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Heartworm Prevention: Does your pet need it?

heartworm prevention pet needed need it boulderholisticvet blog post

Most pet parents want to limit their pet’s exposure to chemicals and medications. I am frequently asked if heartworm prevention is necessary. In this article, I will share my thoughts and opinions about preventing heartworm disease and what I recommend for my patients.

I am exceedingly lucky to live in an area that is NOT endemic with heartworm disease. Most pets that are heartworm positive in Boulder County were infected somewhere else. I use heartworm prevention occasionally in my own dog and never in my indoor kitties.

Life Cycle

If you want to geek out with some parasitology, here is a great image of the life cycle of the heartworm.

The following is a summary of key facts to help you understand the most important points.

  • Monthly heartworm prevention is effective against the L3 and L4 stages. This is when you can prevent the larva from becoming adults.
  • The blood test that is used at your local veterinarian’s office detects the adult worm.
  • The adult worm can live for several years.


When trying to decide if you should give your pet prevention, there are two important questions you should answer.

  1. Do I live in a heartworm endemic area?
  2. Are mosquitoes alive and active in my area?

Is heartworm endemic in your area?

It is important to understand if you are in a heartworm endemic part of the country. If you are unsure, you could always call a few veterinary clinics in your area and ask them how often they see heartworm positive cases. For example, in the 11 years I have been practicing in the Denver/Boulder area, I have seen one heartworm positive dog. I have however seen many heartworm positive dogs that were rescued from other heartworm endemic areas like Texas. You can also consult this map from the American Heartworm Society.

If you live in an area that does not have heartworm disease, skipping heartworm prevention may not pose a risk to your pet. While the possibility for heartworm infection is present in much of the United States, certain areas have a very low incidence of infection. You must assess your own risk tolerance.

For example, I feel completely comfortable skipping heartworm prevention for my pug most summer months. I do, however, test him yearly to ensure his heartworm negative status. Approximately 25% of my clients use heartworm prevention in the summer months while residing in a non endemic area of the United States.

If you live in an area with a high incidence of infection, I would recommend using heartworm prevention. In my practice, I use an ivermectin products like Heartgard. The dose of ivermectin needed to prevent a heartworm infection is very small and safe (more on this later).

Are mosquitoes alive and active in your area?

Depending on where you live in the country, you may experience a winter season. If there are no mosquitoes present, there is no need to use heartworm prevention. For example, Boulder County will experience cold temperatures starting in October that are incompatible with the mosquito life cycle.

Many veterinarians recommend using heartworm prevention year round to prevent intestinal parasites. It can also be helpful for pet parents to establish a routine of giving heartworm prevention at the same time every month. This avoids accidentally missing months of needed prevention.

Travel is another important consideration. If you live in North Dakota and travel to Texas in the winter months with your pet, it is important to use heartworm prevention. If you are unsure if your travel destination is a heartworm endemic area, talk with your veterinarian.

Additional Considerations

Side Effects

Even ivermectin products can have side effects. The most common side effects I have observed are vomiting and diarrhea. This is generally in response to the flavor additives in the chewable formulations. You can purchase Heartgard in an unflavored tablet or have it compounded.

Ivermectin does have slight antibiotic properties. I recommend using a high quality probiotic and feeding a diet that is rich in whole foods while giving heartworm prevention.

MDR1 Gene Mutation

There is a mutation that exists more commonly in Australian Shepherds and Collies (here is a full list of breeds affected) that causes sensitivity to many drugs including ivermectin. However, the dose of ivermectin in heartworm prevention is so small that it is does NOT cause symptoms. You can learn more about this mutation here.

If my pet is healthy, won’t their immune system prevent infection?

Maybe, but not necessarily. Animals with healthy immune function tend to be less prone to parasitic infections. For example, I have two horses in the same herd. They eat the same food, graze on the same grass and keep the same schedule. When I test their stool, my very allergic horse always has a parasite, while my other older horse does not.

You may feed your pet the perfect diet, support them with supplements and they may still have problems with their immune function. This may be due to genetic factors and environmental factors that are out of your control.

Treating heartworm disease is not ideal. I strongly believe that if you live in an endemic area that you can use prevention in non invasive and healthy way. You can read more about the treatment of heartworm infection here.


