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Heartworms in Dogs & Cats: Symptoms & Treatments

Vet looking at dog


You hear about heartworms a lot when you're a pet parent. When you adopt a pup, the shelter might mention they tested negative for heartworms, and your vet likely recommends monthly heartworm prevention. But how serious is it if your pet gets heartworms, and what exactly are they?

Heartworms are parasitic worms that can live in your pet’s heart and lungs, leading to severe health issues. Dogs are the most common hosts for heartworms, but cats and other animals are also susceptible. Heartworms cannot pass from one pet to another, and the disease is not contagious. Your pet can only become infected through mosquito bites. Regular prevention is the best way to protect your pet.

Since you can’t see heartworms, it makes them an especially worrisome problem. Often, you won’t know your dog or cat has developed heartworms until many months after the infection. For this reason, it’s imperative to get the facts on heartworms and how to protect your pets.

What Are Heartworms?

Heartworms, or if you want their fancier name, Dirofilaria immitis, are parasitic worms that can reside in a host’s heart, lungs, and surrounding blood vessels. Heartworms infect various animals, but dogs are the most common hosts.

Adult worms can grow as long as 12 inches and live for 5 to 7 years inside their host. Prevention is the best way to protect your pet from heartworms (more on that below). Luckily, there is only one way for these parasites to infect your furry friend.

How Are Heartworms Transmitted?

Mosquitoes play the primary role in the life cycle of heartworms and how they are transmitted. These pesky insects are also the only way your pet can get heartworms. So, on the positive side, you won’t have to worry about heartworms hanging out in your cat’s litter box or your dog’s water bowl.

Heartworms develop through five stages (L1-L5); once they reach the L3 stage, they can infect a host. When a mosquito bites an infected dog, the L1 larvae enter the mosquito’s blood. 

Within the mosquito, the larvae develop to stage L3, which takes approximately 10 to 17 days, depending on the weather. (The warmer the climate, the faster the development.)

When the infected insect bites a dog, the L3 larvae enter the new host through the skin and enter the dog’s bloodstream. Between 3 and 14 days after the bite, the L3 larvae develop into L4. The L4 larvae begin migrating toward the heart, which can occur anywhere from 57 days to four months.

Once they reach the heart’s bloodstream, they become immature worms, the L5 stage, and are harder to kill. The L5 worms enter the dog’s heart, continuing to develop into mature adults. The entire process takes about six months from when the larvae first enter the dog’s body.

The worms reproduce, and female worms give birth to L1 larvae. Then, the whole cycle repeats.

What Are the Signs of Heartworms?

Heartworms work differently in cats and dogs. Dogs tend to be the perfect host for heartworms, allowing them to live out their lives and wreak havoc on your dog’s pulmonary and circulatory systems. However, these parasites don’t do so well in cats.

Heartworms rarely reach adulthood or sexual maturity in cats, keeping the numbers low. But even immature worms can cause issues for felines, especially since they're hard to diagnose. Therefore, if left unchecked and untreated, your cat could develop respiratory problems from heartworms.

In the early stages, pets might show very few symptoms or none at all. For cats, signs can be apparent or very subtle. For dogs, those heavily infected, very active, or with other health problems might show more obvious clinical signs.

Signs of Heartworms in Cats

  • Coughing
  • Occasional vomiting
  • Appetite loss
  • Asthma-like attacks
  • Weight loss
  • Fainting
  • Seizures
  • Trouble walking
  • Fluid in the abdomen
  • Sudden collapse

Signs of Heartworms in Dogs

  • Mild but persistent cough
  • Low energy and fatigue
  • Loss of appetite
  • Weight loss
  • Fluid in the abdomen
  • Heart failure

If a dog or cat has a high number of heartworms, it can lead to a blockage in blood flow to the heart. This blockage can lead to cardiovascular collapse and a condition called Caval Syndrome. A vet must surgically remove the blockage ASAP, or it is typically fatal.

Signs of Caval Syndrome include:

  • Dark or bloody urine
  • Pale gums
  • Labored breathing
  • Acute anorexia
  • Weakness
Giving dog medicine

How Often Should You Test Your Pet for Heartworms?

The American Heartworm Society recommends “think 12.” Get your pet tested for heartworms every 12 months, and give them preventative medication 12 months a year. Even if your pet is on regular prevention, testing is crucial. Missing a dose, taking it late, or your pet throwing up a pill can all open a window of opportunity for heartworm disease.

Taking your dog to routine annual vet visits is vital to their overall health. Since there are limited signs during the early stages of heartworm disease, regular testing is important. These wellness visits typically include a heartworm test.

Your vet will only need a small drop of blood to run the test. The test looks for female uterine proteins, and you will get results back relatively quickly. With cats, vets typically use an antibody and an antigen test and may also perform an X-ray or ultrasound. These extra measures are because identifying heartworm infection is more difficult with cats.

What If Your Pet Tests Positive for Heartworms?

If your dog or cat tests positive for heartworms, your vet will likely order further testing to confirm the diagnosis. Treatment for heartworms is very different for dogs and cats. The main difference is that melarsomine, the medication to treat heartworm disease in dogs, is unsafe for cats. 

Therefore, stabilization and long-term management, along with continued prevention, is the only treatment plan for felines. Your cat may require regular x-rays every 6 to 12 months, and your vet might administer prednisone for inflammation.

In severe cases, your vet might recommend hospitalization, administer IV fluids, and offer medications for heart and lung problems. In some cases, surgery could be a possibility.

If your dog gets a confirmed diagnosis of heartworm disease, a treatment plan will commence immediately. Your vet will tell you to restrict your dog's exercise to prevent increased heart and lung damage.

It could take several months to stabilize your dog before your vet can start administering the heartworm treatment. Treatment takes approximately nine months, at which point your vet will test again for heartworms. By this time, all the worms should be gone, and you’ll resume regular heartworm prevention. 

How Heartworm Prevention Works

According to Thomas Dock, Public Information Officer and Director of Communications for Noah’s Animal Hospitals, regular heartworm prevention is key. Dock says, “heartworm prevention is one of the safest and most effective medications we have for our dogs and cats when used appropriately.”

He says to view heartworm prevention as working in reverse, de-worming your pet of any larvae they picked up in the previous month. Therefore, missing even one dose could lead to developing adult heartworms and heartworm disease. Missing the dose gives the larvae more time to develop into the L5 stage when they’re harder to kill.

New heartworm prevention products are available that provide an extended window of protection. For example, an injectable moxidectin, like ProHeart® 12, can potentially protect a dog from heartworms for up to 12 months. However, for many pet parents, oral prevention is the most common method, and the recommended frequency is a dose every 30 days.

Protect Your Pet

Making sure your pet is on routine heartworm prevention is a significant part of their wellness plan. To simplify your pet care regimen, you can find several medications that protect against fleas, heartworms, ticks, and other parasites, all in one chewable tablet. Add a reminder to your calendar so you don’t miss a dose, and follow the pill with a couple of tasty treats to make sure your pet eats it.

If you’re worried your pet missed a dose or threw it up, call your vet for instructions. Also, stay current with your annual vet visits and heartworm testing. Although there is no 100% effective method for preventing heartworms, these steps go a long way in keeping your fur babies safe.

For more helpful tips on caring for your dogs and cats, check out the rest of our blog. At NeaterPets, we’re all about helping you raise happy and healthy pets.


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