Serological and molecular survey of canine distemper virus in red foxes (Vulpes vulpes): Exploring cut-off values and the use of protein A in ELISA tests

The wide distribution and ecological plasticity of the red fox (Vulpes vulpes) make it a potential reservoir for many infectious diseases shared with domestic and wild carnivores. One of such diseases is canine distemper, which is caused by an RNA virus and its main domestic reservoir is the dog. However, other carnivores can also participate in its maintenance, as shown by the recent upsurge of reported cases in wildlife in many parts of the world, and by the fact that red foxes may act as true reservoirs for canine distemper virus (CDV). The lack of validated serological tests for wildlife or other non-target species may be a handicap for monitoring this virus. In this study, serological assays were compared in 147 red fox sera using a commercial ELISA validated for its use in dogs and a non-specific modified ELISA with Protein A peroxidase conjugate to detect bound antibodies. In addition, the presence of CDV RNA in brain, spleen, lung, and liver samples from 144 foxes was investigated by a RT-qPCR. Through the comparison of the results of both ELISAs and the use of a finite mixture model of the optical density values obtained by both techniques, we adjusted the cut-off point of the commercial ELISA to obtain the seroprevalence in foxes. The overall seroprevalence detected was 53.7% (79/147) and 57.1% (84/147) by the commercial and modified ELISA, respectively, with a moderate agreement according to Cohen's Kappa statistic (kappa = 0.491, z = 5.97, p < 0.0001). CDV RNA was detected in 30 out of 144 foxes, which resulted in 20.8% of CDV-infected foxes. At individual level, the results obtained by relating the serological status and the presence/absence of RNA in different organs were explained in terms of the pathogenesis of the infection. Our results highlight the convenience of adjusting the cut-off point when using an ELISA assay developed in domestic dogs for its use in foxes. Moreover, Protein A is confirmed to be a good alternative to be used in red foxes, presenting a good reactivity towards its IgG.