Multi-locus DNA sequence analysis, antifungal agent susceptibility, and fungal keratitis outcome in horses from Southeastern United States

Abstract

Morphological characterization and multi-locus DNA sequence analysis of fungal isolates obtained from 32 clinical cases of equine fungal keratitis (FK) was performed to identify species and determine associations with antifungal susceptibility, response to therapy and clinical outcome. Two species of Aspergillus ( A . flavus and A . fumigatus ) and three species of Fusarium ( F . falciforme , F . keratoplasticum , and F . proliferatum ) were the most common fungi isolated and identified from FK horses. Most (91%) equine FK Fusarium nested within the Fusarium solani species complex (FSSC) with nine genetically diverse strains/lineages, while 83% of equine FK Aspergillus nested within the A . flavus clade with three genetically diverse lineages. Fungal species and evolutionary lineage were not associated with clinical outcome. However, species of equine FK Fusarium were more likely ( p = 0.045) to be associated with stromal keratitis. Species of Aspergillus were more susceptible to voriconazole and terbinafine than species of Fusarium , while species of Fusarium were more susceptible to thiabendazole than species of Aspergillus . At the species level, A . fumigatus and A . flavus were more susceptible to voriconazole and terbinafine than F . falciforme . Natamycin susceptibility was higher for F . falciforme and A . fumigatus compared to A . flavus . Furthermore, F . falciforme was more susceptible to thiabendazole than A . flavus and A . fumigatus . These observed associations of antifungal sensitivity to natamycin, terbinafine, and thiabendazole demonstrate the importance of fungal identification to the species rather than genus level. The results of this study suggest that treatment of equine FK with antifungal agents requires accurate fungal species identification.

In vitro susceptibility of Aspergillus and Fusarium associated with equine keratitis to new antifungal drugs

“…Fungal keratitis is one of the most common causes of blindness in horses of the Southeastern USA and is a widespread disease in horses from all states east of the Rocky Mountains. 1,2 The most common causal organisms of EFK are the filamentous fungi Aspergillus and Fusarium. [1][2][3] These fungi represent a component of the normal ocular surface microbiome and are common soil saprobes and plant pathogens, and opportunistic when invading the cornea in EFK.…”

“…1,2 The most common causal organisms of EFK are the filamentous fungi Aspergillus and Fusarium. [1][2][3] These fungi represent a component of the normal ocular surface microbiome and are common soil saprobes and plant pathogens, and opportunistic when invading the cornea in EFK. 2 Once EFK is diagnosed, treatment is generally the same for all horses regardless of fungal species present.…”

“…[1][2][3] These fungi represent a component of the normal ocular surface microbiome and are common soil saprobes and plant pathogens, and opportunistic when invading the cornea in EFK. 2 Once EFK is diagnosed, treatment is generally the same for all horses regardless of fungal species present. Using this approach, treatment results and outcome in EFK remain unacceptable.…”

“…In two recent studies of EFK, 60% of horses did not respond to antifungal medical therapy and required surgery, with up to 37% of eyes resulting in enucleation. 1,2 Available antifungal drugs for EFK are few and limited primarily to natamycin, voriconazole, amphotericin B, itraconazole, and fluconazole. No new antifungal drugs have been introduced for treatment of EFK since the use of voriconazole was described nearly 15 years ago.…”

“…4 Voriconazole is now used worldwide for EFK, but recent studies have indicated development of resistance to voriconazole by filamentous organisms, including species of Fusarium. 2,5,6 Therefore, new effective antifungal drugs and improved selection of these drugs for clinical cases of EFK are desperately needed.…”