Hyalohyphomycoses (other than Aspergillosis and Penicilliosis)
Authors: Torres Harrys A , Kontoyiannis Dimitrios P 
Published: October 2023

Abstract
Hyalohyphomycoses refer to human infections caused by soil-dwelling and plant saprophytic moulds (Ajello, 1986). Hyalohyphomycosis encompasses a loose artificial classification system since it does not refer to a specific taxonomic classification (Rippon, 1988a). Agents of hyalohyphomycosis include nonmelanin-producing, nondematiaceous moulds (Anaissie et al, 1989) whose typical appearance in histopathologic sections consists of colorless, hyaline, or light-colored septate hyphae. These hyphae are either branched or unbranched, and occasionally they are toruloid (Ajello, 1986; Anaissie et al, 1989; Liu et al, 1998). Table 16–1 lists the fungal species reported to cause hyalohyphomycosis. Important human pathogens included in this group are Aspergillus, Fusarium, Scopulariopsis, Pseudallescheria, and Scedosporium (Ajello, 1986; Anaissie et al, 1989). The fungi are identified at the genus and species levels largely on the basis of morphology of their reproductive structures, including phialides (fertile cells), and conidia (spores) (Liu et al, 1998). The general characteristics of the heterogeneous group of fungi-causing hyalohyphomycosis are shown in Table 16–2. Although several nondematiaceous moulds can cause opportunistic fungal infections in humans, this chapter focuses only on those that have been recognized as emerging pathogens. More common causes of hyalohyphomycosis, such as aspergillosis and penicilliosis, are discussed in Chapters 14 and 23, respectively. Epidemiology. Fusarium derives its name from its fusiform conidia (Young et al, 1978). Long been known as plant pathogens, these ubiquitous moulds are common in decomposing organic matter and soil (Anaissie et al, 1988; Walsh and Groll, 1999). Fusarium species cause fusariosis, an increasingly recognized life-threatening mycotic infection worldwide (Boutati and Anaissie, 1997; Walsh and Groll, 1999; Marr et al, 2002). Of note, the epidemiological distribution of fusariosis is not homogeneous, since 50% to 85% of the cases have been reported in the United States (Martino et al, 1994; Girmenia et al, 2000). It is unclear whether this reflects a unique ecological niche for these fungi or is at least partially due to the increased recognition of this infection in the United States. Outside the United States, most cases of fusariosis have been reported in the Mediterranean (e.g., France, Italy) or in tropical countries (e.g., Brazil) (Guarro and Gene, 1995; Girmenia et al, 2000). Similarly, a heterogeneous distribution of cases of fusariosis have been described in different cancer centers, with most cases reported from only a few centers within the United States (Girmenia et al, 2000). In fact, Fusarium species have emerged in some tertiary care cancer centers as the second most common filamentous fungal pathogen after Aspergillus (Walsh and Groll, 1999). Indeed, in a recent small study from a Brazilian cancer center, Fusarium species were identified as the most common mould associated with true fungemia in bone marrow transplant (BMT) recipients (Trabasso et al, 2001). The portals of entry for Fusarium infection include the aerodigestive tract and the skin (via vascular catheters, periungual regions, or burns) (Richardson et al, 1988; Merz et al, 1988; Anaissie et al, 1989; Martino et al, 1994; Walsh and Groll 1999). The predominant mode of infection is inhalation into the lungs or upper airways (Fridkin and Jarvis, 1996). Therefore, not surprisingly, most cases of fusariosis are considered community acquired. However, Fusarium species can also cause nosocomial infections, such as postoperative endophthalmitis or peritonitis associated with dialysis catheters (Young et al, 1984; Fridkin and Jarvis, 1996). The water distribution system has been implicated recently as a reservoir of Fusarium species (Squier et al, 2000; Anaissie et al, 2001). In contrast, others have reported that the most likely source of Fusarium infection is the external environment rather than a nosocomial source (Raad et al, 2002). More research is needed in this controversial area.