Experimental evidence in support of single host maintenance of a multihost pathogen

The rate at which pathogens are emerging appears to be increasing. Therefore, it is important to determine the factors, such as virulence and host specificity, and how they have been affected by the emergence. In the United Kingdom, ranaviruses, which are double stranded DNA viruses (Family Iridoviridae) began to emerge in populations of common frogs (Rana temporaria) in the mid-to-late 1980s, followed closely by emergence in common toad (Bufo bufo) populations. Here we present experimental evidence that a single host may be able to maintain a multihost pathogen. We exposed common frog tadpoles and common toad tadpoles to ranavirus isolates from the mid-1990s at two different doses. Tadpole survival differed significantly between treatments and this was primarily driven by the dose of the exposure. However, at the low dose, common frog tadpoles exposed to isolates from common frogs experienced higher mortality (n = 35/treatment, log-rank: P = 0.0320, Wilcoxon: P = 0.0835, df = 1) than those exposed to common toad isolates. The high dose caused 75% mortality in common frog tadpoles, but common toads never experienced more than 40% mortality. This, and other evidence provided by this study, show that common frogs are likely to be the primary host of the ranavirus in the UK, and that single host maintenance of ranaviruses can occur in anurans.