Heartworm disease is devastating, damaging, and sometimes fatal. Despite ongoing campaigns by veterinary care professionals to bring accurate information to owners, the disease is on the rise. Misconceptions about the disease and its prevention still circulate widely.   

What heartworm disease is and does 

Heartworm disease occurs when heartworm larvae develop into thin, parasitic worms that live inside an animal’s heart, lungs, and in blood vessels surrounding these organs. The infestation damages vessels and organs. 

As the heart pumps blood through the body, worms agitate inside the pulmonary arteries. This causes arterial inflammation, thickens the arterial lining, and creates lesions. Masses of tangled worms, and dead worms floating through arteries, obstruct blood flow. This disease can cause heart, kidney, and liver damage and failure. The longer an animal lives with these parasites, the more likely it is they will suffer lasting damage.

Although it does happen, heartworms in felines usually don’t develop into mature adults as they do in canines. This results in heartworm associated respiratory disease (HARD) caused by immature worms dying.  


Heartworm disease is a cyclical process. It begins when a mosquito draws blood from an infected animal. Inside this blood are tiny larvae—microfilariae. The mosquito transfers the microfilariae to other animals while it’s feeding. These larvae grow into heartworms. The cycle continues. 

There are over 60 species of mosquitoes capable of supporting development of infective heartworm larvae. Additionally, over 20 species are confirmed carriers of infective larvae [1].  

Prevalence of heartworm disease

Positive cases of heartworm disease are on the rise, here in New Mexico and around the world. The Companion Animal Parasite Council shows New Mexico as one of the higher-risk states [2]. The CAPC and the AHS show an increase in number of incidents of heartworm disease and a widening of geographical spread of the disease. 

Increased prevalence has several contributors, including longer timeframes during which mosquitoes can develop microfilariae into an infected state (lengthening transmission seasons), owner compliance and adherence to testing and prevention, and relocation of infected animals. 

Years ago, studies predicted that changing weather patterns would increasingly create more hospitable areas for infective mosquitoes in areas previously unaffected—basically bringing disease into places that had none [1]. According to new reports by the AHS, this projection proves accurate. Heartworm has been diagnosed in all 50 states and, “is considered at least regionally endemic in each of the contiguous 48 states, Hawaii, Puerto Rico, US Virgin Islands, and Guam [3].” Even Alaska has a heartworm season.

Mosquitoes require warmth to develop heartworm larvae. Thanks to an increase in urban-area microenvironments, places that previously may have had disruptions in heartworm risk during cold months are now never at zero risk. Real estate development in areas with previously low incidence continues to raise incidence levels by creating more hospitable environments to hosts by changing water drainage on lands, providing more water sources in urban home sites, and creating urban heat islands [3]. All this means that there are more mosquitoes in more areas for longer periods and infecting more pets.

Diagnosing heartworm disease

Diagnosing heartworm disease begins with a simple, fast blood test. With positive results, further diagnosis is necessary. This can include additional bloodwork, physical examination, ultrasound, or radiographs. Radiographs or ultrasounds can be used to confirm diagnosis and to assess the severity of cardiopulmonary disease caused by heartworms.

Because heartworms don’t usually reach the adult stage in cats, diagnosis can be trickier than it is with dogs and require multiple tests. 

Symptoms of heartworm disease

Symptoms of heartworm infestations don’t appear until the damage has begun. As the disease progresses and creates more damage to vascular and pulmonary systems, more symptoms develop and worsen. Dogs with heartworm disease may first present with a mild cough and reluctance to exercise and can later develop loss of appetite, weight loss, severe fatigue, and a distended belly.

While heartworm disease is less common in cats, it’s usually more severe than in dogs. Clinical signs of heartworm disease or heartworm associated respiratory disease (HARD) in cats include persistent coughing, difficulty breathing, vomiting, lack of appetite, and sudden death. 

Heartworm disease treatment

Treatment for heartworm disease in cats does not exist. 

Treatment for dogs is expensive, lengthy, strict, and can be painful. Over the course of several months, treatment includes administering drugs that will kill microfilariae and adult worms, total exercise restriction, possible hospital stays for monitoring, antibiotics, and pain medications. Some dogs may need lifelong treatment for heart failure. Damage to the heart, lungs, and blood vessels is irreversible even after treatment of heartworm disease. 

Heartworm disease prevention

Prevention is easy, safe, and effective; it’s far less expensive than treatment; and there are several options available. Preventive medications work by killing any heartworm parasites in the blood while they are in the larval stage, before they can mature further. To keep pets safe, and to help prevent the rise and spread of heartworm disease, the American Heartworm Society recommends giving pets heartworm prevention medication twelve months out of the year and testing once every year. 

Heartworm myths, facts, and frequently asked questions

Myth: Pets don’t need heartworm testing while they’re on preventive medications.

Fact: Even while on preventives, pets should be tested once every twelve months. One cause of rising heartworm cases is lack of compliance or adherence to preventive administration. It only takes one late or missed dose to put pets at risk. Pets can and do spit out or vomit up medication without owners being aware. Larvae evolve quickly into stages unaffected by prevention medication. Giving heartworm preventives to pets with adult worms can be harmful. Always pair testing with medication.

Myth: Pets don’t need preventive medications in cold months.

Fact: As mentioned above, there is never zero risk for many places and geographical spread is increasing. Mosquito season, though often defined in months, is difficult to predict. No one can say when the first mosquito will arrive nor when the last will leave. The time periods in which mosquitoes live and infect animals continue to increase. 

Myth: Indoor-only pets don’t need preventive medications.

Fact: Dogs and cats that spend most time indoors are still at risk. Mosquitoes get into and live inside homes and can infect pets. It just takes one. Pets can also escape. Dogs that only go outside to relieve themselves still need protection. 

FAQ: Why do preventives require prescriptions?

A: Preventives are FDA regulated medications; and all medications regulated by the FDA require a prescription to ensure effective and safe use. Requiring a prescription for heartworm preventives also allows veterinarians to determine if there’s risk of them interacting with any other medication a pet may be on. 

FAQ: Does it really matter that much if a pet’s weight doesn’t match the preventive?

A: Yes. Heartworm preventive dosages are based on a pet’s weight. Overdosing is dangerous and underdosing may leave your pet unprotected. If your pet’s weight changes, they may need an updated dosage to keep them safe and protected.  

The biggest, most important takeaways all pet owners should know are: life-altering, sometimes fatal, heartworm disease is easily preventable, and all cats and dogs should have annual testing and year-round preventives. 

If your pet is due for a heartworm test or needs a preventive refill or adjustment due to weight changes, please call us at 505-299-9533 or schedule and appointment through our website.



  1. Noack S, Harrington J, Carithers DS, Kaminsky R, Selzer PM. Heartworm disease – Overview, intervention, and industry perspective. International Journal for Parasitology, Drugs and Drug Resistance. 2021;16:65-89. doi:10.1016/j.ijpddr.2021.03.004
  2. Companion Animal Parasite Council. Parasite prevalence maps. Published March 29, 2019. https://capcvet.org/maps/#/2024/all-year/heartworm-canine/dog/united-states
  3. American Heartworm Society. Heartworm guidelines. Published 2024. https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines
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