Dogs and Heartworms

Why Heartworms are Such a Threat to Dogs

Jack Russell Mix dog laying down
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Heartworm disease is a very serious and potentially deadly condition in dogs. You may have heard veterinary professionals talk about heartworm testing and prevention, but do you understand the seriousness of the disease? As a responsible dog owner, it's best to familiarize yourself with the dangers of heartworms to dogs.

Heartworm disease is caused by an infestation of a parasite known as Dirofilaria immitis.

Though this parasitic nematode is known to affect many animal species, its ideal host is the dog. In short, Dirofilaria immitis invades the heart, lungs and nearby vessels of the dog, eventually leading to death. Heartworms are possibly the most dangerous parasites that affect dogs. While heartworm disease is quite common in dogs, it is also easily prevented with the help of your veterinarian.

Heartworm disease can occur in cats, but it is less common.

How Dogs Get Heartworms

Heartworm disease is transmitted between animals by way of the mosquito. First, a mosquito bites an animal (often a dog) with heartworm larvae (called microfilariae) in its bloodstream and ingests the larvae. These larvae are in their first stage of development (L1). The larvae then mature within the body of the mosquito for about two weeks until they reach the third stage of larval development (L2). When the mosquito prepares bites a dog, L3 microfilariae enter the dog's skin through the tiny wounds from the bite.

They mature for one to three days under the skin of the dog, reaching the L4 stage. These larvae migrate through the dog's body for 50-70 days until they become young adult worms. They then enter the bloodstream and travel towards the heart as they mature to reproductive age. Adult heartworms mate in the vessels of the lung.

At this stage, they are 10-15 cm in length (about the length of a pencil). Within 7 months of that first mosquito bite transmission, Dirofilaria immitis will reach maturity. Mature male heartworms are 15-18 cm long, but females are 25-30 cm (think of angel hair pasta). By this time, microfilariae will be making their way through the bloodstream until they can be ingested by a mosquito, repeating the life-cycle.

How Heartworms Affect a Dog's Body

A single adult heartworm can survive in the dog for five to seven years. Worms generally live in the heart and surrounding vessels of the dog. Damage occurs to the lining of the vessels. Tissue and vessels in the region become inflamed. Blood cells can collect with worms, literally clogging arteries and blocking the valves of the heart. Cardiac output may be decreased, leading to heart enlargement and pulmonary hypertension. All of these issues can also lead to malfunction of other organs in the body, including the liver and kidneys. The more worms present, the greater the complications.

The longer the worms are present, the more severe the damage. The damage caused by heartworms also depends on their exact location in the dog.

Symptoms of Heartworm Disease

The symptoms of heartworms do not typically occur until the disease is somewhat advanced. No signs are seen in the early stages of heartworm disease. This is part of the reason most veterinarians recommend annual heartworm testing. Annual testing is also important for dogs on heartworm prevention (in case the product fails, it's best to catch the disease early).

Coughing is often observed when mild heartworm disease is present. A dog with moderate heartworm disease typically exhibits coughing and exercise intolerance. Once heartworm disease is severe, signs include coughing, exercise intolerance, difficulty breathing, abdominal swelling, collapse and even sudden death.

If your dog is coughing, see your vet right away. A veterinarian can perform simple blood tests to detect the presence of heartworms. A vet can also listen for abnormal heart sounds and perform other diagnostic tests to help determine the severity of heartworm disease. As always, proper communication with your vet with your vet is essential.

Heartworm Prevention

One of the most important things you can do for your dog is to prevent heartworm disease from occurring in the first place. Make sure your dog visits the vet as soon as you first bring him into your life. Keep up with routine wellness visits as recommended. Heartworm prevention will be prescribed by your veterinarian, often in the form of a monthly pill. Some forms of heartworm prevention are topical or injectable. Heartworm prevention works to kill heartworm microfilariae so they cannot mature in your dog's body. It is imperative that you comply with your vet's recommendations regarding heartworm prevention. This is your responsibility as a dog owner. Never stop or skip your dog's regular heartworm prevention unless instructed to do so by your veterinarian.

Heartworm Treatment for Dogs

Though heartworm disease is devastating and potentially fatal, it can often be treated. Unfortunately, heartworm treatment is risky and expensive. Dogs with severe disease may not survive treatment and are often not considered eligible for treatment. This is why heartworm prevention is so important.

Heartworms are killed with the use of an adulticide. The protocol most vets follow is based upon the recommendations set by the American Heartworm Society.

Adulticide therapy has several components. Dogs first undergo diagnostic testing to help determine the severity of heartworm disease. This typically includes lab work and radiographs but may vary depending on the severity of clinical signs, if any.

To begin the adulticide protocol, the dog is first started on heartworm preventive to kill any microfilariae (heartworm larvae) present.

Many vets will use a pre-treatment of antihistamines and anti-inflammatory drugs to prevent a reaction as the larvae die. After the initial dose of heartworm preventive is given, the dog should remain on regular heartworm preventive for the rest of its life (as should all dogs).

The heartworm positive dog is generally started on oral doxycycline or minocycline for the next four weeks. These antibiotics are given to fight bacteria given off by the dying heartworms. It is also thought to weaken the live heartworms.

Thirty days after the first heartworm preventive was administered, the dog will return to the hospital for the first dose of adulticide. A drug called melarsomine (Immiticide) is injected into a muscle along the lumbar spine and the dog is observed for the day in case of a reaction. The same preliminary treatments as before are usually given to prevent reaction (antihistamine and anti-inflammatory injections).

Thirty days later, the dog returns to the hospital and a second melarsomine (Immiticide) injection is given. The dog is typically hospitalized overnight and is given a third melarsomine injection the next day.

Heartworm treatment is risky mainly because of the blood clots that can occur as the worms die. Restriction of the dog's activity is essential throughout treatment and should be strictest during and after adulticide injections. Exercise, excitement and overheating will all increase the likelihood of complications. Vets typically recommend activity restriction for one or two months following heartworm treatment.

When all is said and done, the heartworm treatment protocol may cost as much as $1000-$1500. Even low-cost vet clinics tend to charge $300 or more. When you compare this to the cost of heartworm prevention, it puts things into perspective. The annual cost of prevention ranges from about $35-$250 per year depending on the size of the dog and the brand of prevention chosen. Clearly, prevention is the safer and more affordable option.

Important note: Even after a dog has been treated for heartworm disease, re-infection can occur if heartworm prevention is not used!

Dogs that are not eligible for adulticide therapy may be treated with the so-called "slow-kill method." This is not recommended by the American Heartworm Society and is not considered effective. However, it may be the only option in certain cases.

If you would like to spare your dog (and your wallet) from the above process, then make sure you are giving heartworm prevention all year long. Educate yourself about heartworms and talk to your vet about the best preventive plan for your dog.