of hookworms, roundworms, and ear mites.
Deals when purchased through Care Animal Hospital…
If you purchase 6 months of Revolution® from our facility, you are eligible for an additional month FREE!  Buy 10 months, and you can receive 2 additional months FREE!
Price Matching…
When charges are calculated to include the above free doses, our prices are very comparable to most outside sources.  However, we will price match most competitors, such as Pet Meds and Drs. Foster and Smith.  We’ll even verify the prices for you online!
Manufacturer Guarantee…
If you are not completely satisfied with Revolution® for any reason, the manufacturer will replace the product or refund you the purchase price.  This guarantee applies to current product purchased from a veterinary hospital ONLY.

First Time Clients Receive A Free Wellness Exam. Mention Online Offer*


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