A dog lying in the grass, The Invisible Menace: Heartworm Disease

The Invisible Menace: Heartworm Disease

Heartworm disease, often likened to a silent predator, quietly infiltrates the bodies of beloved pets, posing a grave threat to their health and well-being. This insidious condition, caused by tiny worms transmitted through the bite of infected mosquitoes, hides its presence until it reaches advanced stages, wreaking havoc on the hearts and lungs of dogs and cats. In this blog, we'll explain how heartworms spread and highlight the signs that might indicate their presence. We'll provide essential information on diagnosing, treating, and, most importantly, preventing heartworm disease. Join us as we show you how to protect your pets from this hidden threat, ensuring they stay healthy and happy.

What is Heartworm Disease?

Heartworm disease is a sickness that affects pets, especially dogs and sometimes cats. It's caused by worms that mosquitoes carry. When an infected mosquito bites a pet, it passes the worms into the pet's bloxodstream. These worms then grow inside the pet's heart and lungs, causing serious health problems.

The life cycle of a heartworm involves five stages:

A diagram of heartworm cycle in a dog

  1. Mosquito Infection: It starts when a female mosquito bites an infected animal (usually a dog or a wild animal like a coyote or fox) carrying microscopic baby heartworms, called microfilariae, in its blood.
  2. Development of Heartworms in the Mosquito: Inside the mosquito, these baby heartworms mature into larvae over a period of about 10 to 14 days.
  3. Transmission: When the infected mosquito bites another animal, it injects these larvae into the animal's bloodstream.
  4. Migration to Heart and Lungs: The larvae travel through the infected animal's bloodstream until they reach the heart and lungs. There, they grow into adult worms over several months. The worms can grow up to a foot long and take residence in the heart and lungs of dogs or cats.
  5. Mating and Production of Microfilariae: Once matured, the adult worms mate and produce microfilariae, which circulate in the infected animal's bloodstream, completing the cycle.

Common Misconceptions About Heartworm Disease

There are misconceptions surrounding heartworm disease that can lead to complacency and put pets at risk.

  1. One common misconception is that indoor pets are not susceptible to heartworm infection. However, mosquitoes can still find their way indoors, and all it takes is one bite to transmit the disease.
  2. Another misconception is that heartworm disease only affects dogs. While dogs are more commonly affected, cats and ferrets can also contract heartworm disease, and the consequences can be just as severe.

Symptoms and Signs of Heartworm Disease in Pets

The symptoms of heartworm disease can vary depending on the infection's severity and the animal's species. In the early stages, pets may exhibit subtle signs such as a mild cough or fatigue. This disease is often referred to as the “silent killer” because in its early stages, it often shows no symptoms or only very subtle ones. Dogs and cats infected with heartworms may not exhibit any signs until the disease has progressed to a more advanced stage. By this time, significant damage may have already occurred to the heart, lungs, and other organs.

Below are the symptoms listed by the American Heartworm Society for dogs and cats.


  • Mild, persistent coughing
  • Reluctance to exercise
  • Fatigue after moderate activity
  • Decreased appetite
  • Weight loss

As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen.


Signs of heartworm disease in cats can be very subtle or very dramatic. Symptoms may include:

  • Coughing
  • Asthma-like attacks
  • Periodic vomiting
  • Lack of appetite or weight loss.
  • Occasionally an affected cat may have difficulty walking, experience fainting or seizures, or suffer from fluid accumulation in the abdomen.

Unfortunately, the first sign in some cases is sudden collapse of the cat, or sudden death.

Diagnosis and Testing for Heartworm Disease

Early detection of heartworm disease is crucial for successful treatment and prevention of further complications. We typically perform a series of tests to diagnose heartworm disease, including a blood test that detects the presence of adult heartworms or their offspring. Imaging techniques like X-rays or ultrasounds can tell us the extent of the infection.

Like dogs, cats can be infected with heartworms. There are differences, however, in the nature of the disease and how it is diagnosed and managed. Because a cat is not an ideal host for heartworms, some infections resolve on their own, although these infections can leave cats with chronic respiratory system damage.

While infected dogs may have 30 or more worms in their heart and lungs, cats usually have 6 or fewer—and may have just one or two. But while the severity of heartworm disease in dogs is related to the number of worms, in cats, just one or two worms can make a cat very ill. Diagnosis for cats can be complicated, requiring a physical exam, an X-ray, a complete blood count and several kinds of blood test. An ultrasound may also be performed.

Treatment Options for Heartworm Disease

Treating heartworm disease in dogs can be complex and lengthy, especially in advanced cases. The goal of treatment is to eliminate adult worms and prevent further damage to the heart and lungs. Treatment typically involves a combination of medication, rest, and close monitoring by one of our veterinarians. In severe cases, surgery may be necessary to remove the worms from the heart and blood vessels.

Unfortunately, there is no approved drug therapy for heartworm infection in cats. The drug used to treat infections in dogs is not safe for cats. Nevertheless, cats with heartworm disease can often be helped with good veterinary care. The goal is to stabilize your cat and determine a long-term management plan.

Heartworm in Iowa

A map of the united states showing heart worm incidence

The AHS conducts a heartworm incidence survey every three years, working with veterinary practices and shelters that submit data from heartworm antigen tests run over the course of the previous year. The latest survey was conducted in early 2023 and reflects data from testing conducted during 2022.

Prevention and Proactive Measures Against Heartworm Disease

Prevention is always better than cure when it comes to heartworm disease in dogs and cats. Several options for preventing heartworm infection are available, including monthly preventive medication or shots administered every 6 or 12 months, and yearly testing. Our veterinarians can recommend the most suitable preventive measures based on your pet's lifestyle, risk factors, and overall health. It is essential to consistently follow the prescribed preventive regimen to ensure your pet's protection.

The Importance of Regular Veterinary Check-Ups

Regular veterinary check-ups are essential for maintaining your pet's overall health and well-being, including protection against heartworm disease. During these visits, your veterinarian can perform routine screenings for heartworm infection, assess your pet's overall health, and provide guidance on preventive measures. They can also address any concerns or questions regarding heartworm disease or other potential health issues.

Defending Your Furry Friends: Safeguarding Against Heartworm

In conclusion, defending against heartworm disease is paramount for the well-being of your furry companions. As we've discussed, this silent predator can have devastating consequences if left unchecked. However, armed with knowledge about its transmission, symptoms, and prevention, you can take proactive steps to safeguard your beloved animals. At Advanced Pet Care Clinic, we specialize in heartworm education and medical prevention, as well as treatment for your dog. Contact us now. Together, we can ensure your pets live vibrant, happy lives, free from the threat of heartworm disease.

Warm regards,

Dr. Stevenson & Team

Advanced Pet Care Clinic