Salmonellosis in wild birds

Salmonella bacteria, especially Salmonella enterica, serotype Typhimurium, are commonly found in the intestine of wild birds. These organisms are maintained within bird populations by several mechanisms. The simplest of these mechanisms occurs in raptors since birds that eat other animals risk eating Salmonella-infected prey. Both wild and captive raptors may be temporary or permanent Salmonella carriers or even suffer from clinical Salmonellosis as a result of eating infected prey. A similar infection pathway affects scavenging or carrion eating birds such as vultures, crows, and, most importantly, gulls. For example, gulls are opportunistic scavengers who feed at sites where raw sewage is released. They appear to be relatively resistant to disease but may serve as effective carriers of Salmonella and thus are a source of infection for other animals. In other situations, birds exposed to a contaminated environment may become infected accidentally. This is the case with domestic pigeons and colonial waterbirds. The most significant outbreaks of wild bird Salmonellosis occur, however, in passerines. Thus, although only a few healthy passerines harbor Salmonella in their intestine, these birds often gather in very large numbers at bird feeders. The growth of the “bird feeding industry” has promoted this behavior. Garden bird feeders can become so contaminated with feces that Salmonella contamination may grow to significant levels. If this is accompanied by other stresses such as bad weather or a food shortage, large numbers of these birds may develop Salmonellosis and die. Finches, house sparrows, and cowbirds appear to be especially at risk. Phage and genetic typing suggests that these passerines carry strains of S. enterica Typhimurium that are specifically adapted to songbirds. These infected birds may transmit infection to humans, either directly as a result of handling, or more commonly, as a result of exposure to domestic cats infected by preying on sick and moribund birds.