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Over the past decade, thousands of birds all over the world have died from avian botulism. The paralytic disease, caused by a neurotoxin produced by the bacteria Clostridium botulinum, killed some 7,000 waterbirds in Lake Michigan in 2007 and 2008, and another 4,000 in 2012. In Hawaii in 2008 it killed 183 Laysan Ducks—a quarter of the adults in the local population.

In recent years the prolific killer has raised many concerns, yet scientists know little about how the disease spreads and which species are most vulnerable to it. But one research team from Spain has set out to fix that, with a new study, published today in . The paper measures the sensitivity of 11 different species of waterbirds to botulism, and looks at how different species and their prey contribute to its spread. It’s information that’s “central to determining priority conservation measures against botulism, particularly in the case of endangered species,” the team writes in the study.

The researchers gathered data at three wetlands in the Castilla-La Mancha region of Central Spain, each of which serves as a winter refuge and breeding site for a variety of waterbirds. The scientists took avian censuses during botulism outbreaks in 2011 and 2012 and calculated the mortality rates for each species to figure out their vulnerability to the disease. They also sampled invasive snails, invertebrates, and small fish from the waters to identify possible sources of the bacteria in the birds’ diets, all while monitoring several infected birds at a nearby wildlife rehabilitation center.

Bad Food; Bad Timing

The results showed stark differences across the 11 species. Coots were the most vulnerable—they made up 39 percent of the dead birds—followed by Mallards, Gadwalls, and Black-headed Gulls. On the other hand, Greater Flamingos and Little Grebes were barely affected. These differences appear to be driven by diet and feeding habits. First, many of the species that were prone to infection were feeding on maggots from the bacteria-ridden carcasses of dead birds. Second, the snails that the team sampled can invade new habitats and thrive in sewage-infested waters, which makes them common food items for a number of waterbirds. But at least 30 percent of them were found to carry the offending bacteria. Similarly, have linked invasive mussels with the spread of avian botulism around the Great Lakes. Most of the invertebrates and fish that the team sampled though showed no contamination, so the scientists concluded that the birds that feed on these low-risk prey usually fare well in outbreaks. They also noted that the timing of a bird’s arrival at the wetlands matters: Those that land during an outbreak’s peak in the late summer suffer more mortalities.

Bacterial Resistance

Findings like these can help us cope with future botulism outbreaks, which are likely to become more frequent as climate change alters wetland conditions to favor bacteria and pathogens. Previous studies have found that outbreaks tend to occur when average temperatures are above 70 degrees Fahrenheit, and during droughts.

Knowing which species are most at risk can help prioritize conservation programs, quarantines, and treatment for sick birds and carcass removal. By monitoring the birds at the rehab center, the team in Spain learned that some birds continue to excrete the bacteria for several days after they’ve recovered from botulism. Waiting a few extra days to release treated and recovered birds can cut off a source of botulism toxins.

Yet it’s less easy to control the disease’s other sources. Completely removing invasive snails or mussels may not be realistic due to the logistics involved. But mapping the feeding patterns of birds, like the U.S. Geological Survey , could reveal bacterial hotspots that can be targeted for cleanup and eradication. Taking the botulism-carrying prey off the menu gives the most vulnerable birds a better shot at survival.