Heartworm prevention is one of the most compliance-dependent interventions in veterinary medicine. Monthly dosing, year-round adherence, and correct product selection all matter — and when a dog tests positive despite being on a preventive, the question of whether the product failed or the dose was missed becomes critical.

The FDA CVM adverse-event database provides a window into reported product failures. Among 1.34 million total reports, 297,225 carry a "lack of efficacy" (LOE) designation — meaning the reporter believed the drug did not work as intended. This is a database analysis of what those reports look like, broken down by parasite target, active ingredient, and species.

Important caveat: A "lack of efficacy" report is a suspicion, not a confirmed failure. The FDA does not verify whether the product was administered correctly, whether the diagnosis was accurate, or whether resistance was present. These numbers reflect reporting volume, not proven product deficiency.

The scale of reported preventive failures

The "lack of efficacy" category spans several parasite targets:

Heartworm-related failures (all five heartworm LOE categories combined: ineffective heartworm larvae, lack of efficacy — heartworm endoparasite, ineffective heartworm adults, lack of efficacy — heartworm specific, and ineffective heartworm microfilariae) total approximately 125,160 unique reports. Flea LOE accounts for 62,106, and hookworm LOE for 49,019. These numbers are cumulative over the full 1987–2026 span of the database.

Heartworm failures by active ingredient

Heartworm LOE reports break down as follows by active ingredient (top 15):

Several observations:

1. Ivermectin leads in raw LOE count (36,328). This likely reflects its status as the oldest and most widely dispensed macrocyclic lactone (ML) heartworm preventive, not necessarily a higher failure rate per dose. Ivermectin-based products (Heartgard Plus, Iverhart, and generics) have dominated the market for decades.

2. Combination products appear prominently. The pyrantel pamoate, ivermectin+pyrantel, and milbemycin entries often correspond to multi-parasite products (Heartgard Plus, Trifexis, Sentinel, Interceptor) where the LOE designation may reflect a failure of any component, not just the heartworm fraction.

3. Spinosad appears in 10,016 heartworm LOE reports. Spinosad (Comfortis) is primarily a flea product, but it was co-marketed with milbemycin oxime in Trifexis for heartworm prevention. The LOE entries associated with spinosad likely represent Trifexis reports where the heartworm component was tagged.

4. Melarsomine appears with 4,889 LOE reports. Melarsomine (Immiticide) is the adulticide used to treat — not prevent — heartworm disease. Its LOE entries likely reflect treatment failures or incomplete adulticidal efficacy, a different category from preventive failure.

By species

Heartworm LOE reports are overwhelmingly canine:

The dog-to-cat ratio (roughly 300:1) reflects both the higher testing frequency in dogs and the fact that feline heartworm infection is harder to diagnose and less frequently confirmed. The 421 feline LOE reports may include cases where a cat on a labeled preventive tested positive or showed clinical signs consistent with heartworm disease.

Flea preventive failures

Flea lack-of-efficacy reports total 62,106. The ingredient breakdown reveals a different pattern from heartworm:

Spinosad leads flea LOE reports by a wide margin (23,254). Because spinosad was one of the first oral flea products with rapid adulticidal action, and because it was heavily prescribed in the 2010s, the volume partly reflects market share. However, the high count also aligns with the product's known limitations: spinosad requires frequent dosing, and environmental flea reinfestation can look like product failure.

The isoxazoline class (afoxolaner, fluralaner, sarolaner, lotilaner) collectively accounts for significant flea LOE volume despite being newer entrants. This may reflect both rapid adoption and high expectations for these products.

Hookworm and ascarid failures

Hookworm lack-of-efficacy reports (49,019) and ascarid ineffective reports (19,031) round out the endoparasite LOE picture. Hookworm LOE has attracted attention in the parasitology community because of documented Ancylostoma caninum resistance to benzimidazoles and, more recently, to macrocyclic lactones. The Companion Animal Parasite Council (CAPC) and the American Heartworm Society (AHS) have both highlighted the need for resistance surveillance.

The database does not distinguish between resistance-mediated failures and compliance failures. Both contribute to the reported volume.

What does LOE volume actually signal?

When a preventive product accumulates thousands of LOE reports, it means one or more of the following:

1. The product was not administered correctly. Missed doses, incorrect dosing by weight, or inconsistent timing account for a substantial (but unquantified) share of heartworm LOE reports. The AHS 2024 guidelines emphasize that "reports of lack of efficacy should be evaluated with consideration of compliance history."

