Let’s Talk Heartworms: What Every Dog Owner Should Know
Did you know that heartworm disease is no longer just a problem in the South? It’s now a risk in all 50 states, including the Pacific Northwest, and the prevalence continues to increase each year. Many pet owners may underestimate the danger, but heartworm disease is serious, potentially fatal, and fully preventable.
Even if you live in a region previously considered low-risk, travel with your dog to other areas - or exposure to wildlife like coyotes, wolves, or foxes - can put your pet at risk. Mosquitoes carrying heartworm larvae can travel great distances, and the relocation of infected dogs to new areas has accelerated the spread. One of the largest events in recent history was following Hurricane Katrina, when over 250,000 pets, many infected with heartworms, were adopted and transported across the country.
Because of this widespread risk, year-round heartworm prevention is recommended for all dogs, regardless of location.
What Is Heartworm Disease?
Heartworm disease is a serious, potentially fatal condition caused by a parasitic worm called Dirofilaria immitis. The parasite is spread through the bite of an infected mosquito, which injects tiny larvae, known as microfilariae, directly into your dog’s bloodstream.
Here’s how the infection develops:
- Larvae migration: After entering the bloodstream, the microfilariae travel through your dog’s tissues and blood vessels. Over the course of approximately six months, they mature and grow.
- Adult heartworm development: Once fully grown, the worms settle in the heart chambers - primarily the right ventricle - and in the large vessels that carry blood from the heart to the lungs. Adult heartworms can grow up to 12 inches long in dogs, and a single dog can carry dozens of worms at once.
- Health impact: The presence of adult heartworms causes inflammation, scarring, and obstruction in the heart and lungs. This can lead to serious cardiovascular and respiratory problems, including heart failure, pulmonary disease, and fluid buildup in the abdomen.
Because heartworms develop slowly and early-stage infection often produces no visible symptoms, they can go undetected for months. By the time clinical signs appear, the worms may have already caused significant, sometimes irreversible damage to the heart and lungs.
Understanding this life cycle highlights why early prevention and annual testing are critical to protect your dog’s long-term health.
Signs and Symptoms of Heartworm Disease
One of the biggest challenges with heartworm disease is that early infection often shows no obvious symptoms. By the time signs appear, the worms may have already caused significant damage to the heart and lungs. Knowing what to look for can help catch the disease before it becomes severe.
Early-Stage Signs
In the first months after infection, many dogs may appear completely healthy. However, some subtle signs can occur:
- Mild, persistent cough – Often mistaken for allergies or kennel cough.
- Decreased energy or reluctance to exercise – Your dog may tire quickly on walks or show less enthusiasm for play.
- Slight loss of appetite or weight loss – Small changes in eating habits may be one of the first indicators.
These early signs are often easily overlooked, which is why annual heartworm testing is so important - even for dogs that seem perfectly healthy.
Moderate to Advanced Signs
As the worms mature and multiply, their impact on the cardiovascular and pulmonary system becomes more serious. Common signs include:
- Labored or difficult breathing – Shortness of breath or heavy panting, even at rest.
- Abdominal swelling – Fluid accumulation caused by heart or liver complications.
- Fainting or collapse during activity – Exercise intolerance becomes more pronounced.
- Persistent cough that worsens over time – Often a sign of pulmonary inflammation.
- Difficulty walking or weakness in the hind limbs – Reduced blood flow and heart strain can affect mobility.
In severe cases, untreated heartworm disease can lead to congestive heart failure or sudden death.
Prevention Is Key
The great news is that heartworm disease is completely preventable. Medications exist that kill microfilariae before they mature into adult worms, which protects your dog from serious heart and lung damage.
The American Heartworm Society recommends the “Think 12” approach:
- Test your dog for heartworms every 12 months
- Administer heartworm prevention 12 months a year
Prevention is inexpensive, whereas treating adult heartworm disease can be costly, risky, and stressful for your dog.
Choosing the Right Heartworm Prevention
There are multiple types of heartworm preventives, including monthly oral tablets, chewables, topical treatments, and injectable medications. Each has its own benefits and considerations:
- Safety: Some medications may interact with other drugs or health conditions.
- Effectiveness: Consistent dosing is critical to prevent infection.
- Convenience: Choose a method that fits your lifestyle to ensure year-round compliance.
Your veterinarian can help you determine the best option for your dog based on age, breed, health status, and lifestyle.
Key Takeaways
Heartworm disease is a serious and potentially fatal condition that can affect dogs in every region of the United States. Understanding the risks and taking proactive measures is essential for keeping your dog healthy.
- Heartworm disease is widespread and can be deadly if untreated. Even dogs in regions once considered low-risk are now at risk. Untreated infection can lead to heart failure, lung disease, and even sudden death.
- Mosquitoes, wildlife, and pet relocation have contributed to the spread. Infected mosquitoes, stray animals, and transported pets can introduce heartworms to previously uninfected areas, making year-round prevention important no matter where you live.
- Early-stage infection is often symptom-free. Because dogs may appear healthy for months, annual testing is critical. Catching infection early allows for safer, more effective treatment and prevents severe heart and lung damage.
- Prevention is safe, effective, and far more affordable than treatment. Heartworm medications kill larvae before they mature, protecting your dog from serious disease. Treatment for adult heartworms is expensive, risky, and can be stressful for both pets and owners.
- Partner with your veterinarian to develop the best prevention plan. Your veterinarian can help choose the right medication, dosing schedule, and testing routine for your dog’s age, breed, lifestyle, and health status. Working together ensures your dog stays healthy all year long, even if you travel or live in a previously low-risk area.
Bottom line: With proper testing, consistent year-round prevention, and regular veterinary guidance, heartworm disease is entirely preventable. Protecting your dog now is the best way to avoid costly, stressful treatment later.
If you’re unsure about your pet, schedule an appointment with Willowbrook Veterinary Hospital today.
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