Heartworm Shot for Dogs: ProHeart 12 vs Monthly Preventives Explained
Everything about the heartworm shot for dogs: ProHeart 6 and ProHeart 12 efficacy, cost, side effects, how it compares to monthly chewables and topicals.
If you've ever forgotten to give your dog their monthly heartworm chewable — or watched them spit it out the moment you turned your back — you're not alone. Compliance is the single biggest reason dogs still develop heartworm disease in the United States, despite highly effective preventives being available for decades. The heartworm shot for dogs, marketed under the brand name ProHeart, offers an alternative: a veterinarian-administered injection that protects your dog for six or twelve full months with a single visit.
But how does it actually work? Is it safer or riskier than monthly pills? And what does the full lifecycle of Dirofilaria immitis — the parasite behind heartworm disease — look like inside your dog's body? This guide covers the injection alongside the broader landscape of heartworm prevention so you can make an informed decision with your veterinarian.
Understanding the Heartworm Parasite: Identification and Biology
Heartworms (Dirofilaria immitis) are parasitic nematodes — roundworms — that live primarily in the pulmonary arteries and right side of the heart in infected dogs. Adult females measure 10 to 12 inches (25–30 cm) in length; males are slightly smaller at 5 to 7 inches (12–18 cm). They are thin, white to cream-colored, and resemble cooked spaghetti strands when found during necropsy or surgical extraction.
Heartworm Parasite — Visual Identification
- Adult worms: 5–12 inches long, white/cream, thin and thread-like, found in the right ventricle and pulmonary arteries
- Microfilariae (L1 larvae): Microscopic — approximately 300 micrometers long, circulating in the bloodstream, invisible to the naked eye
- Infective larvae (L3): Deposited by mosquitoes onto the dog's skin, roughly 1 mm, enter through the bite wound
- Location in host: Adults lodge in pulmonary arteries first, then the right atrium and ventricle in heavy infections
- Burden range: A mildly infected dog may harbor 5–15 worms; severe cases can exceed 100 adult worms
The Heartworm Life Cycle: From Mosquito Bite to Adult Worms
Understanding why the heartworm shot for dogs works — and why timing matters — requires knowing the parasite's full life cycle. The American Heartworm Society outlines the following stages:
Dirofilaria immitis Life Cycle — Step by Step
- Microfilariae in bloodstream: An infected dog circulates first-stage larvae (L1 microfilariae) in its blood. These are ingested when a mosquito bites the dog.
- Development inside the mosquito (10–14 days): Inside the mosquito, L1 larvae molt to L2 and then to the infective L3 stage. This development requires ambient temperatures consistently above 57°F (14°C) for at least two weeks. Below this threshold, development stalls.
- Transmission via mosquito bite: When the infected mosquito bites a new dog, L3 larvae are deposited on the skin and migrate into the bite wound.
- Tissue migration (months 1–3): L3 larvae molt to L4 within the dog's subcutaneous and muscle tissues over 50–70 days. They then enter the bloodstream.
- Arrival in pulmonary arteries (month 4–6): Immature adults (L5) lodge in the small pulmonary arteries of the lungs, where they continue to grow.
- Maturation and reproduction (months 6–7): Adults reach sexual maturity. Females release microfilariae into the bloodstream, completing the cycle. A single female can produce thousands of microfilariae daily.
Key takeaway: The window between L3 infection and adult maturation is approximately 6 months. Preventives work by killing L3 and L4 larvae before they mature — which is why consistent, uninterrupted prevention is critical.
How Dogs Get Heartworm: Transmission Routes and Risk Factors
Heartworm is transmitted exclusively through mosquito bites. There is no direct dog-to-dog transmission, no fecal-oral route, and no transmission through shared water bowls or grooming tools. However, the ubiquity of mosquitoes means risk extends far beyond what many owners assume.