2. Environmental parasite pressure overwhelmed the product. This is especially relevant for flea LOE — a product that kills adult fleas on the animal can appear to "fail" if the environment is heavily contaminated with flea eggs, larvae, and pupae.

3. Resistance may be emerging. Documented resistance of Dirofilaria immitis to macrocyclic lactones in the Mississippi Delta region has been a concern since the early 2010s. The FDA issued guidance in 2024 seeking to update efficacy standards for heartworm preventives, acknowledging that the original 100% efficacy benchmark may not fully account for resistant strains.

4. Diagnostic and reporting artifacts. A positive heartworm antigen test in a dog on prevention may be reported as product failure when it reflects a pre-existing infection acquired before the preventive was started, or a false-positive test result. The AHS 2024 guidelines recommend confirmatory testing and careful timing interpretation before attributing a positive test to product failure.

The denominator problem

To contextualize LOE report counts, you need the total number of doses administered — and that number is not publicly available at the ingredient level. A product with 30 million doses sold per year and 10,000 LOE reports has a much lower failure-reporting rate than one with 2 million doses and the same number of reports.

Consider ivermectin: with 36,328 heartworm LOE reports accumulated over roughly three decades of market dominance, the per-dose failure rate is likely very low — but without a dose denominator, any numerical comparison between ingredients is potentially misleading.

Reporting trend over time

LOE reporting volume has grown in parallel with overall adverse-event reporting. The late-2000s and 2010s saw sharp increases in LOE reports, coinciding with:

• Expansion of the pet parasite-prevention market
• Introduction of new isoxazoline and spinosad-based products
• Greater awareness of adverse-event reporting portals among veterinarians and owners
• CAPC and AHS campaigns encouraging practitioners to report suspected product failures

What the FDA is doing

In June 2024, the FDA published final Guidance for Industry (GFI) #276 updating efficacy standards for heartworm preventives. The guidance addresses the concern that the historical 100% efficacy standard — set when ivermectin was introduced in the 1980s — may need revision given the emergence of resistant heartworm strains. The American Heartworm Society's 2024 canine guidelines also discuss LOE reporting and recommend that veterinarians work up each case individually before attributing failure to the product.

Key takeaways for veterinary professionals

• LOE reports are a signal, not a verdict. A high LOE count for an ingredient does not mean the product is ineffective; it means people have reported suspected failures at that volume.
• Compliance is the single biggest modifiable factor. The AHS and CAPC both emphasize that most heartworm "breaks" are attributable to missed or late doses rather than product failure.
• Resistance surveillance is evolving. The FDA's 2024 guidance on heartworm efficacy standards and the ongoing work of the AHS and CAPC indicate that the veterinary community is actively monitoring for macrocyclic lactone resistance.
• Report suspected failures. Whether a product failure is suspected due to resistance, compliance, or an unknown cause, reporting to the FDA (directly or through the manufacturer) strengthens the pharmacovigilance system.

Sources

• FDA Center for Veterinary Medicine. "Adverse Event Reports for Animal Drugs and Devices." https://www.fda.gov/animal-veterinary/product-safety-information/adverse-event-reports-animal-drugs-and-devices
• FDA. "openFDA Animal & Veterinary Adverse Events API." https://open.fda.gov/apis/animalandveterinary/event/
• American Heartworm Society. "Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm Infection in Dogs." Revised 2024. https://heartwormsociety.org/veterinary-resources/ahs-guidelines
• AVMA. "FDA guidance seeks to assure effectiveness of canine heartworm products." July 29, 2024. https://www.avma.org/news/fda-guidance-seeks-assure-effectiveness-canine-heartworm-products
• Companion Animal Parasite Council (CAPC). "CAPC Guidelines." https://capcvet.org/guidelines/
• Bourguinat C, et al. "Macrocyclic lactone resistance in Dirofilaria immitis." Parasites & Vectors, 2011; 4(Suppl 1): S5. https://pmc.ncbi.nlm.nih.gov/articles/PMC3406940/
• Data source: FDA CVM animal drug adverse-event reports (openFDA), downloaded 2026-06-09; analysis run date 2026-06-09. 1,340,077 individual reports analyzed; 297,225 reports contained at least one "lack of efficacy" reaction designation.