Topical treatment of equine sarcoids with imiquimod 5% cream or Sanguinaria canadensis and zinc chloride – an open prospective study.

This open prospective study in 25 horses compared two topical treatments for equine sarcoids and found high complete remission rates, with imiquimod 5% performing slightly better and relapses being relatively uncommon. There was no evidence that treating some lesions produced systemic benefits in untreated distant sarcoids, and local inflammation during therapy was common.

Background and Rationale

• Equine sarcoids are the most common skin tumors in horses and are notoriously difficult to treat, with frequent recurrences regardless of modality.
• Imiquimod 5% is an immune-response modifier (TLR7 agonist) that promotes local anti-tumor immunity and is used for certain dermatologic neoplasms.
• Sanguinaria canadensis combined with zinc chloride is a topical escharotic formulation that induces local tissue destruction and inflammation; it is used empirically against sarcoids.
• The study aimed to evaluate the efficacy and safety of both agents and to test whether topical treatment of some lesions could trigger a systemic immune effect leading to improvement of untreated sarcoids on the same horse.

Study Design and Methods

• Design: Open-label, prospective clinical study without blinding.
• Population: 25 client-owned horses with a total of 164 tumors (sarcoids).
• Tumor allocation: 57 tumors were selected for treatment; 107 were left untreated to monitor spontaneous changes and potential systemic effects.
• Diagnosis: At least one lesion per horse was confirmed as a sarcoid via skin biopsy; remaining lesions were diagnosed clinically as likely sarcoids.
• Interventions:
  • Imiquimod 5% cream (A): applied three times per week.
  • Sanguinaria canadensis + zinc chloride (X): daily application for 6 days (initiation), then every fourth day thereafter.
• Treatment duration: Continued until clinical remission or a maximum of 45 weeks.
• Follow-up: Long-term, with a mean of 34 months, to assess durability and relapse.
• Biopsy timing: Repeat biopsies of treated sarcoids were taken before treatment termination and again at follow-up when owners consented, to corroborate clinical clearance histologically.
• Outcomes:
  • Efficacy: Complete remission (CR) of treated tumors; relapse rate during follow-up.
  • Systemic effect: Any change in untreated distant sarcoids consistent with therapy-induced systemic immunity.
  • Safety: Local and systemic adverse events during therapy.

Key Findings

• Complete remission rates:
  • Imiquimod 5% (A): 84.4% of treated tumors achieved CR.
  • Sanguinaria + zinc chloride (X): 75.0% achieved CR.
• Relapse during follow-up:
  • Imiquimod (A): 7.3% relapsed.
  • Sanguinaria + zinc chloride (X): 21.4% relapsed.
• Untreated tumors:
  • Spontaneous remission occurred in 1.9% of untreated lesions.
  • No evidence of a systemic effect on distant untreated sarcoids was detected.
• Safety and tolerability:
  • Varying degrees of local inflammatory reaction were common with both treatments.
  • Both regimens were judged effective and safe overall within the study context.
• Prognostic observations:
  • Smaller tumors responded more favorably to treatment.
  • Relapse was not observed in sarcoids that had repeat biopsies prior to ending treatment, suggesting histologic confirmation of clearance may reduce recurrence risk.

Interpretation and Clinical Significance

• Both topical regimens can induce high rates of complete remission in equine sarcoids under prolonged, protocolized application.
• Imiquimod 5% showed numerically higher complete remission and lower relapse rates than the Sanguinaria + zinc chloride formulation, suggesting a potential efficacy and durability advantage.
• The absence of improvement in untreated lesions indicates that the benefits of these topicals are local rather than systemic; each sarcoid likely requires direct treatment.
• Expected local inflammation appears to be part of the therapeutic process for both agents and was generally manageable within the study.
• Smaller lesions’ better outcomes underscore the value of early detection and early intervention.
• Repeat biopsy before stopping therapy may help confirm true clearance and minimize relapse, supporting a treat-to-histology strategy when feasible.

Strengths and Limitations

• Strengths:
  • Prospective design with predefined regimens and outcomes.
  • Long mean follow-up (34 months) enabling meaningful relapse assessment.
  • Within-horse untreated controls allowed evaluation of spontaneous remission and systemic effects.
• Limitations:
  • Open-label without randomization or blinding, introducing potential bias.
  • Not all lesions were biopsy-confirmed, leaving some diagnostic uncertainty.
  • Unequal and unspecified distribution of treated tumors between groups limits precise comparative inference.
  • Multiple lesions per horse may create intra-individual correlation not fully addressed in simple percentages.
  • The intensity of local reactions and any analgesia or supportive care protocols were not detailed, which affects generalizability and welfare considerations.

Practical Takeaways for Veterinarians

• Consider topical imiquimod 5% as a first-line option for small to moderate sarcoids given high remission and relatively low relapse rates, recognizing that courses may extend for many months.
• Sanguinaria + zinc chloride can be an alternative when imiquimod is unsuitable, with good remission rates but higher observed relapse; expect and manage local escharotic/inflammatory reactions.
• Treat each lesion directly; do not rely on systemic spillover effects to clear distant sarcoids.
• Set owner expectations for prolonged treatment, frequent local reactions, and the importance of adherence to application schedules.
• When possible, obtain a confirmatory biopsy before declaring completion of therapy to reduce the likelihood of relapse.
• Prioritize early intervention for smaller lesions to maximize response and minimize morbidity.

Future Research Directions

• Randomized, blinded head-to-head trials with standardized dosing, analgesia protocols, and lesion stratification by type/size/location.
• Time-to-remission analyses and quality-of-life/pain assessments during treatment.
• Immune profiling and biomarkers to predict responders to imiquimod or escharotic therapy.
• Evaluation of combination approaches (e.g., surgical debulking plus topical therapy) and sequencing strategies.
• Investigation of adjuvant systemic immunotherapies to augment local treatment and reduce multicentric burden.