Frog virus 3 (FV3) or FV3-like viruses (Iridoviridae) infect a wide range of amphibian species, and they are becoming increasingly and causally associated with amphibian disease outbreaks worldwide. We have established the frog Xenopus laevis as an experimental model to study host defense and pathogenesis of FV3 infection. Although X. laevis adults usually clear FV3 infection within a few weeks, viral DNA has been detected in the kidneys several months after they had been experimentally infected; virus also has been detected in seemingly healthy nonexperimentally infected adults. Based on this information, we hypothesized that covert FV3 infection may occur in Xenopus. We first conducted a survey that detected FV3 by polymerase chain reaction (PCR) in the kidneys (the main site of FV3 infection) in a significant fraction of X. laevis raised in five different locations in the United States. Asymptomatic FV3 carriers also were detected by initiation of an acute systemic FV3 infection in frogs that had been immunosupressed by sublethal γ-irradiation. Finally, we focused on macrophages as a potential site for viral persistence, and we showed that FV3 can infect peritoneal macrophages in vitro as determined by reverse transcriptase-PCR detection of viral mRNAs. Unlike kidney cell lines that are readily killed by FV3, infected macrophages, like uninfected macrophages, survived up to 12 days. Viral transcription also was detected in macrophages from animals up to 12 days after infection. These results suggest that FV3 can become quiescent in resistant species such as Xenopus, thereby making these species potential viral reservoirs.
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