Heartworm disease is caused by a parasite, Dirofilaria immitis. Heartworms are transmitted by mosquitoes and can lead to severe, life threatening disease in both dogs and cats (and ferrets). Through testing and prevention, we can help protect our pets against this serious disease.

In the paragraphs that follow, we will explain the life cycle of the parasite, explain the diseases that this parasite can cause, and finally, we will discuss the diagnosis, treatment and prevention measures that we recommend. If you have any questions about heartworm disease in your pet, please contact our hospital at 525-1937.

Life Cycle

Heartworms are transmitted from one dog to another by bites from any one of more than 70 species of mosquitoes.   There are potentially a half dozen native species of mosquitoes in Minnesota that can successfully transmit the parasite.  Adult Heartworms mate in the heart and subsequently the females discharge thousands of tiny microscopic larvae (called microfilariae) into the blood stream.  Mosquitoes become infected with microfilariae when they take a blood meal from an infected dog.  The microfilariae grow and moult several times within the mosquito and become infective in 10-28 days.  These infective larvae leave the mosquito and enter the dog through the skin when the mosquito takes another blood meal.  It then takes approximately 4-6 months for the larvae to complete their migration to the dog's heart, mature to adult worms and begin producing microfilariae.

Cats become infected by the same larvae that infect dogs.   However, cats are not the natural host for Heartworms and as a result they rarely reproduce in the cat.  As a result, microfilariae are generally not produced and cats often have infections of only a few worms, whereas dogs develop infections of up to a few hundred worms in the heart. 

Heartworm Disease

Heartworm disease is usually a slowly developing and chronic form of disease in which damage to internal organs is often extensive before there are obvious signs of any physical problem or illness.  Many dogs with light infections will exhibit few signs of the infection - particularly if the dogs live a quiet lifestyle or are not physically stressed.

A dog which has had heartworm infection for some time may develop a chronic cough, will tire rapidly on exercise and may collapse from heart failure during or after exercise.  The hair coat often becomes dull and dry and there may be weight loss.   In advanced cases there may also be enlargement of the abdomen, due to fluid buildup.

Since cats do not get a high number of worms in their systems, they rarely exhibit the same symptoms as dogs.  As the worms migrate through their bodies, they can cause symptoms such as coughing, vomiting, respiratory difficulty, and occasionally sudden death.

Testing for Heartworm Infection

Heartworm disease is diagnosed through a blood test.  There are two types of blood tests available.  The most common test is an antigen test, which identifies the presence of adult worms in the body.  The other test is a filter test, in which we look for the presence of microfilariae in the blood stream.  For most patients an antigen test is the best method of heartworm detection.  However, certain types of medication (the daily products) can cause severe reactions if a dog has microfilariae in it's system.  Therefore, if any animal is on daily heartworm prevention, they should receive both the antigen and the microfilariae tests prior to starting preventative each year.

At North Shore Veterinary Hospital, we follow the recommendations of the American Heartworm Society and the Minnesota Veterinary Medical Association - annual testing for all dogs.  Testing is recommended each year, because early identification of this disease is the key to successful treatment.  Since we can not be absolutely certain that our pets ate and fully digested the medication each month, we recommend annual testing even for those pets who are on regular heartworm prevention.   Remember, the small price of an annual test could save your dog's life.

We do not currently recommend annual testing for cats.  Because there is no effective treatment currently available for cats,  early identification of this disease is not as valuable as it is in dogs.  We have the test available and we recommend testing any cat who displays symptoms of heartworm infestation, but until an effective treatment becomes available, we will not require annual testing of cats.

Treating Heartworm Disease

Heartworm disease in dogs may be treated, but the success of treatment is determined in large part by how early the infection is diagnosed.  If a dog is found to have heartworm infection, but is not yet experiencing the symptoms of heart disease, there is a very good chance of successful treatment. 

The most common treatment for adult heartworms is a series of intramuscular injections with an arsenic-based compound (melarsomine hydrochloride).   This compound generally has little or no effect on the dog, but the mass of dead worms killed by the drug and expelled into the lungs can cause severe reactions in some dogs.  It is essential to keep dogs quiet for several weeks following treatment to reduce adverse reactions.  Several weeks following successful treatment of the adult worms,  the dog is given treatment with another drug (ivermectin) to kill the microfilariae in the blood.

Remember, treatment is safest and most likely to be effective when it is instituted early in the course of heartworm infection.  Early detection is the most important part of successful heartworm treatment! 

Unfortunately, at this time, there is no effective treatment for heartworm disease in cats.

Preventing Heartworm Disease

Any dog or cat who is exposed to mosquitoes (and that means all of them - even indoor pets) is at risk for heartworm disease.  As we discussed above, heartworm disease can be very debilitating and is potentially fatal for our pets.   In dogs, treatment is possible, but there are dangers associated with this treatment.  In cats, there is no effective treatment at this time. 

So, what can you do to help your pet?  Prevention is the key to avoiding the effects of this terrible disease.  We use an ivermectin product called Heartgard.  It is available for both dogs and cats.  It is given once a month during the mosquito season.  (We start giving the preventative on May 1st and finish on November 1st.)  Heartgard works by eliminating the heartworm larvae which have infected the animal during the previous 30 days.  This medication is very safe in all breeds of dog and cat.  It is provided in tablets and a chewable form.   The Heartgard product for dogs, Heartgard Plus, also includes medication which controls infection with roundworms and hookworms.  This medication requires a prescription, so all pets must have a current doctor/patient relationship with one of our veterinarians in order to receive this medication.

Almost every pet will take Heartgard chewables eagerly, but in the unlikely event your pet does not like Heartgard, you can return them for a full refund.