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Causes of Heartworms in Dogs, Prevention, and the Symptoms

Heartworm prevention is a year-round task. That's the first thing you should know if you're researching this life-threatening disease. As a pet parent, your dog depends on you to protect them from these fatal parasites that could lead to lung disease, heart failure, and other organ damage. So, before heading out for a game of fetch, take a few minutes to learn what causes heartworms in dogs and why prevention—rather than treatment—is essential.

What are Heartworms?

A heartworm (Dirofilaria immitis) is a variety of parasitic roundworms. These worms mature into adults, mate, and produce offspring all while living inside your dog. During this time they infest muscles, blood vessels, the pulmonary artery, and heart. Adult heartworms, which look like cooked spaghetti, can live inside your dog for 5 to 7 years, leading to a wide range of health problems. They vary in length with female worms reaching up to 12 inches and male worms reaching up to 9 inches.

Once you understand the heartworm life cycle it becomes easier to see why heartworm prevention for dogs is crucial.

How Do Dogs Get Heartworms?

Dogs get heartworms through the bite of an infected mosquito. Infected dogs cannot pass heartworms to other dogs since these parasites need a mosquito to develop to the “infective stage.” Mosquitoes serve as the intermediate host where larvae mature as well as the vector that transports parasites to dogs, cats, ferrets, and other mammals.

Heartworms go through a long series of larval (immature) stages before becoming adults. The first larval stage exists in already infected dogs who have microfilaria (baby worms) in their bloodstream. When a mosquito feeds on an infected dog, first-stage larvae migrate to the mosquito where they complete two more larval stages over a period of 10-14 days. At this point, larvae are at the “infective stage.”

When an infected mosquito bites a dog, they leave a bite wound, creating a path for infective larvae to enter their new host. Once passed on to a dog, the larvae continue their progression into adulthood over 6 months, infecting areas in and around the dog's heart and lungs. Breeding-age adult heartworms then produce a new set of first-stage larvae and the cycle begins again.

In the United States, the warm southern regions have historically seen a larger occurrence of heartworms, but all 50 states have reported the disease. Humans see mosquitos as a nuisance, but when thinking about what causes heartworms in dogs, they can be life-threatening to pets.

Heartworm Prevention for Dogs

It's possible to cure a dog of an infestation of adult heartworms, but the process is time-consuming, expensive, and can be hard on your pet. The typical treatment requires hospitalization while receiving injections of heartworm adulticides, along with several weeks of limited exercise. You should also consider damage to your dog's body before they even begin treatment. Consulting a veterinarian will help you determine the best course of action.

In all cases, heartworm prevention for dogs is key. Since it isn't practical (or possible) to protect your dog from ever getting bitten by a mosquito, the best solution is to prevent potential heartworm larvae from completing their life cycle in your pet's body. Such prevention is simple and inexpensive. Monthly or semi-annual treatments of prescription oral pills aimed at killing heartworms at the larval stages help prevent heartworms from infecting dogs. Topical skin products are also available.

Some assume dogs don't need protection during the winter months, yet it is often recommended to continue prevention year-round to guard against an accidental lapse in protection. Additionally, there is heartworm prevention for dogs that protects against other parasites, making year-round use necessary. Before making a decision, consult your veterinarian for a custom prevention plan for your pet.

Signs of Heartworms in Dogs

It can be difficult to determine whether a dog has adult heartworms by observation. A persistent cough—particularly during exercise—is a classic sign. If you notice a cough combined with a lack of energy during exercise, you may have a more severe case on your hands. Here are a few other signs of heartworms in dogs to be aware of:

  • Lower energy levels
  • Lethargy
  • Changes in appetite
  • Weight loss
  • Breathing difficulty
  • Asthmatic symptoms
  • Fainting or collapsing

While these are signs of heartworms in dogs you should never diagnose your pet without seeing a professional. It can be easy to confuse these symptoms with other ailments that may be less severe. If you do notice any abnormal behavior in your pet, schedule an appointment with their veterinarian. They will administer blood tests and examinations used to make a correct diagnosis. From here, they will advise you on the best treatment plan for your pet.