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Heartworm Awareness

Nobody likes getting bitten by a mosquito. For most of us humans, we move on with our lives and try to cope with the itch. But for dogs and cats, mosquito bites can be more complicated. Heartworms live a life cycle that requires both mosquitos and dogs, and can infect cats as well. When a pet has heartworm disease, it can be difficult to diagnose and treat before it has advanced and caused significant harm to the heart and lungs. Luckily, heartworm disease can be treated and is easily preventable. Learn more about how you can recognize and prevent this disease below.

What is Heartworm Disease?

Also known as dirofilariasis, heartworm disease is caused by a blood-borne parasite known as Dirofilaria Immitis and more commonly called a heartworm. Heartworms are found in the heart, pulmonary arteries, and adjacent large blood vessels of infected dogs and cats. A dog can have as many as 300 worms when diagnosed, while cats usually only have a few. Feline blood vessels are smaller and it only takes a few worms to cause a blockage. By the time clinical signs are observed, the disease is already well advanced. The most obvious symptoms in include soft and dry cough, shortness of breath, weakness, listlessness, and loss of stamina. If you suspect your pet has heartworm disease, contact your veterinarian as soon as possible. The sooner your pet receives treatment, the better prognosis they can have.

The disease is not spread directly from dog to dog, or from dog to cat. Rather, they are spread through mosquitoes. As a result, heartworm disease occurs all over the world, but is most common near large waterways. This includes the Gulf of Mexico, Atlantic Ocean coastlines, Mississippi River, and across the southern Great Lakes. It’s important to note that heartworm disease is endemic in Arizona due to coyotes, meaning this could also occur in Colorado. The greatest risk of infection is when the outside temperature is over 50°F (10°C), but can still occur year round. With varying temperatures in Colorado, mosquitos can still be around in November and December and spread the disease.

Heartworm Life Cycle

Heartworms can be either male or female: the male heartworms are half the size of the females. Female heartworms are more of an issue for our furry friends, as they produce millions of microscopic heartworms, or microfilariae, which live in smaller blood vessels. This parasite requires the mosquito as an intermediate host. When a female mosquito bites an infected dog, the insect ingests the microfilariae. These develop for 10-30 days in the mosquito’s gut, then enter its mouthparts. The microfilariae are now infective larvae and become mature when they enter a dog, through a mosquito bite. The larvae then migrate into the bloodstream and heart. They will mature, mate, and reproduce within 6-7 months. Without treatment, adult heartworms can live for 5-7 years in dogs, and generally last 2-3 years in cats.

Symptoms of Heartworm Disease

It takes several years for dogs to show clinical signs of infection, because the heartworms take time to block the heart, arteries, and  veins. This means that by the time we see clinical signs, the disease is well advanced. For this same reason, it is very rare to see this disease in dogs or cats less than one year old. We won’t start testing puppies and kittens for heartworm until they have reached 6-7 months of age, the earliest age where heartworms mature and become detectable.

The most obvious symptoms of heartworm disease are a soft and dry cough, shortness of breath, weakness, listlessness, and loss of stamina. We also find pets who are faint or disoriented and have abnormal lung and heart sounds. If the pet is experiencing congestive heart failure due to heartworm, the abdomen and legs may swell, the pet will experience weight loss, be in generally poor condition, have anemia, and may even die suddenly during exercise or excitement. Cats may experience overwhelming respiratory failure brought on by an inflammatory reaction to heartworms in the lungs that causes sudden death.

Cats are generally more resistant to heartworm disease, but can still become infected. For every 100 dogs with heartworm disease, about 5 to 15 cats test positive for infection. Cats are not the ideal host for heartworms. Unfortunately, there is insufficient research on heartworm disease in felines, but experts are discovering that cats may get this disease more often than previously thought. In cats, it only takes a few worms to create a major blockage. It has also been seen that respiratory symptoms are more common in cats. This is a result of an inflammatory reaction that can occur when heartworms die in a feline. A common misconception with this disease in cats is that indoor cats cannot contract the disease, as it is spread through mosquitoes. However, ⅓ of cats with heartworm disease are indoor-only cats. It is important to take preventative measures for our feline friends so they can receive the best outcome.


