Heartworm Disease in Dogs

Summary

Heartworm disease is caused by a roundworm parasite, Dirofilaria immitis. Dogs are exposed to the parasite through mosquito bites. Mosquitoes feed on an infected animal and pick up immature worms, then pass them to other animals they feed on later. The larvae mature and migrate through the dog’s body, with adult worms settling in the heart and major blood vessels.

Many cases of heartworm disease are asymptomatic. Mild cases may show only a cough. More severe cases show difficulty breathing and develop into heart, liver, or kidney failure. In rare cases, the worms obstruct the blood flowing through the heart causing an emergency situation called caval syndrome. 

Heartworm disease diagnosis is straightforward with simple tests to detect the presence of heartworm antigens in the blood. 

Treatment for heartworm is difficult and complications are common.  The specific protocol depends on the severity of the disease. A combination of anti-parasitic medications and antibiotics are used to clear the infection. Severe, immediately life-threatening cases require a surgical procedure to help stabilize the dog before antiparasitic treatment can occur. 

Heartworm is considered a preventable disease when using year-round preventative medications. It is recommended that dogs in high-risk areas be tested annually for heartworm disease, even if they are on preventative medication.

Risk Factors

Pet parents who keep their dogs on regular, year-round heartworm prevention as recommended by a veterinarian do not have to worry about heartworm disease but should keep up with routine testing as advised by a vet.

Cases of heartworm have been documented in every US state, but this parasite is most abundant in the southeastern United States and along the Mississippi River Valley. 

Most dogs with mild to moderate heartworm infections have a good prognosis with appropriate treatment. Severe infections can cause heart failure, kidney disease, liver disease, and difficulty breathing, which are all associated with a poorer prognosis. 

Rarely, heartworm infection can cause caval syndrome, where the blood flow through the heart is blocked by a high number of adult worms. This is an emergency. Dogs with pale gums and a rapid heart rate, or dogs that have collapsed suddenly require immediate medical attention.

Possible Causes

Heartworm disease is a parasitic infection caused by the roundworm species Dirofilaria immitis. The severity of symptoms depends on how large the population of parasites in the bloodstream is prior to treatment.

Main Symptoms

The main symptoms of heartworm infection are:

Cough • Unwillingness to exercise • Difficulty breathing

Detailed Characterization

Symptoms of heartworm infection vary significantly depending on the severity of infection. The severity of infection has four stages. 

• Stage 1 (mild) infections do not show any symptoms.

• Stage 2 (moderate) infections show only a cough. 

• Stage 3 (severe) infections have difficulty breathing, unwillingness to exercise, and can have fluid accumulation within the abdomen (ascites). Additional signs of kidney damage, like excessive urination, are often present. 

• Stage 4 infections have caval syndrome, which is an emergency. Caval syndrome results from worms preventing the heart from beating. Dogs with caval syndrome have pale gums, a rapid heart rate, and can suddenly collapse. Dogs with emergency symptoms should be transported to a veterinary hospital immediately.

Testing and Diagnosis

Annual testing is used to screen healthy dogs for heartworm disease. It takes six months from the time a dog is infected until it tests positive, so early, annual identification of positive dogs improves treatment outcomes. Dogs with heartworm infection usually require some or all of the following diagnostics to determine the best treatment:

• Physical examination: to help identify symptoms associated with heartworm infection. 

• Antigen testing: a blood test to detect proteins from the worm, which shows if there is an infection present. 

• Detection of microfilariae: A sample of blood is examined for the microfilarial (pre-larval) stage of heartworms.

• Blood work: indicates overall health and readiness for treatment.

• Diagnostic imaging: X-rays or ultrasound can help determine how developed the infection is.

Steps to Recovery

Heartworm is generally treated with antiparasitic medications and antibiotics. A combination of antiparasitic medications designed to target the specific stages of the heartworm life cycle are usually recommended. Injections of adulticide (adult-targeting) are given over several months, while preventatives to target the larval stages and minimize further spread are given on an ongoing basis. Note: there are no “natural” or herbal medications proven to be effective against heartworm. 

Complications during heartworm treatment are common and most dogs experience moderate to severe pain, lethargy, and appetite loss after each injection of adulticide. Exercise restriction is recommended for at least six months following injections, at which time the dog is retested to see if all the worms have been eliminated.  

A unique feature of heartworms is they have a symbiotic relationship with the bacteria species Wolbachia. As heartworms die off during treatment, these bacteria are released into the bloodstream and cause inflammation. Antibiotics are usually included as part of heartworm treatment as a proactive measure in anticipation of the expected inflammatory response to Wolbachia

Dogs with caval syndrome caused by advanced heartworm disease require immediate surgical removal of the parasites, followed by a heartworm treatment protocol.  Dogs who have developed secondary conditions like kidney or liver disease will require additional treatment as determined by the attending veterinarian.

Prevention

Heartworm is contagious to other dogs, cats, ferrets, and several wild animal species, but not directly. A mosquito becomes infected when it picks up the parasite from an infected host and then passes it to another host 2-6 weeks later. 

Heartworm disease is preventable. Year-round prevention for dogs throughout the United States is strongly recommended, starting as early as 6-8 weeks of age. Several medication combinations are available, and many are also preventatives for other parasite species. 

Managing mosquito populations by using bug spray and removing standing water sources can also reduce transmission of disease. Do not use products containing DEET on pets.

It is also recommended that dogs in high-risk areas be tested annually or bi-annually for heartworm, even if they are on preventative medication.

Is Heartworm Disease Common in Dogs?

Dogs in endemic regions are commonly exposed to heartworm. Veterinary clinics in highly endemic areas such as the Mississippi River Valley and Gulf Coast diagnose hundreds of cases of heartworm disease in dogs every year.

Typical Treatment

Depending on the severity of the heartworm infection, typical treatment usually includes a combination of: 

• Antibiotics • Anti-inflammatories • Repeated adulticide injections • Heartworm prevention

• Exercise restriction • Repeated testing

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