Epidemic and Epizootic Expansions
A small number of opportunistic and pathogenic species or species groups tend to occur in the form of outbreaks, epidemics (in humans), or epizootics (in animals) (Fisher et al. 2012).
Fungal epidemics have already been reported in the early twentieth century. Beurmann and Gougerot (1912) described an expansion of sporotrichosis in France, caused by Sporothrix schenckii. Zhang et al. (2015) demonstrated that each closely related species of Sporothrix has a consistent pattern of outbreaks. Commonsource outbreaks are usually reported, which die out when the environmental conditions no longer support growth of the fungus. An epidemic of sporotrichosis by S. schenckii among miners in South Africa involved more than 3000 cases and subsequently disappeared when the wood that was used in the mines was treated with preservatives (Govender et al. 2015). When nonliving biological material is the source of infection, the epidemic is referred to as a sapronosis. Infected vertebrates do not spread the fungus, and thus sapronoses are caused by non-transmissible opportunistic fungi (Fig. 1.1). Sapronotic outbreaks are linked to a common source of infection, and prevention thus requires physical removal of the infective material.The epidemiological pattern is fundamentally different when fungal pathogens are transmissible between animals including humans. The agent of chytridiomycosis causing global frog decline, Batrachochytrium dendrobatidis, efficiently utilizes the host for its dispersal (Chap. 14). The fungus produces zoosporangia in frog skin, which release massive amounts of zoospores into the environment and contaminate new susceptible hosts. The time of transmission is short, and the environmental phase does not require growth of the thallus but just zoospore dispersal. Transmission can thus be nearly direct, from host-to-host, maximally with a short intermittent phase of motile spores in water.
Contagious individuals potentially infect multiple individuals, and hence direct transmission often leads to exponentially expanding epizootics (Fig. 1.1). The fungus completes its life cycle on the host and is thus considered as a pathogen. This kind of mycosis is transmitted directly between living hosts. According to the official definition from the World Health Organization, zoonoses are diseases and infections that are naturally transmitted between vertebrate animals and humans and vice versa. Among transmissible fungal pathogens, a few species should be considered as zoonotic: Sporothrix brasiliensis (from cats) (Chap. 10) and some species of dermatophytes, e.g., Microsporum canis (from cats), Trichophyton verrucosum (from cattle), and T. benhamiae (from guinea pigs) (Chap. 3).Environmental pathogens have a double life cycle, combining characteristics of both groups above. Part of their life cycle is completed in the environment, while they also have a reservoir in the vertebrate host. Infection takes place by propagules from the environment, and expansions are thus classified as sapronoses. After infection and completion of a pathogenic life cycle, fungal cells should be able to escape from the dead animal body to return to the environmental habitat—although this has rarely been proven. When sapronoses are caused by environmental pathogens, where the environmental habitat is part of the fungus' natural life cycle, the source will be more difficult to eradicate.
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