Hyalohyphomycosis is infection due to nonpigmented (hyaline) septate fungi that form hyphal elements in tissue. These organisms are found worldwide and predominantly affect humans, dogs, cats, and horses. The mode of transmission is usually cutaneous inoculation of environmental organisms, or possibly inhalation.
Examples of genera causing hyalohyphomycosis in humans and other animals include Acremonium, Chrysosporium, Fusarium, Oxyporus, Paecilomyces, Scedosporium, Sagenomella, Phialosimplex, Rasamsonia, Geomyces, Pupureocillium, and Scopulariopsis, among others. (Hyalohyphomycosis does not include the genera Aspergillus or Penicillium or the organisms causing entomophthoromycosis and mucormycosis, previously termed zygomycetes.)
Hyalohyphomycosis is far less common than phaeohyphomycosis, which is discussed in its own section to follow.
Clinical Findings of Hyalohyphomycosis in Animals
In hyalohyphomycosis, lesions range from local cutaneous, subcutaneous, corneal, or nasal mucosal disease to disseminated disease involving the lungs and multiple other organ systems, including osteomyelitis/discospondylitis, bone marrow involvement, and CNS disease.
Patients presenting with cutaneous lesions should be evaluated for systemic disease and immunosuppression. Patients being treated with immunosuppressive drugs may develop cutaneous hyalohyphomycosis and not have lesions at other sites, whereas immunocompetent patients that develop hyalohyphomycosis most often do have disseminated disease, or at least disease that is not confined to the skin.
Horses frequently get fungal keratitis secondary to a superficial corneal injury; disease usually remains confined to the cornea. Histologic lesions of hyalohyphomycosis consist of pyogranulomatous inflammation with intralesional nonpigmented septate hyphae that branch at acute angles, although some fungi can produce yeastlike forms.
Diagnosis of Hyalohyphomycosis in Animals
Cytologic evidence needed
Molecular identification superior to morphologic diagnosis
The causative fungi cannot be identified by their histologic features in tissues; culture isolation and/or PCR assay is required for diagnosis of hyalohyphomycosis. Isolation of a fungus from skin, nasal swab, or exudate should prompt histologic or cytologic evaluation because all of these fungi are common environmental organisms and are often viewed as contaminants.
Treatment of Hyalohyphomycosis in Animals
Species identification to select the most appropriate treatment
Amphotericin B or azoles may be attempted, but many hyalohyphomycete species are resistant
The hyalohyphomycetes have variable susceptibility to many antifungal drugs, including amphotericin B and azoles; molecular identification of species allows the most appropriate empiric treatment selection. Patients on immunosuppressive drugs should be tapered off those drugs, if possible.
Surgical removal with or without azole antifungal treatment is the treatment of choice for local disease. Disseminated disease typically carries a guarded to poor prognosis. Treatment with newer azole antifungals and/or amphotericin B lipid complex may be attempted if speciation is not pursued. Extended treatment times (> 6 months) are often required.
In horses with fungal keratitis, frequent topical treatment with voriconazole or natamycin is recommended combined with appropriate debridement, though prognosis is still guarded.
