Differences in the in vitro susceptibility of mono-eukaryotic cultures of Histomonas meleagridis, Tetratrichomonas gallinarum and Blastocystis sp. to natural organic compounds.

Currently, all pharmaceuticals for the treatment or prophylaxis of blackhead disease (histomonosis) caused by the flagellate Histomonas meleagridis are banned from the market. Consequently, great interest exists on the finding of alternative drugs for the abatement of histomonosis. In this study, carvacrol, Cassia oil, an essential oil (EO) mixture containing thyme and rosemary EO and a Quillaja saponaria saponin were examined using in vitro assays for antiprotozoal and antibacterial activity testing established against cloned xenic cultures of different isolates of Histomonas meleagridis, Tetratrichomonas gallinarum and Blastocystis sp. Whereas similar minimal lethal concentrations (MLCs) of five Histomonas isolates were obtained for both carvacrol and the EO mixture as well as for the saponin, significantly different MLCs were observed for them with Cassia oil, ranging from 0.25 up to 0.50 microl/ml. Testing the Blastocystis isolates, different MLCs were obtained for all substances, whereas the Tetratrichomonas gallinarum isolates showed identical susceptibilities. The effects are independent of the bacteria, underlining the need of well-defined protozoan cultures for these investigations.