Is heartworm prevention in dogs necessary?

Disclaimer: This article, its content, and its related references do not constitute veterinary advice and should not be considered a substitute for veterinary care. If your pet is showing any symptoms that could indicate a medical emergency, please seek immediate emergency care.

Heartworm disease is a common condition in dogs caused by internal parasites spread by infected mosquitoes. A heartworm infection can cause severe disease that impacts the heart, lungs, and vascular system. FDA-approved heartworm prevention products are the only proven way to prevent heartworms in dogs. It’s important for pet parents to discuss heartworm prevention with a veterinarian. If you have a dog, read on to learn:

• How dogs get heartworm disease
• Why dogs need year-round heartworm prevention
• Cost of heartworm prevention compared to heartworm treatment

It’s safer and more cost-effective to use year-round heartworm prevention rather than risk a heartworm infection. Adult heartworms live and reproduce in a dog’s heart and lungs, as well as the blood vessels near those organs, where they can cause permanent heart and lung damage. Treatment for heartworm disease is risky and costly, and it often takes several months to more than a year. Heartworm disease has been diagnosed in dogs in all 50 US states and across Canada, even in colder climates where mosquitoes, the carriers of heartworms, are less common.

Canine heartworm disease is a parasitic infection caused by the worm Dirofilaria immitis and is spread through bites from mosquitoes carrying the worm’s larvae.

After entering the dog’s body, immature heartworms travel through the bloodstream to the large blood vessels in the heart and lungs. After maturing into adult heartworms, the parasitic worms live for several years, during which they grow and reproduce. A single adult female heartworm can produce millions of baby heartworms (microfilaria). Microfilaria circulate through the bloodstream, where they can be picked up by mosquitoes when they take a blood meal, completing the heartworm life cycle.

Dogs with heartworms have, on average, 15 adult worms living in and around their heart and lungs; however, that number can range from a single worm to hundreds. Dogs are the natural host for heartworms, so the parasites are more likely to mature into adults and successfully reproduce in dogs than in other species. Heartworm disease is present in all 50 US states and throughout Canada.

Why heartworm disease in dogs is dangerous

Adult heartworms are approximately the size and shape of a piece of spaghetti, ranging from four to twelve inches in length. They reside in the heart and lungs, and the blood vessels near those organs. This can cause heart, vascular, and lung disease, and can potentially lead to severe lung disease and congestive heart failure. Even if the worms are successfully eradicated, there may be lasting damage, negatively affecting a dog’s quality of life.

The best way to protect dogs against heartworm disease is by using heartworm prevention as directed by a veterinarian. Year-round prevention is recommended by both the Companion Animal Parasite Council and the American Heartworm Society. Mosquitoes that spread heartworms are active year-round in warmer climates and can become active in colder climates if temperatures, even temporarily, get warmer. It only takes a single mosquito bite to result in the spread of heartworms. Infected mosquitoes can also get indoors, so heartworm prevention is important for indoor-only dogs too. Heartworm preventatives do not prevent mosquito bites.

Heartworm preventatives are typically given either orally or topically on a monthly basis. Preventatives kill heartworm larvae as they grow from tiny microfilaria, keeping them from reaching the heart and lungs, as well as the blood vessels near those organs. Puppies as young as eight weeks of age can begin taking heartworm prevention.

Heartworm preventatives must be given at the correct time and dosage to be fully effective. Dogs on prevention can still contract heartworms if they:

• Miss a dose or are given a dose late
• Do not take the correct dosage
• Are given preventatives incorrectly
• Throw up or spit out an oral product
• Get wet or bathe after a topical treatment is applied

Vet-recommended prevention products are highly effective when given as directed, but no preventative is 100 percent effective. However, compliance issues, such as late or missed doses, undermedicating, or incorrectly giving the product, are leading contributors to heartworm infection in dogs.

Because a dog on preventatives can still contract heartworms, it’s important for all dogs to get yearly heartworm tests. This annual testing helps detect infections earlier so treatment can be given as soon as possible. The earlier a heartworm infection is treated, the more likely the treatment is to be successful and the safer it usually is for the dog. Dogs don’t usually show symptoms of heartworm disease until there’s a large presence of heartworms in the body, but this is relative to the size of the dog. Small dogs may develop serious illness with only a very few adult worms, while larger dogs may not show signs until there are many.

Different types and brands of heartworm preventatives vary in price and by the size of the dog, but the cost typically ranges from $70–$200 per year, while the costs associated with a single round of treatment for an average-sized dog are usually on the order of $1,200–$1,800. It’s worth noting that many dogs require more than one round of treatment to fully eliminate the worms.

In addition, heartworm treatment is very unpleasant and carries risks. Many dogs experience severe flu-like symptoms for several days after each injection and are ill enough to require hospitalization. The adult heartworms in the vascular system trigger a significant immune response as they die and break down to be absorbed by the body, a process which takes several months. Anything that elevates the dog’s heart rate, including exercise, increases the risk that dead and dying worms get pushed into small blood vessels, where they can cause potentially life-threatening complications. “An important step for reducing the risk associated with treatment is exercise restriction for several months. This means limiting the dog to only short walks on a leash — something that most owners find very difficult to do, especially since their dog may not be acting sick at all,” states Dr. Jo Myers, a veterinarian on the Vetster platform. “It’s better to use a vet-recommended preventative all year long rather than risk infection.”

There are many different types of heartworm prevention available for dogs. However, not all options are a good fit for all individuals. Talk to a veterinarian about a recommended preventative product based on your dog’s age, size, health status, and lifestyle. Puppies as young as eight weeks of age can be given heartworm prevention. Some preventatives also protect against flea infestations, ticks, and other external and intestinal parasites.

It’s important to get a safe product and to know how to use it correctly. An online vet can help dog owners by discussing the pros and cons of different preventative options and explaining how to use them properly. Depending on local laws and the vet’s discretion, some online vets can prescribe heartworm prevention for your dog.

Can I prevent heartworms in my dog naturally?

There is no scientific evidence that supports the effectiveness of “natural” preventatives. None have been proven to kill heartworm larvae or to prevent mosquitoes from spreading them. Only FDA-approved products have been shown to be both safe and effective at preventing heartworm disease in dogs. Some home remedies, such as essential oils, can be toxic to dogs.

FDA-approved heartworm prevention is made from naturally occurring compounds and ingredients that are effective and safe for the vast majority of dogs. Adverse reactions in dogs correctly taking FDA-approved heartworm prevention are rare. It may be best for dogs with certain neurologic conditions to avoid some preventative products, so talk to a veterinarian first if your dog has a neurologic problem.

What states are at high risk for heartworm disease?

Heartworm disease has been found in all 50 US states and across Canada. States with warm, humid climates have higher heartworm infection rates due in part to increased mosquito populations and activity. However, even in colder climates, mosquitoes can be active for short periods of warmth, so it’s important to keep dogs on year-round heartworm prevention. The American Heartworm Society maintains online heartworm incidence maps that allow you to see up-to-date data on rates of infection in your state.

Do indoor pets need heartworm prevention?

Heartworm disease is spread by mosquitoes carrying heartworm larvae. Mosquitoes can easily get indoors and bite pets that don’t normally go outside. It only takes one bite from an infected mosquito to spread heartworm disease, so it’s important for indoor dogs to receive heartworm prevention too.

Does my dog need heartworm prevention year-round?

Heartworm infestations can occur any time of year. Mosquitoes that carry immature worms are active year-round in warm climates. In colder areas, mosquitoes can come out of hibernation during short periods of warm weather before the official start of “mosquito season,” so monthly prevention used year-round is recommended for all dogs.