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Heartworms In Dogs - Symptoms And Treatments

For over 70 years, the veterinary staff at Metairie Small Animal Hospital has made heartworm prevention for dogs an important part of our approach to preventive care. Heartworm prevention is so important that our veterinarians address the subject with pet owners on the very first visit.

Heartworm Prevention in DogsA heartworm infection can lead to numerous health problems up to and including early death. Therefore, heartworm prevention cannot be ignored. Our team is here to educate you and protect your dog from this terrible disease.

While outdoor playtime certainly offers numerous benefits, it also increases the risk of exposure to heartworms in dogs. This does not mean that dogs are completely safe indoors. It simply means that potential exposure to heartworm infection comes with increased exposure to the outdoors.
Heartworms are spread through dogs via bites from infected mosquitos. Living in Louisiana, the presence of mosquitoes throughout the spring, summer and early fall months means greater chances for your dog to contract heartworms. Once a heartworm infestation occurs, it will become life threatening. Therefore, our goal is to implement a preventive program before your dog is exposed.

What Is Heartworm Disease?

Heartworm disease in dogs is a blood-borne parasitic nematode (roundworm) known as Dirofilaria immitis transmitted by mosquitoes. 

Upwards of 30 species of mosquitoes can act as heartworm transmitters. Mosquitoes ingest immature heartworm larvae, called microfilariae, by feeding on either an infected cat or dog. The microfilariae develop further for 10 to 30 days in the mosquito's gut and then enter parts of the mosquito’s mouth.
When an infected mosquito bites a dog, it injects larvae into the dog. The larvae then mature over a period of several months, eventually ending up in the right side of the heart and the pulmonary arteries. Once this occurs, they mature into adult heartworms in dogs, and can reproduce about six months from the time of infection. At approximately eight months after the infection, heartworms in dogs begin to produce a new crop of microfilariae that will live in the dog's blood for about one month. By the time this occurs, dogs may already show heartworm symptoms, and negative changes are already happening in their bodies.

Heartworm Symptoms - The Four Stages Of Heartworms In Dogs

Heartworm symptoms in dogs are divided into four stages. It is important to understand that individual stages are not always clearly identifiable and some stages can overlap, but the following information will help educate you about the four major stages, as well as their accompanying heartworm symptoms. The four clinical stages of heartworm disease begin when your dog has already become infected and the heartworms are present in the dog’s heart:

  • Stage 1- In dogs, the first stage of heartworm disease will typically be symptom free. In this stage the heartworms are present and settling into the heart. However, in stage one the disease has not yet progressed to the point where the heartworms will have produced a new generation of microfilariae and the dog’s body will not yet have produced antibodies  in an amount sufficient for detection. A blood test is necessary to determine if a dog has heartworms, but be aware that because the test depends on the detection of antibodies, it is possible to have a negative result if the pet was recently infected.
  • Stage 2- Stage two of heartworms in dogs is accompanied by moderate symptoms including intolerance for exercise and a cough. The heartworms have been present long enough in the body for antibody production and probable microfilariae production. During this phase, heartworm disease may be detected with blood tests. 
  • Stage 3- By stage three of heartworms in dogs, the symptoms of the disease will be very noticeable and have a big impact on your dog’s health.  Dogs continue to cough and experience fatigue after exercise, may be reluctant to exercise at all, and can have trouble breathing. During this stage, dogs may also cough up blood. By stage three, the disease can be identified on chest x-rays, as changes to the lungs and vessels can be detected.  
  • Stage 4- Dogs in stage four of heartworm disease have very visible heartworm disease symptoms. These dogs are very ill and their bodies have already experienced significant changes because of the heartworm infection. . The symptoms are similar to Stage 3 but more severe.  Dogs will be reluctant to exercise, tire after exercising, and will exhibit a cough. Dogs with heartworms in this stage may also develop a distended abdomen due to fluid accumulation. This occurs because the heart is no longer functioning properly and cannot pump blood at the rate the body needs.  This is often accompanied by difficulty breathing.  Testing may reveal the impact of the disease in the form of abnormal sounds within the dog’s heart and lungs and an enlarged liver. Even with treatment, this stage of the disease carries a high risk of long term debilitation and possible death.

