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Everything You Need to Know About Heartworm Disease

Heartworm. It sounds like modern dating slang for wearing down your crush enough to finally score a date. (And if that takes off, you heard it here first.) In reality, this unlikely word pairing is a lot more sinister. 

Heartworm is serious business for you and your pet. Scientifically speaking, heartworm disease in dogs is caused by parasitic worms that infect your pet with potentially lethal consequences. And it's everywhere in America — cases have been diagnosed in all 50 states. To save you some stomach-turning Google image results, we’ve broken down the facts, including how to prevent heartworm and what to do if your pet gets infected.

What is it?

As its name suggests, heartworm is a type of parasitic worm called Dirofilaria immitis that can reside in the heart. Thin and thread-like in form, heartworms can grow to a little over one foot in length and live in the heart, lungs, and corresponding blood vessels of certain animals. While dogs are the most common host for heartworms, they can also be found in cats, ferrets, wolves, foxes, bears, sea lions, and more.

How do dogs get heartworm?

Heartworm can only be spread by mosquitoes. The life cycle begins when a mosquito bites an infected host. The mosquito ingests the microfilariae (immature worms) and then after about two weeks the infective larvae move to the mosquito's mouth. The infected mosquito then transmits the parasites into a new host when they bite other animals. It takes about six months for the larvae to mature into heartworms, at which point they’ve already migrated through the new host’s body to the heart or lungs.

How can it impact my pet?

Dogs are considered to be the “definitive host” for heartworm, meaning that the parasites inside a dog can mature into adulthood, mate, breed, and live for 5-7 years. If left untreated, the adult worms can grow to be 12" long and can increase to the hundreds. En masse, they can damage a dog’s circulatory and pulmonary systems (their heart, lungs, and arteries). Because of this, taking preventative measures is the best defense against heartworm—more on that below.

Heartworm disease is different in cats. While they’re still at risk, heartworms fail to thrive in cats, with many not surviving to adulthood or beyond 2-3 years. Since the parasite generally does not reach sexual maturity in cats, it fails to breed, so numbers are generally low, with as few as one to three adult heartworms. They still present a serious problem since the disease can often go undiagnosed, and immature heartworms can cause immense respiratory damage in felines. Additionally, because cats cannot use the heartworm treatments used to treat dogs, their only protection is prevention.

What are the symptoms?

One of the many problems with heartworm disease is its ability to fly under the radar. In its early stages, your pet may not show any detectable signs of heartworm infection. If the infection persists, symptoms include consistent coughing, decreased energy, lethargy, lack of appetite, fatigue, weight loss, dark urine, vomiting, and a swollen belly due to excess fluid. If you notice any signs of heartworm or suspect that your pet may be infected, contact your vet immediately for testing.

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Is it contagious?

No, thankfully. The only way heartworm disease spreads is through mosquito bites. Your dog or cat can’t catch it by coming into contact with another infected dog, cat, or any other animal. 

Let's talk about heartworm prevention

We’re glad you asked. As with other pet-loving parasites, the best way to combat heartworm is prevention with a year-round, FDA-approved topical, tablet, or injectable medication that kills heartworm larvae. Annual heartworm testing via a blood test (usually an antigen test) is necessary as well, as part of a greater veterinary wellness and prevention plan. Because there’s no approved treatment for cats with heartworm infection, periodic testing and prevention is essential to minimize their risk of exposure. Talk to your vet to determine the best heartworm preventive medication based on your pet’s risk factors and lifestyle.

How do I treat my pet if he's infected?

If your pet is tests positive for heartworm, you’ll need to take immediate action. Your pet will likely require more heartworm tests after their initial bloodwork to confirm the diagnosis for sure, since medical treatment can be very costly and strenuous on their body, with several side effects.

For dogs, the goal is to stabilize your pet and kill the parasites completely. A full treatment plan will likely involve exercise restrictions, medications, x-rays, and additional lab tests. Your pet’s vet is your best source of information for treatment options and care.

For cats, while there may not be an approved medication for heartworm disease, there are long-term treatment plans that can help, along with good veterinary care. Contact your vet to discuss the best options for your cat to manage the disease.

Potential Complications

Both cats and dogs with chronic heartworm are at risk for caval syndrome, a complication with a poor prognosis in which a mass of heartworms obstruct the blood flow in the right side of the heart. If untreated, it can lead to cardiovascular collapse and heart failure, so it's imperative to get treatment quickly if you notice symptoms of anorexia, weakness, red urine, pale gums and vomiting.


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