How Do Dogs Get Heartworms? 15 Things to Look Out For
Category_Dog Knowledge

How Do Dogs Get Heartworms? 15 Things to Look Out For

by Wes Chang
A mosquito bite is annoying for a human, for a dog, it can be downright deadly. That's because mosquitoes can carry a parasitic worm called Dirofilaria immitis. These 12 to 14-inch worms can clog your pup's heart, lungs and blood vessels, all the while continuing to mature, mate and reproduce for up to seven years. This isn’t some fictitious horror story; this is the very real heartworm infection that has killed countless millions of dogs. While heartworms sound disgusting, the condition is far worse than the initial impact. As the worms continue to spread throughout your dog's body, they can cause myriad forms of physical damage ranging from severe lung disease to liver disease to heart failure. How do dogs get heartworms? What causes them and how can you prevent them?

A Closer Look at Heartworms

Heartworm prevention is on the minds of most pet owners, and for good reason. The worms themselves live inside the mosquito for a short period of time, which they use to perfect their infective nature. This makes the mosquito the “intermediate host.” Because heartworms continue to develop while inside your dog, dogs are considered the “definitive host.” While our feline friends can contract, and even die from, heartworms, they’re not the target species for this blood-borne parasite and are considered an “incidental or dead-end host.” While there have been recorded cases of dogs with heartworms in all 50 states, it's the most common along the Atlantic seaboard and the southern United States.

The Heartworm Life Cycle

Once a mosquito bites your dog and he becomes infected, the heartworms go straight to work. The adult female heartworm will release her offspring, known as microfilariae, into your dog's bloodstream. When a mosquito bites your newly infected dog, those microfilariae take up residence inside of the mosquito. After around 10 to 14 days, they will become infective larvae. Then, when the infected mosquito bites another dog, those larvae enter the dog through the bite wound. In all, it takes around six to seven months for the infective larvae to grow into adult heartworms. Once developed, those adult heartworms mate with one another, producing even more microfilariae and completing the lifecycle. The FDA provides an illustrated guide to the heartworm life cycle here.

heartworm free pup running through a field

How Do Dogs Get Heartworms? The Four Classes of Infestation

It's important to keep in mind that the only way a dog can contract heartworms is through the bite of an infected mosquito. The disease is not contagious, which means your dog can't catch it if he's simply around a dog who is suffering from the condition unless they are both bitten by an infected mosquito. As long as there is one positive heartworm dog in a community, all dogs are potentially at risk. Want to protect your best friend as much as possible? Knowledge is key! If you know the signs and symptoms of heartworms in dogs, you can take quick action that could potentially save his life. In most cases, the severity of your dog's condition is directly related to how many heartworms he has living inside of him. This is known as his "worm burden". In addition, other factors include the duration of the infestation and your dog's physical reaction to it. You're also more likely to notice more adverse reactions in dogs who are normally active, as heartworms can impede on their respiratory function and make it more difficult for them to play. As such, if your dog has a low worm burden, hasn't been infected long, or isn't normally active, symptoms could take a while to show up, while the opposite also holds true. Once confirming a diagnosis of heartworms, your veterinarian can also reveal which class, or stage, his heartworm disease is in. These range from Class 1 (mildest symptoms) to Class 4 (most severe symptoms). Let's briefly review each class. Class 1: No Symptoms Dogs in Class 1 might not show any signs of a heartworm infestation at all. Or, they might have mild symptoms that are virtually undetectable. Class 2: Mild to Moderate Symptoms Dogs in Class 2 do show some kind of symptoms, but they are not severe in nature. This might include a mild cough or fatigue after moderate activity. Class 3: More Severe Symptoms Dogs with Class 3 heartworm disease will normally undergo a chest x-ray to determine the extent of internal damage. Symptoms can affect their internal organs as well as their outward appearance. Class 4: Most Severe Symptoms Class 4 heartworm disease is also called Caval Syndrome. When dogs reach this stage, they have such a massive worm burden that there's a physical blockage of worms that prevents blood from flowing back to their heart. This syndrome is life-threatening and the only way to treat it is to quickly remove the heartworms via high-risk surgery.