The current standard for testing for heartworm disease is the blood antigen test. This test is recommended at least one time yearly to detect the adult female worm.

I have seen protocols that are circulating the internet that recommend using a PCR (DNA test) test four times a year to detect the L3 stage instead of using heartworm prevention. I don’t currently recommend this protocol as this PCR test has not been validated. This method may miss the L3 stage completely and lead to the maturation of adult heartworms.


Honestly, the treatment for adult heartworms is just plain yucky. It requires a lot of cage rest and the side effects can be bad. However, when dogs are truly heartworm positive, it can save their life.

Some pet parents elect to not treat their pet’s heartworm disease. This is common especially in older dogs. In these cases, a slow kill method can be used. These dogs take a monthly dose of heartworm prevention in combination with a round of doxycycline (antibiotic). While this method is not supported by the American Heartworm Society, I have seen it’s appropriate application in practice.


Good news for kitties! Heartworm disease is way less common in cats compared with dogs. Cats are also better at clearing these infections on their own. The rare kitty that is diagnosed with heartworm disease is generally very sick with respiratory symptoms.

I would love to hear from you! Do you use heartworm prevention for your pet? Leave a comment or question in the section below!

Sending love to you and your furry friends!

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13 thoughts on “Heartworm Prevention: Does your pet need it?”

  1. I’m in DFW Tx does my dog needs the heart worms medicine I’m not crazy about giving my dog any meds I believe only in the rabies shot.

    1. Hi Anna. I hear you on not wanting to give your pup any meds and there is a high prevalance of mosquitoes in your area. For this reason, we would recommend year round heartworm prevention for your dog. JoJo, Veterinary Nurse

  2. Hi, my 1 year old husky was diagnosed in May with myasthenia gravis and megaesophagus. I live in NW Arkansas and we live in the country and have mosquitoes. I normally give my dogs (4) all together an annual heart worm prevention shot. But I’m concerned this may have adverse affects for Lil Fred due to the auto-immune disease. He is due for his shot at the end of this month. He had a six month shot in February. what is your advice?

    1. Hi Jill, we really appreciate your question and want to say what a great job you are doing as an advocate for Lil Fred (cute name!). In our practice, we use Heartgard Plus and for our most sensitive pups we have had the single ingredient of Ivermectin compounded into a capsule form. There are many online (and local) compounding pharmacies that offer this option. This would be a great thing to discuss with Lil Fred’s veterinary team at his next visit! We’d love to know whatyou and your veterinary team decide. In love, JoJo

  3. We live in in western colorado. Do you recommend heartworm for our area and, if so, which month should I begin giving it to her. She has been diagnosed with 4th stage breast cancer but is doing very well.

  4. This is from what Dr. Angie wrote above. Is this correct? Most summer months? Thank you.
    “For example, I feel completely comfortable skipping heartworm prevention for my pug most summer months.”

    1. Hi Karen,

      This is a great question!

      It truly depends on location. Here in Colorado, we do not have too many mosquitos or heartworm positive cases, even in the summer months. However, in lower altitude areas, this would not be true and heartworm prevention would be really important year-round.

      It also wouldn’t be wrong for Colorado dog parents to give heartworm prevention during summer months. It’s very dependent on the pet parent’s comfort level, lifestyle, and approach to their pup’s health care.

      I hope this helps!


  5. I also live in Colorado. When do you get your Heartworm test done for your dog? Should I even bother in the winter or wait until spring or summer?

      1. What is the safest heartworm prevention medication? And flea/tick medication? My vet started us on Simparico Trio but I’m seeing a lot of information on serious adverse effects and I don’t want to use it any longer. But I live in Coastal NC, we definitely have a lot of mosquitoes, so I must use something. I have a 50lb standard poodle, 1.5 years old, just got her in May and I love her more than the world!

        1. Hi Ami,

          Awww, I love how much you love your girl! She’s so lucky to have you!

          We generally use Heartgard here in Colorado. That said, different products work better depending on what part of the country you’re in (some bugs are more resilient in certain areas!). I would ask your veterinarian about Heartgard!!

          With love,

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