The winter thaw is on the horizon, and as we move toward the warmer weather of spring we’re preparing for a new season of creeping, crawling, and flying bugs. Of these, perhaps the most annoying, and potentially deadly, is the mosquito.

With their bite, mosquitoes can transmit a dangerous parasite to pets known as heartworm. Found in all 50 states, complications from heartworm infection can include exercise intolerance, shortness of breath, collapse, and even sudden death in dogs and cats. It’s important to make sure we understand the dangers of heartworm disease in pets, first.

What is Heartworm Disease?

Heartworm disease is caused by a mosquito borne parasite, Dirofilaria immitis. Transmitted by over 30 species of mosquito, the mature heartworm lives in the pulmonary arteries and heart of pets and wildlife. The disease causes permanent and life threatening damage to your pet’s heart and other organs.

How Does My Pet Get Heartworm?

The heartworm lifecycle takes a long time to complete. The basic steps to infection are:

  • A mosquito bites an infected animal (pet or wildlife) and takes up the heartworm larvae, or microfilariae, into its body
  • The microfilariae develop inside the mosquitoes body over 10-30 days
  • The mosquito bites a healthy pet, injecting the microfilariae into the pet’s body.
  • The microfilariae develop further in the pet’s bloodstream over the course of several weeks, finally making their way to the pet’s heart and lungs. There, they develop into adult heartworms capable of reproduction.
  • After about 6-7 months of infection, the adult heartworms reproduce and send new baby microfilariae into the pet’s bloodstream which can now be taken up by a new mosquito and transmitted to a new pet.


Cats are resistant hosts for heartworm and usually only have 1-3 adult worms when infected. This makes diagnosis in cats extremely challenging if not impossible. Even immature worms cause heartworm associated respiratory disease (HARD).

Dogs are ideal heartworm hosts, typically with upwards of 30 mature worms when infected. In dogs, diagnosis may be made with a combination of the following tests:

  • Blood antigen test
  • Blood antibody test
  • Echocardiogram
  • Chest radiographs

Treating Heartworm Disease

In dogs, treatment includes injections that slowly kill off the adult heartworms over the course of several weeks. Most dogs need to be hospitalized and monitored during treatment, and at the very least must be on exercise restriction so as not to exacerbate these problems.

Sadly, the same drug is not approved for use in cats. Cats have a strong reaction in their lungs as the heartworms respond to the medication and die. Some veterinarians choose to treat the infected cat with the drug anyway, and hope for the best. Others prefer to treat for HARD and see if the cat outlives the worms (which typically live 2 -3 years in the cat, compared to 5 – 7 years in the dog). Either way, dead and dying worms can still cause an acute respiratory emergency in cats at any time, and sudden death is always a risk.

Heartworm Prevention

If you are feeling bleak at this point, take heart (ahem). Heartworm prevention is safe, effective, and inexpensive when compared to treatment of heartworm or heartworm related complications.

We have several options in veterinary medicine, and so it’s a great start to talk to us about your needs and your pet’s lifestyle and we can prescribe a preventive that works for you. There are topicals and oral medications available.

All preventives kill the microfilariae circulating in the bloodstream. It takes about 51 days for these baby worms to develop to maturity, and it’s important to interrupt this process each and every month. This is why giving your pet their heartworm medication each month is so important.

At Dupont Veterinary Clinic, we want to help protect your cat and dog from these parasites for life.  Give us a call or schedule an appointment so we can keep them healthy and happy together!