An Outbreak of Ranaviral Disease in a Survival Assurance Population of Boreal Toads (Anaxyrus boreas boreas): Diagnosis and Implications for Captive Amphibian Management
Ranaviruses are emerging pathogens that can cause mass mortality in wild and captive amphibians.1 In captivity, the possibility of subclinical infections, a potentially broad host range (including reptiles and fish), and nonspecific lesions can complicate diagnosis and control of ranaviral disease.2 Between April and July 2010, 90% mortality occurred in a breeding colony of 39 boreal toads (Anaxyrus boreas boreas) housed in a multi-species room in an aquarium. Clinical signs included skin vesicles, ulcers, and hyperemia. Histologically, early outbreak lesions included hematopoietic, renal, epidermal, and hepatic necrosis, and multiple tissues often contained intravascular necrotic and karyorrhectic cell debris. Cytoplasmic inclusion bodies were identified most frequently in hematopoietic cells, circulating leukocytes, renal tubular epithelial cells, and hepatocytes. Transmission electron microscopy of kidney and skin revealed icosahedral viral particles consistent with Ranavirus. Chronic lesions from late-outbreak mortalities included deep ulcerative dermatitis with bacterial and fungal invasion and scattered visceral abscesses. Inclusion bodies were not observed. Ranavirus infection was confirmed by TaqMan real-time PCR3 on six antemortem oropharyngeal swabs and fourteen postmortem tissue samples. Sequence data from three genome regions (neurofilament triplet H1-like protein [NF-H1], major capsid protein [MCP], and DNA polymerase)4 were consistent with a novel Ranavirus genotype. Two months after the outbreak Ranavirus DNA was still detected by real-time PCR in samples from four of five healthy, surviving toads. These findings illustrate the potential for introduction of novel, highly pathogenic viruses into naïve captive and wild populations and emphasize the importance of biosecurity in management of captive amphibian conservation programs.
The authors would like to thank Kristin Benson, Jennifer Burchell, Yvonne Cates, Isamara Navarrete, Tammy Tucker, Scott Streiker, and Bethany Bohnsack. This work was funded by National Leadership Grant LG-25-08-0066 from the Institute of Museum and Library Services.
1. Chinchar VG, Hyatt A, Miyazaki T, Williams T. Family Iridoviridae: poor viral relations no longer. Curr Top Micr Immunol. 2009;328:123–170.
2. Pessier AP, Mendelson JR, eds. A Manual for Control of Infectious Diseases in Amphibian Survival Assurance Colonies and Reintroduction Programs. IUCN/SSC Conservation Breeding Specialist Group. Apple Valley, MN. 2010.
3. Pallister J, Gould A, Harrison D, Hyatt A, Jancovich J, Heine H. Development of real-time PCR assays for the detection and differentiation of Australian and European ranaviruses. J Fish Dis. 2007;30(7):427–438.
4. Holopainen R, Ohlemeyer S, Schütze H, Bergmann SM, Tapiovaara H. Ranavirus phylogeny and differentiation based on major capsid protein, DNA polymerase, and neurofilament triplet H1-like protein genes. Dis Aquat Organ. 2009;85:81–91.