Ribeiroia - Evidence For Emergence

Evidence For Emergence

Dating back over 200 years, there have been reports of amphibian malformations suggesting that the general phenomenon is not a new occurrence in amphibian populations (Ouellet 2000). Despite the historical occurrence of amphibian malformations, the real question is whether such malformations are increasing in prevalence and severity in wild amphibian populations. Limited historical data on malformations and the absence of baseline malformation data has complicated attempts to assess whether malformations are emerging. However, research over the past decade has shown that malformations are emerging in some amphibian populations. For example, the background rate of malformations of northern leopard frogs in Minnesota increased from 0.4% in 1958-1963 to 2.5% in 1996-1997 (Hoppe 2000), with recently observed malformations more severe and more diverse. Similarly, Johnson and Lunde (2005), in a comparison of recent (1990–2000) and historical (1899–1989) publications, found that recent reports document:

1. a wider range of more severe malformations,
2. a greater number of affected amphibian species,
3. a larger number of affected sites, and
4. a higher frequency range of affected individuals at reported sites.

For both of these studies, however, the mechanisms driving the apparent increase in malformations were not identified (see Factors influencing emergence). In a more rigorous study, Johnson et al. (2003) confirmed that historical malformations at six of eight sites (1947–1990) were associated with Ribeiroia infection. Moreover, three of these sites continued to support limb malformations at frequencies of 7-50% in one or more species. One pond in particular (Jette Pond in western Montana) showed increased prevalence of malformations in Pseudacris regila from 20% in 1960 to 46% in 2000, which was correlated with an increased intensity of Ribeiroia infection. Although more research is necessary, these studies provide evidence that malformations, especially those linked to Ribeiroia infection, are emerging in some amphibian populations.