Heartworm Injection for Dogs: ProHeart 12, Cost & Prevention Guide

What Is the Heartworm Injection for Dogs?

The heartworm injection for dogs is a long-acting, veterinarian-administered preventive that protects against Dirofilaria immitis — the parasitic nematode responsible for heartworm disease. Unlike monthly chewables or topicals that rely on owner compliance, injectable heartworm prevention delivers sustained protection through a controlled-release microsphere formulation of moxidectin.

Two injectable products are currently FDA-approved and widely available in veterinary clinics:

• ProHeart 6 (moxidectin 0.17 mg/kg): Provides 6 months of continuous protection per injection. Manufactured by Zoetis.
• ProHeart 12 (moxidectin 0.5 mg/kg): Provides 12 full months of protection in a single injection. Also manufactured by Zoetis and approved by the FDA in 2019.

Both formulations use biodegradable microspheres that slowly release moxidectin into your dog's system over the protection window. Your veterinarian administers the injection subcutaneously (under the skin), typically between the shoulder blades, in a quick office visit that takes roughly five minutes.

Understanding Heartworm: Parasite Identification and Life Cycle

What Heartworms Look Like

Adult Dirofilaria immitis worms are long, thin, and pale white to cream-colored — resembling cooked spaghetti strands. Females measure 25–31 cm (10–12 inches) in length; males are slightly smaller at 12–20 cm (5–8 inches). They reside primarily in the pulmonary arteries and the right side of the heart, where they can live 5–7 years in an untreated dog.

A heavily infected dog may harbor 50 to over 200 adult worms, though infections with fewer than 10 worms are common in areas with moderate transmission seasons. Even low worm burdens cause progressive damage to the pulmonary vasculature.

How the Heartworm Life Cycle Works

Heartworm transmission requires a mosquito intermediate host. The American Heartworm Society describes the cycle in these stages:

1. Microfilariae circulation: An infected dog's bloodstream contains baby heartworms (microfilariae) released by adult female worms.
2. Mosquito ingestion: A mosquito bites the infected dog and ingests microfilariae during its blood meal.
3. Larval development (L1 → L3): Over 10–14 days inside the mosquito — and only when ambient temperatures remain above 57°F (14°C) — the microfilariae molt twice to become infective third-stage larvae (L3).
4. Transmission to new host: The mosquito bites a new dog, depositing L3 larvae onto the skin. The larvae enter through the bite wound.
5. Tissue migration (L3 → L4 → immature adult): Over 2–3 months, larvae migrate through subcutaneous tissue and muscle, molting to the L4 stage and then to immature adults.
6. Pulmonary artery colonization: Immature adults enter the bloodstream and travel to the pulmonary arteries, where they mature into reproductive adults approximately 6–7 months after initial infection.
7. Reproduction: Adult females release microfilariae into the bloodstream, and the cycle repeats when another mosquito feeds.

This prepatent period — the roughly 6-month gap between infection and detectable adult worms — is why heartworm testing protocols matter and why a single missed preventive dose can leave a window of vulnerability.

How Dogs Get Heartworm: Transmission Routes and Risk Factors

Mosquitoes are the only natural transmission route for heartworm. Dogs cannot catch heartworm directly from other dogs, from shared water bowls, or from contact with infected animals. However, any of the approximately 70 mosquito species found in the United States can serve as a vector, and only a handful need to carry infective L3 larvae to sustain transmission in an area.

Environmental and Geographic Risk Factors

Heartworm has been diagnosed in all 50 U.S. states. Historically, the Southeast, Gulf Coast, and Mississippi River Valley carry the highest incidence, but urban heat islands, irrigation, and climate shifts have expanded risk into previously low-incidence regions. According to the American Veterinary Medical Association, even indoor-only dogs are at risk — mosquitoes readily enter homes through open doors, torn screens, and garages.

Risk amplifiers include:

• Proximity to standing water (ponds, retention basins, marshes, even clogged gutters)
• Outdoor lifestyle or frequent hiking/camping in wooded or riparian areas
• Living in regions with year-round temperatures above 57°F (14°C)
• Travel to high-incidence states without current preventive coverage
• Lapsed or inconsistent preventive use — the single biggest risk factor

Heartworm Symptoms by Severity

Early heartworm infection is clinically silent. Most dogs show no outward signs until adult worms have been present in the pulmonary arteries for months. The Merck Veterinary Manual classifies heartworm disease into four severity classes:

