Pathogenesis of Frog Virus 3 (Ranavirus, Iridoviridae) Infection in Wood Frogs (Rana sylvatica)

Abstract

Wood frogs ( Rana sylvatica) are highly susceptible to infection with Frog virus 3 (FV3, Ranavirus, 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, inoculated orally with a fatal dose of 10 pfu/frog, and euthanized at 0.25, 0.5, 1, 2, 4, 9, and 14 days postinfection (dpi). Mild lesions occurred sporadically in the skin (petechiae) and bone marrow (necrosis) during the first 2 dpi. Severe lesions occurred 1 to 2 weeks postinfection 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 was first detected (polymerase chain reaction) in liver at 4 dpi; by dpi 9 and 14, all viscera tested (liver, kidney, and spleen), skin, and feces were positive. Immunohistochemistry (IHC) first detected viral antigen in small areas devoid of histologic lesions in the oral mucosa, lung, and colon at 4 dpi; by 9 and 14 dpi, IHC labeling of viral antigen associated with necrosis was found in multiple tissues. Based on IHC staining intensity and lesion severity, the skin, oral, and gastrointestinal epithelium and renal tubular epithelium were important sites of viral replication and shedding, suggesting that direct contact (skin) and fecal-oral contamination are effective routes of transmission and that skin tissue, oral, and cloacal swabs may be appropriate antemortem diagnostic samples in late stages of disease (>1 week postinfection) but poor samples to detect infection in clinically healthy frogs.

Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens

“…While the majority of X. laevis -FV3 research has bypassed the skin barrier via intraperitoneal injection of virus into the host (18, 227–229), water-bath exposure of healthy tadpole and adult X. laevis to FV3-infected frogs in the same tank revealed that healthy individuals become infected with FV3 within 3 h of exposure (230). A key symptom of FV3 infection in susceptible developmental stages or frog species is the formation of skin lesions, skin shedding, and epidermal cell necrosis (231, 232). It is proposed that loss of the skin barrier during FV3 infection allows for increased pathogen entry and ultimately leads to mortality in susceptible hosts, stressing the overall importance of the skin barrier and barrier integrity.…”

Pathogenesis of Bohle Iridovirus (Genus Ranavirus) in Experimentally Infected Juvenile Eastern Water Dragons (Intellagama lesueurii lesueurii)

“…16 Although this is the first report of inclusion bodies in lungs of ranaviral-infected dragons, there are reports of inclusion bodies in keratinocytes adjacent to areas of necrosis in skin lesions of BIV-like infected boreal toads (Anaxyrus boreas boreas), in circulating leukocytes of a FV3-infected eastern box turtle (Terrapene carolina carolina), and in the cytoplasm of skin epidermal cells of FV3infected wood frogs euthanized 14 dpi. 1,13,16 To identify the progression of BIV infection in EWDs at different time points, we collected cloacal swabs and tissue samples for PCR, virus isolation, histopathology, and IHC. In our study, Ranavirus sp DNA was detected in all cloacal swabs, as well as liver and kidney tissue samples collected at the first time point (3 dpi) in the early stages of infection, which contrasts to an FV3 pathogenesis study in wood frogs (Rana sylvatica) where DNA was not detected in multiple tissues until shortly before death.…”

“…Inclusion bodies have previously been described in multiple organs of ranavirus-infected fish, tortoises, turtles, and amphibians 1,13,16,17,22,25,29,35 and, up until now, only in the liver of lizards. 8,24 While inclusion bodies are commonly associated with ranaviral infections, 8,17,18 they are not consistently reported.…”

“…Alternatively, the positive virus isolation and PCR result from kidney and liver could reflect the presence of virus in blood cells (viremia), as it disseminated systemically from its site of entry in the digestive tract. The presence of nonspecific staining of the keratin layer and the absence of IHC staining in positive PCR tissues could be addressed by a more specific and sensitive technique such as ISH 16 (as seen in Fig. 4b,d).…”

Hopping Over Red Leg: The Metamorphosis of Amphibian Pathology

“…1 In this issue of Veterinary Pathology, Forzán et al, using experimental infections of a Ranavirus, Frog Virus 3 (FV3), in wood frogs (Rana sylvatica), provide a new and wellcharacterized model that advances our understanding of the other major EID in amphibians. 8 The ranaviruses were discovered in the mid-1960s as part of efforts to characterize the herpesvirus-associated renal carcinomas, but for a long time their association with natural disease was limited to American bullfrog (Lithobates catesbeianus) tadpoles with generalized edema and hemorrhagic lesions grossly resembling red leg syndrome. 22 In the 1990s, Ranavirus-associated mass mortality events in native salamanders and frogs from North America 4,9 and common frogs (Rana temporaria) 5 from Europe significantly raised the profile on these viruses.…”