E45 - Confirmed or Suspected Alloimmune Thrombocytopenia in Mule Foals

Abstract: Background – Alloimmune thrombocytopenia is a risk for neonatal mule foals as evidenced by experimental induction. There are little data on the prevalence and clinical findings of naturally occurring alloimmune thrombocytopenia in mule foals.

Hypothesis/objectives – To monitor platelet counts in mule foals over time and document development of thrombocytopenia, flow cytometry results, clinical signs and treatment. The hypothesis was that alloimmune thrombocytopenia is common among mule foals, but can be subclinical.


Animals – Eighteen neonatal mule foals.

Methods – Prospective longitudinal study. Signalment, immunoglobulin source and plasma concentration, clinical signs of and treatment for thrombocytopenia, comorbidities, and serial platelet counts were recorded. Thrombocytopenia was defined as platelet counts < 100,000/µL. Results are expressed as median (range).

Results – Twelve of 18 foals (67%) developed thrombocytopenia, with lowest platelet counts of 11,000 (4,000-50,000)/µL on day 1-5 of life. Six of 12 thrombocytopenic foals (50%) developed clinical signs consistent with thrombocytopenia, and 3/6 (50%) received specific treatment. Eight of 11 tested thrombocytopenic foals were positive for platelet bound antibodies on flow cytometry. Clinically affected foals had lower platelet counts than those without clinical signs (9,900 µL vs 78,000/µL, P = 0.01). Platelet counts were documented to return to reference values by 12 (7-50) days. No foals were euthanized or died due to thrombocytopenia.

Conclusions and clinical importance – Based on this study, approximately 67% of mule foals develop thrombocytopenia, however up to 50% remain subclinical, even with severe thrombocytopenia. Affected foals have a good prognosis and platelet counts eventually normalized.