According to the American Veterinary Medical Association, heartworm has been diagnosed in all 50 U.S. states. Risk factors include:
- Outdoor exposure: Dogs spending time outdoors during dawn and dusk (peak mosquito feeding times) face higher risk
- Geographic region: The Southeast, Gulf Coast, and Mississippi River Valley have the highest incidence, but cases occur nationwide
- Lack of prevention: Dogs not on any preventive are at direct risk; even one missed monthly dose can leave a gap
- Standing water: Properties with birdbaths, clogged gutters, ponds, or low-lying areas serve as mosquito breeding grounds
- Wildlife reservoirs: Coyotes, foxes, and wolves can carry heartworm, maintaining environmental infection pressure even if neighboring dogs are treated
Heartworm Symptoms by Infection Severity
Heartworm disease is often clinically silent in its early stages. By the time symptoms appear, adult worms have typically been present for months, and damage to the pulmonary vasculature may already be significant.
| Severity | Worm Burden | Clinical Signs | Prognosis |
|---|---|---|---|
| Asymptomatic (Class I) | 1–10 worms | None, or occasional mild cough. Often discovered on routine antigen test. | Excellent with treatment |
| Mild (Class II) | 10–25 worms | Persistent cough, exercise intolerance, mild weight loss, abnormal lung sounds on auscultation | Good with treatment; some permanent pulmonary changes possible |
| Moderate (Class III) | 25–50 worms | Chronic cough, dyspnea (labored breathing), weight loss, syncope (fainting), right-sided heart enlargement visible on X-ray, proteinuria | Guarded; treatment carries higher complication risk |
| Severe — Caval Syndrome (Class IV) | 50–100+ worms | Acute collapse, hemoglobinuria (dark red/brown urine), severe dyspnea, shock. Worms physically obstruct the tricuspid valve. | Poor without emergency surgical extraction; often fatal within 24–48 hours |
The Heartworm Shot for Dogs: ProHeart 6 and ProHeart 12
The heartworm shot for dogs is a sustained-release injectable suspension of moxidectin, the same active ingredient found in some monthly topical and oral preventives. Manufactured by Zoetis, it is available in two formulations:
- ProHeart 6: Provides 6 months of continuous protection per injection. FDA-approved for dogs 6 months of age and older.
- ProHeart 12: Provides 12 months of continuous protection per injection. FDA-approved for dogs 12 months of age and older. Approved in the U.S. in 2022 after years of use in Australia and other markets.
How the Injection Works
The moxidectin is encapsulated in biodegradable polymer microspheres. After subcutaneous injection (typically between the shoulder blades), the microspheres slowly release moxidectin into the bloodstream over the labeled duration. This maintains a consistent drug level that kills L3 and L4 larvae before they can mature into adults.
Cost Breakdown (2025–2026 Pricing)
| Product | Duration | Typical Cost (Small Dog, 11–25 lb) | Typical Cost (Large Dog, 51–100 lb) | Annual Cost |
|---|---|---|---|---|
| ProHeart 6 | 6 months | $55–$80 per injection | $90–$140 per injection | $110–$280 |
| ProHeart 12 | 12 months | $95–$130 per injection | $150–$220 per injection | $95–$220 |
| Monthly oral (e.g., Heartgard Plus, Interceptor Plus) | 30 days | $8–$12/month | $14–$22/month | $96–$264 |
| Monthly topical (e.g., Revolution Plus) | 30 days | $18–$28/month | $25–$40/month | $216–$480 |
Costs vary by veterinary clinic, geographic region, and whether the injection is bundled with an exam. The annual heartworm antigen test ($35–$65) is required regardless of which preventive you choose.
Safety Profile and Side Effects
ProHeart's early history was contentious. The original ProHeart 6 was voluntarily withdrawn from the U.S. market in 2008 following reports of adverse reactions, including immune-mediated hemolytic anemia, liver failure, and deaths. After extensive reformulation and clinical trials, it was reintroduced in 2017 with updated safety data. ProHeart 12 followed with its own FDA approval in 2022.
