A global overview of the conservation, management and research of the world's waterbird flyways

On a world-wide basis, avian botulism is the most significant disease of waterbirds. Type C botulism has been reported in waterbirds from every continent except Antarctica, and outbreaks with one million or more waterbird deaths have been reported by the USA, Canada, and Russia. Unfortunately, population impacts of avian botulism have not been well studied. Species that are numerous, geographically widespread, and have a high reproductive potential (e.g. Mallard Anas platyrhynchos) may be able to withstand sporadic high losses, while populations of other, less common or endangered species whose populations are disproportionately exposed to botulism, may not be as resilient (e.g. Northern Pintail Anas acuta, Black-faced Spoonbill Platalea minor). The effect of botulism on local or regional waterbird populations is also significant. The 1996 botulism outbreak at the Salton Sea, California, killed nearly 15% of the western population of American White Pelicans Pelecanus erythrorhynchos. Because the occurrence of avian botulism is largely controlled by environmental factors and not dependent on waterbird density, this disease has the potential to cause significant population declines in some species, seriously impeding conservation efforts. INTRODUCTION During the last century, avian botulism killed many millions of birds, especially waterfowl and shorebirds, and was the most significant disease of waterbirds in total mortality. Large outbreaks of “duck sickness”, later recognized to be type C botulism, were first documented in the western USA and Canada in the early 1900s (Hobmaier 1932). Millions of waterfowl died in three widely separated regions of North America – the deltas of the Great Salt Lake, Utah, the southern San Joaquin Valley, California, and the Elfros region of Saskatchewan, Canada. Outside North America, the first outbreaks of botulism in wild birds were recorded in Australia in 1934 (Pullar 1934), in Russia (formerly USSR) in 1957 (Kuznetzov 1992), and in Europe in 1963 (Jensen & Price 1987), first in Sweden and shortly after in Denmark (1965), Great Britain (1969), and The Netherlands (1970). Botulism was also confirmed in South Africa (1965), New Zealand (1971), Japan (1973), Argentina (1979) and Brazil (1981). To date, type C botulism has been diagnosed in wild waterbirds in at least 28 countries and on every continent with the exception of Antarctica. More than a million deaths from type C avian botulism have been reported during single outbreaks in some wetlands in North America and Russia (Table 1), and outbreaks with losses exceeding 50 000 birds have been relatively common. Even though most of the large outbreaks of avian botulism occurred in North America, the global importance of the disease and its potential to cause massive and even catastrophic losses of birds are evident. ETIOLOGY AND HOST RANGE OF THE DISEASE The agent which causes avian botulism is a neurotoxin produced by the bacterium Clostridium botulinum, a strict anaerobe that forms dormant spores in adverse conditions. Botulinum toxin is produced only after the spores germinate during vegetative growth of the bacteria. Birds primarily acquire the disease by ingesting toxin-laden food items; thus, the disease is usually a food poisoning and not an infection. It can also be caused by “toxico-infections”, when botulinum toxin-producing bacteria colonize the intestinal tract of an individual or secondarily infect a wound. Seven different neurotoxins are produced by strains of C. botulinum, designated types A to G (Smith & Sugiyama 1988). Most botulism outbreaks in birds are caused by type C toxin, but sporadic die-offs among fish-eating birds, primarily in the Great Lakes of North America, have been caused by type E toxin. Botulinum toxin causes a flaccid paralysis in birds, with loss of motor control, flight and ambulation. A common sign of botulism in birds is paralysis of the nictitating membrane. In the final stages of the disease, birds are unable to lift their heads; thus, the disease has also been called “limberneck”. Death usually results from respiratory failure or drowning. All birds are probably susceptible to botulinum toxin, with the exception of vultures and possibly other scavenging birds, which may have an innate resistance to the disease. Upon review of the literature and over 2 000 diagnostic records at the USGS National Wildlife Health Center (NWHC), confirmed reports of type C botulism were found for 264 species of birds representing 39 families; most of these (22) were waterbird families. Foraging behavior appears to be the most significant risk factor for avian botulism (Rocke & Friend 1999). Filter feeding and dabbling waterfowl, such as Mallard Anas platyrhynchos, Green-winged Teal A. crecca, and Northern Shoveler A. The global importance of avian botulism