Heartworm Disease in Dogs

Written by Small Door's medical experts

Heartworm disease is a common infection in dogs. Heartworms are spread by mosquitoes, and dogs can contract this parasitic infection after being bitten by an infected mosquito. While this is a potentially fatal disease, it is also preventable and can be treated if diagnosed early. Read on to learn more about how to prevent and treat heartworm disease in your dog.

In this article: 

What is heartworm disease in dogs? 

Heartworm, also known as Dirofilaria immitis, is a large parasite resembling a very long, thin worm. Heartworm disease is when the parasite grows inside your dog, causing inflammation of the nearby blood vessels when the immune system gets stimulated. Heartworms take root and reproduce in a pulmonary artery, which supplies blood from your dog’s heart to its lungs. This can lead to life-threatening blockages in your dog’s normal blood flow. 

What do heartworms do to a dog's heart?

Once infected, heartworms begin to migrate to the blood vessels, lungs, and heart, where they grow into adult worms. The heartworms can cause inflammation and stress on your dog’s heart. The damage to the heart can lead to heart failure.

How common is heartworm disease in dogs?

Heartworm disease in dogs is a very common infection worldwide. In the United States, more than 1 million dogs have heartworm disease, the American Heartworm Society estimates.

Since it is transmitted by mosquitoes, it is most often reported in countries with either tropical, semitropical, or temperate climates. Dogs and coyotes are the most common animals infected by heartworms.

Signs & symptoms of heartworm disease in dogs

Signs and symptoms of heartworm disease in dogs may include the following: 

  • Coughing

  • Shortness of breath or difficulty breathing (when active or at rest)

  • Blue or purple discoloration of the gums or skin

  • Coughing or spitting up blood

  • Nosebleeds

  • Fainting or collapse

  • Exercise intolerance

  • Swelling of the abdomen or chest due to fluid accumulation

  • Weight loss 

  • Sudden death

Live and dead worms can end up causing blockages, scarring, and damage to areas in or near the heart and lungs, which make it difficult for the heart and lungs to function. In addition, the constant battle being fought by a dog’s immune system can cause inflammatory proteins to collect elsewhere, such as in the kidneys.

Yet, it should also be noted that while an infected dog can show one or several of the aforementioned symptoms, many infected dogs can be asymptomatic, meaning they will not show any outward signs of infection. 

What causes heartworm disease in dogs?

The heartworm, a parasite also known as Dirofilaria immitis, causes heartworm disease in dogs. A parasite is a tiny organism that lives and grows in a host and feeds off that host.

How is heartworm disease spread?

Heartworm disease is spread through the bite of infected mosquitoes. Approximately 70 different species of mosquitoes are carriers of this parasite. 

What is the life cycle of the heartworm?

The life cycle of the heartworm consists of the following: 

  • A mosquito bites an infected animal and ingests the heartworm microfilariae (also known as the early life stage of a heartworm – i.e. a “baby” heartworm) as it feeds.

  • These microfilariae then grow into larvae in the mosquito (i.e. “teenager” heartworms).

  • The mosquito then transmits the larvae into a healthy dog through biting and feeding. 

  • The larvae, which have now been transmitted into a dog, will then grow into adult heartworms in the lung and heart vessels, reaching up to 12 inches in length.

  • Adult heartworms can also reproduce within the dog and give birth to microfilariae, which can live for up to two years inside a dog. 

How to diagnose heartworm disease in dogs

Your veterinarian can diagnose heartworm disease by administering one or several of the following diagnostic tests: 

  • Heartworm antigen testing: This is a blood test that detects the presence of mature female heartworms.

  • Direct blood smear: This is a blood test that analyzes a drop of blood under a microscope and checks for any live microfilariae swimming in the blood sample. 

  • Difil test and Knott’s test: These are tests that can identify the concentration of microfilariae in a blood sample. These are used to detect more subtle infections that may not show up on a direct blood smear. 

  • Radiograph and ultrasound: Radiographs and ultrasonography (echocardiogram) can provide your veterinarian with a visualization of your dog's heart and lungs. This is important because these images can show any inflammation or enlargement of the right side of the heart, as well as within the arteries connected to the heart and lungs, all of which can be indicative of heartworm infection. 

  • Complete blood count (CBC), chemistry, and electrolyte tests: These blood tests will check your dog’s red and white blood cells, platelets, overall organ function (such as the kidneys and liver), and electrolyte balances, all of which can identify whether there are any additional abnormalities that may be related to a heartworm infection. 

  • Urinalysis: Your veterinarian may recommend testing your dog’s urine, as this can be helpful and show whether they are losing protein. 

Treatment for heartworm disease in dogs

The first step in treatment involves staging the progression of infection. There are four different categories of disease progression, which can vary based upon the age and size of your dog, whether there are any other concurrent health conditions, and how many heartworms or microfilariae are suspected to be present at the time of diagnosis. The disease categories are: 

  • Class I: This is the lowest risk category. Dogs in this category are usually not showing any symptoms, and diagnosis can be an incidental finding in their annual heartworm screening test results. At this stage, there are usually only mild changes, if any, observable on bloodwork and radiograph results. 

  • Class II: This category is for moderately affected dogs, meaning they usually have minimal diagnostic changes, and may only be showing minor symptoms such as occasional coughing or exercise intolerance. 

  • Class III: Severely infected dogs will fall into this category. Usually, Class III dogs will present with severe symptoms, such as a significant cough, trouble breathing, and weight loss. At this stage, there will be noticeable damage to your dog’s organs due to the heart needing to pump harder than normal to compensate for blood vessel blockages caused by the accumulations of worms.

  • Class IV: Also known as caval syndrome, this is when infection is so severe that a dog will collapse due to shock. If possible, treatment at this stage will involve emergency surgery, during which heartworms will be removed through a surgical incision made through the jugular vein. 

