Update (July 3, 2026): FAT CAT Trial — Clopidogrel vs Aspirin for Secondary ATE Prevention in Cats, and 2024 Absolute Benefit Re-Analysis

Bottom line.

• The FAT CAT (Feline Arterial Thromboembolism: Clopidogrel vs. Aspirin Trial) was a multicenter, double-blind, randomized, positive-controlled study demonstrating that clopidogrel is superior to aspirin for secondary prevention of cardiogenic arterial thromboembolism (ATE) in cats.
• Clopidogrel-treated cats had a significantly longer median time to recurrent ATE or cardiac death (346 days vs 128 days) and lower likelihood of recurrent ATE (48% recurrence vs 75% with aspirin).
• A 2024 viewpoint analysis (Rishniw, JAVMA) reassessed the absolute risk reduction: clopidogrel reduces ATE risk by approximately 35% relative (95% CI 5-45%), translating to an absolute risk reduction of ~3.5% over the 10% baseline ATE rate in cats with moderate-to-severe HCM — a number needed to treat of approximately 23-29 cats to prevent 1 ATE event.
• The ACVIM 2020 consensus recommends clopidogrel for cats at increased ATE risk (moderate-to-severe LA enlargement, SEC, low LA appendage velocity, low LA FS%); aspirin is no longer recommended as first-line thromboprophylaxis.
• This is a clinician-facing evidence summary. It is not a treatment protocol; confirm drug selection and monitoring against current ACVIM guidelines and a veterinary formulary.

Clinical facts

• Trial design: FAT CAT was a multicenter, randomized, double-blind, positive-controlled study (clopidogrel vs low-dose aspirin) in cats that had survived at least one cardiogenic ATE event and been stable for ≥1 month.
• Primary endpoint: Time to recurrent CATE or cardiac death (composite endpoint).
• Outcome — clopidogrel arm: Median time to composite endpoint 346 days (95% CI 146-495 days).
• Outcome — aspirin arm: Median time to composite endpoint 128 days (95% CI 58-243 days).
• ATE recurrence: 48% of clopidogrel-treated cats vs 75% of aspirin-treated cats had a recurrent ATE event during follow-up.
• Tolerability: Both drugs were well tolerated; no dose-related toxicity identified.
• Clopidogrel palatability: Cats find clopidogrel unpalatable in tablet form — compounding into capsules or use of pill pockets is typically required for reliable administration.

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What the evidence shows

FAT CAT trial findings

Hogan et al. (2015) conducted the first and to date largest controlled trial of antiplatelet therapy for secondary ATE prevention in cats.¹ Cats were randomized to clopidogrel or low-dose aspirin after surviving a primary ATE event. Clopidogrel was associated with a significantly longer median time to the composite endpoint of recurrent CATE or cardiac death (346 days vs 128 days). Recurrent ATE occurred in 48% of cats on clopidogrel vs 75% on aspirin — a relative risk reduction of approximately 35%. These findings led directly to the ACVIM 2020 consensus recommendation endorsing clopidogrel as the preferred antiplatelet agent for cats at risk of ATE.

Interpreting the absolute benefit (2024 JAVMA analysis)

A 2024 viewpoint by Rishniw in JAVMA offered a detailed re-analysis of the FAT CAT data in the context of primary prevention — the scenario most commonly confronted in clinical practice, where clopidogrel is prescribed to cats with moderate-to-severe HCM who have not yet experienced an ATE event.² The analysis identified two key assumptions: first, that the secondary prevention benefit observed in FAT CAT translates to primary prevention (unproven but clinically assumed); and second, that aspirin has little or no effect on ATE occurrence in cats (supported by earlier uncontrolled case series showing no observable aspirin benefit). Taking the baseline ~10% ATE risk in cats with moderate-to-severe HCM and applying a 35% relative risk reduction, the absolute risk reduction is approximately 3.5%, with a number needed to treat of 23-29 cats to prevent one ATE event. The viewpoint does not argue against clopidogrel use, but emphasizes that clinicians should discuss the moderate effect size and practical administration challenges with clients when making the decision.

ACVIM guideline position

The ACVIM 2020 consensus explicitly recommends clopidogrel in cats considered at risk of ATE (moderate-to-severe LA enlargement, low LA FS%, low LA appendage velocity, SEC; Level of Evidence: medium).¹ For cats at very high risk, the guidelines allow for combination antiplatelet therapy (clopidogrel plus low-dose aspirin or a PO factor Xa inhibitor) at low level of evidence. Aspirin alone is no longer recommended as first-line thromboprophylaxis in cats.

How this fits clinical practice

The FAT CAT trial remains the most rigorous controlled evidence base for feline thromboprophylaxis. Clopidogrel is the correct choice over aspirin. The practical message from the 2024 re-analysis is nuance rather than contraindication: the benefit is real but moderate (NNT ~23-29), and clinicians should have an honest conversation with clients about realistic expectations — clopidogrel does not eliminate ATE risk, it reduces it. The added challenge of administration palatability means that adherence should be discussed proactively. For cats at very high ATE risk (severe LA enlargement, spontaneous echo contrast, prior ATE), the risk-benefit calculation favors treatment more strongly.

Always confirm drug selection, monitoring intervals, and specific ATE risk criteria against current ACVIM guidelines and a veterinary formulary.

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References

1. Hogan DF, Fox PR, Jacob K, et al. 2015. Secondary prevention of cardiogenic arterial thromboembolism in the cat: the double-blind, randomized, positive-controlled feline arterial thromboembolism; clopidogrel vs. aspirin trial (FAT CAT). J Vet Cardiol 17(suppl 1):S306-S317. https://www.sciencedirect.com/science/article/abs/pii/S1760273415000958
2. Rishniw M. 2024. How much protection does clopidogrel provide to cats with hypertrophic cardiomyopathy? J Am Vet Med Assoc 262(10). https://avmajournals.avma.org/view/journals/javma/262/10/javma.24.04.0269.xml

Changelog

• 2026-07-03: First published.