Clinical Signs and Lesions
Clinical signs of chytridiomycosis due to Bd in juvenile and adult anurans may include erythema of ventral surfaces, abnormal posture such as splayed limbs, depression, slow righting reflex, abnormal skin shedding and tetanic spasms upon handling (Berger et al.
2009). Signs generally occur in the terminal stages of disease and correlate with heavy infections, severe skin pathology and loss of plasma electrolytes due to disturbance of epidermal ion transport (Voyles et al. 2009). Therefore most of the course of infection remains subclinical, and dead frogs are often in good body condition. With Bsal, there is also lethargy and excessive skin shedding, but widespread multifocal ulceration is a marked and typical sign (Martel et al. 2013; Van Rooij et al. 2015).Both Batrachochytrium species occur only within epidermal cells, but sporangia of Bd infect the superficial epidermal layers (including the stratum granulosum), whereas Bsal grows in all layers (Berger et al. 2005; Martel et al. 2013). Bd generally has a predilection for ventral skin areas and feet, but Bsal occurs over all the skin including the dorsum. Histopathology of Bd infection includes hyperkeratosis, disordered epidermal cell layers, spongiosis, erosions and occasional ulcerations. Hyperplasia may occur resulting in diffuse or focal thickening, whereas other regions of epidermis may be thin and eroded. Bacteria often colonize the layers of sloughing keratin and grow within “empty” sporangia. A mild inflammatory response may occur as a slight increase in mononuclear cells in the dermis and epidermis but is often negligible (Berger et al. 2009). The deeper infection by Bsal leads to more erosive and ulcerative changes without hyperkeratosis or hyperplasia (Van Rooij et al. 2015).
Electron microscopy revealed a zone of apparently condensed host cytoplasm, up to 2.5 μm thick, around some Bd sporangia that appeared fibrillar and excluded organelles (Berger et al. 2005). It is possible this host reaction provides protection of sporangia from antimicrobials or immune response. Keratinization appears to occur prematurely in infected cells, compared with uninfected cells in the same epidermal layer (Berger et al. 2005).
Specific internal lesions are not observed in frogs with severe chytridiomycosis, besides some terminal changes. In an Australian survey, concurrent diseases were diagnosed in 12% of frogs with severe chytridiomycosis, but most frogs had no other specific lesions (Berger et al. 2004).
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