Can You Give Heartworm Preventative to a Dog with Heartworms? What You Need to Know

Heartworm disease is a scary diagnosis for any pet owner. If your furry friend has been diagnosed with heartworms, you might be wondering if you can still give them heartworm preventative medication. The answer isn’t as straightforward as you might hope, and giving preventatives to an infected dog can actually be dangerous in some cases.

As someone who’s researched this topic extensively and spoken with multiple veterinarians, I want to share what I’ve learned about this critical pet health issue. Let’s dive into the facts about heartworm disease and clarify when preventatives can or cannot be given to infected dogs.

The Dangerous Truth About Heartworm Disease

Heartworm disease is no joke. It’s caused by parasitic worms (scientific name Dirofilaria immitis) that are transmitted through mosquito bites. These nasty worms take up residence in your dog’s heart, lungs, and blood vessels, causing severe health problems and potentially death if left untreated.

The adult heartworms look like strands of cooked spaghetti (gross, I know) with males reaching about 4-6 inches and females growing to a whopping 10-12 inches in length! A typical infected dog carries around 15 worms, but that number can range anywhere from just 1 to a horrifying 250 worms.

Why You SHOULDN’T Give Heartworm Preventative to an Infected Dog

Here’s the crucial information that every dog owner needs to understand

Heartworm preventatives DO NOT kill adult heartworms

This is the most important fact to remember. Preventatives are designed to kill the immature larvae (microfilariae) before they can develop into adults. They aren’t designed to treat an existing adult heartworm infection.

In fact, according to the FDA, giving a heartworm preventative to a dog that’s already infected with adult heartworms can be harmful or even deadly. Here’s why:

• If microfilariae are present in your dog’s bloodstream (which happens in 80-90% of infected dogs), the preventative may cause these microfilariae to suddenly die all at once
• This mass die-off can trigger a shock-like reaction in your dog
• In some cases, this reaction can be fatal

So no you generally should NOT give heartworm preventative to a dog with heartworms unless specifically directed to do so by your veterinarian as part of a treatment protocol.

The Progression of Heartworm Disease

Heartworm disease progresses through four classes or stages:

1. Class 1: No symptoms or mild symptoms like an occasional cough
2. Class 2: Mild to moderate symptoms including occasional coughing and tiredness after moderate activity
3. Class 3: More severe symptoms including persistent cough, tiredness after even mild activity, breathing difficulties, and signs of heart failure
4. Class 4: Also called “caval syndrome” – this is life-threatening with a massive worm burden physically blocking blood flow to the heart

Without proper treatment, heartworm disease damages your dog’s heart, lungs, liver, and kidneys, eventually causing death. This isn’t something to mess around with or try to treat yourself!

Proper Testing Before Starting Prevention

This is where many pet owners make mistakes. Before starting ANY heartworm preventative, dogs that are 7 months or older should be tested for heartworms first.

Why? Because a dog might look perfectly healthy on the outside while harboring heartworms on the inside. If you start a preventative without testing, those adult worms will continue to damage your dog’s body until symptoms become severe enough to notice.

Testing typically involves:

• An antigen test that detects proteins released by adult female heartworms
• Sometimes a test for microfilariae in the bloodstream
• Additional tests like x-rays or ultrasounds may be needed in some cases

The Proper Treatment for Heartworm Disease

If your dog tests positive for heartworms, the FDA-approved treatment involves:

1. Melarsomine dihydrochloride (trade names Immiticide and Diroban) – This arsenic-containing drug is injected deep into back muscles to kill adult heartworms in dogs with class 1, 2, or 3 disease
2. Advantage Multi for Dogs (containing imidacloprid and moxidectin) – This can be used to eliminate microfilariae from the bloodstream

I won’t sugarcoat it – heartworm treatment is rough. It’s expensive, requires multiple vet visits, and can cause serious complications including potentially life-threatening blood clots in the lungs. The treatment involves blood tests, x-rays, hospitalization, and a series of injections.

Your veterinarian will likely develop a comprehensive treatment plan that might include:

• Restricting your dog’s activity during treatment
• Medications to reduce inflammation
• Antibiotics before treatment (research suggests certain bacteria live inside heartworms)
• A specific schedule for the adulticide injections
• Follow-up testing to ensure all heartworms are gone

Prevention is Always Better Than Treatment!

The best approach is to never let your dog get heartworms in the first place! There are many FDA-approved preventative products available by prescription from your vet. Most are given monthly, either as:

• Topical liquids applied to the skin
• Oral tablets (both chewable and non-chewable options exist)
• There’s even an injectable option given every 6-12 months (by a vet only)

Some preventatives do double-duty by also protecting against other parasites like intestinal worms, fleas, ticks, and ear mites.

The American Heartworm Society recommends the “Think 12” approach:

• Give your dog 12 months of heartworm prevention (year-round!)
• Get your dog tested for heartworms every 12 months

Where is Heartworm Disease Most Common?

While heartworm disease has been reported in all 50 states, it’s most common in certain regions:

• Along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey
• Along the Mississippi River and its major tributaries

But don’t get complacent if you don’t live in these areas! Climate change and pet transportation mean heartworm disease can show up anywhere.

My Personal Experience with Heartworm Disease

I remember when my neighbor’s Lab was diagnosed with heartworms. They had missed just two months of preventative during a move across country, and that was enough for infection to occur. The treatment was rough – their dog had to be kept extremely calm (nearly crate-rest) for weeks, and the injections were painful and expensive.

The worst part was seeing how guilty they felt for missing those preventative doses. Don’t put yourself through that! Set calendar reminders, have your vet send notifications, whatever it takes to stay on schedule.

FAQs About Heartworm Disease and Prevention

Is heartworm disease contagious between dogs?

No! Heartworm disease is not contagious. A dog cannot catch heartworms from being near an infected dog. The disease is only spread through the bite of an infected mosquito.

How long do heartworms live in dogs?

Inside a dog, heartworms typically live 5-7 years if not treated.

Can indoor dogs get heartworms?

Absolutely! Mosquitoes can easily get inside homes, so even indoor dogs need heartworm prevention.

Can humans get heartworms from their pets?

People cannot get heartworms from their pets. In rare cases, people can get heartworms after being bitten by an infected mosquito, but humans aren’t natural hosts for heartworms, so the larvae usually die before becoming adults.

How often should my dog be tested for heartworms?

Annual testing is recommended for all dogs on heartworm prevention. Your vet might recommend more frequent testing in certain situations.

Final Thoughts: Always Consult Your Veterinarian

If your dog has heartworms or you suspect they might, PLEASE consult your veterinarian immediately. Do not start, stop, or change any medication without professional guidance. The stakes are simply too high to risk making a mistake.

Remember: prevention is always cheaper, safer, and better than treatment. Keep your furry friend on year-round prevention, and you’ll likely never have to worry about the dangers of heartworm disease.

Have you had any experiences with heartworm disease in your pets? What preventatives do you use? Share your thoughts in the comments below!

Disclaimer: While I’ve researched this topic extensively, I’m not a veterinarian. Always consult with your vet before making medical decisions for your pet.