Causes and Susceptible Species
Salmonellosis refers to disease caused by bacteria in the genus Salmonella. There are many species and strains of Salmonella and a large number of them are at least potentially pathogenic to a variety of vertebrates. In many instances, however, salmonellae may inhabit the alimentary canal without causing overt disease; note, for example, the frequent reports of Salmonella in pet turtles. The following discussion will focus on salmonellosis in songbirds caused by Salmonella enterica serovar typhimurium, by far the most frequently recognized problem with regard to Salmonella and wildlife in New York.
In recent times salmonellosis has generally emerged in mid-to-late winter in flocks of redpolls and pine siskins that have fled boreal forests to the north. During outbreaks, salmonellosis is sometimes confirmed in other species such as goldfinches and evening grosbeaks, but such so-called spillover has to date remained a very minor feature of these episodes. Note: all of the aforementioned species typically feed in long bouts at feeding stations (as opposed to the come and go habits of chickadees and nuthatches).
The disease is transmitted through fecal contamination of food. How outbreaks begin is poorly known. Possible mechanisms include the presence of carrier birds in either affected or unaffected species, and the possibility of low levels of contamination in commercially packaged birdseed. In addition to outbreaks in redpolls and siskins, salmonellosis is sporadically confirmed in house sparrows without any noticeable seasonal component. Predators and scavengers of diseased songbirds may be susceptible to infection but there is little evidence of significant morbidity in those species beyond an occasional diagnosis in house cats.
Outbreaks of salmonellosis in redpolls and pine siskins tend to occur in those winters in which there are large winter movements of these species into the northern United States.
Many outbreaks may be recorded over broad geographic regions and the total mortality by winter's end may be large. It seems plausible that any benefit these species gain by access to bird feeders may be cancelled by losses to salmonellosis.
Diagnosis is tentatively made from the characteristic esophageal lesions in the esophagus. If further confirmation is desired, the organism can be cultured and identified in alimentary canal samples (contents, lesion fragments).
S. enterica sv typhimurium principally infects parts of the alimentary canal. In the avian species mentioned above, the most severe lesions are usually in the esophagus. These lesions are sites of thickening and necrosis that appear as relatively firm yellowish masses that can often be palpated and visualized through the skin. Sick birds may appear weak, and they may tend to sit around feeding stations with fluffed-up plumage. At death most individuals are thin despite evidence of continued feeding.
Preventing the Spread of Salmonellosis
When an outbreak of salmonellosis is detected at a bird feeding station, the traditional recommendation is to halt feeding for a minimum of two weeks. Spilled seed and seed husk debris should be cleaned up, and the feeders disinfected with 10% household chlorine bleach (1 part bleach/ 9 parts water) before redeployment. This strategy will disburse the birds, separating uninfected susceptible birds from sick birds and the contaminated feeder environment.