To diagnose heartworm disease, your veterinarian will run one or more simple blood tests. First, we will perform the Heartworm Antigen Test, which looks for adult female heartworms. This is generally recommended during annual wellness exams at Harmony Veterinary Center as part of the yearly lab work for your pet and is a general precautionary measure. If this comes up positive, further tests can determine the severity of the disease and if the dog is healthy enough to undergo treatment. This may include tests to look for the smaller microfilariae, Complete Blood Count (CBC) tests, radiographs (X-Ray), and a cardiac ultrasound. Learn more about testing for heartworm in dogs here. The process of testing for heartworm disease in cats can be a little more complex. Please call us at 303-432-8551 if you have further questions or concerns on how this process would look for your specific animal.


Prompt treatment is essential and, according to the American Heartworm Society, most dogs that test positive for heartworm can be successfully treated. An injectable drug, melarsomine (or Immiticide), an aresenical compound, is given in a series of injections. Most dogs receive one injection, go through a 30 day period of rest, then 2 more injections are given 24 hours apart. Depending on the severity of the infection, additional medications such as antibiotics, pain medications or anti-inflammatories may also be given to support the patient. Dogs must be kept quiet and cannot exercise for one month following the final injection of heartworm treatment. Complete rest is essential during treatment. The adult worms die in a few days and decompose, and are carried to the lungs. The pieces get lodged in small blood vessels and reabsorbed in the body.

Dogs will be tested for microfilariae and adult heartworms at one month and nine months after the last injection. The prognosis after treatment is generally good: most dogs display renewed vigor and vitality, improved appetite, and healthy weight gain. After treatment, your pet will start on heartworm preventative to protect against the disease in the future. Dogs with severe heartworm may need antibiotics, pain relief medications, special diets, diuretics to remove fluid in the lungs, or drugs to improve heart function. In addition, some dogs may require lifetime treatment for heart failure, including diuretics, heart medications such as ACE-inhibitors, beta blockers, or cardiac glycosides, and special low-salt diets. If your dog has heartworm disease, talk to your veterinarian about developing a long-term healthcare plan to keep them as happy and healthy as possible.

Unfortunately, there is no drug approved for heartworm treatment in cats. When the heartworms die and travel to the pulmonary arteries and lungs, sudden death of the patient can occur. This leaves two options for cat owners. First, you and your veterinarian may decide to treat the symptoms of heartworm disease with the goal that the cat outlives the worms. Remember, heartworms live in a cat for 2-3 years, so this option is possible. Cats can be treated with oxygen and corticosteroids/cortisone during a crisis to relieve the reaction in the pulmonary arteries and lungs. This will reduce clinical signs and improve quality of life, but the threat of acute crisis or sudden death still exists. Another option is surgical removal of the heartworms. This is recommended for cats with severe signs and must be performed by a specialist. Up to 40% of cats may die during or after this procedure, so it is typically reserved for those cats who have severe disease and a poor prognosis without surgery.


Heartworm disease is highly contagious for dogs in warmer months, and prevention is strongly recommended for all furry friends that travel to high-risk areas. At Harmony Veterinary Center, we primarily use HEARTGARD® Plus, which can be purchased for dogs. It consists of Ivermectin & Pyrantel and helps prevent heartworm disease. It also treats and controls three species of hookworms and some roundworms. Made with beef, this combination of medications is given by mouth as a flavored chewable treat that is given monthly. It is generally well tolerated at label doses: contact your veterinarian if you notice side effects such as an upset stomach, vomiting, diarrhea, dilated pupils, unsteadiness when walking, or a dazed demeanor. It should not be given to cats or puppies younger than 6 weeks, or pets who do not have a current negative heartworm test. Make sure to talk to your veterinarian about any medications that your pet is already taking, or if they have allergies/sensitivities to any drugs. For dogs with sensitivity to beef, we have alternatives. We encourage you to discuss these with your veterinarian.

It is never recommended to give your dog preventative medications meant for cats, or vice versa. Each contain a combination of compounds that can be toxic when given to the wrong species. 

No pet should have to endure this disease. Ensure that everyone looking after your furry friend understands what heartworm disease is and how to identify possible signs. Pets with these signs should be taken into a veterinary clinic or emergency pet hospital as soon as possible. If you have any questions please contact us at 303-432-8551 or at