Review - The Risk of Pathogen Spillover from Wild Birds

Wild birds can carry pathogens, such as Campylobacter and Salmonella, that can become contaminants for food. Most of our concern is in the fields with crops that will receive minimal processing, but also in instances where birds gain access into our processing facilities. A recent review of the research shows that we probably overemphasize the risk, but more research is needed in understanding transmission from birds to our food systems.

"We conclude that current data do not provide sufficient information to determine the likelihood of enteric pathogen spillover from wild birds to humans and thus preclude management solutions. The primary focus in the literature on pathogen prevalence likely overestimates the probability of enteric pathogen spillover from wild birds to humans because a pathogen must survive long enough at an infectious dose and be a strain that is able to colonize humans to cause infection."

From their paper:

"Campylobacter spp. 14.8%(64/431)of North American breeding birds had Campylobacter spp. prevalence data (1+ observations) meeting our inclusion criteria 1–9 (Data S2). The species with the most observations meeting our inclusion criteria 1–9 were rock pigeon [N= 3659from 15 studies, range 6–1800 individuals tested, 0.1–70% reported prevalence, estimated prevalence 16± 5.3%(SE)],European starling[N= 2094 from 12 studies, range 1–957 individuals tested, 0–75% reported prevalence, estimated prevalence 28 ± 6.0% (SE)],...."

"Salmonella spp. were the most studied bacteria with 33% (141/431) of North American breeding birds having prevalence data (1+ observations) meeting our inclusion criteria 1–9. The species with the most observations of data meeting inclusion criteria 1–9 were herring gull [N = 12470 from 10 studies, range 1–5324 individuals tested, 0–22% prevalence,estimated prevalence 8.2± 2.2%(SE)],house sparrow [N = 5581 from 19 studies, range 2–1124 individuals tested, 0–21% prevalence, estimated prevalence 2.5 ± 0.7% (SE)], rock pigeon [N = 5458 from 30 studies, range 4–1800 individuals tested, 0–100% prevalence, estimated prevalence 4.0 ± 0.9% (SE)..."

So birds can carry pathogens, and we need to keep them out of food processing facilities, but with field crops, further work is needed before we go to the levels of habitat destruction.

Biological Reviews
https://onlinelibrary.wiley.com/doi/full/10.1111/brv.12581
Are we overestimating risk of enteric pathogen spillover from wild birds to humans?
Olivia M. Smith  William E. Snyder  Jeb P. Owen
First published: 31 January 2020 https://doi.org/10.1111/brv.12581


ABSTRACT
Enteric illnesses remain the second largest source of communicable diseases worldwide, and wild birds are suspected sources for human infection. This has led to efforts to reduce pathogen spillover through deterrence of wildlife and removal of wildlife habitat, particularly within farming systems, which can compromise conservation efforts and the ecosystem services wild birds provide. Further, Salmonella spp. are a significant cause of avian mortality, leading to additional conservation concerns. Despite numerous studies of enteric bacteria in wild birds and policies to discourage birds from food systems, we lack a comprehensive understanding of wild bird involvement in transmission of enteric bacteria to humans. Here, we propose a framework for understanding spillover of enteric pathogens from wild birds to humans, which includes pathogen acquisition, reservoir competence and bacterial shedding, contact with people and food, and pathogen survival in the environment. We place the literature into this framework to identify important knowledge gaps. Second, we conduct a meta‐analysis of prevalence data for three human enteric pathogens, Campylobacter spp., E. coli, and Salmonella spp., in 431 North American breeding bird species. Our literature review revealed that only 3% of studies addressed the complete system of pathogen transmission. In our meta‐analysis, we found a Campylobacter spp. prevalence of 27% across wild birds, while prevalence estimates of pathogenic E. coli (20%) and Salmonella spp. (6.4%) were lower. There was significant bias in which bird species have been tested, with most studies focusing on a small number of taxa that are common near people (e.g. European starlings Sturnus vulgaris and rock pigeons Columba livia) or commonly in contact with human waste (e.g. gulls). No pathogen prevalence data were available for 65% of North American breeding bird species, including many commonly in contact with humans (e.g. black‐billed magpie Pica hudsonia and great blue heron Ardea herodias), and our metadata suggest that some under‐studied species, taxonomic groups, and guilds may represent equivalent or greater risk to human infection than heavily studied species. We conclude that current data do not provide sufficient information to determine the likelihood of enteric pathogen spillover from wild birds to humans and thus preclude management solutions. The primary focus in the literature on pathogen prevalence likely overestimates the probability of enteric pathogen spillover from wild birds to humans because a pathogen must survive long enough at an infectious dose and be a strain that is able to colonize humans to cause infection. We propose that future research should focus on the large number of under‐studied species commonly in contact with people and food production and demonstrate shedding of bacterial strains pathogenic to humans into the environment where people may contact them. Finally, studies assessing the duration and intensity of bacterial shedding and survival of bacteria in the environment in bird faeces will help provide crucial missing information necessary to calculate spillover probability. Addressing these essential knowledge gaps will support policy to reduce enteric pathogen spillover to humans and enhance bird conservation efforts that are currently undermined by unsupported fears of pathogen spillover from wild birds.