Heartworm Disease

Heartworm disease is caused by an internal parasitic worm called Dirofilaria immitis. The intermediate host is a mosquito, and this means that the heartworm stays inside the mosquito for a short period of time to become infective before transiting to other animals. The definitive host is a dog which means that, inside the dog, the worm grows into adults, mates, and produces microfilariae, the offspring of the heartworm. The heart, lungs, and other related vessels are where the adult heartworms live. An animal cannot get infected just by being near an already infected animal, meaning that the disease is not contagious. It can only spread out via mosquitos.

IMG Credit: American Heartworm Society

CANINE

Lifecycle & infection process

1. When the mosquito bites an infected dog and ingests the microfilariae in the bloodstream, it becomes a heartworm intermediate host.

2. During the next 10 to 14 days, inside the mosquito’s body, the microfilariae grow into infective larvae.

3. The mosquito bites another dog and transfers the larvae through the saliva puddle into the bloodstream.

4. In this newly infected dog, the larvae grow in the tissues and migrate to the heart to fully mature and become adult heartworms. This process takes about 6 to 7 months.

5. The adult worms mate together, and the fertilized female heartworms release microfilariae into the bloodstream.

This cycle repeats over and over, infecting more dogs and other animals. The lifespan of a worm inside a dog is from 5 to 7 years.

Symptoms

For canines, the severity of the disease is associated with the quantity of worms living in the dog, the infection period, and the body’s response to the worms. Also, this can be classified into 4 stages: stage 1 where there are no symptoms or have only mild symptoms including coughing; stage 2 where the dog coughs occasionally and feel tired; stage 3 which involves serious symptoms like frequent cough, trouble breathing, and heart failure; and stage 4 which involves caval syndrome, a life-threatening syndrome caused by a large mass of heartworms that blocks blood from flowing back to the heart.

FELINE

Lifecycle & infection process

The heartworm’s lifecycle infection process of felines is very similar to that of canines.

1. When the mosquito bites an infected dog or a host and ingests the microfilariae in the bloodstream, it becomes a heartworm intermediate host.

2. During the next 10 to 14 days, inside the mosquito’s body, the microfilariae grow into infective larvae.

3. The mosquito bites a cat and transfers the larvae through the saliva puddle into the bloodstream.

4. In this infected cat, the larvae grow in the tissues and migrate to the heart to fully mature and become adult heartworms. This process takes about 7 to 8 months. However, because cant is not a natural host and the heartworms do not develop well inside it, many larvae do not reach maturity.

5. The adult worms mate together, and the fertilized female heartworms release microfilariae into the bloodstream. Compared to dogs, very few microfilariae are released, so it is hard to detect.

The lifespan of a worm inside a cat is from 2 to 4 years.

Symptoms

For felines, there are two main points when they show symptoms: when the still growing heartworms immigrate to the arteries and when the matured worms die. First, when immature heartworms arrive at the lungs and die there, an inflammatory response occurs in the lungs of the cat. This is called Heartworm Associated Respiratory Disease, abbreviated as HARD. Second, as the matured worms die, they release toxins in the bloodstream, causing lung damage which can lead to problems in the respiratory system and even sudden death.

While canines have some specific symptoms for heartworm disease, some felines may have nonspecific symptoms that are similar to other feline diseases. This includes vomiting, a decrease in activity level, an increase in appetite level, or a decrease in weight. In addition, some cats do not even show mild symptoms and signs of sickness and die suddenly due to infection.

---

Reference

Blagburn, Byron, et al. Pfizer Atlas of Veterinary Clinical Parasitology. Pfizer Animal Health, 1999.

“Heartworm in Dogs.” American Heartworm Society, 24 Jan. 2022, www.heartwormsociety.org/heartworms-in-dogs.

“Keep the Worms Out of Your Pet’s Heart! The Facts about Heartworm Disease.” U.S. Food and Drug Administration, 22 Aug. 2019, www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease.