Abstract
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.
Highlights
Infectious diseases are emerging at an increased rate (Woolhouse and Gowtage-Sequeria 2005, Jones et al 2008) providing opportunities for pathogens to infect and cause disease in additional host species (Benmayor et al 2009)
Viruses isolated from common frogs were associated with different disease syndromes: RUK 11 was isolated from a R. temporaria adult presenting substantial internal haemorrhages and RUK 13 was isolated from a frog presenting superficial skin ulcers, unlike those obtained from B. bufo where only one disease syndrome has been observed (Hyatt et al 2000)
Virulent ranavirosis had been circulating in UK common frog populations for close to ten years when the ranaviruses used in this study were cryobanked, after isolation from wild UK amphibians
Summary
Infectious diseases are emerging at an increased rate (Woolhouse and Gowtage-Sequeria 2005, Jones et al 2008) providing opportunities for pathogens to infect and cause disease in additional host species (Benmayor et al 2009). Host range and virulence are dictated both by evolution and by ecology; being shaped by historical interactions and by the more contemporary process of emergence (Woolhouse et al 2005). On the other hand, introduced pathogens offer opportunities to investigate the determinants of host range and virulence. The prospects for such studies have increased as the globalization of human activities has led to an increase in such novel introductions
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