Salmonella and Salmonellosis in Wild Birds

Salmonella enterica is an important bacterial pathogen in humans and warm-blooded animals. Wild bird species represent both a potential reservoir for zoonotic infection and as a susceptible host to infection by host-adapted variants. Historically, wild birds were considered to be a major source of Salmonella infection in livestock, but in recent years, it has been more apparent that birds are more likely to act as a reservoir for recycling infection on farms rather than as the primary source of infection. Birds may also transmit infection to humans directly from feces or indirectly through fecal contamination of foods, including peanut butter. While many bird species can be infected with Salmonella, the rates of infection are variable, and most cases lead to intestinal carriage rather than disease. In this case, fecal shedding of Salmonella bacteria from birds can represent a risk for transmission to humans. As such, care is needed when in contact with fecal material such as that found on bird tables or feeders. In recent years, there have been emergences of Salmonella Typhimurium genotypes associated with high mortality in songbirds or passerine birds, resulting in 'die offs' in Europe, Israel, New Zealand and the US. Additionally, S. typhimurium DT2 and other variant Copenhagen genotypes are associated with high mortality disease in pigeons. These genotypes show evidence of evolution towards adaptation to specific hosts, with pseudogenes leading to loss of functional metabolic pathways and specific virulence factors. These 'signatures of adaptation' are common in host-adapted Salmonella serovars and suggest these S. typhimurium isolates are evolving to adapt to specific avian hosts.