Introduction
Heartworm disease is a serious and potentially fatal condition caused by parasitic worms called Dirofilaria immitis that infect the heart, lungs and associated blood vessels of dogs, cats and various other species. The disease is spread through the bite of an infected mosquito and can cause severe lung disease, heart failure and damage to other organs in both dogs and cats. However, while heartworm disease is treatable in most dogs, it is often considered untreatable in cats due to the risks associated with current adulticide treatment protocols. This article provides an overview of heartworm disease and its effects in dogs and cats, and explores why the disease is treatable in dogs but often not treated in cats.
Sources:
https://oakcreekclinic.com/blog/?p=heartworm-disease-in-dogs-and-cats-the-basics-for-pet-parents
https://foxrunanimalhospital.com/heartworm-disease-in-dogs-and-cats/
Heartworm Lifecycle
Heartworms are spread by mosquitoes. When a mosquito bites an infected dog, it ingests the heartworm larvae called microfilariae. These then undergo further development inside the mosquito and become infective larvae. When the mosquito bites another dog, the infective larvae are then transmitted to the new host. In the new dog, these larvae migrate through the tissues and mature into adult worms which travel to the heart and reside in the pulmonary arteries and heart.
The fascinating Heartworm Life Cycle Illustration from the American Heartworm Society shows the process step-by-step. As noted, once inside a dog, the larvae mature, with adult female worms releasing new microfilariae into the dog’s bloodstream to continue the cycle. This allows transmission every time a mosquito bites the infected dog.
Prevalence
Heartworms are far more prevalent in the dog population than in cats. Studies show that approximately 28% of dogs test positive for adult heartworm infection, compared to only 4% of cats (Hays, 2020). The 2020 heartworm prevalence survey conducted by the American Heartworm Society also found higher rates in dogs, with 28% of dogs and 4% of cats infected (American Heartworm Society, 2020). The higher prevalence in dogs is likely due to factors like dogs spending more time outdoors and differences in mosquito feeding habits between species. But while infection rates are lower in cats, heartworm disease should not be overlooked as cats are vulnerable to potentially severe complications.
Symptoms
Heartworm disease can cause a variety of symptoms in both dogs and cats. In dogs, the most common early symptom is a mild, persistent cough. As the disease progresses, dogs may develop fatigue during exercise, difficulty breathing, and weight loss. Severely infected dogs may cough up blood and have fluid accumulation in the abdomen. In cats, the symptoms are often nonspecific and can include vomiting, weight loss, difficulty breathing, and coughing. Cats are also prone to sudden death from heartworms due to the worms blocking blood flow in the pulmonary arteries. The severity of symptoms depends on the number of worms present, with dogs generally showing more obvious symptoms than cats. However, both dogs and cats infected with heartworms can develop life-threatening complications.
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Diagnosis
Diagnosing heartworm disease in both dogs and cats involves multiple tests. The most common initial test is a simple blood test that looks for the presence of adult heartworm antigens. However, this test can have false negative results, especially in cats. Therefore, further testing may be required to confirm diagnosis.
In cats, diagnosis relies more heavily on imaging such as chest X-rays and ultrasound to look for signs of heart and lung disease that could indicate heartworm infection. Cats may also undergo antibody testing, which looks for the body’s immune response to the parasite rather than testing directly for the parasite itself. This can help detect exposure to heartworm larvae (VCA). Overall, reaching a heartworm diagnosis in cats typically requires assessing the results of multiple tests together (Cornell).
