Emergence of Azole-Resistant Aspergillus fumigatus Strains due to Agricultural Azole Use Creates an Increasing Threat to Human Health

Abstract

Aspergillus fumigatus, a ubiquitously distributed opportunistic pathogen, is the global leading cause of aspergillosis and causes one of the highest numbers of deaths among patients with fungal infections [1]. Invasive aspergillosis is the most severe manifestation with an overall annual incidence up to 10% in immunosuppressed patients, whereas chronic pulmonary aspergillosis affects about 3 million, primarily immunocompetent, individuals each year [2]. Three triazole antifungals, namely itraconazole, voriconazole, and posaconazole, are recommended first-line drugs in the treatment and prophylaxis of aspergillosis [3]. However, azole resistance in A. fumigatus isolates is increasingly reported with variable prevalence in Europe, the United States, South America, China, Japan, Iran, and India [4]–[9]. For example, about 10% of strains of A. fumigatus from the Netherlands are itraconazole resistant, and in the United Kingdom, the frequency increased from 0%–5% during 2002–2004 to 17%–20% in 2007–2009 [10]–[13]. In the ARTEMIS global surveillance program involving 62 medical centers, 5.8% of A. fumigatus strains showed elevated MICs to one or more triazoles [5]. Similarly, the prospective SCARE (Surveillance Collaboration on Aspergillus Resistance in Europe) study involving 22 medical centers in 19 countries identified an overall prevalence of 3.4% azole resistance. Azole-resistant A. fumigatus (ARAF) ranged from 0% to 26% among the 22 centres and was detected in 11 (57.9%) of the 19 participating European countries [4 and P.E. Verweij, personal communication]. Interestingly, almost half (48.9%) of the ARAF isolates from the SCARE network in European countries were resistant to multiple azoles and harbored the TR34/L98H mutation in the cyp51A gene [4 and P.E. Verweij, personal communication]. Indeed, multi-azole resistance in A. fumigatus due to the TR34/L98H mutations has become an emerging problem in both Europe and Asia and has been associated with high rates of treatment failures [12]–[14]. Azole antifungal drugs inhibit the ergosterol biosynthesis pathway, specifically the cytochrome p450 sterol 14-α-demethylase encoded by the cyp51A gene, which leads to depletion of ergosterol and accumulation of toxic sterols. The majority of ARAF isolates contain alterations in the target enzyme and the mutated target showed reduced or no binding to the drugs [15]. While most mutations in ARAF isolates were single nucleotide substitutions in the target gene (cyp51A), mutations at other genes such as the cdr1B have also been reported. For example, in the United Kingdom the frequency of ARAF isolates without cyp51A mutations has been reported to be more than 50% [16].