There are few organic substrates in man's environment which are free from fungi. Some of these saprophytic fungi can adapt to the parasitism of man and animals. The fungi which can cause systemic mycoses in animals are normally saprophytes which grow in soil which is enriched buy humus, decaying vegetation, or nitrogenous waste such as fecal material of animals and birds.
Twedt, David C. Jr. and Graham, David L.
"Systemic Mycoses in Small Animals,"
Iowa State University Veterinarian: Vol. 34
, Article 4.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol34/iss3/4