Equine practitioners routinely manage horses with acute or ongoing hemorrhage—whether from trauma, guttural pouch mycosis, periparturient complications, or perioperative bleeding. In these situations, adjunctive hemostatic therapies such as formalin, aminocaproic acid (ACA), and Yunnan Baiyao are frequently administered to stabilize patients while definitive treatment is pursued. Despite their widespread use, objective evidence supporting these agents in horses has been limited.
This work* evaluated how these commonly used drugs influence coagulation in horses that are healthy, using both standard laboratory assays and point-of-care viscoelastic testing.
Study Overview
Eight healthy adult mares were enrolled in a prospective, randomized crossover design. Each horse received:
- Formalin: 50 mL of 10% buffered formalin diluted in 1 L saline IV
- Aminocaproic acid: 40 mg/kg IV
- Yunnan Baiyao: 4 g orally, plus the traditional “red pill”
Each treatment was administered during a separate study week with a two-week washout period. Horses served as their own controls.
Blood samples were collected at baseline, one hour post-administration, and at each drug’s typical redosing interval. Coagulation was evaluated using standard laboratory testing and VCM Vet™ viscoelastic analysis, allowing direct comparison of each agent’s effect on clot formation and stability.
Key Findings
Formalin
- Moderate, significant increases in clot firmness and stability
- Findings support long-standing clinical impressions that low-dose formalin can enhance hemostasis
- Decreases in platelet count, red blood cell indices, and hematocrit were also observed, consistent with known hemolytic and platelet-activating effects
Aminocaproic Acid
- Significant increases in clot strength and early clot development
- Shortened partial thromboplastin time (PTT) and decreased fibrinogen, suggesting a shift toward hypercoagulability
- Mechanism aligns with aminocaproic acid’s role in inhibiting fibrinolysis and stabilizing formed clots
Yunnan Baiyao
- No significant changes in viscoelastic parameters, platelet count, fibrinogen, prothrombin time, or PTT
- In healthy, non-bleeding horses, Yunnan Baiyao did not produce measurable effects on coagulation
Safety
- No major adverse effects were observed with any treatment at the doses used
Clinical Takeaways for Practitioners
- Aminocaproic acid appears to be the most consistently effective adjunctive agent, particularly when fibrinolysis is suspected.
- Low-dose formalin may improve clot quality but should be used judiciously due to its narrow therapeutic margin and potential hematologic effects.
- Yunnan Baiyao showed no measurable impact on coagulation in healthy horses, prompting reconsideration of its routine use in non-bleeding patients.
- Point-of-care viscoelastic testing remains a valuable tool for rapid, whole blood assessment in emergency settings.
These findings help refine evidence-based hemostatic strategies in equine practice while reinforcing the essential role of supportive care and appropriate diagnostics.
Looking Ahead
These results reflect drug effects in healthy horses and may not fully represent responses during active hemorrhage, systemic disease, or repeated dosing.
Ongoing research is focused on:
- Evaluating these agents in clinically bleeding horses
- Understanding cumulative or repeated dose effects
- Integrating pharmacologic therapies with cornerstone treatments such as transfusion and fluid resuscitation
While hemostatic agents can support clot formation, they remain adjuncts—not substitutes—for definitive hemorrhage control.
* This article was presented as a research abstract at the 2025 ACVIM Forum in Louisville, Kentucky. Its authors are Drs. Ana D. Nobrega, Ina Mersich, Rebecca C. Bishop, Annette M. McCoy, Scott M. Austin, Anne M. Barger, and Pamela Anne Wilkins.
