Clinical signs, pathology and dose-dependent survival of adult wood frogs, Rana sylvatica, inoculated orally with frog virus 3 (Ranavirus sp., iridoviridae)

Amphibian populations suffer massive mortalities from infection with frog virus 3 (FV3, genus Ranavirus, family Iridoviridae), a pathogen also involved in mortalities of fish and reptiles. Experimental oral infection with FV3 in captive-raised adult wood frogs, Rana sylvatica (Lithobates sylvaticus), was performed as the first step in establishing a native North American animal model of ranaviral disease to study pathogenesis and host response. Oral dosing was successful; LD50 was 10^{2.93 (2.42–3.44)} p.f.u. for frogs averaging 35 mm in length. Onset of clinical signs occurred 6–14 days post-infection (p.i.) (median 11 days p.i.) and time to death was 10–14 days p.i. (median 12 days p.i.). Each tenfold increase in virus dose increased the odds of dying by 23-fold and accelerated onset of clinical signs and death by approximately 15%. Ranavirus DNA was demonstrated in skin and liver of all frogs that died or were euthanized because of severe clinical signs. Shedding of virus occurred in faeces (7–10 days p.i.; 3–4.5 days before death) and skin sheds (10 days p.i.; 0–1.5 days before death) of some frogs dead from infection. Most common lesions were dermal erosion and haemorrhages; haematopoietic necrosis in bone marrow, kidney, spleen and liver; and necrosis in renal glomeruli, tongue, gastrointestinal tract and urinary bladder mucosa. Presence of ranavirus in lesions was confirmed by immunohistochemistry. Intracytoplasmic inclusion bodies (probably viral) were present in the bone marrow and the epithelia of the oral cavity, gastrointestinal tract, renal tubules and urinary bladder. Our work describes a ranavirus–wood frog model and provides estimates that can be incorporated into ranavirus disease ecology models.

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