Our Creature Comforts Veterinary Service team knows that most pet owners do not fully understand heartworm disease and prevention. We’re taking this opportunity to share what you should know about heartworm in dogs and cats by answering your most frequently asked questions.

Question: What are heartworms?

Answer: Heartworms are parasites that live in the heart, lungs, and nearby blood vessels of their hosts. They prefer canine hosts, including domestic dogs and wildlife, but can also infect cats and a few other mammal species. Adult heartworms can grow up to a foot long, damage your pet’s vital organs, and cause heartworm disease.

Q: How do pets get heartworms?

A: Heartworms can reproduce inside their host, producing immature larvae that circulate in the host’s bloodstream. Mosquitoes then bite these infected pets and harbor the larvae for 10 to 14 days before transmitting them by biting another pet. If that pet is treated monthly with a heartworm preventive, their next dose will kill these larvae, but if not, the larvae will find their way to the heart, mature to adults, and restart their life cycle. Infected, untreated pets serve as a reservoir for other mosquitoes to spread infection.

Q: Are dogs and cats equally susceptible to heartworms?

A: Dogs and cats are not equally susceptible—dogs are intended heartworm hosts, but cats are not. Heartworms thrive and reproduce easily in dogs, but most heartworm larvae do not survive in cats. However, although cats are a lower risk and host fewer worms, infected cats can suffer more serious consequences than dogs. Predicting which cats will clear infections on their own and which will suffer from heartworm disease is impossible, so all cats are considered at equal risk and should be placed on heartworm prevention medication.

Q: Will I know if my pet has heartworms?

A: At first, probably not. The worms mature and reproduce relatively slowly, so you won’t notice clinical signs until the infection is advanced. Detecting infection in dogs before this point is critical, because once you see clinical signs, the damage cannot be reversed. In cats, disease signs can develop at any time, or your cat may be completely asymptomatic. When signs do become noticeable, they may include:

  • Coughing
  • Wheezing or difficulty breathing
  • Weight loss
  • Poor appetite
  • Chronic vomiting

Q: Should my pet be tested for heartworms?

A: We recommend screening all dogs for heartworm with a blood test during their annual wellness visit, and anytime you adopt a pet with an unknown history. Your pet should also be tested prior to starting a new heartworm prevention regimen. Cats are not routinely screened, but can be tested as needed for suspected infection with blood and imaging tests.

Q: How do heartworm preventives work?

A: Heartworm preventives are given monthly, typically in an oral or topical formula. They work retroactively, killing the immature larvae already in your pet’s body before they can mature into adults. The medications are available from our veterinary service and are recommended for all pets.

Q: Do I need to give heartworm preventives during the winter?

A: For maximum effectiveness, we recommend giving heartworm preventives year-round. Although mosquito activity slows down in the winter, the threat is never completely eliminated, and skipping only one month can leave your pet vulnerable to infection. Heartworm preventives also protect your pet against intestinal parasites, including roundworms, which can harm other household pets and your human family members. 

Q: What happens to my pet if they become infected?

A: Dogs who test positive for heartworms can be treated with a specific protocol that kills the adult worms. The one compound that can kill the adults is given as an injection series over several months. Other aspects of heartworm treatment in dogs include:

  • Day hospitalization for monitoring during treatment injections
  • Antibiotics to kill heartworm-associated bacteria
  • Steroids to reduce allergic responses or other negative treatment reactions
  • Preventive medications to kill larvae and reduce spread to other pets

The injections used to treat dogs are not safe for cats. Infected cats can only be stabilized and managed with medications, and must be monitored closely throughout their infection. Heartworms in a cat usually die in two to three years, but can cause respiratory and heart problems during that time. Because cats cannot be safely treated, prevention is crucial.

Talk to a member of our Creature Comforts Veterinary Service team if you have additional questions about heartworm, or call us to schedule your pet’s next wellness visit, heartworm test, and parasite prevention consultation.