Heartworm in Dogs and Cats

By Nancy Otten, Animal Health Technologist

What is heartworm?

The heartworm is a type of nematode (roundworm) called Dirofilaria immitis.  It is a parasite of dogs and cats, but it also can infect a variety of other animals.  It causes a serious and potentially fatal condition by living in the arteries of the lungs and in the right side of the heart. 

The heartworm has a very interesting lifecycle.  Adult heartworms can live 5 to 7 years in the dog and 2 to 3 years in the cat.  In this time they can produce millions of larvae (microfilariae.)  These larvae are unable to mature to adulthood unless they spend part of their lives in a mosquito.  When a mosquito feeds on an infected animal’s blood, they take in some of these young microfilariae.  When in the mosquito they develop into the infective stage of the parasite.  When the mosquito feeds again, they transfer these infective microfilariae into the host.  The microfilariae then take 6 to 7 months in dogs and 7 to 8 months in cats to develop into adult heartworms.  Infected dogs can have anywhere from 1 to 250 heartworms in their hearts and lungs.  Cats can have anywhere from 1 to 3 worms, usually in their lungs.

What does it do to dogs and cats?

In dogs, the number of heartworms and the level of activity will influence the clinical signs; the larger the number of worms and the more active the dog, the more severe the heart and lung changes.  Clinical signs can include coughing, exercise intolerance, difficulty breathing, fainting, and abnormal heart and lung sounds.  High numbers of worms in the arteries of the lungs cause severe inflammation and in the heart cause the organ to become enlarged and weakened due to the increased workload.  This leads to congestive heart failure.  The worms in the heart decrease the output of oxygen-rich blood causing liver and kidney problems due to poor perfusion.   

In cats, the clinical signs are very non-specific and may mimic other feline diseases.  Changes in the heart are minimal.  Their primary response is in the lungs.  The clinical signs may include vomiting, coughing, gagging, difficulty breathing, rapid breathing, lethargy and anorexia.  Occasionally, the clinical signs appear very suddenly and become fatal just as quickly.  We don’t see very heavy infestations in cats.  Fewer microfilariae survive to adulthood, presumably because of a strong immune response.  Also, it is more common in cats to have single sex infections.  In clinical studies, 100% of dogs exposed to infective microfilariae develop heartworm disease.  Cats are less susceptible to the disease with 61-90% of exposed animals developing the disease.  Cats also don’t seem to be a reservoir for the disease.  Only 20% of heartworms in cats will produce microfilariae.      

How do veterinarians diagnose heartworm?

An accurate patient history, a thorough physical exam, and an examination of the clinical signs may be enough for the vet to suspect heartworm.  Chest x-rays may confirm their suspicion.  In both infected dogs and cats, the pulmonary arteries in the lower lobes of the lungs will appear enlarged.  In the dog, the main pulmonary artery and the right side of the heart may also be enlarged.  The vet can also perform two types of tests:  an antigen test and a microfilarial test.  In an antigen test, specific antigens (proteins) produced by adult female heartworms can be detected.  This test will not detect the infection of worms less than 5 months old.  A microfilarial test will detect the presence of microfilariae.  This test is not suitable for cats because only 20% of infected cats will produce microfilariae.      

How do we treat heartworm?

Unfortunately, there is no approved treatment for heartworm in cats.  Supportive therapy is administered which means that the symptoms are treated rather than the disease.  One or more of the following may be administered:  IV fluids, oxygen therapy, cage confinement, bronchodilators, etc.  The lifespan of heartworm in the cat is only 2 to 3 years so the treatment plan is to make the cat as comfortable as possible until the worms die naturally.

Fortunately, there is treatment for dogs infested with heartworm.  The first stage of treatment is to kill the adult worms using an adulticide.  The drug administered kills the worms slowly over a period of a month.  These dead worms can cause some complications.  As the worms are decomposing, they break up and travel to the lungs through the bloodstream.  In the lungs they become lodged in the small blood vessels and cause blockages until the body absorbs them.  These blockages can be dangerous so to reduce damage, a strict exercise restriction for one month must be enforced.  A month after the administration of the adulticide, a microfilaricide is administered to kill the microfilariae.   

How is heartworm prevented?

Heartgard and Interceptor are products that kill microfilariae in the bloodstream.  They are tablets given once per month to prevent the microfilariae from developing into adult heartworms.  Revolution does the same but it is applied topically once per month.  These products allow Whistler dogs to visit heartworm prevalent areas safely.  Upon their return to Whistler, they simply need to take one of these products to prevent potential heartworm development.

Is heartworm a problem in Whistler?

Heartworm is not a problem in Whistler due to our climate.  The development of the microfilariae in the mosquito to the stage where they are infective is temperature dependent.  The warmer the temperatures, the quicker they develop.  For example, in a laboratory environment where the temperature is maintained at 18 degrees Celsius, it would take the microfilariae 30 days to develop to the infective stage.  At a steady 27 degrees Celsius, it would take 10 to 14 days.  There is another way that their development is temperature dependent.  If the temperature drops below the developmental threshold of 14 degrees Celsius (for a couple of hours at night, for example,) that would be enough to retard maturation.  Even if the average daily temperature supports continued development, our cool evenings prevent it.  There have been two confirmed cases of heartworm in Whistler but both dogs were from Mexico.  No Whistler dogs have contracted the disease.