Heartworm in dogs and cats, a serious and potentially life-threatening condition, is transmitted through mosquito bites. Since mosquitoes can find a way into the home, all pets, regardless of lifestyle, are susceptible to contracting heartworm. The good news is that heartworm preventives are readily available for dogs and cats. And on top of prevention, recognizing the signs of an ongoing heartworm infestation is crucial for obtaining early treatment — which helps prevent life-long heart and blood vessel damage from heartworm.
Though only a veterinarian can conclusively determine whether or not a dog or cat has heartworm, there are certain heartworm risk factors that pet parents can watch out for. These risk factors are related to age, consistency of heartworm preventives, and so on.
To find out if a dog or cat is at risk for heartworm, take the following test, put together by Sploot’s veterinarians:
This complete guide covers everything you need to know about heartworm in dogs and cats, how it’s transmitted, the heartworm symptoms to watch out for, and more.
What is Heartworm in Dogs & Cats?
Heartworms, scientifically known as ‘Dirofilaria immitis’, are parasitic worms that pose a significant threat to dogs and cats. In both cats and dogs, heartworms live in the blood vessels, heart, and lungs; the worms derive their sustenance from the host’s blood and can grow up to 8 inches in length, potentially causing obstructions, inflammation, and damage to the sensitive vital systems that they inhabit.
Heartworms can affect any breed, sex, and age of dogs and cats. Heartworm symptoms are, however, rarely observed in puppies and kittens less than 6 months of age. The reason behind this is that heartworm larvae take 6-7 months to mature — meaning that by the time symptoms appear, the puppy or kitten would have been older than 6 months, and the infection is already well on the way.
Cause of Heartworm in Cats & Dogs
Heartworms in cats and dogs is transmitted only through the bite of an infected mosquito.
Mosquitos can become infected by ingesting larvae from the bloodstream of infected animals. This means that cats and dogs CANNOT pass heartworm directly to their own species or to another susceptible species — but if a mosquito gets infected by an infected pet, the disease can be transmitted.
Note: Heartworm is technically ‘zoonotic’, meaning it can, on rare occasions, be transmitted to people. However, humans are considered ‘suboptimal hosts’ because the human body is not a hospitable environment for heartworms to mature. Heartworms would typically die before they reach the human heart.
Symptoms of Heartworm in Dogs & Cats
Symptoms associated with heartworm in dogs and cats include the following:
- Shortness of breath
- Persistent coughing
- Lethargy (lack of energy)
- Unwillingness to exercise
- Weight loss
- Reduced appetite
- Vomiting
- A swollen abdomen (more common in heavy infestations — caused by heart failure and subsequent fluid accumulation)
Diagnosing Heartworm in Dogs & Cats
Prompt diagnosis gives way to early treatment — and this is important for both dogs and cats because long-standing heartworm in dogs and cats take longer to treat and may have already inflicted permanent damage to a dog or cat’s blood vessels.
Note: Although cats are known to have a lower worm burden (i.e. lower worm count) compared to dogs, a smaller number of worms can still do serious damage to a cat’s body.
Veterinarians employ a number of methods to diagnose heartworm. After taking note of clinical signs of heartworm in dogs and cats from a thorough physical exam, veterinarians follow up with more specific tests.
Heartworm Antigen Testing
The most common test for heartworm in both dogs and cats is a blood test to detect the presence of heartworm antigens (i.e. proteins released by heartworms). This test can identify adult female heartworms in the pet's bloodstream. This is the type of test used in annual heartworm testing for pets, a preventive care practice that helps pets stay heartworm-free.
For cats that are exhibiting symptoms of heartworm infestation, heartworm antibody testing may also be used for increased diagnostic accuracy. Antibodies are proteins released by the pet’s body in response to heartworm; therefore, antibody testing detects the presence of both male and female worms.
Heartworm Antibody Testing
Alongside heartworm antigen (+/- antibody) testing, the dog’s or cat’s blood sample may be examined under a microscope and screened for heartworm microfilariae, which are the early-stage larvae of heartworms.
Imaging Tests
In addition to the above tests, veterinarians may also use imaging tests to locate heartworms and/or get a sense of the degree of the infestation.
Note: Only licensed veterinarians can diagnose heartworm. Symptoms alone are not reliable. Early infections may not produce any symptoms at all. Furthermore, symptomatic heartworm in dogs and cats can easily be mistaken for other respiratory conditions unless proper testing is done.
How to Treat Heartworm in Dogs & Cats
Generally speaking, prevention is the best, healthiest, and most cost-effective way to keep pets heartworm-free. However, when prevention fails, early treatment is key
Although dogs and cats are infected by the same type of heartworm (D. immitis), heartworm treatment is different for cats and dogs. This is mainly because there is currently no medication available to treat heartworm in cats — meanwhile, there is an anti-heartworm injectable medicine for dogs. In any case, heartworm treatment is generally strenuous on a pet’s body.
