Phagocytosis is an elementary function of neutrophils, monocytes and tissue macrophages. The process on its own as well as its efficiency is significantly limited in case of Streptococcus equi infections in horses. Strangles diagnosis and poor effectiveness of treatment is due to difficulties in identification of asymptomatic hosts. The aim of this study was to compare the course of neutrophil and monocyte phagocytosis in clinically healthy horses after Streptococcus equi infection versus control group. The phagocytosis activity was determined by means of PhagoTest^® and flow cytometry. The blood was obtained from 13 mares recovered from streptococcal infection caused by S. equi and 12 mares which never demonstrated clinical symptoms of this disease. A statistically significant difference was shown in blood monocytes concentration in control group. There were no statistically significant differences between the amount of lymphocytes and neutrophils. Lower phagocytosis activity (fluorescence) was observed among neutrophils (P = 0.049) and monocytes (P = 0.006) in recovered mares. Moreover, the percentage of eosinophils was also lower in this group, while the chemotaxis was enhanced. Limited immunological response in the recovered mares may be due to asymptomatic carrier state of S. equi. Therefore, the determination of neutrophil and monocyte phagocytic activity may be useful in diagnosis of asymptomatic S. equi infections.

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