The bacterial infection spreading through Spain’s waterbirds is reaching serious levels. Last season 80 percent of gadwalls and black-winged stilts died. Both species of bird is now classed as endangered.
Botulism is a major killer of waterbirds, including some endangered species. In earlier studies, some also published in Applied and Environmental Microbiology, these investigators had found that eutrophication (the ecosystem’s response to the addition of artificial material) of some of these wetlands, due to effluent from waste water treatment plants, was encouraging the growth of Clostridium botulinum and other bacterial pathogens of birds.
To investigate the matter further, researchers have been studying the Navaseca lake, which is located in the La Mancha region. They found that one way the disease spreads is through maggots, which the birds feed on (with the maggots growing on the dead carcasses of other birds.) This causes the feeding birds to die from the neurotoxin, and the cycle repeats.
In terms of where else the bacterium comes from, the investigators also found high numbers of a freshwater snail. The species — Physa acuta — entered the area a few years ago and has adapted due to the high levels of sewage. This is despite the area being a UNESCO “biosphere reserve.” The snail is found in anthropogenic reservoirs (which means “of environmental pollution and pollutants”), occurring in warm water discharges from power stations and in some rivers
The snails are a food source for several species of waterbird, including mallards, gulls, and coots. Other birds, such as flamingos and grebes, are largely unaffected because they do not eat the snails or the maggots. The researchers also wonder whether these birds have a degree of genetic resistance.
In terms of factors promoting the spread of the disease, higher temperatures are a factor since this makes the pollution-ridden conditions worse and encourages snail growth. For this reason, patterns of bird deaths are higher during the summer. The lower water levels also encourage the growth of the bacteria, which grow under anaerobic conditions.
The research findings are published in the journal Applied and Environmental Microbiology. The research paper is titled “Differences in the vulnerability of waterbird species to botulism outbreaks in Mediterranean wetlands: an assessment of ecological and physiological factors.”
