Heartworm Disease: Why Your Pet Really Does Need Monthly Prevention


Heartworm disease, spread by mosquitoes, has been documented in all 50 states, and is considered endemic in 48 states. Both temperature and humidity play an important role in the life cycle of heartworms.

When a mosquito bites a heartworm-positive dog, the mosquito ingests the microfilaria (maggot stage of heartworm). Inside the mosquito, the microfilaria matures from a baby into a larva (little worm) and then injects the L3 larval stage back into dogs, infecting them with heartworms.

Below is a diagram of the life cycle of heartworms. Once infected, the worms can live five to seven years if they are never treated. You can also see that the monthly preventative is only effective at killing the developing larva (see blue “Prevention Effective” area). This is why it is so important to give prevention every month.

heartworm life cycle

Watch a video of a heartworm case from one of our recent patients that shows microfilaria within the blood of a heartworm-positive dog.

Due to temperature and humidity playing an important role in the life cycle of heartworms, the transmission of heartworm disease is often limited to six to eight months in Chicago. For this reason, many people only give prevention during the “warm” months of the year. Only giving heartworm prevention during the “warm” months can place your dog at risk of developing this deadly disease. Below is an incidence map from the American Heartworm Society from 2016.  As you can see, Chicago sees on average 26 to 50 cases/year per clinic.

heartworm incidence in U.S.

How does heartworm prevention protect my dog?

When a dose of heartworm prevention is given to a dog (either topically or orally) it acts as a dewormer and kills off any of the possible larval stages that mosquitoes have transmitted to dogs. The medication is usually in and out of the dog’s system within 24 hours and does not stay in the dog’s body for 30 days. Heartworm prevention does not kill adult heartworms. It only kills the larval stages. This is why it is important to be given every 30 days because within 30 to 45 days post-exposure the larva can develop into juvenile adults, which the prevention is not effective at killing.

Why do I need to give prevention 12 months of the year, since the transmission of heartworm disease is limited to 6 to 8 months in Chicago?

There are two main reasons we recommend year-round prevention. First, there have been some studies that have shown that a single dose of heartworm prevention does not always kill off 100% of the larva, but repeated doses do kill 100%. Another reason monthly prevention is recommended is because most heartworm preventions also prevent intestinal parasites that can be spread to humans. Your dog can be exposed to these parasites by simply walking in areas where other animals (dogs or feral cats) defecate. When used monthly, your dog will also be dewormed against these parasites—roundworms and hookworms most commonly.

What if I miss a dose? Do I need to re-test before starting back on prevention?

If you miss a dose of prevention or are late giving it, just give it as soon as you remember. The annual heartworm test performed at annual wellness visits test for adult heartworms, and a single missed dose could lead to possible heartworms, but due to the life cycle it would take six to seven months for that test to show up positive. Therefore, we recommend if you miss a dose just start it back up as soon as possible and be sure to have your annual test completed within the next year to be sure that your dog did not get exposed to heartworms during the period in which he/she was unprotected.

Is heartworm disease considered contagious?

Not really. A dog can’t spread heartworms to another dog directly. A mosquito is always needed for transmission. With that in mind, having a heartworm-positive dog in the area does increase the risk for exposure as many mosquitoes may be infected with heartworm, waiting to transmit the disease to their next victim.

Drew Sullivan, DVM, Director
Medical District Veterinary Clinic

How to Ensure Your Pet Has the Longest, Healthiest Life Possible

May 6 through 12 is National Pet Week! This annual celebration sponsored by the American Veterinary Medical Association (AVMA) offers an opportunity for veterinarians to join with clients to promote responsible pet ownership and honor the many amazing pets in our lives.

This year’s theme is “Barks, Purrs, Tweets, Neighs … Pets Speak Love Many Ways.” (The above image shows the 2018 poster contest winner.)

Celebrating the special bond between people and pets reminds us of the importance of pet health care and the need for owners and veterinary caregivers to partner to keep four-legged family members as healthy and happy as possible.

To celebrate the 35th anniversary of National Pet Week, AVMA has shared seven specific values that every pet owner should consider to ensure that their pet lives the longest, healthiest life possible.

1. Choose well, commit for life

Select the pet that’s right for your family’s lifestyle and make a commitment to that pet for its life. Even if you have already welcomed a pet into your home, your veterinarian can help you better understand the social and health care needs of your individual pet.

2. Socialize now—new doesn’t have to be scary

Learn about how to appropriately prepare your pet to enjoy a variety of interactions with other animals, people, places and activities. Everyone will be more comfortable!

3. Nutrition and exercise matter

With an estimated 53% of dogs and 58% of cats in the United States considered overweight or obese, and humans plagued by this issue as well, the AVMA encourages pets and their owners to get proper nutrition and regular exercise—together! This not only improves cardiovascular health, maintains a healthy weight, and supports good mental health for both owner and pet, but it also strengthens the human-animal bond. For tips on walking, running, or starting another exercise program with your pet, visit avma.org/Walking. Visit avma.org/nutrition for more information on your pet’s healthy weight.

Step It Up! is the Surgeon General’s initiative to promote walking. The program encourages brisk walking to reduce the risk of heart disease and diabetes in people. Dog owners know there is no better motivator for a walk than their canine companion. Recent scientific studies show that dog owners may get more exercise and are less likely to be obese than those who don’t own or walk a dog. Owners that walk their dog also had greater mobility within their homes. Other studies have shown that all pets, not just dogs, have been shown to lower heart rates and blood pressure as well as promote quicker recovery times from stressful events.

The Centers for Disease Control recognizes that keeping pets healthy keeps people healthy too. Visit cdc.gov/healthypets for more information.

4. Love your pet? See your vet!

Everybody loves their pet, yet a majority of cat owners and nearly half of dog owners do not take their pet to the veterinarian unless it is visibly sick or injured. Pets often hide signs of illness. Reular check-ups are vital to catching health problems early. Early treatment means better health for your pet. It can also save money!

5. Pet population control: Know your role

Do your part to prevent pet overpopulation. Talk to your veterinarian about when you should have your pet spayed or neutered. Avoid unplanned breeding through spay/neuter, containment, or managed breeding. To learn more, visit the AVMA webpage on spaying and neutering your pet.

6. Emergencies happen. Be prepared.

Include your pets in your family’s emergency plan. The AVMA offers a step-by-step guide to assembling emergency kits and plans for a variety of pets and animals.

7. Give them a lifetime of love

Thanks to better care, pets are living longer now than they ever have before—but as pets get older, they need extra care and attention. Regular veterinary examinations can detect problems in older pets before they become advanced or life-threatening, and improve the chances of a longer and healthier life for your pet.

Visit the AVMA’s special page for senior pets to find out what is “normal” and what may signal a reason for concern about an aging pet. Contrary to popular belief, dogs do not age at a rate of seven human years for each year in dog years.

To honor and celebrate all of the amazing dogs and cats that are a part of our Medical District Veterinary Clinic family, we invite you to bring in your favorite pictures, so we can post them around the clinic. You can also email them to gethelp@medicaldistrictvet.com. We can’t wait to see all of the adorable photos!

Amber Slaughter, DVM
Medical District Veterinary Clinic