Zoospore-based in vitro susceptibility of Pythium insidiosum to triamcinolone acetonide and interactions with antimicrobial agents.

Pythiosis is a severe infectious disease caused by the oomycete Pythium insidiosum, affecting both humans and animals and often associated with poor therapeutic outcomes. Although triamcinolone acetonide has been used as an adjunctive therapy for equine pythiosis, its antimicrobial activity remains controversial. This study evaluated in vitro susceptibility of 15 clinical P. insidiosum isolates to triamcinolone acetonide, alone and in combination with azithromycin, itraconazole, or amorolfine hydrochloride, using zoospore-based broth microdilution assays. Triamcinolone acetonide and itraconazole did not exhibit in vitro activity at the concentrations evaluated, whereas azithromycin and amorolfine retained anti-P. insidiosum activity. The interactions between triamcinolone acetonide and each antimicrobial agent could not be classified because none of the tested combinations inhibited P. insidiosum growth. Scanning electron microscopy demonstrated preserved hyphal morphology following exposure to triamcinolone acetonide. Our findings suggest that triamcinolone acetonide may have interfered with the activity of azithromycin and amorolfine. However, further studies are required to confirm this observation. Therefore, caution is warranted when considering combined therapeutic strategies involving triamcinolone acetonide and antimicrobial agents for the treatment of pythiosis.