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Importance of Heartworm Prevention in Dogs and Cats

Updated on April 21, 2018

What is Heartworm?

Dirofilaria immitis. Better known as 'heartworm', is a species of roundworm that hosts in dogs. In short, it is a parasite. The adult worm is able to reside in the lungs and heart of dogs, which causes major damage to the organs. Unfortunately, this species of roundworm is able to infect more than just dogs. Domestic animals like cats and ferrets are also susceptible to heartworm, and wild animals, including wolves, foxes, and sea lions, have also been known to carry the disease.

If left untreated heartworm is deadly.

The life cycle of the heartworm
The life cycle of the heartworm | Source

The Life Cycle of the Worm

So how do dogs and cats get heartworm in the first place? To understand how animals become infected the life cycle of the heartworm needs to be evaluated. First a mosquito will take a bite from an infected animal. This causes the mosquito to become infected with the disease. The within the mosquito larvae develop, and these larvae are transferred to the victim of the mosquito when it takes its next blood meal. This stage typically take between 10 and 28 days. Now the host animal has been infected with heartwrom. The larvae migrate to the heart where they will grow into adults and reproduce. Over the course of a few years the damage these worms cause can be deadly.

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Where is Heartworm Found?

Now that we know how animals contract this disease and how its life cycle work, where is it found? This disease is most prevalent in the Southern area of the United States. However, don't think you are off the hook just because you don't live in Georgia. Heartworm disease has been reported in all 50 states, as well as every content but Antarctica. Why is that? Think about it. How do dogs and cats become infected? Mosquitoes. If you have mosquitoes in your area, then there is a chance that heartworm is also present.

It is sad but it often makes me laugh when people say their pet is not susceptible to heartworm because they don't go outside. Have you ever had a mosquito end up in your home? I know I have. Where does your dog do its 'business'? Outside, right? Have you ever been bitten while walking to the mailbox. I know I'm not the only one. Just because your dog or cat isn't spending hours each day outside doesn't mean they can't contract the disease. I know I can't be the only poor soul that has been eaten alive by a mosquito that has been stuck in my house. Think just because its winter that you are safe from mosquitoes? Maybe. That is going to depend upon where you live. Mosquitoes tend to inhabit warmer climates, and they stay active year round in places that stay warm year round.

Heartworm in Cats

Thankfully for cats, they are not the natural host for heartworms. However, that means that when they become infected there is a greater chance of death. This is because the heartwrom has not adapted to living in the body of a cat and does not always behave the same way as it does in dogs. Infected cats typically have fewer adult heartworms, because the worms are ill adapted to living in the cate, but the worms do not tend to stay in the heart. Heartworms that do not reside in the lungs will travel to other organs; including the lungs, liver, and brain. Currently there is no approved method for treating adult heartworms in cats. This means that if a cat has adult heartwroms it will most likely die from complications. Typically, an infected cat will die when the heartworms inside of it will die, assuming that the cat does not die of major organ damage first. In a cat the longevity of the heartwrom is significantly shorter than it is in dogs. 2-3 years is currently the known lifespan of the heartwrom in a cat, compared to the 5-7 years it can live in a dog.

If left untreated heartworms can kill an otherwise health dog.
If left untreated heartworms can kill an otherwise health dog. | Source

Heartworm in Dogs

As the dog is the natural host for the heartwrom it takes many years and many, many worms before problems are seen. Also, as the natural host for the worm dogs are very susceptible to the disease, with nearly 100% of dogs exposed to larvae becoming infected. Thankfully, there is a way to treat heartworm in dogs. If the dog is found to be healthy enough for treatment there is one drug that has been approved by the FDA to be used on adult heartwroms on dogs: Immiticide. This is an arsenic based drug that will kill the adult worms. This treatment takes several weeks and is used in combination with other drugs to kill the worm in all of its life stages in the dog. Once the worms have been killed this dog must rest so that its body can absorb the dead worms. If the dogs exerts itself too much it risks the worms traveling to various parts of the body, including the lungs, and causing severe damage and possibly death.

In severe infections surgery may be used to remove heartworms. This course of treatment is usually seen when many adult heartwroms inhabit the heart.

An Informative Video of the Heartworm Life Cycle


How do veterinarians test for heartworm? There are two different blood tests that can be done to determine if heartwrom in present. One can be done in your veterinary office, it is called a snap test. The other must be sent out to a professional laboratory, typically this test will also look for other vector borne diseases like lyme disease. Both of these blood test are checking for an antigen produced by an adult female heartworm. While they are both good tests, the test sent out to the laboratory is a little more sensitive then the in office snap test. If an animal is found to have heartworms x-rays can be used to determine the severity of the infection to determine the best course of treatment.

