vet checking dog for heartworm
Pet Health Questions and Answers

What Causes Heartworm in Dogs

Heartworm is an awful disease. It’s no wonder that we, as dog owners, worry about it so much.

Whether you’re concerned about your dog catching heartworm or spreading it to others, we’re here to ease your mind. First, know this:

Heartworm in dogs is caused by larvae called microfilariae, which are carried by mosquitos. When a mosquito bites an infected animal, they extract the larvae, and it lives in their system for 14 days before it can be passed to other animals the mosquito bites.

Infected animals cannot pass heartworms directly to other pets or humans. 

Let’s dive deeper into how heartworm is spread and which other animals can develop heartworm.

How Does a Dog End Up with Heartworm?

Heartworm is caused by mosquito bites. If mosquitos can survive in your climate, your dog is at risk.

When these parasitic insects feast upon Fido’s blood, they leave certain larvae – known as microfilariae – behind. These larvae will proceed to burrow into your dog’s body, where they can live for up to seven years and eventually grow as large as 12 inches in length.

Now, not all of these larvae will automatically become life-threatening. The fact is, your dog may already have a number of them in their body already. Over time, however, more of these parasites will find their way into Fido’s body during every mosquito season.

If enough of these larvae are present and your dog’s immune system cannot fight them off, at least one may grow into an adult parasite that wraps around the canine heart (hence the name) and other internal organs. This will ultimately cause a great deal of discomfort as these body parts systematically fail, leading to a tragic and painful demise for your dog.

mosquitos cause heartworm in dogs

Can Any Dog Get Heartworm?

Any dog can develop heartworm. Thankfully, a mosquito must go through a whole process to carry heartworm and infect your dog. This includes:

  • As larvae are involved, the mosquito guilty of causing the heartworm infection must be female.
  • These larvae that cause heartworm will not develop within the mosquito unless it’s particularly hot – usually over 55 degrees Fahrenheit.
  • The mosquito must then target a dog that has already been infected by heartworm. In biting this pooch, the mosquito will extract the larvae (aka microfilariae) that will then be passed on.
  • The mosquito will provide a home for these larvae, which develop in about fourteen days.

When this time elapses, however, any dog is fair game for infection – the bug will be looking for something to snack on.  If it’s your pooch that is chosen, it could take up to seven months before you notice any ill effects.

Is Heartworm Contagious?

The disease is only passed on by mosquito bites. A dog cannot pass heartworm onto another canine through play, sharing food or toys, or any other concern that would arise from an airborne virus.

Naturally, however, an area that plays host to a sizable mosquito population could see an epidemic of heartworm if necessary precautions are not taken.

Can Other Animals Suffer from Heartworm?

Yes, very much so – heartworm is not limited exclusively to domestic dogs. Canines provide a natural home for the parasites that cause this disease, however, so any members of this animal family are particularly at risk. 

This could include coyotes and foxes, so if you live close to such wildlife, you will have to be particularly vigilant about taking preventative measures.

Cats, meanwhile, are also at risk of heartworm. Symptoms can vary in cats from subtle to dramatic, according to the American Heartworm Society. These symptoms include:

  • Weight loss
  • Coughing
  • Asthma-like attacks
  • Vomiting
  • Lack of appetite
  • Difficulty walking
  • Fainting
  • Seizures
  • Fluid acclimation in the abdomen

Cornell University notes that heartworm in cats often affects the lungs rather than the heart and that heartworm can lead to heartworm-associated respiratory disease (HARD).

They also state that sometimes, the first and only symptom of heartworm in cats is sudden death.

Unfortunately, heartworm in cats is harder to detect, and sometimes blood testing shows false negatives.

Ferrets can also get heartworm. They are as susceptible as dogs, according to the FDA, but display similar symptoms to cats.

Can a Human Get Heartworm?

It’s possible for a human to contract heartworm from a mosquito bite, but it’s extremely rare – and even if we do, the larvae do not live long enough within human bodies to complete the cycle of sickness.

There is no way that a dog could pass the condition on to their owner either, so there is no need to quarantine a canine who has the condition. This, in turn, means that there is no reason to prolong their suffering – seek medical attention immediately if you suspect that your dog has been exposed to this awful disease.

dog heartworm on dog heart

How Common is Heartworm in Dogs?

According to the FDA, heartworm disease is most common along the Atlantic and Gulf coasts and along the Mississippi River. However, they also note that heartworm occurs in all 50 states.

The American Heartworm Society tracks infections every three years, and 2019 had the highest case rate since 2001.

It’s important to note that every protected dog is one less contributor to the spread. Even where the risk is relatively low, if everyone decided not to use heartworm prevention medication, their dogs would be able to catch and spread heartworm—potentially making for a dangerous, high-risk situation.

The best way to keep case rates down for your dog and everyone’s is to use preventative medication. It’s also the best way to keep your dog protected!

What Should I Do if I Suspect My Dog Has Heartworm?

Seek professional advice! Heartworm is not something to trifle with, and if you have any reason at all to suspect that your dog is living with this condition, they should be rushed to a vet ASAP.

Any animal healthcare professional will be able to run many blood tests on your dog to confirm or deny if they have been struck down with heartworm. It’s much better for all concerned if the infection can be avoided in the first place. Heartworm treatment can be intrusive, painful, long, and expensive.

Related article: Signs of Heartworm in Dogs

To learn more about heartworm treatment, see our article about treating heartworm in dogs.