RESUMEN
UNLABELLED: Avian botulism kills thousands of waterbirds every year, including endangered species, but information about the differences between species in vulnerability to botulism outbreaks and the capacity to act as carriers of Clostridium botulinum is still poorly known. Here, we estimated the vulnerability to botulism of 11 waterbird species from Mediterranean wetlands by comparing the number of affected birds with the census of individuals at risk. The capacity of different species to act as carriers was studied by detecting the presence of the C. botulinum type C/D botulinum neurotoxin (BoNT) gene in fecal samples and prey items of waterbirds in the wild and by the serial sampling of cloacal swabs of birds affected by botulism. We found differences among species in their vulnerabilities to botulism, probably related to feeding habits, season of arrival, turnover, and, possibly, phylogenetic resilience. The globally endangered white-headed duck (Oxyura leucocephala) showed mortality rates in the studied outbreaks of 7% and 17% of the maximum census, which highlights botulism as a risk factor for the conservation of the species. Invasive water snails, such as Physa acuta, may be important drivers in botulism epidemiology, because 30% of samples tested positive for the BoNT gene during outbreaks. Finally, our results show that birds may excrete the pathogen for up to 7 days, and some individuals can do it for longer periods. Rails and ducks excreted C. botulinum more often and for longer times than gulls, which could be related to their digestive physiology (i.e., cecum development). IMPORTANCE: Botulism is an important cause of mortality in waterbirds, including some endangered species. The global climate change may have consequences in the ecology of wetlands that favor the occurrence of botulism outbreaks. Here, we offer some information to understand the ecology of this disease that can be useful to cope with these global changes in the future. We have found that some species (i.e., coots and dabbling ducks) are more vulnerable to botulism and have a more relevant role in the onset and amplification of the outbreaks than other species (i.e., flamingos and grebes). Feeding habits can explain these differences in part; in addition to the well-known role of necrophagous fly maggots, we found here that water snails are frequent carriers of Clostridium botulinum This is relevant, because these water snails can thrive in eutrophic and polluted wetlands, exacerbating other changes driven by climate change in wetlands.