Treating Heartworm in Dogs
1. Anti-Heartworm Injectable
The primary goal of treatment is to kill the adult heartworms residing in the heart and lungs of dogs. This is usually done using an injectable drug. The dog will need 3 injections, and the treatment in total will typically take one year. For some infections, retreatment may be necessary, extending the total time of treatment further.
2. Antibiotics & Corticosteroids
Antibiotics and steroids are not standalone treatments: they are used alongside anti-heartworm injectables. Antibiotics are prescribed to clear away Wolbachia, a type of bacteria associated with heartworm, which can be released into the pet’s bloodstream when the heartworms start to die. In some cases, corticosteroids may be given to reduce inflammation and the dog's reaction to the dying heartworms.
3. Surgical Removal
Heartworm removal surgery is rarely needed but might be used as a last resort treatment— specifically, in cases where dogs have severe heartworm infestations with a high worm burden that cannot be effectively treated with traditional methods. This surgery may be recommended if the infestation causes significant damage to the heart and lungs or if there is a risk of life-threatening complications.
Treating Heartworm in Cats
1. Steroids & Medication to Ease Respiratory Problems
Medications are only meant to relieve the symptoms of heartworm in cats; these cannot address the source of the problem. Corticosteroids may be prescribed to cats in order to reduce inflammation caused by the presence of heartworms. Additional medication can also be given to help ease respiratory issues and manage signs of congestive heart failure.
2. Surgical Removal
In some cases, if the heartworm burden in cats is localized, surgical removal of the worms may be considered. This is done with the aid of ultrasound.
How to Prevent Heartworm in Cats & Dogs
We cannot emphasize enough that an ounce of prevention goes a long way when it comes to heartworm in cats and dogs. In addition, as touched on earlier, medication for treating heartworm is not available for cats — this further emphasizes the importance of dutiful prevention.
Note: Make sure to stay on top of yearly, bi-annual, or monthly preventives. The Sploot Vets app makes this easier than ever before, with heartworm reminders (and other routine care reminders) delivered straight to your phone. In addition, the Sploot Vets app allows pet parents to order medication refills with an option for same-day or next-day delivery.
1. Monthly Oral or Topical Heartworm Preventives — for Dogs & Cats
Both dogs and cats have their own set of heartworm preventives that are given on a monthly basis. These can be either orally or topically administered. These generally require a veterinarian’s prescription.
Both dogs and cats can get heartworm preventives that also prevent other parasites such as intestinal worms in dogs, intestinal worms in cats, fleas, and ticks.
Note: Make sure to refrain from using heartworm preventives for dogs on cats and vice versa as these medications are species-specific.
2. Yearly or Bi-Annual Heartworm Preventives — Currently for DOGS ONLY
Long-term heartworm preventives for dogs usually come in the form of injectables. These injectable medications can be viable for 6 months or 12 months, depending on their indications. These can only be administered by licensed veterinarians.
3. Regular Heartworm Testing
Along with other heartworm preventive measures, our veterinarians recommend regular heartworm testing for dogs. If a dog receives consistent heartworm preventives, heartworm testing can be done yearly. This yearly test will confirm that the preventives are working as intended. On the other hand, if heartworm preventives have not been given consistently, heartworm testing every 6 months is recommended to allow for early detection.
On the other hand, heartworm testing for cats can be more nuanced. Veterinarians will give specific instructions for routine heartworm testing based on clinical observations.
When is Heartworm Season?
Heartworm season aligns with the times of the year when mosquitos are most active. In general, mosquito activity peaks during the summer season which then correlates to a higher chance of heartworm infection. However, mosquitos can also be active anytime from early spring to early fall.
Essentially, heartworm in dogs and cats is a year-round threat and preventives are NOT only given during specific seasons.
Do Heartworm Preventives ALWAYS Need to Be Given Monthly?
For cats, heartworm preventives are given on a monthly basis. For dogs, heartworm preventives can be given monthly, every 6 months, or every 12 months depending on the type of heartworm preventive used.
Why Aren’t Heartworms a Significant Threat to People?
Heartworm can be transmitted to people too — but developing a serious infection is very rare. Heartworms aren’t considered a significant threat to people because the human body is not hospitable to heartworms. (Thankfully!) Heartworms usually die before they reach the human heart. This is why humans are considered ‘suboptimal’ hosts for heartworm.
What Do Heartworms Look Like in Dog Poop or Cat Poop?
Heartworm showing up on dog poop is a common misconception. Heartworms do not live in a dog's or cat's gastrointestinal tract. Therefore, they do not show up in cat or dog poop. The types of dog worms and cat worms that typically show up in stool include whipworms and roundworms, as well as segments of tapeworm that may resemble grains of rice.
Final Thoughts About Heartworm in Dogs & Cats
We hope this guide on heartworm in dogs and cats has been informative! If you have specific questions about heartworm in cats and dogs, we're here to help!
Sploot Veterinary Care is a primary and urgent care veterinary clinic with unparalleled availability. Our clinic doors are open 365 days a year, for extended hours, across all of our convenient clinic locations. Easily book an appointment online or through the Sploot Vets app. You can also call our team to arrange an urgent drop-off appointment if needed.
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