Salmonella enterica serotype Typhimurium is commonly found in the intestinal tract of a variety of wild bird species. The most significant outbreaks of wild bird salmonellosis occur in passerine birds.
Passerines can serve as reservoirs of disease, or birds can suffer significant mortality secondary to infection
Finches, house sparrows (Passer domesticus), and brown-headed cowbirds (Molothrus ater) appear to be particularly at risk (Fig 1), carrying strains of Salmonella specifically adapted to songbirds.
In one study, Salmonella serotype Typhimurium was isolated from 15% of healthy house sparrows (Tizzard 1979). Salmonella has been isolated from approximately 90% of sick or dying sparrows (Woebeser 1969) (Fig 2).
Symptomatic disease is most likely to develop when concurrent stressors, such as reductions in food availability and weight loss, are present. Affected birds appear disoriented and are reluctant to fly. Birds then collapse, and quickly die. The entire clinical course lasts only 2 to 3 hours (Kirkwood 1995).
Animal care workers can be exposed to salmonellosis by handling infected birds
Direct contact with songbirds and their feces is an important source of disease, however salmonellosis outbreaks have also been reported in humans that do not work directly with birds. One outbreak was traced back to a human kitchen where food was prepared in a large, barn-like building where sparrows could freely enter and roost.
Salmonellosis secondary to songbird fecal contamination has also been documented in horses and cattle, although one study set in California dairies concluded that wild birds were not important reservoirs of Salmonella spp. (Kirk 2002). Equine infections in one veterinary hospital, were traced back to barns where sparrows foraged in horse feed (Woebeser 1969).
Domestic cats can contract Salmonella
Small animal veterinarians are also at risk of contracting salmonellosis through exposure to domestic cats infected by preying upon sick birds. Domestic cats are believed to be responsible for killing millions of passerines annually. Illness secondary to eating wild songbirds can be a regular springtime event in cats allowed outdoors in the northeastern United States. After a 2 to 5 day incubation period, cats with “songbird fever” can exhibit anorexia, lethargy, vomiting, bloody diarrhea , and fever for 2 to 7 days. Disease can be fatal in the immunosuppressed, and even in the immune competent complete convalescence can take up to 3 weeks (Tizard 2004, Scott 1988).
Bird feeder use can perpetuate salmonellosis in songbird populations
Bird feeders can attract large numbers of birds to feeders. Feeders are then contaminated when birds pass relatively large amounts of feces in and around feeder sites. Feeders must be regularly cleaned and disinfected. Tizard (2004) has recommended that feeders be sanitized every 2 weeks using a utility sink or bucket—not the kitchen sink or bathtub! Additionally feeders that promote fecal contamination, such as platform feeders, should not be selected (Fig 3).
In addition to contamination of bird feeders, bacterial contamination of commercial seeds may explain some outbreaks of salmonellosis in passerine birds.
Salmonella has been isolated from caged passerines
Salmonella typhimurium var. copenhagen is a common cause of granulomatous ingluvitis in European finches such as the snowfinch (Montifringilla nivalis) and chaffinch (Fringilla coelebs). High mortalities caused by Salmonella enterica serovar Typhimurium were also recently reported in 17 canary flocks from different regions of Tehran, Iran (Madadgar 2009). Necropsy and histopathology revealed necrotic hepatitis and congestive septicemia.
It is important for animal health care professionals to recognize the potential hazards of working with passerine birds so that appropriate measures can be taken to minimize the risk of contracting disease. Wild bird isolates of Salmonella enterica serotype typhimurium may not represent a large zoonotic risk to the general population, however the vast majority of sick and dying songbirds harbor this microbe.
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