Reported adverse reactions in current formulations include:
- Most common: Injection site swelling or tenderness (typically resolves within 48–72 hours), lethargy for 1–2 days, transient vomiting or diarrhea
- Less common: Hypersensitivity reactions (facial swelling, hives), elevated liver enzymes
- Rare but serious: Immune-mediated reactions, anaphylaxis
Your veterinarian will screen your dog's health history before recommending the injection. Dogs with a history of severe allergic reactions, immune-mediated disease, or significant liver dysfunction may be better candidates for monthly oral alternatives.
Heartworm Prevention Products Comparison
| Product | Type | Active Ingredient(s) | Duration | Additional Parasite Coverage | Minimum Age |
|---|---|---|---|---|---|
| ProHeart 12 | Injectable | Moxidectin | 12 months | Heartworm only (hookworm treatment in some labels) | 12 months |
| ProHeart 6 | Injectable | Moxidectin | 6 months | Heartworm + treats hookworm | 6 months |
| Heartgard Plus | Oral chewable | Ivermectin + Pyrantel pamoate | 30 days | Heartworm + roundworm + hookworm | 6 weeks |
| Interceptor Plus | Oral chewable | Milbemycin oxime + Praziquantel | 30 days | Heartworm + roundworm + hookworm + whipworm + tapeworm | 2 weeks |
| Simparica Trio | Oral chewable | Sarolaner + Moxidectin + Pyrantel | 30 days | Heartworm + roundworm + hookworm + fleas + ticks (5 species) | 8 weeks |
| Revolution Plus | Topical | Selamectin + Sarolaner | 30 days | Heartworm + fleas + ticks + ear mites + roundworm + hookworm | 8 weeks |
Decision Framework: Injection vs. Monthly Oral vs. Topical
- If your dog reliably takes monthly chewables without vomiting or spitting them out → Monthly oral products like Heartgard Plus or Interceptor Plus remain the gold standard with the longest safety track record. Choose this if you want maximum flexibility to stop treatment if adverse effects occur.
- If you frequently forget monthly doses or your dog refuses oral medication → The heartworm shot (ProHeart 6 or 12) eliminates compliance gaps. Once injected, protection is continuous. This is also ideal for dogs in foster/rescue situations where caregiver transitions can disrupt schedules.
- If your dog needs broad-spectrum parasite coverage (fleas, ticks, intestinal worms) → Monthly all-in-one products like Simparica Trio offer heartworm + intestinal parasite + flea/tick coverage in a single chewable. You could pair ProHeart 12 with a separate flea/tick product like Bravecto or NexGard for similar breadth.
- If your dog has a history of seizures or is on medication for epilepsy → Discuss with your veterinarian. Isoxazoline-class drugs (found in Simparica Trio, NexGard, Bravecto) carry an FDA advisory regarding seizure risk in dogs with pre-existing neurological conditions. The heartworm shot (moxidectin only) avoids this class entirely.
Heartworm Treatment Protocol: What Happens After a Positive Test
Prevention is straightforward; treatment is not. If your dog tests positive for heartworm, the American Heartworm Society recommends a multi-phase protocol that typically spans 3 to 4 months and costs between $1,000 and $3,000+ depending on disease severity and your region.
Standard Adulticide Treatment Protocol
- Confirmatory testing (Day 0): A positive antigen test is confirmed with a second, different test (often a microfilariae test or a different antigen assay). Chest X-rays, CBC, and chemistry panel assess organ function and disease severity. Cost: $150–$350 for full diagnostic workup.
- Pre-treatment stabilization (Days 1–30): Doxycycline (10 mg/kg twice daily for 30 days) is prescribed to kill Wolbachia, a symbiotic bacterium living inside heartworms. Eliminating Wolbachia weakens the worms and reduces inflammatory reactions when they die. Strict exercise restriction begins immediately. Heartworm preventive (moxidectin or ivermectin) is also started to prevent new infections and eliminate circulating microfilariae.
- Adulticide injections (Days 30, 60, 61): Melarsomine dihydrochloride (Immiticide) is administered via deep intramuscular injection into the lumbar muscles. The standard protocol uses three injections: one on Day 30, then two injections 24 hours apart on Days 60 and 61. This "split-dose" protocol is safer than the older two-injection method, reducing the risk of pulmonary thromboembolism from mass worm die-off.
