Heartworm Disease – Understanding the Disease Transmission


Have you ever been pestered by a mosquito? Have you ever found a mosquito in your house (or found the evidence of mosquito bites)? My guess is that your answers are yes! We do seem to have mosquitoes all over the place in North Carolina, and it will get worse as we enter into the warmer months. Mosquitoes can transmit heartworm disease to dogs and cats. It only takes 1 bite from 1 mosquito. Since mosquitoes can be found anywhere in our area, even in our houses, every cat and dog needs to be protected against heartworms year round.

Have you every familiarized yourself with the heartworm life cycle? Understanding this life cycle helps to further emphasize the importance of not skipping doses. Heartworm prevention is not effective against all of the life cycle stages of the heartworm. There is only a small window of the life cycle when the prevention is effective.

  • A heartworm positive canid most likely has microfiliariae circulating in the bloodstream. These microfiliariae are produced by the adult heartworm(s) in the infected animal.
  • The female mosquito ingests the microfilariae while taking a blood meal from the animal.
  • Over the next couple of weeks, the microfiliarae go through 2 molts to become larval stage 3 (L3s) – the infective stage. The L3s then hang out in the mouth parts of the mosquito.
  • When the mosquito feeds on a dog or cat, the L3 form is deposited on the skin of the animal. The L3 migrates through the skin at the site of the mosquito bite.
  • Within 1 to 3 days, the L3 molts to an L4 stage in the subcutaneous tissues.
  • The L4s spend several weeks migrating through the tissues.
  • Within 50 to 70 days of infection, the L4s transitions to the sexually immature stage (2 to 3 cm in length), and they move to the vascular system.
  • The young adults reach the pulmonary arteries by 70 days. For dogs, this is where final maturation and mating occurs. For cats, many infections are aborted at this point, which can actually still cause inflammation in the vessels and airways. In a small portion of cats, worms may still go on to mature and mate.
  • At 4 months post infection, the worms are approximately 10 to 15 cm in length. Once they reach maturity at 6.5 months in dogs, males can be 10 to 15 cm long and females can be 25 to 30 cm long. Since maturation is less common in the cat, it is not uncommon for a cat to have a single or few worm infection. Mature worms are slightly smaller in cats than dogs (an adult female is around 21 cm)
  • After mating, the female heartworms begin producing microfilaria. Since cats have much smaller worm burdens, microfilaremia is less common and short lived. If a cat has a single sex infection, the cat will not produce microfilaria.
  • The adult worms can live 5 to 7 years in the heart of a dog. Adult worms are thought to be able to live 2 to 4 years in a cat.

Although mosquitoes are seasonal in North Carolina, it is strongly recommended that dogs and cats still be maintained on year-round heartworm prevention. Here are some strong reasons to consider:

  • Predicting the exact beginning and end of mosquito season is tricky.
  • Some mosquito species are effective at overwintering indoors.
  • Staying in the habit of administering the preventive every month facilitates consistency, especially for the high mosquito-prevalence months.
  • Remember that many heartworm preventive products are also deworming your pets and even protecting against fleas.

Next time, we will talk more about the signs and symptoms an animal may exhibit when infected with heartworms. For more information about heartworm disease, also visit our ‘Paws to Protect‘ section on our website.


American Heartworm Society

Dr. C. Noureddine, DVM, MS