Sadly determining if a cat has heartwrom is harder than it is with dogs. Cats must exhibit several other symptoms for a long period of time in order for a heartwrom diagnosis to be made. This is because cats typically have much fewer heartworms than dogs. An infected cat, as it is not the primary host of the parasite making it much harder for the parasite to survive to adult hood, typically only has 3-5 heartworms. The heartworm test specifically is testing for an antigen produced by adult female heartworms. If all of the heartworms in the cat are male it will never test positive for heartworm, even though it is infected. Essentially, the more adult female heartworms the more likely the test will come back positive so all of the 3-5 worms in the cat must be female in order for the test to come back as positive. Because of this, many veterinarians do not recommend regular heartworm testing for cats.

Preventing Heartworm

There are many products available that prevent heartworm infections. Many are chewable tablets that are given once each month. These tablets typically contain ivermectin, which is a compound that kills heartworms in their larval stage. Sentinel Spectrum, TriHeart, Trifexis, Iverhart, and HeartGard are all tablets given once each month. Some even contain does of pyrantyl, which is used to rid dogs and cats of intestinal worms.

If you dog will not take the tablet orally there are alternative methods. Revolution is a topical treatment that is most often used in puppies and dogs that will not take the tablets. As an added bonus it also protects against fleas, ticks, and mites. As another bonus, Revolution is one of the few products that is safe to use on cats for heartworm prevention.

If a monthly topical solution or tablet does not sit well with your dog, or if you simply can't remember to give the dose an injectable heartowrm prevention can be considered. In the United States ProHeart 6 is the only approved injectable heartwrom prevention. And even better, it only needs to be given once every 6 months. In other countries ProHeart 12 is available for year round prevention of heartworm. Both forms of ProHeart work by using a slow release of chemicals to prevent heartworms from reaching the adult stage.

Things to Consider

  • In the United States heartwrom prevention mediations are controlled substances and can only be prescribed by a veterinarian.
  • Most veterinarians will only allow a patient to receive heartwrom medication if they test negative for heartwrom.
  • Many veterinarians recommend yearly heartworm testing even if animals regularly take heartwrom prevention.
  • Many veterinarians recommend using heartwrom prevention year round, regardless of the presence of mosquitoes.
  • It only takes one bite for a dog or cat to become infected with heartworm.
  • Heartworm has been reported in humans in rare cases.
  • It is considerably cheaper to administer heartwrom prevention than it is to treat a dog or cat with heartwrom.
  • Heartworm disease in cats often mimics other diseases, making diagnosing it difficult.
  • Talk with your vet's office to see if there are any discounts if you buy in bulk. Some manufacturers offer price breaks if you buy 6 or 12 months of product at once. Some will even offer additional mail-in or on-line rebates. Yes, it is more up front but it saves money in the long run and your dog will be on the product for life anyway so why not get it?


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    • Ann1Az2 profile image


      6 years ago from Orange, Texas

      Yes, that's the one! I couldn't remember the name of it. Unfortunately, when I use it on my cats, it makes them lethargic for a day or two and I don't like their reaction to it. I try to control the flea situation by killing them in the yard and around the baseboards (where the cats can't get to it) in my house. My cats don't go outside, but we bring fleas in on our shoes and other cats and dogs in the neighborhood bring them into my yard. Fleas are a real problem down here, but the topical solutions they have really do not agree with my cats. I've tried Frontline, too, and I think the fleas are immune to it. I brush my cats everyday and use a flea comb and as I say, try to control fleas in the yard and the house. I use lemon water to clean with most of the time.

      None of this gets rid of the fleas entirely, but it does keep them from getting really bad.

    • isharkbait profile imageAUTHOR


      6 years ago from Virginia Beach, VA

      Revolution. It can also be used on puppies and dogs under five pounds.

    • Ann1Az2 profile image


      6 years ago from Orange, Texas

      No, the topical treatments are not specifically for heartworms - they are for fleas, but, at least for cats, anyway, they contain a heartworm preventative.

    • Ann1Az2 profile image


      6 years ago from Orange, Texas

      I live in the South, so I'm very familiar with heartworms. Dogs that stay outside are especially susceptible to them. But even, if they are inside, they still have to go outside and there are always mosquitoes. Many of the same topical medicines used for fleas also contain heartworm preventative. We also have a huge problem with fleas down here. So you can kill two birds with one stone.


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