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6 Dangers of Skipping Heartworm Prevention for Dogs

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Heartworm disease affects dogs in many parts of the world and continues to spread to new regions. In the U.S., canine heartworm disease has been identified in all 50 states (1, 2). This is especially disappointing, because heartworm disease is preventable and prevention does not have to be expensive.

Heartworm disease is transmitted through the bite of an infected mosquito. After a lengthy period of development inside a dog’s body, adult heartworms take up residence in the heart and blood vessels of the lungs. Heartworm prevention kills the worms during their development so that they never cause damage to your pet. The consequences of skipping heartworm prevention for dogs can be steep—even costing your canine companion her life.

Here are just some of the dangers of skipping heartworm prevention for dogs.

6 Dangers of Skipping Heartworm Prevention for Dogs

No Added Protection Against Intestinal Parasites

Many heartworm preventives are also monthly dewormers for your dog. Frequent deworming of your dog is recommended because some of these worms can be passed to humans. Depending on which heartworm preventive you and your veterinarian choose for your dog, it may cover hookworms, roundworms, whipworms and tapeworms. For instance, Interceptor® Plus (milbemycin oxime/praziquantel) offers five-worm protection in a monthly chew. If you skip your pet’s heartworm prevention, your dog will miss out on this added benefit of treating and controlling intestinal worms.

See important safety information below for Interceptor® Plus.

Even One Skipped Dose Puts Your Dog at Risk

dog lies in grass

Monthly heartworm preventives should be given every 30 days. If you skip or delay giving your dog even one dose, she will be at risk of infection. 

The way that heartworm prevention works is by killing a larval stage of the worm. These larvae get into your dog from the bite of an infected mosquito. If you don’t give your dog the medication every 30 days, the larvae will grow to a point where the preventive won’t be able to kill them anymore.

If the larvae are not killed during this brief window, they are not susceptible to any known treatment until they are adults in about six to seven months. At that point, they are causing damage to your dog’s lungs. 

You Won’t Know Your Dog Is Infected for Months

To make matters worse, you won’t know your dog has heartworm disease for at least six months after a skipped dose.

Heartworms cannot be detected by any current test until there is at least one adult female worm in your dog. The most common blood test performed by veterinarians looks for proteins produced by adult female heartworms, called antigens. Since it takes at least six months for heartworms to reach adulthood, it takes at least that long to know if your dog is infected after missing a dose. 

One of the first signs of heartworm disease you may notice in your dog is that she gets tired after a short walk or a few rounds of fetch. Veterinarians call this exercise intolerance, but what it means is that your dog isn’t herself. That playfulness that you love is decreased or gone.

Heartworms cause exercise intolerance because they damage the lungs and are physically in the blood vessels and heart, which makes it difficult to pump blood and oxygen to the rest of the body. Without enough oxygen, your dog is tired all of the time. She can’t do all of the activities she enjoys because of a preventable disease. 

Heartworm Treatment is Expensive

vet looks at X-ray of dog

Heartworm treatment requires multiple injections given by your veterinarian. These injections, along with the necessary tests and additional medications, are expensive—treatment can cost well over $1,000. 

Even before the first dose of heartworm treatment is given, your veterinarian will need blood work, an EKG (electrocardiogram), and chest X-rays to make sure your dog is healthy enough to undergo treatment. Since there are no tests to count how many worms are living in your dog’s lungs, these other diagnostic tests are able to tell your veterinarian how much damage they have caused. Depending on the extent of lung damage, your veterinarian may also recommend an echocardiogram.

Heartworm prevention, on the other hand, is in the range of $8-$20 per month, depending on both the size of your dog and type of prevention you choose. This means that it is often less expensive to give prevention to your dog every month of every year of her life than undergo treatment once. Plus, your dog will need heartworm prevention during and after treatment, or else she will be susceptible to new infection.

Heartworm Treatment is Painful and Long

dog rests in crate

Heartworm treatment is also taxing on dogs—it’s painful, long, and arduous. From the time your dog is diagnosed with heartworm disease until several months after treatment, she will need to be on strict rest. This means no running, no playing, and no active adventures. This restriction could last four to six months, because heartworm treatment takes time.   

The recommended treatment protocol starts with a one-month course of an antibiotic that will weaken the adult heartworms, then the first injection of the medication (melarsomine) to kill the adult heartworms. Melarsomine is derived from an arsenic compound, so accurate dosing and proper administration are imperative to avoid potentially serious side effects. The first injection is followed at least one month later by two consecutive doses, and then it’s still another six to eight weeks before your dog can play again (3). For many dogs, and their human family members, this long restriction is very difficult emotionally, logistically, and physically.

Some dogs are too sick to undergo this treatment and for these dogs, a risky surgical procedure is their only option. 

Heartworms Can Be Fatal

Left untreated, heartworms can cause enough lung and heart damage to kill your dog. Some dogs who are fortunate to only have a few heartworms may outlive their parasite. But, it takes five to seven years for heartworms to die naturally, during which time they are continuing to cause damage. 

Even once treated, dogs who previously had heartworm disease may later suffer complications that shorten their lives. It is not worth the risk. Protect your dog year-round with a monthly dewormer that includes heartworm prevention.

Interceptor Plus Indications

Interceptor Plus prevents heartworm disease and treats and controls adult roundworm, hookworm, whipworm, and tapeworm infections in dogs and puppies 6 weeks or older and 2 pounds or greater.

Interceptor Plus Important Safety Information

Treatment with fewer than 6 monthly doses after the last exposure to mosquitoes may not provide complete heartworm prevention. Prior to administration of Interceptor Plus, dogs should be tested for existing heartworm infections. The safety of Interceptor Plus has not been evaluated in dogs used for breeding or in lactating females. The following adverse reactions have been reported in dogs after administration of milbemycin oxime or praziquantel: vomiting, diarrhea, decreased activity, incoordination, weight loss, convulsions, weakness, and salivation. For complete safety information, please see Interceptor Plus product label or ask your veterinarian.

Disclaimer: The author received compensation from Elanco US Inc., the maker of Interceptor Plus, for her services in writing this article


  1. Drake, J., Parrish, R.S. Dog importation and changes in heartworm prevalence in Colorado 2013–2017. Parasites Vectors 12, 207 (2019). https://doi.org/10.1186/s13071-019-3473-0
  2. Drake, J., Wiseman S. Increasing incidence of Dirofilaria immitis in dogs in UA with focus on the southeast region 2013-2016. Parasites Vectors 11, 39 (2018). 10.1186/s13071-018-2631-0
  3. American Heartworm Society. (2018). Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs [PDF file]. Retrieved from https://www.heartwormsociety.org/images/pdf/2018-AHS-Canine-Guidelines.pdf

Interceptor is a trademark of Elanco or its affiliates.

© 2020 Elanco.  PM-US-20-0719

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