Objectives: Bovine mammary protothecosis is a serious pathology that entails high economic losses in the dairy industry. The disease, the frequency of which has recently been increasing worldwide, is caused by unicellular, achlorophyllous, yeast-like algae of two species: Prototheca zopfii and Prototheca blaschkeae. The objective of this study was to investigate the in vitro activity of a panel of conventional antifungal drugs against Prototheca spp. isolates.
Methods: A total of 144 P. zopfii genotype 2 and P. blaschkeae strains isolated from milk of mastitic cows were subjected to drug susceptibility testing by Etest methodology.
Results: Five out of ten antifungal drugs tested exhibited no activity against Prototheca spp. isolates. The best activity against Prototheca spp. was demonstrated by amphotericin B (MIC₉₀ of 1.5 mg/L). The MICs differed significantly (P < 0.01) between P. zopfii genotype 2 and P. blaschkeae, with the latter species being more susceptible to amphotericin B and azoles. Marked differences (P < 0.05) in azole and amphotericin B activities were noted among Prototheca spp. isolates originating from different European countries. Based on the correlation coefficients, a considerable cross-interaction was found among MICs of azoles and between MICs of azoles and amphotericin B for Prototheca spp. (P < 0.03).
Conclusions: This study represents the largest, cross-European evaluation of antifungal activity against Prototheca spp. to date. The activity of amphotericin B against Prototheca spp. validates its potential use as a therapeutic agent against bovine protothecosis. For laboratory testing of drug activity against Prototheca spp., the Etest method is encouraged, due to its technical simplicity, rapidity and high intra- and inter-laboratory reproducibility.