Heartworm Prevention Year-Round

What Is Heartworm Disease?

Heartworm disease is caused by a parasitic worm called Dirofilaria immitis that lives in the heart, lungs, and associated blood vessels of infected animals. Left untreated, heartworms can grow to 12 inches in length, live for 5–7 years in dogs (and 2–3 years in cats), and accumulate in large numbers — dogs have been found with over 250 worms. The damage they cause to the heart and pulmonary arteries is serious, progressive, and potentially fatal.

Heartworm is transmitted exclusively by mosquitoes. An infected mosquito takes a blood meal and deposits microscopic larvae (microfilariae) into the host's bloodstream. These larvae migrate to the heart and vessels over several months, maturing into adult worms. Because heartworm treatment in dogs is expensive, risky, and requires extended confinement, prevention is always the right choice.

Who Is at Risk?

Any dog or cat exposed to mosquitoes is at risk — and mosquitoes exist nearly everywhere on Earth. Factors that increase exposure include:

Living in or traveling to areas with warm, humid climates
Time spent outdoors, especially at dawn and dusk when mosquitoes are most active
Spending time near standing water, wooded areas, or fields

Indoor pets are not fully protected — mosquitoes routinely enter homes. Year-round prevention is the only way to close this gap. Talk to your vet or book a vet appointment at Happy Paws to find the right prevention product for your pet.

Heartworm Prevention Options

Prevention is simple, effective, and inexpensive compared to treatment. Options include:

Monthly oral chewables (dogs): Flavored chewables based on ivermectin or milbemycin are the most popular. They kill larvae deposited in the past month before they mature.
Monthly topical treatments: Applied to the skin, these are useful for dogs that resist oral products and for cats.
6-month injectable (dogs): A subcutaneous injection administered by a vet that provides 6 months of protection — ideal for owners who struggle with monthly compliance.
12-month injectable (dogs): A once-yearly injection that provides full-year protection with a single vet visit.

For cats, heartworm prevention is also important — though treatment options are very limited for feline heartworm (there is no approved adulticide treatment for cats). Monthly topical preventives are the standard approach.

The Annual Heartworm Test

Before starting or restarting heartworm prevention — and annually thereafter — dogs should be tested for existing heartworm infection. Giving preventive medication to a heartworm-positive dog can cause a severe reaction. The test is a simple blood test performed in-clinic with results in minutes.

Test before starting any new preventive product
Test annually even if on continuous prevention (to catch any breakthrough infections)
Retest if there was a gap in prevention longer than 30 days

Schedule your dog's annual heartworm test alongside their yearly wellness exam. Use My Pets on TailRounds to track test results and prevention history so nothing falls through the cracks.

What Happens If Heartworm Is Found?

Heartworm treatment in dogs is a multi-month process involving:

Strict exercise restriction (activity accelerates worm-related lung damage)
A series of deep muscle injections of melarsomine (an arsenic-based compound that kills adult worms)
Multiple follow-up visits and blood tests
Months of cage rest after treatment

Prevention costs a fraction of treatment. Year-round prevention is not optional — it is essential. If your dog is currently unprotected, start today. Find a clinic near you to get tested and start prevention.