Heartworm disease in dogs and cats is a serious and potentially fatal condition that affects pets around the world. Heartworm is caused by Dirofilaria immitis, a parasitic worm transmitted through mosquito bites. Once inside the body, the larvae grow into long adult worms that live in the heart, lungs, and major blood vessels. Understanding how heartworm disease develops and how preventable it truly is can help keep pets safe year-round.
For additional information on other parasites, check out our blog pages for more information.
For scientific and epidemiological reference, the American Heartworm Society provides up-to-date guidelines.
How Heartworm Disease in Dogs and Cats Begins
Mosquitoes are responsible for spreading heartworm disease. When an infected mosquito bites a dog or cat, it injects heartworm larvae into the skin. Over the course of several months, these larvae migrate into the bloodstream and develop into adult worms, sometimes reaching 6 to 18 inches in length. The risk of heartworm is especially high in warm climates like Oklahoma, where mosquitoes are active for much of the year.
Dogs are natural hosts for heartworms, meaning the parasite survives easily and reaches full maturity. This is why heartworm in dogs is extremely common without prevention.
Symptoms of Heartworm Disease in Dogs
In the early stages, heartworm disease in dogs may cause very subtle symptoms, such as an occasional cough or decreased stamina. As the number of worms increases, the disease becomes more serious. Signs may include:
- Persistent coughing
- Reduced ability to exercise
- Weight loss
- Fatigue
- Fluid buildup in the abdomen
- Pale gums
- Fainting episodes
- Collapse
Severe, untreated heartworm disease in dogs can lead to congestive heart failure or sudden death.
Heartworm Prevention for Dogs
Heartworm disease is almost entirely preventable with monthly or long-acting medications. Options include:
- Monthly oral preventatives
- Monthly topical preventatives
- Six-month injectable prevention
Many monthly preventatives also protect against intestinal parasites, offering broader protection. Because no medication is 100% perfect and doses may occasionally be missed, annual heartworm testing is strongly recommended.
You can browse prescription preventatives through our online pharmacy:
Shop Heartworm Prevention
Heartworm Disease in Cats
Heartworm disease in cats is very different from the disease in dogs. Cats are not natural hosts, which means:
- Fewer worms reach adulthood
- Worms are often smaller
- Cats rarely have baby heartworms (microfilaria) in the bloodstream
- Standard canine heartworm tests may not detect infection
Because heartworm testing in cats is more complex, veterinarians often use a combination of tests such as antigen tests, antibody tests, and imaging.
Symptoms of Heartworm in Cats
Even a single heartworm can cause significant illness in cats. Signs of feline heartworm disease may include:
- Coughing
- Wheezing or asthma-like symptoms
- Vomiting
- Sudden collapse
- Sudden death
Unlike dogs, there is no approved treatment for adult heartworms in cats. This makes prevention even more important.
The American Heartworm Society provides additional feline-specific details. We also offer preventatives for cats in our online pharmacy.
Why Prevention Matters
Treatment for adult heartworms in dogs is expensive and carries potential risks, especially in advanced infections. Preventing the disease is far safer and more cost-effective than treating it.
For cats, monthly heartworm prevention is strongly recommended because treatment for adult worms does not exist. Preventative medications are the only effective protection.
Pet owners can learn more about parasite care in our related posts on intestinal parasite prevention and common canine and feline parasites.