Heartworm Disease

Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart of dogs and cats. Our clinic has personally diagnosed heartworm in 5 dogs just in the last year.  While this disease is treatable once diagnosed, treatment is very expensive and can be quite rough on those treated.  Fortunately, this disease is preventable.  But first, let’s talk a little more about what heartworm is, how it is transmitted and how it affects dogs and cats.

Transmission:  Heartworms (Dirofilaria immitis) are spread by mosquitoes.  The juvenile form of the worm is picked up by the mosquito when it bites an infected animal.  These juveniles then grow and develop inside the mosquito until it bites another animal that they then infect.  The young worms migrate into the blood stream of their new host and eventually to the right side of the heart and arteries of the lungs; there they mature into adult worms, breed, and release new juvenile worms (microfilaria) into the blood stream for mosquitoes to pick-up.  This entire process takes about 6 months to occur, which is why we recommend testing dogs for heartworms once a year, generally in the spring to make sure they were not infected the previous mosquito season.

Signs of Heartworm Disease: Many dogs with heartworms will not show any signs for a long time.  They are often picked up during routine screening tests, and we are able to treat them, before the worms make them very sick.  The signs most commonly seen are coughing and weakness during exercise.  Severely affected animals can even go into heart failure or liver failure and may require surgical removal of the worms to survive.

Diagnosis:  A simple blood test exists for dogs that detects proteins from adult worms in the blood.  This is the initial test that we run in the clinic on a routine basis.  If a dog shows up positive on this test, the next step is to look at a blood sample under the microscope.  If there are juvenile worms (microfilaria) present in the blood at that time, they can be seen with a microscope.  This confirms the diagnosis.  If these are not seen, additional blood is sent to a laboratory for secondary testing to confirm the positive result.

Treatment:  Once a dog has a confirmed diagnosis of heartworm disease, they are started on a treatment schedule.

The following treatment protocol is the current recommendation of the American Heartworm Society, and it allows for the best chance of curing the disease while minimizing the risk to treated dogs.

Day 1:  A monthly heartworm preventative is administered to kill off the circulating juvenile worms.  Antibiotics (doxycycline) are started that kill bacteria associated with the adult worms and lessen the effect on the immune system when the worms are later killed.  Steroids are also started to decrease the immune response and prevent an allergic response.

Day 30:  Heartworm preventative is again administered to prevent an additional infestation.

Day 60:  The dog is continued on preventative.  At this time he/she is given the first shot of melarsomine, an arsenic derivative that will kill the adult worms; the dog is monitored throughout the day for reactions to this drug. Another round of steroids is also started to help decrease the amount of inflammation that occurs in response to the heartworms dying.

Day 90-91:  Monthly preventative is given.  The second and third injections of melarsomine are performed on consecutive days, with continuous monitoring for a reaction to the dying worms.

Day 120:  A blood sample is examined for circulating juvenile worms in the bloodstream.  Heartworm preventative is continued on a monthly basis.

Day 271:  Another blood test is run to make sure the body is completely cleared of adult worms as well as juveniles.  Dogs should be continued on monthly heartworm preventative throughout the year for the rest of their lives following treatment.

As you can see, this treatment protocol is prolonged and can be very tough on the dog’s body systems.  The cost of treating a large dog can be over $1,000 as the melarsomine injections are very expensive and the amount needed is based on the animal’s body weight.

Heartworm Disease in Cats:  You may have noticed that only dogs were mentioned in the testing and treatment sections.  While heartworm is more rare in cats than dogs, it is still possible for them to be infected (even kitties who always stay indoors are occasionally exposed to mosquitoes).  Infected cats may only have one or two adult worms present and so testing becomes more difficult, as a very small infestation may not show up on the regular test used for dogs.  Cats suspected of disease are generally tested for their body’s response to the worms rather than the worms themselves.  Cats that have been exposed to heartworm but did not develop an infestation will show up positive on these tests as well, and so it becomes difficult to distinguish those truly infected.

There is no approved treatment for heartworm positive cats.  The melarsomine injections used in dogs are too much for the cat’s immune system to handle.  Rather, cats not showing signs are put on a preventative to prevent re-infection and then we must wait for the adult worms to die on their own (which can take up to seven years.)  Severely affected cats can even require surgical removal of the worms.

While your cat may be at lower risk for heartworm disease than your dog, due to the difficulty in diagnosing and treating this disease in cats, we recommend that all cats be kept on heartworm preventative at least throughout mosquito season (April-December.)

Prevention:  Here is the good news.  Preventing heartworm is easy and much less expensive than treating it.  Preventatives work by killing any larvae that get into the body before they can mature and cause disease.

There are a variety of options for preventing heartworm infection in both dogs and cats, including monthly tablets and chewables, monthly topicals and a six-month injectable product available only for dogs. All of these methods are extremely effective, when administered properly on a timely schedule.  We recommend maintaining all of your pets on heartworm preventative throughout the year, but at least from April to December when heartworm risk is greatest.  Most of these products have promotions for buying a year’s worth at a time and if maintained year round, we only require heartworm testing every other year.


Revolution: Revolution is a product we use for dogs and cats that is applied once a month topically to the back of the neck.  Revolution kills and prevents fleas and ear mites, prevents heartworm, and controls hookworm and roundworm infections.  For dogs, Revolution can also repel ticks.

Heartgard Plus: Heartgard plus is a flavored chewable product for dogs that is given orally once a month.  Heartgard plus prevents heartworm disease, and treats and controls roundworm and hookworm infections.  Heartgard plus does not protect against fleas.

ProHeart 6:  Proheart 6 is an injectable that last for 6 months for dogs.  Proheart prevents heartworm for a continuous 6 months and is ideal for clients that are on the go where a monthly prevention may be forgotten.  ProHeart does not have any intestinal parasite prevention and does not protect against fleas.

Note: The flavoring used in Heartgard can be enough to set off an allergic response in dogs with food allergies.  If your dog has food allergies ask us about topical (or injectable) alternatives for heartworm prevention.

Please visit https://www.capcvet.org for maps of various infectious animal diseases including heartworm in your area.