Heartworm is a ubiquitous disease that kills dogs and cats throughout the United States, even though the disease is completely preventable when owners ensure their pets receive safe, inexpensive, monthly medications. Many pet owners are unaware of heartworm’s dangers, or are misinformed about how the disease is transmitted or prevented, leading thousands of pets to become unnecessarily infected every year. During Heartworm Awareness Month, learn how to protect your pet from this devastating disease by reading our Colony Veterinary Hospital team’s guide to heartworm transmission, treatment, and prevention.

Heartworm disease in pets

Heartworms are different from other worms that infect pets, including roundworms, hookworms, or whipworms, which live in an animal’s intestines, and are relatively easy to diagnose and treat. Heartworms, which live in a pet’s heart chambers and blood vessels, are more difficult to recognize and eradicate. As a result of where heartworms take up residence within a pet’s body, they can wreak serious havoc on an infected animal’s health. Heartworm disease develops months or years after initial infection, because the larvae need time to grow, mature, and reproduce. 

Heartworm transmission and infection in pets

Mosquitoes transmit heartworms to pets, unlike most other parasites, which pets usually contract from their environment. A mosquito must first bite an infected pet or wild mammal, ingesting the immature larvae circulating in the affected animal’s blood. The larvae mature for a few weeks inside the mosquito, who then transmits them to a new host when the bug bites another pet.

After transmission, the larvae eventually travel to the infected pet’s heart, where they mature for several months and can grow up to a foot in length. Once they reach adulthood, heartworms begin to reproduce, and the infected pet now serves as a disease reservoir. Traditional heartworm geographic hot spots are in locations with large, year-round mosquito populations, but pet adoption and transport from southern states has allowed heartworms to spread northward and westward, and infection has now been detected in all 50 U.S. states.

Heartworms affect dogs and cats differently

Both dogs and cats can contract heartworms, but dogs are considered the preferred hosts, and heartworms thrive in canine species (e.g., dogs, foxes, wolves), who may host hundreds at a time. Because cats are not preferred heartworm hosts, few of these parasites survive to adulthood within the feline body. Despite hosting only one or two worms in most cases, infected cats can develop severe asthma-like respiratory disease, lose weight, vomit, have seizures, or die suddenly from heart complications.

Dogs with heartworms can live years with their disease before exhibiting signs. Once a pet owner observes their four-legged friend’s heartworm disease signs, significant damage has already occurred. Dogs’ potential heartworm signs include:

  • Coughing
  • Exercise intolerance
  • Fatigue
  • Decreased appetite
  • Weight loss
  • Abnormal lungs or abdominal fluid buildup

Heartworm testing and diagnosis in pets

Your veterinarian will perform several blood tests—alone or in combination—to determine whether your pet has heartworm disease. Tests may include:

  • Heartworm antigen — Used most often in dogs, detects adult female heartworms.
  • Heartworm antibody — Used most often in cats, detects antibodies the immune system has created to fight heartworms.
  • Blood microfilaria test — This test examines a pet’s blood on a microscope slide to identify immature heartworm larvae.

Heartworm tests are recommended annually for all dogs, but are not run routinely on cats, who often solely live indoors, but can still come in contact with infected mosquitoes. These tests aren’t perfect, and sometimes your veterinarian will run multiple tests at once or repeat tests several times to detect infection. Your veterinarian may also take X-rays and ultrasound images to examine your pet’s heart and detect worms.

Why heartworm prevention is better than treatment for pets

Heartworm preventives easily kill immature parasites in your pet’s body—up to six weeks or so after infection. Popular heartworm preventive brands include Heartgard, Interceptor, and Revolution. These drugs come in topical or oral forms, are typically given once per month, and work retroactively to kill larvae that were transmitted to your pet in the weeks before administration. Most parasite preventives are inexpensive, easy to administer, and often also protect your pet from several other parasites.

If a mosquito bites an unprotected pet, the heartworms can mature to a stage at which they are no longer easily killed, requiring more costly and extensive treatment. Heartworm treatment for dogs is expensive—approximately $1,500 for most pets—and often includes:

  • A series of painful muscle injections over several months
  • Day hospitalization for observation and pain control after injections
  • Complete cage rest to prevent the heart from overworking 
  • Antibiotics to kill heartworm-associated bacteria before the injection series is begun
  • Steroids to avert allergic reactions from the toxins the dying worms emit
  • Surgery to physically remove worms in severe cases

Unfortunately, no safe heartworm treatment exists for cats, who often go undiagnosed. However, if their veterinarian detects heartworm disease, a cat will be closely monitored and given supportive care for their respiratory and heart issues. An infected cat’s veterinarian may recommend surgery if heartworm disease has severely affected the pet.

Because heartworm is difficult to detect in cats and they can die suddenly from complications, prevention is the only sure way to protect your feline friend. For dogs, prevention is considerably less expensive, painful, and stressful than heartworm treatment, preventing the potential for long-term heartworm-associated damage. 

Protecting your pet from heartworms is easy, and to help determine which preventive medicine will be most effective for your pet, our Colony Veterinary Hospital team offers multiple options. Contact us to schedule your pet’s heartworm test or wellness visit, or to discuss all your four-legged friend’s parasite prevention needs.