Treatment Protocol: What Happens If Your Dog Tests Positive

Heartworm treatment is a multi-month medical protocol — not a single injection. Your veterinarian will follow the American Heartworm Society's recommended adulticide protocol:

Step-by-Step Treatment Timeline

1. Confirmatory testing (Day 1): A positive antigen test is confirmed with a microfilariae test and thoracic radiographs to assess lung and heart damage. Cost range: $150–$400 for initial diagnostics.
2. Pre-treatment stabilization (Weeks 1–4): Your dog begins doxycycline (an antibiotic that weakens heartworms by targeting their symbiotic Wolbachia bacteria), a monthly preventive to kill incoming larvae, and strict exercise restriction. Prednisone may be added for inflammation.
3. Adulticide injections (Weeks 4–8): Melarsomine dihydrochloride (Immiticide) is administered via deep intramuscular injection into the lumbar muscles. The standard protocol uses three injections: one injection on Day 1, then two injections 24 hours apart on Day 30–31. Dogs are typically hospitalized or closely monitored during this phase.
4. Recovery and recheck (Weeks 8–16): Strict crate rest and leash-only bathroom breaks continue for 6–8 weeks post-injection. Dead worms fragment and are absorbed by the lungs; excessive activity during this window can cause fatal pulmonary embolism.
5. Post-treatment antigen test (Month 9): A follow-up antigen test confirms all adult worms have been eliminated. If positive, the protocol may need to be repeated.

Total treatment cost: $1,000–$3,500 depending on disease severity, dog size, and geographic location. Caval syndrome surgical extraction can exceed $6,000–$10,000 at a specialty center.

This is why the heartworm injection for dogs — and all preventives — represent such a cost-effective investment compared to treatment.

Prevention Products Compared: Injection vs. Oral vs. Topical

Choosing a heartworm preventive depends on your dog's size, your budget, and your own compliance habits. Here is a direct comparison of the major FDA-approved options available through veterinary clinics in 2026:

Why Choose the Heartworm Injection for Dogs?

The ProHeart injectables solve the single biggest failure point in heartworm prevention: missed doses. A 2024 survey by the American Heartworm Society found that fewer than 50% of dog owners on monthly preventives administered all 12 doses on schedule. With ProHeart 12, there is no monthly pill to forget — one visit per year covers your dog.

Advantages of injectable prevention:

• Guaranteed compliance — the medication is always in your dog's system
• No vomiting or spitting out pills (a real problem with oral chews)
• No topical application mess or bathing restrictions
• Cost-competitive with 12 months of premium oral preventives

Limitations to consider:

• Heartworm-only protection — you will still need a separate flea/tick/intestinal parasite product
• Requires a veterinary visit (though often bundled with annual wellness exams)
• If an adverse reaction occurs, the medication cannot be removed — it must metabolize over the full protection period
• Not approved for dogs under 6 months of age

Safety Profile and Side Effects

ProHeart 6 and ProHeart 12 underwent extensive FDA safety trials. Reported adverse reactions are uncommon but can include:

• Mild (most common): Injection site swelling or tenderness, transient lethargy, mild appetite decrease — typically resolving within 24–48 hours
• Moderate: Vomiting, diarrhea, hives or facial swelling (allergic response)
• Rare but serious: Anaphylaxis, seizures, immune-mediated hemolytic anemia

Your veterinarian will screen your dog with a heartworm antigen test before the first injection. Administering moxidectin to a dog with existing adult heartworms can trigger a severe reaction as microfilariae die rapidly. This pre-screening is non-negotiable.

Seasonal Risk Calendar: When Heartworm Transmission Peaks

Heartworm transmission requires sustained temperatures above 57°F (14°C) for at least 10–14 consecutive days so that L3 larvae can develop inside the mosquito. Here is a regional breakdown:

The American Heartworm Society and the AVMA both recommend year-round prevention regardless of geography. The reasoning: missed spring or fall doses during unseasonably warm stretches create infection windows, and year-round products also cover intestinal parasites that have no seasonal break.

Environmental Control: Reducing Mosquito Exposure at Home

No preventive is 100% effective, so reducing your dog's mosquito exposure adds a meaningful layer of protection.