- Post-treatment monitoring (Days 61–120): Strict crate rest and leash-only activity continue for at least 6–8 weeks after the final injection. Dead worms fragment and pass through the pulmonary vasculature; excessive activity can dislodge worm fragments and cause fatal pulmonary embolism. A follow-up antigen test at 6 months post-treatment confirms success.
Environmental Control: Reducing Mosquito Exposure
No preventive is 100% effective — reported efficacy for most products ranges from 95% to 99.9% — so reducing mosquito exposure provides a meaningful secondary layer of protection.
Yard and Property Management
- Eliminate standing water weekly: Empty birdbaths, plant saucers, clogged gutters, tarps, and old tires. Mosquitoes breed in as little as a bottle cap of water, with eggs hatching in 48–72 hours in warm weather.
- Larvicides for unavoidable water features: Bti (Bacillus thuringiensis israelensis) products, such as Mosquito Dunks, are safe for use around dogs and kill mosquito larvae without harming fish, birds, or beneficial insects. A single dunk treats 100 square feet of water surface for 30 days and costs approximately $5–$8.
- Yard barrier sprays: Permethrin-based yard treatments (applied by professional pest control services) reduce adult mosquito populations. Important: Permethrin is highly toxic to cats. If you have cats in the household, avoid permethrin yard sprays or keep cats indoors for 24–48 hours after application until surfaces are fully dry.
- Landscaping: Trim dense vegetation, maintain short grass, and improve drainage in low-lying areas to reduce mosquito resting habitat.
Indoor and Personal Protection
- Window and door screens: Repair tears and ensure tight seals. Mosquitoes can enter through gaps as small as 1/8 inch.
- Dog-safe repellents: Products containing K9 Advantix (permethrin + imidacloprid) repel mosquitoes in addition to killing fleas and ticks. Again, permethrin products must never be used on or around cats.
- Limit outdoor time at peak hours: Mosquitoes are most active at dawn and dusk. When possible, schedule walks for mid-morning or early afternoon.
Seasonal Risk Calendar: When Heartworm Threat Peaks
While the American Heartworm Society and the AVMA recommend year-round prevention regardless of geography, understanding seasonal peaks helps explain why compliance matters most during these windows.
| Region | Peak Transmission Season | Year-Round Risk? | Notes |
|---|---|---|---|
| Southeast (FL, GA, LA, MS, AL) | March–November (peak June–September) | Yes — year-round transmission documented in South Florida and Gulf Coast | Highest incidence in the U.S. |
| South Central (TX, OK, AR) | April–October | Near year-round in coastal Texas | Irrigated areas extend mosquito season |
| Midwest (IL, IN, OH, MO) | May–October | Low but present in winter | Mississippi River corridor is a hotspot |
| Northeast (NY, MA, CT, PA) | June–September | Minimal December–February | Urban heat islands extend mosquito activity |
| Mountain West (CO, UT, MT) | June–August | Very low in winter | Elevation reduces but doesn't eliminate risk |
| Pacific Northwest (WA, OR) | June–September | Low | Increasing cases linked to warmer summers and irrigation |
| Southwest (AZ, NV, Southern CA) | Year-round in irrigated and low-desert areas | Yes in many areas | Monsoon season (July–September) creates breeding sites |
Source: American Heartworm Society incidence maps and peer-reviewed surveillance data. Climate change is extending mosquito seasons in most U.S. regions.
Zoonotic Potential: Can Humans Get Heartworm From Dogs?
Heartworm is not directly transmissible from dogs to humans. You cannot catch heartworm from touching, being licked by, or living with an infected dog. Human infection is possible but extremely rare and biologically abnormal.