After being diagnosed, dogs will begin treatment, which can include one or several of the following: 

  • Corticosteroids: Steroids can help with any lung or cardiac-related inflammation caused by the heartworms, as well as help alleviate any treatment-related pain. 

  • Cardiac medications: Some cardiac medications, such as furosemide (a diuretic) or sildenafil (a pulmonary vasodilator), may be administered to help prevent any adverse cardiac events caused by the heartworms. 

  • Strict exercise restriction: This is an extremely important step of treatment, as any exertion during treatment can result in dying worms causing a pulmonary thromboembolism, or blockage, which can be a life-threatening emergency. This means no long walks and no running. Often, crate rest is required to achieve this. 

  • Oxygen therapy: Supplemental oxygen therapy may be needed depending on the progression of disease and how badly the lungs and heart are affected by the time treatment is initiated. 

  • Doxycycline: This antibiotic may be prescribed prior to heartworm treatment being initiated, as there are often rickettsial (or bacterial) organisms called Wolbachia living inside of heartworms. When the heartworms are killed with treatment, the Wolbachia release an inflammatory protein into the bloodstream.Administration of doxycycline can help remove the Wolbachia from the heartworms prior to the initiation of heartworm treatment, and it can also reduce the number of heartworms present.Doxycycline is also beneficial because it can kill young heartworms as well. 

  • Immiticide injections: Also known as melarsomine dihydrochloride, this is the only known product currently available to treat adult and some immature heartworms.This medication is a Food and Drug Administration (FDA)-approved arsenic compound and is administered via injection in either a two-dose or three-dose protocol. Brand names are Immiticide and Diroban. The injection is given in the muscle and can cause some soreness or discomfort at the site of injection. Because of this, your veterinarian may also prescribe a pain or anti-inflammatory medication to help with any discomfort. 

  • Non-arsenic therapy: This is a combination treatment of a macrolide antibiotic along with doxycycline. This method is not typically considered as a first choice and is often selected only if a dog has already had one or more courses of Immiticide treatment, if they have other serious health conditions, or have had an adverse reaction to previous Immiticide treatment. Additionally, at times there have been shortages of Immiticide available nationwide, especially within shelters, which can necessitate non-arsenic therapy as a treatment option. 

Your dog will need to be hospitalized during the day of treatment so that your veterinarian can monitor them for any signs of discomfort or reaction. The medical staff will monitor your dog for signs and symptoms of any adverse events, including fever, gastrointestinal upset, lethargy, pain, or any cardiac events (such as thromboembolism).

Is there a cure for heartworm disease in dogs?

Heartworm disease can be cured if proper treatment is administered and the infection is caught while it is still at a treatable stage. 

Is heartworm disease in dogs contagious for humans or other pets?

Heartworm disease is not contagious to humans, and your dog cannot pass it onto another dog or animal. It can only be transmitted via mosquito bites to other mammals, including coyotes, cats, and ferrets. 

What is the cost of treating heartworm disease in dogs?

In addition to fees associated with an exam and diagnostic testing, the cost to treat heartworm disease can vary, depending on the severity of symptoms and disease process at the time of diagnosis. Costs can vary from a few hundred dollars to a few thousand dollars. 

Recovery and management of heartworm disease in dogs

Recovery of heartworm disease can be a long process. Depending on the treatment method administered, treatment and recovery can be ongoing for several months (from a few months up to as many as 15 months) as your dog’s body works to clear itself of the dying heartworms. Continued exercise restriction and follow-up diagnostics will be required to ensure the parasitic infection is resolving and your dog is not experiencing any adverse cardiac or lung events. 

How to prevent heartworm disease in dogs

Heartworm disease in dogs can be prevented by having your dog take preventative medication that will kill any larval heartworms in the event your dog is infected. These FDA-approved medications are available in oral, topical, and injectable forms, which can be administered either monthly, biannually, or annually, depending on the formulation and medication. Your veterinarian will be able to recommend a preventative medication that’s most appropriately suited for your dog’s healthcare needs and lifestyle. 

What is the best preventative medication for heartworm in dogs? 

Recommended oral brands for heartworm preventatives in dogs include Simparica Trio or Heartgard. These are flavored chewables that can be fed to your pet. Topical brand options include Revolution or Advantage Multi. These involve applying a small tube of liquid to the dog’s skin (typically on the skin between the shoulder blades while parting the fur). Either of these options are administered consistently on a once-monthly basis, all-year round. 

Does my dog still need heartworm testing if they are on preventatives?

Yes, it is recommended by the American Heartworm Society that all dogs undergo annual heartworm testing every 12 months regardless of whether they are on preventatives. As infected dogs often will not show any clinical symptoms of disease, it is important to test annually to confirm that they are negative. This can help catch any potential breakthrough cases.

It is also important to test for infection prior to administering heartworm prevention because if a preventative is unknowingly given to an infected dog, it can lead to a fatal reaction. 

As it takes a minimum of five months for heartworm larvae to migrate after infection, puppies younger than five months of age are not tested for heartworm disease because even if infected, they are too young to test positive. 

Is there a vaccine for heartworm disease in dogs?

There is no vaccine for heartworm disease at this time. However, there are several different types of preventative medications available. Some of these preventatives are available in injectable forms, such as Proheart6 and Proheart12, which can provide up to 6 or 12 months of protection against heartworm infection. 

Summary of heartworm disease in dogs 

Although heartworm disease can become severe or even fatal, preventative measures can help ensure your dog does not develop infection if they’re bitten by an infected mosquito. Following through with annual blood test monitoring and administering consistent preventative medications can help save your dog’s life. Your veterinarian can help you come up with a proactive preventative plan against heartworm disease.

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