Is Your Dog Infected? Watch For These Signs

Keeping these four classes in mind, let's take a look at 15 symptoms that could point to a heartworm infestation in your dog. Occasional Cough While your pup might have food allergies or sneeze around dust, an occasional cough that persists even in clear conditions might be a red flag. In Class 1 infestations, it's often the first and only sign you'll notice. The cough will get more persistent as more worms crowd your dog's heart and lungs. Tiredness Does your pup seem especially tired after moderate activity? Especially when this fatigue is combined with a cough, you could be dealing with a Class 2 infestation. Nosebleeds and Unusual Bruising Another common allergic reaction, nosebleeds, or unusual bruisings can occur in dogs with heartworms. Check your pup's nostrils and skin for signs of any that you might have missed. Reluctance to Exercise In keeping with extreme tiredness, you might notice that your once-active dog now prefers to lie indoors rather than play fetch. If yours seems unwilling to exercise or even go for a walk, it's worth having your veterinarian chem him out. Sickly Appearance As pet parents, we just "know" when our fur babies don't look right. If your pup is looking sickly, heartworms might be to blame. If this appearance change is coupled with tiredness and a cough, you're likely in a Class 3 heartworm infestation. Decreased Appetite It's common for dogs with heartworms to refuse their daily kibble. If yours is suffering from the condition, quick medical attention can help sustain him until his appetite levels regulate again. Weight Loss You can't gain any weight if you don't eat! A dog with heartworms will often appear skinnier than normal, due to the fact that the parasites are overtaking them in a way that makes simple digestion a task. In severe cases, a pup’s stomach may appear swollen or distended due to fluid accumulation. Difficulty Breathing Trouble breathing is another sign of a Class 3 infestation. You might also notice other signs of congestive heart failure, such as pacing before bedtime and an elevated respiratory rate. Swollen Belly As the disease progresses, many dogs will get a swollen belly. This is due to the fact that the heartworms cause excess fluid to build up in his abdomen. Protruding Chest You might also notice that your dog's chest protrudes or has a bulging appearance. While the worms themselves can create the problem, it's exacerbated by extreme weight loss. Pale Gums One of the trademark symptoms of Caval Syndrome, pale gums are a common indicator of cardiovascular collapse. Worried you'll miss this sign? Don't be. If your dog is suffering from Caval Syndrome, there will be other, more obvious signs that he's in distress. Abnormal Lung Sounds Does your dog make strange sounds when he's breathing? Abnormal lung sounds can be another sign of heartworms, though they are also linked to other upper respiratory conditions. Bloody, Dark Brown Urine We all know that clear, golden urine is an indicator of good health and proper kidney function. That's why dark brown urine or urine with blood in it should be taken seriously. Call your veterinarian if you notice this symptom with your dog, as it's another telltale sign of Caval Syndrome. Unconsciousness As worm burdens increase and blood flow is further prevented, this loss of blood to the brain can cause your dog to collapse and lose consciousness. Again, this symptom is most closely related to Caval Syndrome. Official Tests Rather than monitor your dog at home, it's always best to bring him in for a visit to your veterinarian's office. With a simple blood test, the veterinarian can screen your dog for heartworms, though it's best for your dog to undergo a second test to confirm the results. Veterinarians will perform these blood tests on both dogs currently suffering from heartworms, as well as once a year for dogs on preventative heartworm medication. Other tests commonly administered to dogs with heartworm disease include:
  • Complete blood cell count
  • Blood chemistry panel
  • Chest x-rays
  • Urinalysis
Seizures and Blindness Sometimes, the heartworm parasites get lost and accumulate in other places besides your pup's heart and lungs. If they wind up in his brain, they can lead to seizures. In the same vein, those that wind up in his eyes can lead to blindness.

Healthier Dogs, From the Inside Out

You'd do anything if it meant securing the health and wellbeing of your beloved pal. Now that you know the answer to the question, "How do dogs get heartworms?", you're ready to take the next steps toward preventing the disease.

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