When humans are bitten by an infected mosquito, Dirofilaria immitis larvae occasionally survive long enough to migrate to the lungs, where they die and form a small granuloma (coin lesion) visible on chest X-ray. This is typically asymptomatic and discovered incidentally. The worms cannot reach maturity in humans and do not reproduce. According to the CDC, human dirofilariasis cases number fewer than 100 reported instances in U.S. medical literature. No treatment is usually necessary beyond monitoring.
Heartworm can spread to other canids in your household (other dogs, foxes, coyotes) through the same mosquito vector. Cats are also susceptible to heartworm disease, though they are atypical hosts and typically carry fewer adult worms. Ferrets are highly susceptible and should receive heartworm prevention. Consult your veterinarian about protecting all susceptible pets in your household.
How Often Should You Treat Your Dog for Heartworm?
The answer depends entirely on which preventive you choose:
- ProHeart 12 (injection): One injection per year, administered by your veterinarian. Schedule the annual visit alongside your dog's heartworm antigen test and wellness exam.
- ProHeart 6 (injection): One injection every 6 months. Many clinics schedule these at the annual wellness exam and the 6-month follow-up.
- Monthly oral preventives: Administered once every 30 days, year-round. Set a recurring calendar reminder. If you miss a dose by more than 2 weeks, consult your veterinarian — a gap of 45+ days may require retesting before resuming.
- Monthly topical preventives: Applied to the skin every 30 days. Avoid bathing or swimming for 48 hours before and after application to maintain efficacy.
Regardless of the product, the American Heartworm Society recommends annual antigen testing even for dogs on year-round prevention. No preventive is 100% effective, and early detection of breakthrough infections dramatically improves treatment outcomes.
Frequently Asked Questions
How do I know if my dog has heartworm?
Early-stage heartworm is almost always asymptomatic. The only reliable way to detect it is through an annual antigen blood test performed by your veterinarian. This test detects proteins produced by adult female worms and is accurate approximately 6–7 months after initial infection. If your dog shows a persistent cough, exercise intolerance, weight loss, or fainting episodes, schedule a veterinary visit promptly — these may indicate a moderate to advanced infection.
Can the heartworm shot cause an allergic reaction?
As with any medication, allergic reactions are possible but uncommon. The most frequent side effects are localized injection site swelling and mild lethargy lasting 1–2 days. Serious reactions (anaphylaxis, immune-mediated hemolytic anemia) are rare. Your veterinarian will review your dog's medical history before recommending ProHeart. Dogs with prior severe drug reactions or active immune-mediated disease may be directed toward monthly oral alternatives instead.
Is the heartworm shot safe for puppies?
ProHeart 6 is approved for puppies 6 months of age and older. ProHeart 12 requires the dog to be at least 12 months old. Puppies younger than 6 months should be started on a monthly oral preventive (many are approved from 6–8 weeks of age) and transitioned to the injection once they reach the appropriate age.
Can heartworm spread to other pets in my home?
Heartworm cannot spread directly from dog to dog or dog to cat. It requires a mosquito as an intermediate host. However, any unprotected pet in the same household is at risk from the same mosquitoes that infected the first dog. Cats and ferrets should also receive heartworm prevention. There is no risk to pet birds, reptiles, or fish, as D. immitis only infects mammals.
What if my dog is already heartworm-positive — can they still get the shot?
No. The heartworm shot (and all heartworm preventives) are designed for prevention only, not treatment. Administering a preventive to a dog with adult heartworms can trigger a severe reaction as microfilariae die rapidly. Your veterinarian must perform a negative antigen test before initiating any preventive, including ProHeart. If your dog tests positive, they will need the full adulticide treatment protocol described above before starting prevention again.
Does pet insurance cover the heartworm shot?
Most standard pet insurance policies classify heartworm prevention as routine/wellness care and do not cover it under accident-and-illness plans. However, many insurers offer optional wellness add-ons (typically $15–$30/month) that reimburse a portion of preventive costs, including ProHeart injections. Heartworm treatment, on the other hand, is typically covered under illness plans if the infection occurs after the policy's effective date and waiting period.
Aaron Whyte
All our authors care for dogs every day — read more of their work on the authors page.