The severity of heartworms in dogs is directly dependent upon:

  • The number of worms present in a dog’s body
  • The duration of the infection
  • The response of the infected dog, in fighting off the infestation.


As heartworm disease progresses through each stage, treatment methods become increasingly invasive. This is a big reason why early detection plays a major role in the options and ability for your dog to recover. Remain aware of any changes in your dog’s behavior. Keep an eye out for any changes in behavior that align with the symptoms of heartworm disease and if you do find that your dog is displaying symptoms that could be indicative of heartworm, it is important to make a veterinary appointment right away. 
Other heartworm symptoms include:

  • Anemia
  • Fainting Spells
  • Right Sided Chronic Heart Failure
  • High Blood Pressure
  • Rapid Heart Beat


It is important to understand that the symptoms listed above are indicative of advanced stage heartworm disease. Unlike medications that are used to prevent heartworm in dogs, the medications that are used to kill an advanced stage heartworm infection carry a higher rate of potential side effects, can be painful for the dog and are costly to the owner. In addition, the treatment will require considerable downtime for your dog’s normal exercise routine while the dog recovers from the infection.

Heartworm Prevention For Dogs

Heartworm prevention is available in several forms including monthly chewable tablets and injections that last for 6 months. The chewables are readily accepted by most dogs as a treat. It is very important that the chewable be given once a month on the same day of the month to make sure that your dog is adequately protected. Side effects of the chewable medication are rare. However, as with any medication, please call if you notice any changes in your dog’s health or behavior.  The injectable heartworm preventative lasts provides 6 months of protection and must be administered by a veterinarian.  Always speak with a veterinarian about which heartworm prevenattive is best for you pet.

What You Need To Know About Heartworm Treatments

The first thing to understand is that there is a significant difference between heartworm prevention and heartworm treatment. Prevention is simple to do and is effective in protecting your dog against heartworm disease. Treatment options are used for dogs that are already sick because they have become infected. 

The first step in heartworm treatments is obtaining a diagnosis. The first step in diagnosis of heartworm in dogs is to perform a blood test. If the blood test comes back positive, then the following tests may also be performed to determine the stage and severity of the disease in order to decide on the most appropriate treatment plan:

  • A urinalysis, or the testing of a dog’s urine
  • An antigen test determines the presence of adult female heartworms
  • Radiographs, or X-Rays to view the size and shape of a dog’s heart. This is helpful because many dogs with heartworm develop enlarged pulmonary arteries, or have obstructions in the arteries leading to the lungs
  • Ultrasounds allows us to directly view the internal structures of the heart and surrounding vessels, in order to assess the condition and function of the heart

During heartworm treatments, most patients are hospitalized to receive an adulticide, which is a medication that kills adult heartworms. This is typically done in two stages over a period of two months.  For the third and final stage, the veterinarian will look for signs of microfilariae in the bloodstream and base the final treatment on those results.  
For more severe cases, such as dogs experiencing thromboembolic complications (in which a blood clot that has formed breaks loose and travels through the bloodstream to clot another vessel), hospitalization may be necessary for a longer period of time while heartworm treatments are administered. In some extreme cases, a surgical procedure may be necessary to remove adult worms from the right heart and pulmonary artery by way of the jugular vein. This procedure is recommended if the infestation consists of a high number of adult worms.

Ask Your Veterinarian About Heartworm Medicine For Dogs

It is important to consult your veterinarian when making preventive care decisions for your dog. This is true for a variety of reasons. There are many over the counter products on the market today that range from ineffective to outright dangerous. Our veterinarians are trained and qualified to help you make the best decisions regarding preventive care and treatment of any health conditions your dog may develop, especially when it comes to parasitic infections.

Schedule An Appointment To Diagnose Or Prevent Heartworm In Dogs

Heartworm is an easily preventable disease. If your dog is not currently using a veterinarian-recommended heartworm prevention medication, please schedule an appointment right away. There is no reason for your dog to be exposed to heartworm disease when prevention is so simple.

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