Date of this Version
Clark, L. 2014. Disease risks posed by wild birds associated with agricultural landscapes. In K.R. Matthews, G.M. Sapers, and C.P. Gerba, editors. The Produce Contamination Problem, Second edition. Academic Press, Elsevier Inc., Boston, MA. 139-165. doi: 10.1016/B978-0-12-404611-5.00007-5.
There are over 1,400 cataloged human pathogens, with approximately 62% classified as zoonotic (Taylor et aI., 2001). While most evidence of direct transmission of pathogens to humans involves domestic and companion animals, the reservoir for most zoonoses is wildlife, yet there are relatively few well-documented cases for the direct involvement of transmission from wildlife to humans (Kruse et aI., 2004). In part, this absence of evidence reflects the mobility of wildlife, the difficulty in accessing relevant samples, and the smaller number of studies focused on characterizing wildlife pathogens relative to the human and veterinary literature (McDiarmid, 1969; Davies et aI., 1971; Hubalek, 2004). Because humans generally do not have direct contact with wild birds, exposure to pathogens is via indirect routes, i.e., environmental. This indirect exposure route (sapro-zoonotic) makes identifying the wild-bird source of the pathogen all the more difficult. Thus, most assessments implicating birds in carriage or transmission of pathogens of zoonotic importance are based on reasonable inference. The first step in this process is documentation that birds are hosts or can carry or transmit the pathogen. The second step involves a demonstration that the bird species involved is associated with agricultural food production. Usually the evidence provided is actual census information, behavioral observations, or evidence of the presence of feces. The third step involves demonstration that a pathogen or parasite that could originate from a bird associated with agricultural production or processing is the causative agent of human disease. The best evidence would be genetic, but even other diagnostic methods would suffice. Unfortunately this last step is rarely documented in the literature. However, with better diagnostic technologies and better understanding of the disease ecology, it is feasible that documenting the actual risks posed by wildlife to human health will become easier, and we will be better able to identify control points for pathogen management originating from wildlife.