Zoonotic disease risk and intensive animal agriculture

Evidence quality

Evidence quality: Moderate (Jones et al., 2013; Rohr et al., 2019). Systematic reviews and interdisciplinary syntheses identify plausible mechanisms and documented associations linking agriculture to zoonotic emergence, but the evidence does not support attributing every outbreak or pandemic to animal farming (Jones et al., 2013; Rohr et al., 2019). The relevant processes occur across wildlife, livestock, human, and environmental systems, and their relative importance varies by pathogen, species, production system, and location (Jones et al., 2013).

Human-animal interfaces and emergence

Zoonotic emergence requires a pathogen, susceptible hosts, and opportunities for cross-species exposure and onward transmission (Jones et al., 2013). Agricultural expansion can alter habitats and increase contact among wildlife, domesticated animals, vectors, and people (Jones et al., 2013; Rohr et al., 2019). A multidisciplinary systematic review found examples in which agricultural intensification or environmental change was associated with increased emergence risk at the wildlife-livestock-human interface, while also concluding that the evidence base did not adequately capture the full biological, environmental, economic, and social complexity (Jones et al., 2013).

Relevant interfaces include farms, animal transport and supply chains, and markets or other settings where live animals, workers, and the public may interact (Espinosa et al., 2020; Rohr et al., 2019). These settings do not all carry the same risk, and a high rate of animal contact does not by itself establish that spillover will occur (Jones et al., 2013). It increases the number or intensity of opportunities that must be managed through surveillance, infection control, and species-specific measures (Espinosa et al., 2020; Rohr et al., 2019).

Amplification in livestock populations

Once a transmissible pathogen enters a susceptible animal population, production conditions can affect its spread (Espinosa et al., 2020). An economic and epidemiological review describes intensive farming as capable of amplifying disease because of high animal density, genetic proximity, immunodeficiency, and live transport (Espinosa et al., 2020). This supports the inherited claim that dense, genetically similar animal populations can facilitate pathogen spread (Espinosa et al., 2020). It does not imply that every intensive farm produces a novel pathogen or that extensive and backyard systems are risk-free; those systems can create different forms of wildlife contact and may have different surveillance or biosecurity capacity (Espinosa et al., 2020; Jones et al., 2013).

Some analyses describe the combination of animal production, pathogen emergence, and the economic costs of disease control as an “infectious disease trap” (Espinosa et al., 2020). In that framing, animal farming can contribute both to opportunities for emergence and to amplification after introduction, while meat production and trade can transmit economic consequences across sectors (Espinosa et al., 2020). This is an analytical model, not evidence that animal agriculture is the sole cause of pandemics (Espinosa et al., 2020).

Biosecurity, surveillance, and residual risk

Biosecurity includes measures intended to prevent pathogen entry, limit spread within a facility, and reduce exposure of workers or visitors (Youssef et al., 2021). A systematic review of farm-level interventions found that hand hygiene, sanitation, personal protective equipment, vaccination, ventilation, and routine practices may be beneficial, but the small and heterogeneous literature produced uncertain estimates and generally medium risk of bias (Youssef et al., 2021). Biosecurity therefore reduces some risks but does not remove all spillover pathways, particularly those involving wildlife, environmental change, supply chains, or pathogens not yet detected (Jones et al., 2013; Youssef et al., 2021).

A broader synthesis estimated that agricultural drivers had been associated with more than one quarter of emerging human infectious diseases and more than half of emerging zoonotic diseases since 1940, while emphasizing that agriculture and infectious disease affect each other through multiple pathways (Rohr et al., 2019). This supports treating animal agriculture as a significant, plausible driver of zoonotic risk, not as a complete explanation for pandemic emergence (Rohr et al., 2019).

The inherited bottom line also states that reducing scale and improving oversight reduce risk (Jones et al., 2013; Youssef et al., 2021). Improved surveillance and evidence-based oversight are supported risk-management measures (Rohr et al., 2019; Youssef et al., 2021). Evidence for reducing production scale as a standalone intervention is less direct because scale is correlated with density, movements, biosecurity capacity, and the type of wildlife-livestock interface (Jones et al., 2013).