Yard Management

• Eliminate standing water weekly: Empty birdbaths, plant saucers, tire swings, clogged gutters, and any container holding water for more than 5 days (the minimum time for mosquito eggs to hatch and develop).
• Treat ornamental ponds: Use Bacillus thuringiensis israelensis (Bti) dunks — sold as Mosquito Dunks — which are safe for pets, fish, and wildlife. One dunk treats 100 square feet of water surface for 30 days. Cost: approximately $8–$12 for a 6-pack.
• Maintain landscaping: Trim overgrown vegetation and keep grass short to reduce mosquito resting areas. Mosquitoes avoid direct sunlight and wind.
• Consider barrier sprays: Professional permethrin-based yard treatments can reduce mosquito populations by 85–90% for 21–28 days. Warning: Permethrin is highly toxic to cats. If you have cats that access treated areas, discuss pyrethrin-free alternatives with your pest control provider. Keep dogs off treated surfaces until fully dry (typically 2–4 hours).

Indoor Protection

• Repair torn window and door screens promptly
• Use fans on patios and in kennel areas — mosquitoes are weak fliers and avoid air movement above 1 mph
• Run air conditioning during peak mosquito hours (dawn and dusk) to keep windows closed
• Consider pet-safe indoor mosquito traps (UV light + fan designs) for garages and mudrooms

Zoonotic Potential: Can Humans Get Heartworm from Dogs?

Humans are dead-end hosts for Dirofilaria immitis. In rare cases, a mosquito carrying infective L3 larvae can bite a human, and the larva may migrate to the lungs, where it dies and forms a small granuloma (a coin-shaped lesion visible on chest X-ray). This condition — human pulmonary dirofilariasis — is not dangerous and typically requires no treatment, but it can be mistaken for a lung tumor on imaging.

Importantly, humans cannot develop mature adult heartworms, cannot produce microfilariae, and cannot transmit the parasite. Heartworm is not considered a zoonotic disease in any clinically meaningful sense. Your infected dog does not pose a direct heartworm risk to you or your children.

That said, an infected dog does serve as a reservoir for mosquitoes to pick up microfilariae and transmit them to other dogs — which is why treating positive dogs and maintaining prevention in all household dogs matters for community-level control.

Heartworm Injection for Dogs: Frequently Asked Questions

How do I know if my dog has heartworm?

You cannot reliably detect heartworm by observing symptoms — early infection is silent. Your veterinarian performs a simple blood-based antigen test that detects proteins produced by adult female worms. This test takes about 10 minutes in-clinic and costs $35–$75. The American Heartworm Society recommends annual testing even for dogs on year-round prevention, because no preventive is 100% effective and early detection dramatically improves treatment outcomes.

How often should I treat my dog for heartworm?

If you use ProHeart 12, your dog receives one injection annually — typically timed with their yearly wellness exam. If you use ProHeart 6, injections are given every 6 months. For monthly oral or topical products, dosing must occur on the same date every month without gaps. Annual antigen testing is recommended regardless of which preventive you choose.

Can the heartworm injection be given to puppies?

ProHeart 6 and ProHeart 12 are approved for puppies 6 months of age and older. For puppies younger than 6 months, your veterinarian will typically start a monthly oral preventive (such as Heartgard Plus, which is approved for puppies as young as 6 weeks) and then transition to the injectable at the 6-month mark.

Can heartworm spread to other pets in my household?

Heartworm does not spread directly from dog to dog or from dogs to cats. However, cats and ferrets are also susceptible to heartworm infection via mosquito bites, and the disease can be even more dangerous in cats because there is no approved adulticide treatment for feline heartworm. If you have cats, discuss feline heartworm prevention (such as Revolution Plus or Interceptor) with your veterinarian.

What if I switch from a monthly chewable to the heartworm injection?

Your veterinarian will administer a heartworm antigen test to confirm your dog is negative, then give the first ProHeart injection. The injection should be given within 30 days of the last monthly dose to avoid a gap in coverage. There is no required washout period — the medications do not interact negatively.

What if my dog has an adverse reaction to the injection?

Monitor your dog for 24–48 hours after the injection. Mild injection-site swelling and brief lethargy are common and self-resolving. Contact your veterinarian if you observe vomiting, facial swelling, hives, difficulty breathing, or collapse. For severe reactions outside clinic hours, call the ASPCA Animal Poison Control Center at (888) 426-4435 (24/7, consultation fee applies) or proceed to your nearest emergency veterinary hospital.

Is the heartworm injection for dogs worth the cost?

At $120–$200 per year for ProHeart 12, the injection is comparable to or less expensive than 12 months of premium combination oral preventives ($200–$320+ annually). When you factor in the compliance guarantee — and the $1,000–$3,500 cost of adulticide treatment — the injection is the most cost-effective option for owners who sometimes forget monthly doses. Consult your veterinarian to determine which preventive protocol best fits your dog's health profile and your household's lifestyle.