Heartworm

Heartworm disease is a serious and potentially fatal disease in pets in Singapore and many other parts of the world.

It is caused by foot-long worms (heartworms) that live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body [1].

Heartworm disease affects dogs, cats and ferrets, but heartworms also live in other mammal species, including wolves, coyotes, foxes, sea lions and—in rare instances—humans [2].

Heartworm disease is a serious, progressive condition, passed by infected mosquitoes.

The earlier it is detected, the better the chances the pet will recover. There are few if any, early signs of disease when a dog or cat is infected with heartworms, so detecting their presence with a heartworm test administered by a veterinarian is important.

The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins.

TESTING

Testing procedures and timing differ somewhat between dogs and cats.

Annual testing is necessary in endemic areas, even when your pets are on heartworm prevention year-round, to ensure that the prevention program is working.

If you miss just one dose of a monthly medication—or give it late—it can leave your pets unprotected [1].

Even if you give the medication as recommended, your pet may spit out or vomit a heartworm pill—or rub off a topical medication.

Heartworm preventives are highly effective but not 100 percent effective.

If you don’t get your pet tested, you won’t know your pet needs treatment.

Guidelines on when to test [1]:

DOGS

Puppies < 7 months old

Start on heartworm prevention without a heartworm test. It takes at least 6 months for a dog to test positive for heartworm antigen after an infected bite.

For endemic areas, tested 6 months after your initial visit, tested again 6 months later, and yearly after that to ensure they are heartworm-free.

Adult dogs, those >7 months old and previously never been given preventive medication.

Test before starting heartworm prevention medication.

Tested 6 months and 12 months later and annually after that.

CATS

Any age

Heartworm infection in cats is harder to detect than in dogs because cats are much less likely than dogs to have adult heartworms.

The preferred method for screening cats includes the use of both an antigen and an antibody test (the “antibody” test detects exposure to heartworm larvae).

Your veterinarian may also use x-rays or ultrasound to look for heartworm infection.

Cats should be tested before being put on prevention and re-tested as the veterinarian deems appropriate to document continued exposure and risk.

Because there is no approved treatment for heartworm infection in cats, prevention is critical.

If there has been a lapse in prevention (one or more late or missed doses), dogs should be tested immediately, then tested again six months later and annually after that.

Heartworm Lifecycle

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4

5

3

4

5

Heartworm Lifecycle. Diagram adapted from American Heartworm Society.

Heartworm FAQs

Do I need a prescription for my pet’s heartworm preventive medication? If so, why?

Yes. You need a prescription letter to purchase heartworm-prevention medication in Singapore.

IVERMECTIN is the active ingredient in many heartworm prevention medications.

It has been listed as a controlled drug by HSA and NPARK (AVS) after 2021 due to inappropriate use by some humans during the COVID-19 pandemic.

HSA lists that this medication is to be used by or on the order of a licensed veterinarian.

This means heartworm preventives must be purchased from your veterinarian or with a prescription through a pet pharmacy.

Before you start your pet on any heartworm preventive medication, the veterinarian typically performs a heartworm test to make sure your pet does not already have adult heartworms, as giving preventives can lead to rare but possibly severe reactions that could be harmful or even fatal.

If the heartworm testing is negative, prevention medication is prescribed.

What other information is available for heartworm disease in cats?

Heartworm disease in cats is presented differently from heartworm disease in dogs.

The cat is not a normal host for heartworms, and most worms in cats do not usually develop to the adult stage.

If cats have adult heartworm infestation, they typically have just one to three adult worms.

Many cats affected by heartworms have no adult worms and these immature worms can still cause severe damage to the respiratory system.

These conditions are collectively known as heartworm-associated respiratory disease (HARD).

The medication used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only means of protecting cats from the effects of heartworm disease.

Heartworm prevention for cats typically involves using monthly medications prescribed by a veterinarian. These medications are designed to kill the heartworm larvae. Some common forms include topical treatments or oral medications.

Can canine heartworm infect humans?

Heartworms are rarely transmitted from dogs to humans.

There were only 81 cases of heartworm in humans reported from 1941 to 2005 in the USA [3].

Heartworms get into the bloodstream of both humans and dogs through mosquito bites.

But in humans, heartworm L3 larvae never fully develop into adult heartworms.

As young heartworms die within the lung or other tissue structures, your body can have an inflammatory reaction resulting in structural damage.

The symptoms of heartworm infections in animals and humans differ because of how they develop in the bloodstream.

Some possible symptoms and signs of heartworm infections in humans can include [3]:

abnormal cough
coughing up blood
pain in your chest
wheezing
chills
fever
buildup of fluid around your lungs (pleural effusion)
round lesions that show up on chest X-rays (“coin” lesions).

Video Credit: Heartworm in Nontraditional Species (Lions, Tigers, and Bears, Oh My!), Dr. Bianca Zaffarano DVM; American Heartworm Society.

NEED MORE INFORMATION?

If you are interested in learning more about heartworm prevention for your pet, you can contact us at 6243 3282 to schedule an appointment.

Author Contributions

Dr. Denise Ng BSC BVMS

Conflict of interest

The author declares that there is no conflict of interest.

Funding

The author received no specific grant from any funding agency in the public, commercial or not-for-profit sectors for the preparation of this review article.

Attribution:

1. American Heartworm Society. Heartworm Basics

https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics. Accessed 6 Dec 2023.

2. Centers for Disease Control and Prevention. Dirofilariasis FAQs.

https://www.cdc.gov/parasites/dirofilariasis/faqs.html Accessed 6 Dec 2023.

3. Jewell Tim, Healthline. Can humans get heartworms from dogs? (1 Aug 2018) https://www.healthline.com/health/heartworms-in-humans#for-pet-owners. Accessed 5 Dec 2023