Heartgard for Dogs: Year-Round Prevention in Cypress, TX

If you've ever wondered whether your dog really needs heartworm prevention in January, you're not alone. Every winter I hear some version of "the mosquitoes are gone, so we can take a break, right?" Here in Cypress, TX, that assumption gets dogs killed. I've been practicing at Hearthstone Animal Clinic since 2007, and I still see heartworm-positive dogs every single month, including February.

This post breaks down exactly how Heartgard works, why year-round dosing matters in our climate, what it costs, and what happens if you skip doses. If you're reading this at 11pm trying to remember when you gave the last chewable, start here.

What is Heartgard and how does it actually work?

Heartgard (and its newer formulation, Heartgard Plus) is a monthly chewable that prevents heartworm disease in dogs. The active ingredient is ivermectin, which has been used safely in veterinary medicine for over 40 years.

Here's what most owners don't realize: Heartgard doesn't prevent your dog from getting bitten by an infected mosquito. It kills heartworm larvae (called microfilariae) that are already in your dog's bloodstream, before they can mature into foot-long adult worms that live in the heart and pulmonary arteries. The drug works retroactively, clearing out any larvae your dog picked up in the previous 30 days.

Why timing matters

Heartworm larvae take about six months to mature into adults. Heartgard is only effective against larvae in a specific early stage of development. If you skip two or three months, larvae can age past the point where ivermectin kills them. Once that happens, you've got worms growing toward your dog's heart, and no monthly chewable will stop them.

This is why the American Heartworm Society recommends giving Heartgard every 30 days, year-round, with no gaps. According to their heartworm prevention guidelines, even a single missed dose can leave your dog vulnerable.

Why does Cypress, TX have such a high heartworm risk?

Texas is the number-one state in the country for heartworm cases. The Houston metro area, including Cypress, ranks among the highest-risk regions in Texas. Three factors make our area particularly dangerous:

• Year-round mosquito activity: We rarely get sustained freezes long enough to kill the mosquito population. A few cold nights don't reset the clock.
• High humidity and standing water: Our climate is perfect mosquito breeding ground. Bayous, detention ponds, even a forgotten bucket in your backyard.
• Wildlife reservoirs: Coyotes, foxes, and unprotected stray dogs carry heartworm and keep the parasite circulating in our local mosquito population.

I've had clients move here from Colorado or Arizona, where heartworm is rare, and assume their old prevention schedule still applies. It doesn't. If your dog lives in Cypress, 12 months of protection is the minimum.

How much does Heartgard cost?

For most dogs, Heartgard Plus costs between $10 and $18 per month, depending on your dog's weight. A six-month supply for a medium-sized dog (26-50 lbs) runs around $70 to $85 at most veterinary clinics, including ours at Hearthstone Animal Clinic.

Compare that to treating a heartworm infection: the full protocol runs $1,200 to $1,800 for uncomplicated cases, and significantly more if your dog needs hospitalization or has heart damage. The math isn't close.

Heartgard vs. Heartgard Plus

Standard Heartgard prevents heartworm only. Heartgard Plus adds pyrantel, which treats roundworms and hookworms. For most dogs, I recommend the Plus version because intestinal parasites are common and the price difference is minimal.

What are the side effects of Heartgard?

Ivermectin at heartworm-prevention doses is extremely safe for the vast majority of dogs. In 25 years of practice, I can count on one hand the number of dogs who've had a true adverse reaction to Heartgard.

That said, side effects can include:

• Vomiting or diarrhea (usually mild, resolves in 24 hours)
• Lethargy or decreased appetite
• Drooling, especially if your dog doesn't like the taste

A note about MDR1 gene mutations

Certain breeds (Collies, Australian Shepherds, Shetland Sheepdogs, and some herding breed mixes) can carry a gene mutation that makes them sensitive to high doses of ivermectin. At heartworm prevention doses, Heartgard is considered safe even for these breeds. The FDA-approved dose in Heartgard is much lower than the toxic threshold. If you're worried, we can discuss genetic testing, but in practice this is rarely an issue.

How does Heartgard compare to other heartworm preventives?

Heartgard is one option among several. Here's how I think about the main alternatives:

Heartgard Plus vs. NexGard Plus

NexGard Plus is a newer all-in-one chewable that combines heartworm prevention, flea and tick control, and intestinal parasite treatment in a single monthly dose. If your dog also needs flea and tick prevention, NexGard Plus simplifies things. If your dog tolerates Heartgard well and you're already using a separate flea/tick product, there's no urgent reason to switch.

Heartgard Plus vs. topicals

Some owners prefer topical preventives because their dog is a picky eater. The downside: topicals can wash off if your dog swims frequently, and some dogs hate the greasy feeling. Most dogs eat Heartgard like a treat.

What about injectable heartworm preventives?

There are injectable options on the market that last 6 or 12 months. I don't carry or recommend these at Hearthstone. The monthly chewables give me more control over dosing and make it easier to adjust if your dog's weight changes. Monthly also means if you notice a problem, we catch it faster.

What happens if my dog misses a dose?

If you're a week or two late, give the dose immediately and continue on your normal schedule. If it's been more than six weeks, give the dose and schedule a heartworm test in six months. The test won't show a new infection right away because it takes that long for worms to mature and produce detectable antigens.

This is the part most owners get wrong, and it's not their fault. The heartworm lifecycle is confusing. Here's the short version: Heartgard kills yesterday's larvae, not tomorrow's. A missed dose doesn't just leave a gap. It creates a window where larvae can mature past the drug's reach.

Signs your dog might already have heartworm

Early infection shows no symptoms. By the time you notice something, the disease is usually advanced:

• Persistent cough, especially after exercise
• Fatigue or reluctance to play
• Weight loss despite normal appetite
• Swollen belly (fluid accumulation from heart failure)

If your dog shows any of these signs, especially if prevention has lapsed, don't wait. We can run a heartworm test in 10 minutes.

When should you contact your veterinarian about heartworm?

Contact your Cypress veterinarian if:

• Your dog has missed two or more months of heartworm prevention
• You're adopting a new dog with unknown history
• Your dog shows any symptoms listed above
• You're unsure which preventive is right for your dog's lifestyle
• Your dog's annual heartworm test is overdue

I recommend annual testing even for dogs on consistent prevention. No drug is 100% effective, and dogs occasionally spit out chewables when you're not looking. A yearly test catches problems before they become emergencies.

Keep your dog protected year-round

Heartworm disease is slow, silent, and preventable. A monthly chewable like Heartgard Plus costs less than a single emergency visit and takes 30 seconds to give. If this was my dog, I wouldn't skip a single month, and I don't.

If your dog is overdue for heartworm testing, needs to start prevention, or you have questions about which product fits your situation, schedule a visit at Hearthstone Animal Clinic. We're here in Cypress, TX at (281) 859-9244, and we can usually get your dog seen the same week. Heartworm prevention is the easiest win in veterinary medicine. Let's make sure your dog stays protected.