Pathogenesis of frog virus 3 (Ranavirus sp, Iridoviridae) infection in wood frogs, Rana sylvatica (Lithobates sylvaticus)

Wood frogs, Rana sylvatica, are highly susceptible to infection with Frog Virus 3 (FV3, Ranavirus sp, Iridoviridae), a cause of mass mortality in wild populations. To elucidate the pathogenesis of FV3 infection in wood frogs,... [ view full abstract ]

Wood frogs, Rana sylvatica, are highly susceptible to infection with Frog Virus 3 (FV3, Ranavirus sp, Iridoviridae), a cause of mass mortality in wild populations. To elucidate the pathogenesis of FV3 infection in wood frogs, 40 wild-caught adults were acclimated to captivity for six months, inoculated orally with a fatal dose of 104.43 pfu/frog, and euthanized at 0.25, 0.5, 1, 2, 4, 9 and 14 days post-infection (dpi). Mild lesions occurred sporadically in the skin (petechia) and bone marrow (necrosis) during the first 2 dpi. Severe lesions occurred 1-2 weeks post-infection and consisted of necrosis of medullary and extramedullary hematopoietic tissue, lymphoid tissue in spleen and throughout the body, and epithelium of skin, mucosae and renal tubules. Viral DNA (PCR) was first detected in liver 4 dpi; by dpi 9 and 14 all viscera tested (liver, kidney and spleen), skin and feces were positive. Immunohistochemistry (IHC) first detected virus in small areas devoid of histologic lesions in the oral mucosa, lung and colon 4 dpi; by 9 and 14 dpi IHC staining along with mild to severe necrosis was found in multiple tissues. Based on IHC staining intensity and lesion severity, the skin, oral and gastrointestinal mucosae and renal tubular epithelium were important sites of viral replication and shedding. Direct contact (skin) and fecal-oral contamination are likely effective routes of transmission. Skin or cloacal swabs can be sources of ante-mortem diagnostic samples in late stages of disease (> 1 week post infection). Oral, cloacal and skin swabs are probably poor samples to detect infection in clinically healthy frogs.