Ranaviruses such as frog virus 3 (FV3) are large
double-stranded DNA (dsDNA) viruses causing
emerging infectious diseases leading to extensive morbidity and mortality of amphibians and other ectothermic vertebrates worldwide. Among the hosts of FV3, some are highly susceptible, whereas others are resistant and asymptomatic carriers that can take part in disseminating the infectious virus. To date, the mechanisms involved in the processes of FV3 viral persistence associated with
subclinical infection transitioning to lethal outbreaks remain unknown. Investigation in Xenopus laevis has revealed that in asymptomatic FV3 carrier animals,
inflammation induced by heat-killed (HK) Escherichia coli stimulation can provoke the relapse of active
infection. Since
Toll-like receptors (TLRs) are critical for recognizing microbial molecular patterns, we investigated their possible involvement in
inflammation-induced FV3 reactivation. Among the 10 different TLRs screened for changes in expression levels following FV3
infection and HK E. coli stimulation, only TLR5 and TLR22, both of which recognize bacterial products, showed differential expression, and only the TLR5
ligand flagellin was able to induce FV3 reactivation similarly to HK E. coli Furthermore, only the TLR5
ligand flagellin induced FV3 reactivation in peritoneal macrophages both in vitro and in vivo These data indicate that the TLR5 signaling pathway can trigger FV3 reactivation and suggest a role of secondary
bacterial infections or microbiome alterations (stress or pollution) in initiating sudden deadly disease outbreaks in amphibian populations with detectable persistent asymptomatic ranavirus.IMPORTANCE This study in the amphibian Xenopus laevis provides new evidence of the critical role of macrophages in the persistence of ranaviruses in a quiescent state as well as in the reactivation of these pathogens into a virulent
infection. Among the multiple microbial sensors expressed by macrophages, our data underscore the preponderant involvement of TLR5 stimulation in triggering the reactivation of quiescent FV3 in resident peritoneal macrophages, unveiling a mechanistic connection between the reactivation of persisting ranavirus
infection and bacterial
coinfection. This suggests a role for secondary
bacterial infections or microbiome alterations (stress or pollution) in initiating sudden deadly disease outbreaks in amphibian populations with detectable persistent asymptomatic ranavirus.