Across the world, large-scale livestock farming has expanded rapidly in recent years. However, new scientific evidence shows an association with increased human health issues in both farmers and neighboring populations. A commentary by Smit and Heederik  recently published in GeoHealth explores how viruses, bacteria and air contaminants derived from livestock farming cause respiratory health problems in humans. Daniela Ceccarelli, one of the journal’s editors, asked Lidwien Smit, co-author of the commentary, some questions about the health risks associated with intensive farming and how these could be addressed with improved agricultural planning.
What are the major livestock-associated risks to human health?
Infectious diseases that can be transmitted from animals to humans, so-called zoonoses, are a threat. For example, avian influenza, Salmonella and Campylobacter infections are livestock-associated zoonoses. While large-scale epidemics in the general population due to airborne transmission are rare, infected dairy goat farms caused a large Q-fever outbreak in the Netherlands about a decade ago.
Some people are also concerned about antibiotic resistant bacteria; while this is a risk for everyone through foodborne pathogens, scientists are still gathering evidence to determine whether living near a farm puts you at increased risk [Moyer, 2016]. I think the most important health threat for neighbors comes from inhalation of air pollutants. Like traffic-related pollution, this can cause serious respiratory and cardiovascular effects.
How does agriculture contribute to air pollution?
Livestock farms – particularly poultry and swine barns – emit large amounts of dust particles from manure, bedding material, straw, animal feed, feathers, skin flakes and hair. The dust may be contaminated with bacteria and viruses that are mostly harmless for humans, although pathogenic microorganisms such as avian influenza virus or Coxiella burnetii, the bacterium causing Q-fever, can under certain circumstances be found in the air near farms.
Farm operations also emit a mixture of gases such as ammonia, an irritant gas that is formed by enzymes in animal waste. Ammonia is primarily emitted by cattle farms, and by the application of manure to agricultural land. Ammonia reacts with combustion-derived gases in the atmosphere (primarily from industrial and traffic emissions) to form secondary inorganic aerosols, which contributes to fine dust air pollution.
Do farmers experience greater health threats than the general population?
Obviously, farmers are exposed to much higher exposure levels, and contact with farm animals is a major risk factor for carrying antibiotic resistant bacteria. Long-term exposure to barn dust can also cause chronic respiratory conditions. That said, farmers are a working-age population group (aged 16-65) often with a relatively healthy lifestyle. Moreover, they may have built up immunity in the past, for instance to the bacterium that causes Q-fever. However, new inhabitants of livestock farming areas, and also vulnerable groups such as children, the elderly, and people with chronic illnesses may be at higher risk to become ill, despite the lower ambient exposure levels.
What specific research studies have you been involved with?
In recent years, I coordinated a large-scale epidemiological study among 2500 neighboring residents of livestock farms. We conducted the study in the southeast of the Netherlands, a region that is both densely populated and characterized by a large number of intensive swine, poultry, cattle and goat farms.
Participants attended a medical examination that included lung function tests, blood and feces collection, and a questionnaire.
Ambient ammonia concentrations turned out to be associated with worse lung function [Borlée et al, 2017], while people living closer to poultry and goat farms were at increased risk of pneumonia. As a direct result of our research, the Dutch government now plans to reduce the emissions from poultry barns by 50% over the next ten years.
How can scientific knowledge help improve agricultural policy making?
First of all, it is of major importance that the scientific community, policy makers, farmers and other stakeholders are aware that agriculture is one of the major sources of air pollution. Clearly, more stringent environmental regulations need to be enforced, while taking into account animal welfare and the economic viability of (family) farm operations. Better knowledge of exposure-response relationships in the general population will help to inform whether the risk of living near a farm is acceptable or not.
Where are additional research efforts needed in this field?
Despite clear associations between lung health and farm proximity, it needs to be elucidated which of the multi-pollutant mixture of farm emissions are actually causing the observed respiratory effects. That will help to implement better preventive measures. Furthermore, we need to know more about the effects of farm-related pollution on the most vulnerable people.
—Daniela Ceccarelli, Editor for GeoHealth and Department of Bacteriology and Epidemiology, Wageningen Bioveterinary Research, The Netherlands; email: [email protected]; and Lidwien Smit, Institute for Risk Assessment Sciences, Utrecht University, The Netherlands