Introduction
Cryptococcosis is a primary or opportunistic, life-threatening fungal infection of worldwide distribution caused by members of the genus Cryptococcus, viz., Cryptococcus neoformans and Cryptococcus gattii species complexes (Kwon-Chung et al.
2017). The major environmental sources of Cryptococcus comprise soil contaminated with avian (most often pigeon) guano (C. neoformans) or plant debris and decaying wood, especially in tree hollows protected from light (C. gattii) (Sorrell 2001; Spickler 2013). The presence of virulent isolates of C. neoformans in pigeon excrement was reported by Passoni (1999).In humans, C. neoformans is responsible for 98% of cryptococcal infections in patients with AIDS (Litvintseva et al. 2005). Most cryptococcal infections are caused by C. neoformans var. grubii, while C. neoformans var. neoformans causes disease exclusively in immunocompromised individuals (Litvintseva et al. 2005). In contrast, the sibling species C. gattii causes cryptococcosis predominantly but not exclusively in immunocompetent individuals (Casadevall and Perfect 1998; Byrnes et al. 2010).
Both species of Cryptococcus may additionally cause cryptococcosis in a wide range of animals, from lower invertebrates such as soil-dwelling amoebae, nematodes, cockroaches, and mites to higher mammals (Steenbergen et al. 2001; Mylonakis et al. 2002). Naturally occurring cryptococcosis has also been documented in amphibians, reptiles, birds, and mammals (Refai et al. 2014). Cryptococcosis has been recorded in cattle (Emmons 1952), horses and dogs (Barclay 1979; Sutton 1981), cats (Trivedi et al. 2011), foxes and ferrets (Lewington 1982; Morera et al. 2011), goats (Chapman et al. 1990), monkeys (Pal et al. 1984), cheetahs and pigs (Ajello 1958), llamas and alpacas (Stephen et al. 2002), koalas and other marsupials (Krockenberger et al. 2002), and various marine mammals (Duncan et al. 2006; MacDougall et al.
2007; Rotstein et al. 2010). Interestingly, the clinical manifestations of cryptococcosis in various animals may differ from those in humans according to animal species and site and extent of colonization. For example, in outbreaks of mastitis in ruminants, the clinical signs may include anorexia, decreased milk production, and enlargement of the supramammary lymph nodes (Lin and Heitman 2006).Moreover, some animals act as reservoirs or passengers for Cryptococcus spp., representing a constant source of infection in the environment. Cryptococcus spp. have been isolated from asymptomatic animals as well as clinical cases. C. neoformans var. grubii has been shown to be a transient colonizer of the nasal mucus of cats, dogs, koalas, and ferrets (Malik et al. 1995, 1997, 2002; Malik et al. 2006a; Sorrell et al. 1996; Lester et al. 2004; Morera et al. 2014; Danesi et al. 2014a). Furthermore, nasal colonization of wild and domestic animals has been suggested to be a good indicator of the environmental presence of C. gattii (Duncan et al. 2006).
Among animals, cryptococcosis is most commonly observed in cats (Pennisi et al. 2013), with the involvement of the upper and/or lower respiratory tract, subcutaneous granulomata (typically contiguous with the sinonasal cavity), and disseminated infections (Sorrell 2001; Duncan et al. 2006; McGill et al. 2009; Headley et al. 2016). Clinically, multifocal skin involvement is a typical marker for widespread hematogenous dissemination to multiple tissues. Animals, including wild and domesticated cats and wildlife, share urban and rural habitats with people, and often one or more species serve as potential sentinel hosts for human exposure (Danesi et al. 2014b).
The main epidemiological findings from human and animal studies of cryptococcosis are summarized in Fig. 12.1.
12.2
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