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Heartworm in Dogs and Cats

Every dog or cat owner is aware that a heartworm preventative should be a part of any pet’s healthcare routine, but just how much do you know about heartworm disease?

What is heartworm disease?

Heartworm disease is a serious and potentially fatal condition that affects both dogs and cats. The disease is caused by a parasitic roundworm (Dirofilariaimmitis), which is transmitted to the pet by mosquitoes.

As the worm larvae develop, they inhabit the heart and blood vessels of the animal&rs quo;s lungs where they can cause serious and lasting damage. Adult heartworm can grow up to 14 inches (36cm) and an infected dog may be host to up to 250 worms. The presence of heartworm can also cause blood clots, allergic reactions, and inflammation of the lung tissue.

Heartworm basics

  • Heartworms are spread by mosquitos – one bite is all it takes
  • Heartworm disease affects both dogs and cats
  • Indoor and outdoor pets are at risk of heartworms
  • Heartworm disease can be hard to treat, but easy to prevent
  • Early signs of heartworm may be difficult to detect
  • Heartworms affect the lungs as well as the heart

How is heartworm disease transmitted?

The heartworm parasite (Dirofilariaimmitis) is spread by mosquitoes. When a mosquito bites an infected animal, it takes with it the microscopic heartworm offspring known as microfilariae. Once inside the mosquito’s digestive system, the microfilariae develop into infective larvae. When this mosquito bites another animal, the now infective microfilariae are transferred and enter the animal’s body through the bite wound. A single mosquito bite can transfer enough microfilariae to cause heartworm disease in a dog or cat.

Once the heartworm larvae have been transmitted to your pet they will develop under the skin before moving to the bloodstream. The heartworms then develop further before moving through the tissue to the lungs where they mature into breeding adults.

Risk factors for heartworm disease

As heartworm is spread from host to host by mosquitoes, any pets in areas where mosquitoes are found will be at risk. Mosquitoes are more prevalent in warm climates, in areas of flooding and near stagnant water. Be aware that mosquitoes can emerge in brief warm periods in cold climates, and that even small amounts of standing water such as in bird baths or gutters can be breeding grounds for mosquitoes.

Many owners may think that pets that live indoors are not at risk from heartworm, but as it is easy for mosquitoes to enter the home, ‘inside’ cats and dogs may still be infected.

The number of infected hosts in the area will also increase the risk of heartworm being transmitted. This includes not only other pets but also wild animals such as ferrets, foxes, coyotes, raccoons and wolves.

How does heartworm prevention work?

It is important to note that heartworm medications do not treat existing heartworm, they can only prevent the development of heartworm disease. Nor will these medications protect your pet from the initial infection.

Heartworm preventatives kill off heartworm at the larva stage. This means that any microfilariae that have entered your pet’s bloodstream in the 30 days previous will be eliminated before they can grow into adults. This breaks the lifecycle of the heartworm, preventing them from breeding and also preventing them from migrating into the heart, lungs and other organs.

Testing for heartworm disease

You may notice that your heartworm medication advises that your pet is tested for existing heartworm before you administer the product. This is because the available heartworm medications can only prevent heartworm larvae from developing, they cannot treat heartworm disease itself.

As symptoms of the disease don’t often develop for several years, getting your dog or cat tested by a vet is the only way to confirm the existence of heartworm. Dogs, in particular, may have adverse reactions to medication if heartworm disease is already present, even if they are not showing symptoms.

Getting your pet tested is reasonably simple and inexpensive. In most cases, a blood test can diagnose heartworm disease, but your vet may also perform additional diagnostic tests such as urine tests or x-rays.

Due to the lifecycle of the heartworm, tests will not be effective until approximately 6 months after your pet has first been infected.

Symptoms of heartworm disease

Heartworm disease presents differently in dogs and cats, so it is important that you can recognize the symptoms that may arise.

Symptoms may not develop for several years after infection, at which point irreversible damage may have been done. Early detection by regular blood tests is key to providing your pet with the best chance at eliminating heartworm and returning to full health.

The severity of symptoms will depend on the number of worms present, the location of the worms (lung, heart, kidneys) and the activity level of your cat or dog.

Heartworm symptoms in dogs

  • Coughing
  • Lethargy
  • Loss of appetite
  • Difficulty breathing
  • Fainting

Heartworm symptoms in cats

  • Coughing
  • Difficulty breathing
  • Gagging
  • Vomiting

Treatment options and prognosis for heartworm disease

Once a dog or cat has developed heartworm disease, treatment can be costly and potentially dangerous to your pet. The success of the treatment relies on eliminating the heartworm without causing damage to the host animal.

Heartworm treatment for dogs

While treatment for canine heartworm has developed somewhat in recent years, it is not without risk. Your vet will need to administer a series of injections, often spaced up to a month apart. Your dog will need to be hospitalized for the injection, then strictly confined for several days or even weeks. As the adult worms die and decompose following the injection, the fragments can block your dog’s blood vessels. By confining your dog, you are reducing its movement and therefore reducing the risk of these blockages. Your vet may also administer medications to treat inflammation.

The successful recovery of your dog depends on a number of factors, including the stage of heartworm development and the overall health of your dog. But generally speaking, with the right care, the prognosis for dogs is good.

Heartworm treatment for cats

Unfortunately, the drug therapy used to kill adult heartworm in dogs is not safe for use on cats. While it is sometimes used, this treatment can lead to lung failure and even death. This is why preventative measures are so important.

Currently, the only option is to treat the symptoms of heartworm disease until the heartworms die off naturally, which can take up to 2-3 years in cats. This treatment may include corticosteroids to treat inflammation, diuretics to remove fluid from the lungs, and in acute situations, the administration of oxygen.

In some parts of the world, surgical extraction is being trialed as a treatment option for cats. As the trials improve and become more widespread, this may be a more successful method for eliminating heartworm.

Heartworm disease FAQs

Are heartworms contagious?

Heartworm disease or the heartworms themselves are not directly contagious. This means that your cat or dog cannot become infected simply by being in contact with another infected animal. Heartworms are only spread by mosquitoes.

Can humans contract heartworm disease?

While humans are just as at risk of mosquito bites as their pets, humans are not a natural host for heartworms. The heartworm microfilariae may be transmitted to a human with a mosquito bite, the larvae will not complete their life cycle and are eliminated by our immune system before they can develop.

What happens if I miss a dose of heartworm medication?

If the dose is less than a week late, give the missed dose immediately and then resume your normal heartworm preventative schedule. If the missed dose is more than a week late you will need to consult your veterinarian. Your vet will likely advise you to give the missed dose, but also to schedule a heartworm test in 6 months’ time.

Got a question that’s not answered here? Read our heartworm FAQs.


Heartworm in Dogs and Cats

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