Treatment in Dogs
The only FDA-approved drug for treating heartworm infections in dogs is called melarsomine dihydrochloride (trade name Immiticide). Treatment involves a series of injections of melarsomine to kill the adult worms. Most protocols include a 30 day regimen involving the macrocyclic lactone (preventive) drug, doxycycline, and prednisone before the injections to weaken the worms. On days 60 and 90, the dog receives an injection of melarsomine to kill the adult worms. These injections are administered by a veterinarian in hospital. The dog needs to be confined for one month after the melarsomine injections to allow the dead worms to be absorbed. Complications can arise after treatment when the dead worms begin to break up and cause inflammatory debris. This is usually managed by administering corticosteroids. Full clearance of the worms is confirmed with additional heartworm antigen testing. Using only melarsomine has a 95% cure rate when used correctly. VCA Animal Hospitals notes that severe cases may require additional medications like antibiotics, diuretics, and pain killers to manage complications.
Treatment Difficulties in Cats
Unlike dogs, unfortunately, there is no safe and effective approved drug for treating heartworm infections in cats. The drug commonly used for treating heartworm in dogs, called melarsomine (Immiticide®), has been used experimentally in cats but is not considered safe (1). Melarsomine can cause severe inflammation in the lungs of cats that can be fatal.
Another major challenge with treating heartworms in cats is that the adult worms cannot be reliably identified or extracted. Heartworms in cats tend to have lower worm counts than in dogs. The adult worms reside deep in the heart and pulmonary arteries where they are difficult to access and remove surgically (2).
There are currently no approved drugs that can safely eliminate adult heartworms or microfilariae from cats. Supportive veterinary care is essential, which may include medications to manage heart failure, blood thinners, antibiotics, and steroids to reduce inflammation (3). However, these treatments only help manage symptoms and do not cure the infection.
Prevention
The best way to prevent heartworm infection in both dogs and cats is through consistent, year-round administration of heartworm preventative medication. There are several heartworm prevention options available including oral and topical medications.
According to the American Heartworm Society, heartworm preventatives are extremely effective when administered properly. These medications work by eliminating the immature heartworm larvae before they develop into adult heartworms that can cause damage to the heart, lungs, and arteries.
The American Heartworm Society recommends year-round heartworm prevention for all dogs and cats. Annual testing is important even for pets on prevention to ensure the medication is working properly. For dogs, an antigen test is recommended annually. For cats, heartworm prevention is considered the best practice since there is no approved treatment. https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics
There are a variety of oral, topical, and injectable heartworm prevention options on the market. Pet owners should consult with their veterinarian to determine the most appropriate prevention plan for their pet based on lifestyle, age, health status, and geographic risk level.
Prognosis
The prognosis for heartworm disease depends greatly on whether the animal receives treatment and at what stage of disease treatment begins. Dogs have a good prognosis if treated early before any lasting damage occurs. With treatment, most dogs make a full recovery and live a normal lifespan.
Cats have a grave prognosis if heartworm disease goes untreated. Because safe approved treatment options do not exist for cats, the heartworm infection is usually fatal within 2 to 4 years. Even with supportive care, most cats will succumb to complications like chronic respiratory disease, heart failure, or blood clots due to the heartworm infection. The lifespan of an infected cat is significantly shortened without the hope of a cure.
One study showed the median survival time for infected cats was only 2.3 years from the time of diagnosis compared to 6.6 years for uninfected cats https://pubmed.ncbi.nlm.nih.gov/32904770/. The prognosis for heartworm positive cats is unfortunately poor compared to treated dogs who often fully recover.
Conclusion
There are significant differences in how heartworm disease affects dogs compared to cats. While heartworm infection is treatable in dogs, it is often untreatable in cats, leading to potentially fatal outcomes. The lifecycle and symptoms of heartworm also vary between the two species.
Prevention through monthly heartworm medications is crucial for both dogs and cats. Since treatment is so challenging in cats, preventing infection in the first place is the best way to protect cats’ health and avoid potentially fatal heart and lung damage from adult heartworms.
With proper prevention, along with annual testing and prompt treatment if needed, heartworm can be effectively managed in dogs. For cat owners, priority must be placed on continuous heartworm prevention. Through responsible prevention and care, pet owners can protect their dogs and cats from the dangers of heartworm disease.