Histofarcin Skin Test and an In-Vitro Antifungal Effects of Croton Macrostachyus against Mycelial Form of Histoplasma capsulatum Variety Farciminosum isolated from  Horse in Adama City, Ethiopia

Background: Epizootic Lymphangitis (EL) is a chronic debilitating disease of equines caused by Histoplasma capsulatum variety farciminosum (HCF). It is one of the major equine diseases with economics and equines welfare consequences. An experimental study design was conducted from Nov, 2021 to June, 2022 to assess the histofarcin skin test and evaluate the antifungal efficacy of Croton macrostachyus against the mycelium form of HCF. The histofarcin antigen used for histofarcin skin test was prepared from field isolates of HCF, identified from clinical cases of EL in cart horses from Adama City. Result: Then, histofarcin skin test was evaluated on 60 horses, 15 clinically infected cases, and 45 healthy horses. Each horse was injected with 0.1ml of the histofarcin skin test antigen intradermally in the middle of the neck. The thickness of the skin was measured before the injection and 24 hrs post injection using Bar Knight Mc Lintock(McLK)-P0005) caliper. The sensitivity and specificity of histofarcin skin test were 100% and 95% respectively. An in vitro growth inhibitory effect of Croton macrostachyus against the mycelia form of HCF was evaluated. Two-fold serial dilutions of the methanol extracts of the plant were prepared and assessed. A culture media with no antifungal agent and media containing ketoconazole served as a negative and positive control, respectively. Conclusion: The results showed that Croton macrostachyus have an inhibitory effect at concentrations ranging from 2.5mg/ml–10mg/ml. Thus, the minimum inhibitory concentration of the plant extract was found to be 2.5mg/ml. In conclusion, this study demonstrated that histofarcin skin test is a potential diagnostic tool for screening EL. Also, the methanol extracts of Croton macrostachyus showed good in-vitro growth inhibitory activity against the mycelial phase of HCF. Therefore, Validation of the performance of the histofarcin skin test assessment is necessary. Also, further investigation of the effects of C. macrostachyus on the yeast form of HCF is needed in addition to Phytochemial analysis (fractionation) and evaluation of the toxicity of the plant.