The Weird and Insidious World of Heartworm in Dogs

Heartworm disease in dogs is a parasitic, blood-borne disorder caused by an invasive nematode. Here's what you need to know.


In Jane Austen’s Pride and Prejudice (1813), during his first torturous attempt to propose marriage to Miss Elizabeth Bennet, Mr. Darcy states that “disguise of every sort is my abhorrence.” I’ve come to feel much the same way about heartworm in dogs. Parasitic roundworms in dogs of every sort are my abhorrence. Heartworm disease in dogs is a parasitic, blood-borne disorder caused by an invasive nematode called Dirofilaria immitis, which is transmitted by mosquitoes.

Because of its lengthy maturation, during which a dog presents no clear heartworm symptoms, the Dirofilaria roundworm has an effective period of disguise to wreak havoc inside a dog. Prevention is your best line of defense against the appearance of heartworm symptoms.

How do dogs get heartworms?

Here’s the good news: Heartworm is not spread through contact, poop, air, or water. Transmission of heartworm depends on two things, to wit, sufficiently warm temperatures and mosquitoes. The bad news is that wherever mosquitoes flourish and thrive, the potential for a dog to get heartworms exists. From its birth as a microfilaria to maturity as an adult roundworm, the heartworm’s lifecycle takes as many twists and turns as a romantic novel.

Baby heartworms travel through a dog’s bloodstream. They can block and restrict bloodflow, but they cannot become adults without the intercession of a mosquito. When a mosquito bites an infected dog, they ingest the microfilaria. Inside the mosquito, the microfilaria develops into an infective larva, a process that can take anywhere from 10 to 14 days. This mosquito bites another dog, and the infective larva passes into the dog’s bloodstream, where it takes up residence and grows into a mature roundworm in the heart.

For one worm to grow from an infective larva to full maturity inside a dog can take from six to seven months. When there are adult male and female heartworms present, they can reproduce, and the process can begin again. Depending on the number of infective larvae transmitted to dogs by mosquitoes, a single dog may contain as many as 250 to 300 fully grown worms. Because the life cycle is so gradual, by the time you notice heartworm symptoms in a dog, chances are good that there is already a full-blown infestation.

Heartworm symptoms

As we’ve noted, by the time you notice heartworm symptoms in dogs, you and your dog will not be fighting an invasion, but dealing with an occupation. Heartworms cause disease by blocking the flow of oxygenated blood to the lungs, liver, and kidneys. The severity of heartworm symptoms depends on the number, size, and maturity of the worms that are present.

Because of their effects on the lungs, liver, and kidneys, the signs of heartworm in dogs are associated with those organs. Coughing and shortness of breath indicate damage to the lungs. This can also affect a dog’s energy level, making dogs tired and weak after a small amount of exertion. Decreased flow of blood to the liver and kidneys affects a dog’s ability to process nutrients and filter toxins from the blood. Symptoms of heartworms that indicate damage to the liver and kidneys include loss of appetite, which leads to weight loss, anemia, and jaundice.

The damage that heartworm symptoms can cause in dogs is not impossible to reverse, but it is both costly and time-consuming to cure a dog who is dealing with an advanced heartworm invasion. Treatment is possible, but prevention is the best and most effective way to protect your dog, particularly if you live in an area where mosquitoes are prevalent.

Heartworm treatment and prevention

A veterinarian will use blood tests, X-rays, or ultrasounds to determine the extent of the heartworm invasion force. Depending on the number of fully grown adult heartworms in a dog, a full course of treatment can take a month or longer. Prevention really is the most effective way to keep potentially deadly heartworm symptoms from manifesting. Dirofilaria immitis nematodes take so long to grow to maturity, it is rare for puppies or dogs under one year of age to become infected and exhibit symptoms. Starting a preventative course of treatment is highly recommended in areas that are prone to mosquitoes and before their peak seasons begin.

Can people be hosts for heartworms?

Heartworms are not zoonotic parasites. A dog can no more pass heartworms to other dogs than it can to humans. Mosquitoes are the only way that heartworms spread, and although the Dirofilaria immitis roundworm can be transmitted to humans by mosquitoes, they cannot grow to maturity inside humans. The nematodes known as heartworms are slow-growing and insidious. They can take half a year or more to grow to maturity, and it may take several years for symptoms to become severe enough to notice. If you live in an area where mosquitoes are common, cancel the heartworm apocalypse before it starts! Make sure your puppies and dogs have the right preventative medications!

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