For years, wildlife has been routinely listed in audit checklists and corrective action reports as a “potential contamination source.” What does the science tell us? A far more nuanced story. 

The growing body of research on fresh produce food safety and wildlife is proving to be a complicated and variable topic. How wildlife interacts with the fresh produce environment is a dynamic ecological component. As a result, wildlife can be challenging for fresh produce food safety professionals to know how to manage.  

When a risk profile is contextual, species-specific, and environmentally influenced by other interacting factors, that makes a complicated recipe for food safety management. 

Wildlife as Carriers: Capacity vs. Risk 

Numerous studies demonstrate that many wildlife species have the capacity to carry zoonotic pathogens such as Shiga Toxin Producing E. coli (STEC), E. coli O157:H7, and Salmonella. For example, feral swine in California’s Central Coast has shown measurable prevalence of O157 and non-O157 STEC in a number of longitudinal studies since the early 2000s. Rodents in agricultural areas have demonstrated variable Salmonella prevalence, and low overall carrying of pathogenic E.coli.  Birds, heavily dependent on species, environment, and habitat, have been found to carry STEC serogroups and Salmonella at variable levels. Birds highlight a unique variability across species and situations, with many types of birds posing little to no risk, some contributing reduced risk from other birds/pests/rodents (e.g., falcons, owls), and some birds representing risk when considering what they are interacting with (i.e., livestock, landfills, compost piles) prior to interacting within produce fields. 

Overall, prevalence in wildlife populations is generally low and highly variable, often in the 0–5% range for many species studied, but with spikes in prevalence that should not be ignored. Environmental presence of foodborne pathogens in research studies in the leafy green areas have repeatedly shown that there can be a period of time when pathogen presence in the environment is high, and during those spikes wildlife may also harbor more pathogens.   

The presence of a pathogen in a wildlife species does not necessarily equate to elevated crop risk, especially since wildlife still must impact the crop production areas. Wildlife risk, like overall environmental risk, is dynamic and episodic. As a result, risk cannot be determined simply by presence or pathogen carriage percentages, it must consider more variables like: 

• Animal density and aggregation behavior (i.e., bird flocking vs. solitary species) 

• Interaction of wildlife with higher-load environments (CAFOs, AFOs,  compost sites, landfills, waterways) 

• Wildlife and water movement patterns between livestock operations and cropland 

• Seasonal timing relative to production windows and potential increases in overall microbial presence/load 

• Environmental persistence and die-off dynamics in the environment, and in fresh produce fields 

Feral Swine: A Clearer Risk Profile 

Among wildlife studied in leafy green regions, feral swine consistently have been found to carry higher pathogen prevalence compared to other wildlife species. Investigations following the 2006 spinach outbreak in the California Central Coast found feral swine carrying E. coli O157 and found evidence of pig movement between cattle areas and produce fields. Numerous leafy green area surveys since then have identified O157, STEC, and Salmonella at varying levels in wild pigs sampled, with study positivity rates for E.coli O157 varying from 1.9%, 4.2%, and 4.7% in three wildlife studies. Importantly, wild pigs move across landscapes, across fence lines, and interact with both livestock and surface water. As such, they have potential to spread pathogens within different areas of the ecosystem, including the potential to make it into fresh produce fields.  

Rodents: Lower Prevalence, Contextual Risk 

Large rodent surveys across fresh produce farms and adjacent cattle operations have generally found very low O157 prevalence (often near 0%), but a more modest Salmonella detection (typically 2–7%). Rodents do not appear to be primary reservoirs of O157 in these agricultural systems. However, researchers have found that risk (and pathogen prevalence) increases when rodents interface with CAFOs, manure storage, compost facilities, and urban waste streams. Similarly rodent pressure changes based on population cycles, reproductive rates based on the ecosystems, and climate. Again, context determines risk.  

Birds: Species Matter More Than Presence 

Wild birds introduce additional complexity because fencing and terrestrial exclusion methods are ineffective. Research across thousands of birds in California and Arizona shows that overall pathogen prevalence is low, large flocking species (e.g., blackbirds, cowbirds, geese) likely present more risk due to fecal load and aggregation behavior. When small songbird (passerine) birds have been studied, it has been found that they contribute minimal fecal mass, and a low overall pathogen prevalence. Laboratory and field studies show that E. coli declines quickly in bird feces, particularly under desiccation and UV exposure. In most cases, small bird droppings likely present limited hazards under typical environmental conditions.  

However, that is not to say that all birds and in all situations are low risk. Temporal events, such as migratory geese or a murder of crows occupying a field, can shift risk dramatically. This is especially true depending on what environment they have been interacting with (i.e., CAFOs, livestock, watersheds). 

Once again, these findings supports that science shifts us from “bird presence” as an audit compliance checkbox to species-specific evaluation, seasonal monitoring, density-based risk assessment, adjacent land assessment (i.e., livestock birds will interact with, landfills, rivers/waterways, compost piles, etc.). 

Habitat Removal: A Cautionary Tale 

One of the most important wildlife learnings over the past 15+ years is ecological – reaffirming the need to consider food safety risks within ecosystems and not as singular factors to be controlled. After the 2006 outbreak in the Central Coast leafy green region, large-scale vegetation removal in the Salinas Valley was implemented to create buffers between wildlife, livestock, and leafy green fields. The overall assumption, and best intentions, was that removing habitat would reduce wildlife-mediated pathogen transfer. Subsequent analysis showed a more complicated response with removal of non-crop vegetation not showing the protective effect it was hypothesized to impart, also finding that riparian removal showed no clear protective effect, and finally finding that adjacent grazed land was associated with higher risk than ungrazed natural habitat.  

Long story short, wildlife ecology impacts pathogen dynamics within agricultural settings in non-linear ways. Simplistic mitigation ignoring the overall system and environment can create unintended consequences. 

What This Means for Food Safety Professionals Managing Risks 

Overall, the wildlife body of evidence supports these core conclusions: 

• Wildlife is part of the shared agricultural ecosystem. 

• Pathogen prevalence is generally low, but variability is high and must consider the wildlife species, the environment they interact with, and the likelihood of crop interactions. 

• Wildlife risk is highly contextual and can be shaped by livestock density, habitat, season, and environmental conditions. 

• Ecological disruption does not necessarily improve food safety outcomes. Crops are grown in, and become part of, the agricultural ecosystem and mitigation and management systems need to consider that complexity. 

This reinforces a broader systems insight that food safety cannot be managed through isolation alone. Wildlife risk to fresh produce crops is neither zero nor absolute. It is dynamic. 

The path forward for fresh produce risk management is not wildlife elimination, but instead an ecosystem-informed risk management system, grounded in surveillance, targeted mitigation, and collaboration across adjacent land users. 

Western Growers’ Nexus is a collaborative learning center focused on the intersection of sustainability, ecosystems, agriculture, and food safety—bringing science, systems thinking, and cross-sector leadership together to advance practical, risk-based solutions for modern fresh produce production. 

 

 

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