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Updated: Nov 11, 2020

We receive so many questions from owners asking about heartworm disease and heartworm prevention, so today we consulted with Dr. Staci Dennis to set the record straight! Dr. Dennis responded to some of the most commonly asked or mentioned comments we receive from owners regarding heartworm disease and prevention.


What are heartworms?

“Heartworms are a worm that affects the heart and lungs. They are spread by mosquitoes, and are so much easier to prevent than to treat. There are other parasites that are easily treatable, but heartworms is not one of them. There are so many things that we have no prevention or control over, so we should take advantage of the fact that heartworms have a means of prevention."

~Dr. Staci Dennis, DVM, DABVP

Heartworm disease is transferred to pets through mosquitoes, but how? Not all mosquitoes are capable of carrying larval heartworms, but ones that are capable must also exist in warm climates so that the larvae is able to mature. Unfortunately, Texas’ climate makes it one of the top States in the U.S. for heartworm disease. Once a mosquito has given a pet heartworms, other mosquitoes that bite the pet afterward are now capable of carrying the larvae themselves, which creates a heartworm endemic in affected areas. The result of heartworm disease can involve worms up to 14 inches in length growing and living in your pet’s heart.

Signs of heartworm disease can range from no symptoms at all to a life-threatening situation. Some of the symptoms associated with heartworm disease are:

  • Coughing

  • Shortness of breath/ panting

  • Easy tiring/ intolerance of exercise

  • Fluid accumulation in the abdomen or chest

  • Nose bleeds

  • Sudden death

Thankfully, heartworms are easy to prevent. Dogs and cats are able to start heartworm prevention as early as four to six weeks of age. There are some differences in age requirements for starting heartworm prevention based on product and species. We recommend giving heartworm prevention to your pet as instructed, every 30 days, all year-round. Once dogs reach one year of age, they are required to have an annual heartworm test in order to refill heartworm prevention. The annual heartworm test ensures that we are still heartworm negative, as giving heartworm prevention to heartworm positive dogs can cause adverse reactions.

Frequently-heard reasons for declining heartworm prevention

“My pet is indoor only” “My pet never goes outside” “My pet only goes outside to potty”

It only takes one bite from a mosquito carrying microfilaria, or “baby heartworms,” for an animal to become infected. This means that your pet can become heartworm positive even if they never leave their backyard, and if they are only outside for a couple of minutes at a time. Additionally, it is extremely difficult to ensure that no mosquitoes ever make it inside of your home, meaning your pet can be affected indoors, as well.

“Prevention makes my pet sick”

Like with any other type of drug, in human and animal medicine, there is no guarantee as to how an individual will respond to a new medication. Heartworm prevention is taken by lots of pets with rare reports of adverse side effects, and is considered to be one of the safest medications we can give our pets. Contact your veterinarian in the event you feel your pet is having an adverse reaction to heartworm prevention.

“I’m really bad at remembering to give it”

Luckily, since heartworm prevention is typically given every 30 days, most owners are able to give it on the same day every month, which can make it easier to remember. Putting a reminder in your phone, on your calendar, or putting a note next to your pet’s food or medications can help remind you to give the prevention.

“None of my other dogs have ever had heartworms”

Not all mosquitoes are capable of transmitting heartworms, but it only takes one bite from one affected mosquito for a dog to contract heartworms. While there is nothing we love hearing more than “my dogs have never had heartworms,” that merely suggests that your dogs are extremely lucky. There is currently no data that supports the idea that some pets are inherently more immune or resistant to heartworms than others.

“My previous dog had heartworms and he seemed to be just fine”

Signs of heartworm disease can range from no symptoms at all to life-threatening symptoms. Many dogs do not show any symptoms during the early stages of heartworm disease, but that is not to suggest that their bodies aren’t being negatively affected by heartworm disease. More advanced heartworm disease can include heart failure, fluid build-up in the chest, and exercise intolerance.

“It’s too expensive”

The average cost of heartworm prevention typically runs between $5-$15 per month. For the price of 2 lattes per month, you can protect your pets from heartworm disease, whereas heartworm treatment can be upward of $1000. Contact your veterinarian’s office for an estimate on heartworm prevention.

“I only give prevention during the hotter months of the year”

Due to the hot climate in Texas, we recommend giving pets heartworm prevention year-round. Texas is one of the states with the highest number of heartworm cases each year, and this is largely due to the fact that Texas never truly freezes, making it possible for mosquitoes to affect pets throughout the entire year.

“I prefer to buy heartworm prevention online/through a pharmacy”

While heartworm prevention may be purchased online, we strongly recommend purchasing heartworm prevention through your veterinarian, for several reasons - the main reason being that products sold by a veterinarian are purchased from a reputable distributor. Did you know that most major heartworm prevention companies will often pay for heartworm treatment in the event that a dog tests positive for heartworm disease? These companies require proof of veterinary records that indicate hearworm prevention has been purchased consistently without any lapse in patient reminders. This may exclude prevention purchased online. Additionally, distributors and veterinary clinics can assure that products have been stored correctly, and are not altered by extreme temperatures that may affect the effectiveness of the drug.

“I don’t think my pet would ever come up as positive for heartworms, but if they do, I’ll just treat it when it happens”

Preventing heartworms is far less expensive for owners, and far less painful for the dog. Heartworm treatment often costs upward of $1000, lasts 4-6 months, requires 6-8 weeks of strict crate rest and isolation, and requires multiple injections of an arsenic-based drug to treat the heartworms. Additionally, heartworm disease can cause damage to the heart that cannot be reversed even after the heartworms are treated.

At this time, there is no treatment for cats diagnosed with heartworm disease, so prevention for cats is imperative.

“My dog’s heartworm test just came back negative”

We love seeing negative heartworm tests! Let's keep it that way by keeping your pet on a heartworm prevention.



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