Pets in all 50 states have tested positive for heartworms, including those states with a historically lower risk. This means all pets are now at risk for heartworm infection and developing heartworm disease, which is spread from one infected pet to another through mosquito bites. Avoiding mosquitoes may be nearly impossible, but monthly heartworm preventives can kill heartworm larvae and prevent infection. Fremont Animal Hospital wants pet owners to better understand heartworm disease risks, and to learn why we recommend prevention for all pets.

Heartworm transmission and lifecycle

Adult heartworms live and reproduce inside their canine hosts, including dogs, coyotes, foxes, and wolves, and produce heartworm larvae (i.e., microfilariae) that circulate through the host’s bloodstream. A mosquito that bites an infected animal picks up these larvae, which take only a few weeks to mature into an infective stage. Then, when the mosquito bites an unprotected dog or cat, they transfer these larvae onto the pet’s skin, and the larvae enter the pet’s body through the tiny bite wound. Over six to seven months, these larvae grow into foot-long adults that take up residence in the pet’s heart and lungs, and their life cycle begins again. 

Heartworm infection in dogs

Dogs are the heartworms’ ideal host, and a dog can have anywhere from 30 to more than 100 worms inside their most vital organ. Without treatment or prevention, the worms will continue to grow in number as the native worms reproduce, and from the pet’s additional exposure to infected mosquitoes. The more worms, the more likely the pet will suffer severe, long-term heart and lung damage. 

In the early stages, heartworms do not usually cause clinical signs unless the dog has other complicating health issues. But, as the infection worsens, your pet will show signs indicating decreased heart function, including:

  • Cough
  • Exercise intolerance
  • Decreased appetite
  • Weight loss
  • Fluid in the chest or abdomen
  • Heart or respiratory failure

Close-up of tangled roundworms on a metallic surface in a veterinarian's lab. One worm is prominently arched and slightly blurred in the foreground, suggesting a scientific context.

Heartworm infection in cats

Cats are not ideal heartworm hosts, so most larvae do not survive until adulthood. Infected cats who host adult worms have fewer than six, and often only one or two, at one time. However, cats may have more severe disease than dogs, because their bodies mount an inflammatory attack on the foreign invaders that most often leads to respiratory problems. The worms can also migrate to other areas, including the nervous system. 

Many cats are asymptomatic and go undiagnosed, but those who develop signs may show:

  • Coughing, wheezing, or trouble breathing
  • Vomiting
  • Weight loss
  • Trouble walking or seizures
  • Sudden collapse or death

Heartworm diagnosis and treatment in dogs and cats

Adult female heartworms can be easily detected in dogs using a routine heartworm antigen blood test that is recommended annually. If the worms are younger than seven months, they may not be detected initially, but will be revealed by repeat testing 6 to 12 months later. Infected cats may not host any adult worms, or they may host only adult male worms, so the antigen test is less effective. A combination of an antigen and an antibody test, which checks for larvae exposure, is more effective for cats.

Infected dogs can be treated, but the process usually takes several months and involves considerable expense. Dogs need a series of oral medications and injections to clear adult worms, must be strictly exercise restricted during treatment, and should be started on a preventive regimen to avoid reinfection. Unfortunately, a safe treatment protocol for cats does not exist. Cats may spontaneously clear the infection on their own, or may need supportive care and medication management for two to three years until the worms die. 

Pet heartworm prevention

Heartworm prevention is the only way to ensure pets avoid heartworm infection and the damage and long-term complications associated with heartworm disease. Prevention is administered monthly in a prescription-only oral or topical formulation, and usually costs less than $150 for a one-year supply—compared with more than $1,500 for heartworm treatment. 

Prevention should be provided year-round to all pets, including those who live exclusively indoors, because mosquitoes can easily come inside. Studies show that one-quarter of infected indoor cats never went outside, but were still infected. Skipping the winter months is not recommended, because mosquito activity is unpredictable, and protection gaps leave your pet vulnerable to infection. Plus, heartworm preventives also control intestinal parasites, which are a year-round problem.

Heartworm disease can be devastating, can permanently damage your pet’s body, and can make a sizable dent in your wallet. Prevention is the best, most cost-effective strategy to protect your pet from heartworms. Our Fremont Animal Hospital team can help you find the right product, so contact us to schedule your pet’s next wellness visit, heartworm test, or heartworm prevention consultation.