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Heartworm Disease

What is heartworm disease?

Heartworm disease has been diagnosed in all 50 states and is a serious, sometimes fatal, disease. It is caused by worms that can grow up to a foot long and live in the right side of the heart and the arteries of the lungs. Heartworm disease is most often considered a disease of dogs, but it can affect cats as well. Foxes, coyotes, and wolves can also be infected by heartworms and serve as a reservoir or source of infection for pets.

Heartworm Incidence 2013


How are heartworms spread?

Heartworms are spread through the bite of a mosquito. When a mosquito bites a dog infected with heartworms, it ingests microscopic heartworm larvae. The larvae mature into juvenile heartworms, which can now affect new dogs and cats when they are bit by the mosquito. Once the juvenile heartworms enter a dog or cat, it takes 4-6 months to mature into adult heartworms. While dogs are the true hosts of heartworms, cats are considered to be accidental hosts. This means that heartworms do not usually reproduce in a cat, and cats tend to be infected with only 1-3 heartworms at a time.

Heartworm Life Cycle


What are the signs of heartworm disease?

Dogs: When dogs are first infected with heartworms, they may show few if any signs. As time passes and the heartworms mature and reproduce, the signs become more pronounced. Signs may include mild cough, a reluctance to exercise, becoming fatigued easily, decreased appetite, and weight loss. More severe signs include a swollen belly, heart failure, blood clots, and sudden death.

Cats: Since cats are accidental hosts, they do not have the same clinical signs as dogs. Often, clinical signs in cats are due to a type of allergic reaction to the heartworms. Signs may include coughing, asthma-like attacks, periodic vomiting, lack of appetite, or weight loss. Unfortunately, sometimes the first signs in cats are collapse and death.


How is a heartworm infection diagnosed?

Since heartworms can cause such severe disease, the earlier an infection is detected the better. Adult heartworms can be identified by performing a blood test. Since it takes up to 6 months for adult heartworms to be fully mature, dogs must be at least 7 months of age to test. Heartworms are much more difficult to detect in cats, and often involves at least two blood tests. Sometimes radiographs (x-rays) or an ultrasound is needed to detect heartworms in a cat.


How is heartworm infection prevented?

We recommend that all dogs be on heartworm prevention all year. Dogs can be started on heartworm prevention as young as 6 weeks of age. There are several different preventatives available including Heartgard Plus, Trifexis, and Revolution.

  • If a dog is less than 7 months of age, then we recommend starting heartworm preventatives immediately. Your dog should then be tested for heartworms at one year of age. If preventatives are maintained consistently all year around, then we recommend testing for heartworms every 3 years.

  • If a dog is over 7 months of age and has not been on heartworm preventatives previously, it should be tested for heartworms prior to starting preventatives. If negative, your dog can immediately start preventatives. Your dog should be tested again for heartworms 6-12 months after starting preventatives because the adult worms can take 6 months to fully mature and be detected by the heartworm test.


If your dog remains on heartworm prevention year-round then we recommend testing every 3 years. While heartworm prevention is very effective, it is not 100% and on rare occasions, dogs are diagnosed with heartworm infection even after being on preventative. If your dog misses a dose of heartworm preventative or you are late in administering, then we recommend continued heartworm prevention and performing a heartworm test approximately 6 months after the missed or late dose.


Cats can also be given heartworm prevention starting as young as 6 weeks of age. Since the risk of infection is lower in cats, we recommend that cats be on heartworm prevention at least during the months of the year when mosquitoes are most prevalent (April- November).


Another way to protect your dog and cat from heartworm infection is to limit their exposure to mosquitos. Dispose of sources of standing water, such as old tires or buckets, because these are breeding areas for mosquitoes.  Limit your pet’s time outside in the evening hours when mosquitoes are most active. Use mosquito repellents if your dog or cat will be outside when mosquitoes are active, such as Skin So Soft or the flea and tick preventative Vectra.


What happens if my dog is heartworm positive?

If your dog is found to be infected with heartworms then we recommend treatment as soon as possible to minimize the damage that a heartworm infection can do to the heart and lungs. The treatment protocol occurs over the course of 6-9 months.  During this time, your dog must be on strict exercise restriction.

  • Day 1:  Your dog will be given a dose of heartworm prevention to kill any juvenile worms that are present. In addition, your dog will be started on an antibiotic called doxycycline. The antibiotic treats a bacteria that is associated with the adult heartworms and aids their viability and reproduction. Eliminating this bacteria will make the adult heartworms easier to kill.

  • Day 30: Your dog will be treated with medication to kill the adult heartworms called melarsomine. We recommend hospitalization for the day to monitor your dog for any adverse reactions to the medication. In addition, your dog will be sent home with steroids to help minimize the body’s reaction to the dying adult heartworms. Heartworm preventative is also administered.

  • Day 60 & 61: Your dog is given two more doses of melarsomine 24 hours apart. Your dog will be hospitalized overnight to monitor for any adverse reactions. Your dog will again be sent home with steroids and will be given heartworm prevention.

  • Day 90: A blood test will be performed to test for juvenile heartworms. This test is performed in our clinic and takes 15-20 minutes.  Your dog should continue to receive heartworm prevention every 30 days.

  • Day 240: Your dog will be tested for adult heartworms. If your dog is negative, the heartworms were eliminated and heartworm prevention should be continued year-round.


There is no approved treatment for adult heartworms in cats. The melarsomine that is used in dogs is too potent for a cat’s immune system to tolerate. When a cat is diagnosed with heartworms, we recommend administering heartworm prevention and waiting for the heartworms to die on their own, which can sometimes take up to 7 years. In severely affected cats, surgery may be necessary to remove the adult worms.


Where can I find more information on heartworm disease?

The American Heartworm Society provides the most up to date information regarding heartworms in dogs and cats.

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