In vitro and in vivo Sensitivity of a Flagellated Protozoan, Histomonas meleagridis, to Metronidazole and Nitarsone

Histomoniasis or blackhead disease is a life-threatening disease of turkeys that is caused by a flagellated protozoan, Histomonas meleagridis. Currently metronidazole and nitarsone are used commonly against blackhead disease in Pakistan. Initially we tested the sensitivity of H. meleagridis collected from field outbreak in turkey to metronidazole (Flagyl®) and nitarsone (Histostat-50TM®) using in vitro culture conditions. H. meleagridis at 200 and 500 ppm of nitarsone showed reduced growth in comparison to respective untreated culture. However, there was no inhibition of growth H. meleagridis treated with metronidazole at 200 ppm, while reduced growth was seen at 500 ppm. To test the sensitivity of H. meleagridis strain to nitarsone and metronidazole in vivo, poults offered Flagyl® and Histostat® premix diet or a control diet were inoculated cloacally with H. meleagridis. The Histostat® treated group of birds showed significant difference while metronidazole-treated group showed non-significant difference compared to that of infected control group especially at 200 ppm when measuring weight gain and liver and cecal lesions scores. Histomonas meleagridis were re-isolated from the Flagyl® fed chicks and subjected to the in vitro assay. Isolated H. meleagridis maintain their resistance to metronidazole at 200 ppm. This study demonstrates that field isolate of H. meleagridis has acquired partial resistance to metronidazole. H. meleagridis strains currently present in Pakistan seems to be more sensitive to nitarsone than metronidazole; hence metronidazole should be replaced with nitarsone for better prevention and control of black head disease in poultry.