Increasing agricultural production to feed >11 billion people by 2100 raises several challenges for effectively managing infectious disease. Of many factors examined in this article linking agricultural expansion to infectious disease, one is conversion of natural habitat to cropland or rangeland. Land conversion increases contact between wild animals, livestock and humans.
As natural ecosystems are converted to crop land or range land, interactions among humans, and domesticated and wild animals, could increase. … These interactions are crucial because 77% of livestock pathogens are capable of infecting multiple host species, including wildlife and humans, and based on published estimates from the 2000s, over half of all recognized human pathogens are currently or originally zoonotic, as are 60–76% of recent emerging infectious disease events [Rohr 2019: 451].
“As natural ecosystems are converted to crop land or range land, interactions among humans, and domesticated and wild animals, could increase” [Rohr 2019: 451].
Land conversion pushes humans and livestock up against wilderness areas, increasing contact between species with previously little to no contact. The jumping of a pathogen to a new host species is called “spillover.”
Spillover appears to be a function of the frequency, duration and intimacy of interactions between a reservoir and novel host population. For example, influenza is believed to have jumped from horses to humans soon after domesticating horses and then made additional jumps to humans from other domesticated animals, such as poultry and swine [Rohr 2019: 451].
Furthermore, agricultural intensification tends to involve greater concentrations of a single variety of a single species, increasing the risk that any new disease will spread quickly in the population.
A central tenet of epidemiology is that the incidence of many infectious diseases should increase proportionally with host density because of increased contact rates and thus transmission among hosts. Hence, increasing human and livestock densities could cause increases in infectious diseases unless investments in disease prevention are sufficient to prevent these increases [Rohr 2019: 451].
Industrial-scale confined livestock production is
vulnerable to devastating losses of animals to disease. For instance, in just the last 25 years, an influenza A virus (H5N1) and a foot-and-mouth outbreak led to the destruction of more than 1.2 million chickens and 6 million livestock in China and Great Britain, respectively, and a ‘mad cow disease’ epizootic led to the slaughter of 11 million cattle worldwide [Rohr 2019: 449].
Increased agricultural production tends to be accompanied by new irrigation infrastructure and increased pesticide, fertilizer and antibiotic use, all of which increase infectious disease risk. Dams (often created for irrigation schemes) increase risk of mosquito-borne disease. Antibiotic overuse for livestock fosters resistance among pathogens that can also infect humans. Greater pesticide use leads to resistance among disease vectors such as mosquitoes to insecticides, while also weakening immune systems among exposed humans and wildlife hosts, increasing infection rates/severity. Nutrient enrichment caused by fertilizer can also contribute to the spread of infectious disease, for example, through mosquitos or snail vectors.
Finally, the urbanization and globalization associated with agricultural intensification/expansion elongates food supply chains, which increases movement of people and goods over borders, spreading food-born illness, flu and other infections.
These analyses revealed that agricultural drivers were associated with 25% of all diseases and nearly 50% of zoonotic diseases that emerged in humans since 1940. These values are even higher if we include the use of antimicrobial agents as an agricultural driver of human disease emergence, given that agricultural uses of antibiotics outpace medical uses in the developed world nearly nine to one [Rohr 2019: 451].
The authors recommend numerous measures for improving agricultural production while limiting infectious disease, including reducing antibiotic use for livestock, conserving biodiversity, improving and diversifying livestock and crop genetic material, investing in urban agriculture, social investments, and inter-disciplinary research and collaboration.
Rohr, Jason R., et al., 2019, Emerging human infectious diseases and the links to global food production, Nature Sustainability 2, https://www.nature.com/articles/s41893-019-0293-3.