Dog heartworm disease is something no pet owner wants to deal with—thankfully, it's a condition that you can easily prevent. It's also a disease that dog owners should be well educated on, as quick treatment if it does occur is of utmost importance. We've taken the most frequently asked questions about dog heartworm disease and answered them as thoroughly and accurately as possible so that you can ideally prevent your dog from ever having to experience this.
If you're looking for a highly trained veterinarian in Colorado Springs, CO, we'd love to help you care for your dog and other pets that you have. The first order of business is to get your dog seen right away to get them on heartworm prevention if they're not already, so please call us at (719) 212-1857.
What is heartworm disease and how can it affect my dog?
Heartworm disease is a serious and potentially fatal disease that occurs in many parts of the world in which the dog is afflicted with foot-long heartworms, or parasites, which live in the right side of the heart. These parasites cause problems with the blood flow and the sealing of the valves—opening and shutting on the right side of the heart. And the most common thing that we see is respiratory issues because blood goes from the right side of the heart to the lungs. If heartworm disease affects those pulmonary arteries, which it often does, and those worms sometimes try to migrate into the lungs, that migration can cause a lot of respiratory signs and damage to other organs. The dog will often have difficulty breathing and will likely be coughing. However, signs like these typically only show up after the infection has been established and progressed to cause a high level of damage; most positive dogs show no outward signs at all.
For many years, Colorado was not seen as a state with high risks for heartworms. However, since 2017, we have seen a large increase in the number of positive cases throughout the state. In 2025, 1 in 300 dogs tested for heartworm disease tested positive, with 180 cases in El Paso County alone. Many rescue dogs coming up from areas like Louisiana, Mississippi, and Texas can bring heartworms up with them, and with our changing weather patterns over the last several years, it has become even easier for these to spread among our pets at home. Recommendations have also changed from seasonal heartworm prevention to year round protection, as our winters have seen temperatures swing from below freezing to above 50 degrees within a single month.
How would my dog catch heartworms?
The only way for dogs to get heartworm disease is through a mosquito bite—they are a mandated intermediate host. A mosquito bites a dog that has heartworms and picks up immature larvae from that dog. This larva has to live and develop in the mosquito for about two weeks, and then it turns into infective larva. So if that mosquito, which now has heartworms, bites a dog that's not on prevention, it injects the infective larvae into that dog. This is how a dog will develop heartworms. An infected mosquito can travel up to 2 miles before it bites another dog, which also contributes to the spread of heartworms.
Can dog heartworms be prevented?
Yes, absolutely. In most areas, there's almost nothing you can do to completely get rid of mosquitoes, at least effectively enough to eliminate heartworm disease. Even dogs housed mostly indoors can encounter mosquitos that make their way into our homes. Hence, our way of preventing heartworm disease in dogs is by putting them on heartworm prevention. These preventatives kill the baby heartworms before they can develop into adults. There are plenty of kinds of heartworm prevention methods that we tailor to your pet and their lifestyle. Injectable preventatives such as ProHeart, can be given at your pet's annual vet visit, eliminating the need for monthly reminders about medication. For dogs that love taking their monthly treat, oral preventatives such as Simparica Trio are flavored chewables, which makes giving medication much easier, and combine heartworm and intestinal parasite prevention with tick and flea prevention as well. We recommend the use of the newer generation of heartworm medications, as resistance to the older generation of medications has started to emerge all over the country. Your vet will talk to you about what works best for your pet and your lifestyle.
What are the signs in my dog that would indicate they may have heartworms?
Mild or low worm burden cases of heartworm disease are asymptomatic, so we will screen dogs every year for heartworm antigen. Even if they're asymptomatic and have been on heartworm prevention, tests screen for cases of resistance as well as major tick-borne diseases in Colorado, so we can catch any cases that might pop up. Early diagnosis is linked to a much better outcome for most patients, as we want to treat them before advanced damage is done to the heart and lungs.
When an infected dog starts showing symptoms, they typically depend on the stage of the infection:
- In the early stages, you can see lethargy, coughing, and shortness of breath.
- Once your dog has adult worms that live inside the heart, they can do a lot of damage over time. Again, the dog will be coughing, lethargic, and possibly experience exercise intolerance, so they cannot play or go for walks as long as they usually would.
- If you have a severe worm burden in the mid to late stages, you can have a dog with congestion in the lungs or the liver, so you could see bulging ribs or abdomen. The dog could also develop a heart murmur because of the worms affecting the function of the heart.
- In more severe, late-stage cases, infected dogs can have edema or swelling of the extremities, and the abdomen will fill up with fluid. Those are much more advanced cases.
How will a veterinarian diagnose if my dog has heartworms?
The only way that we can easily detect heartworms is by doing a blood test. The female heartworm releases a protein (antigen) that we can pick up on in the dog's blood. The vast majority of infections are going to be circulating heartworm antigens in the blood. A small blood sample allows us to detect adult worms, and then we know that the dog is infected.
It's vital to understand the life cycle of a heartworm. When the mosquito bites the dog, it takes five to six months for that worm to develop into an adult worm, and the adult heartworm is what we're able to detect in the blood. If your dog were infected within the last six months, the heartworm test today would not be able to pick up that heartworm. That's why if they've been off prevention and are an adult dog, we test them now and then again in six months to ensure they're negative.
Why is early detection and diagnosis of heartworms so important?
Early diagnosis and detection are essential because early detection of adult heartworms often can allow us to intervene and treat the heartworms before they do significant damage to the heart and lungs. And if we can do that, we can usually have long-term success with minimal to no long-term complications.
How is heartworm disease treated?
Although somewhat manageable in the early stages, the treatment for heartworm disease is certainly no fun for the dog. We start dogs on antibiotics to kill off a bacteria that lives in the heartworms, making them weaker and more susceptible to treatment. We typically do three spinal muscle injections—one on the first day, then one a month later, and one a day later. These injections can be uncomfortable, so we manage their pain and ensure we reduce inflammation with medication. We also have them on strict activity restriction for a prolonged period of time because as these worms die off, they can shed little pieces off into the dog's heart and the bloodstream, which increases with increased stress on the heart. High activity increases the risk that these pieces can end up within vessels in the lungs, causing a condition called pulmonary thromboembolism (restricted blood flow to a major portion of the lungs), which is an emergency condition. Heartworm treatment can be challenging and expensive, which is one of the reasons why we focus so heavily on prevention.
How soon should I bring in my dog to see a veterinarian for heartworm prevention?
Your dog should come in as soon as possible. If you have a puppy, we recommend bringing them in as soon as they come home with you. We recommend starting puppies on heartworm prevention at eight weeks, or, again, as soon as you have them. Any dog over six months should have a heartworm test before starting for heartworm prevention. All veterinarians will highly recommend bringing any rescues, adoptees, or adult dogs that are currently not on prevention in for a screening, to start heartworm prevention.
Heartworm prevention for dogs that aren't infected is designed to prevent the disease, as even puppies are exposed to mosquitoes. When mosquitoes bite a dog and deposit baby worms into their skin, the worms are susceptible to being destroyed by these heartworm preventatives before they enter the bloodstream. This is why it's important that we start preventatives early, before the worms can do any damage.
The American Heartworm Society is another helpful resource on dog heartworm disease. If you live in or near Colorado Springs, CO, and are looking for quality pet care, come visit us at North Powers Animal Hospital where your pet is our priority. Contact us at (719) 212-1857 to set an appointment, or email us at [email protected]. Our staff would love to talk with you!