Quantitative microbial risk assessment for spray irrigation of dairy manure based on an empirical fate and transport model

Authors: Burch, Tucker; Spencer, Susan; STOKDYK, JOEL - Us Geological Survey (USGS); KIEKE, BURNEY - Marshfield Clinic Research; LARSON, REBECCA - University Of Wisconsin; FIRNSTAHL, AARON - Us Geological Survey (USGS); RULE, ANA - Johns Hopkins University; Borchardt, Mark

Submitted to: Environmental Health Perspectives
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2016
Publication Date: 8/16/2017
Publication URL: https://handle.nal.usda.gov/10113/5801870
Citation: Burch, T.R., Spencer, S.K., Stokdyk, J.P., Kieke, B.A., Larson, R.A., Firnstahl, A.D., Rule, A.M., Borchardt, M.A. 2017. Quantitative microbial risk assessment for spray irrigation of dairy manure based on an empirical fate and transport model. Environmental Health Perspectives. https://doi.org/10.1289/EHP283.

Interpretive Summary: Land application of livestock manure by irrigation with center pivot or traveling gun equipment is becoming more common in the United States. Manure irrigation is economically beneficial particularly to large livestock operations as it reduces manure hauling by truck and increases flexibility for applying manure multiple times to agricultural fields throughout the growing season. However, one potential downside is that any pathogens present in irrigated manure will be sprayed into the air and could be transported downwind, causing illness in people and livestock. We measured air concentrations of microorganisms during 21 manure irrigation events on three Wisconsin dairy farms that use center pivot and traveling gun irrigation methods. Results show that microorganism concentrations in air decline with increasing distance downwind from the irrigated site, but microorganisms can still be detected 700 feet downwind depending on wind speed and the initial concentration of microorganisms in manure. We further show that the risk for acute gastrointestinal illness for exposure to airborne pathogens 500 feet downwind from dairy manure irrigation is in the range of 1 in 10,000 to 1 in 100 per irrigation event. The level of risk depends primarily on pathogen type, pathogen prevalence on dairy farms, downwind distance from the irrigation equipment, and the number of irrigation events during a growing season. Our risk estimates are in the same range as risk levels established as acceptable in the United States for disease transmission from drinking municipal water or swimming at a beach. Our study is the first to use measured concentrations of airborne pathogens during manure irrigation to estimate human health risk. The findings will be useful to policymakers and public health officials for establishing safe setback distances from irrigated dairy manure.

Technical Abstract: Background: Spray irrigation for land-applying livestock manure is increasing in the United States as farms become larger and economies of scale make manure irrigation affordable. However, human health risks from exposure to zoonotic pathogens aerosolized during manure irrigation are not well-understood. Objectives: 1) Estimate human health risks due to aerosolized zoonotic pathogens downwind of spray-irrigated dairy manure and 2) determine which factors (e.g., distance, pathogen concentration in source manure, weather conditions) control risk estimates. Methods: We sampled downwind air concentrations of manure-borne fecal indicators and zoonotic pathogens during 21 full-scale dairy manure irrigation events at three farms. We fit these data to hierarchical, empirical models and used model outputs in a quantitative microbial risk assessment to estimate risk (probability of acute gastrointestinal illness) for individuals exposed to spray-irrigated dairy manure containing Campylobacter jejuni, enterohemorrhagic Escherichia coli, or Salmonella spp. Results: Median risk estimates from Monte Carlo simulations ranged from 10-5 to 10-2 and decreased with distance from the source. Risk estimates were most sensitive to pathogen prevalence in dairy manure. Airborne microbe concentrations were positively associated with wind speed, which was the most important weather condition. Conclusions: Median risk estimates were the same order of magnitude or lower than acceptable levels of risk in the United States for exposures to drinking water or recreational water. Reducing pathogen prevalence and concentration in source manure would most effectively mitigate risk. Maximizing distance from irrigated manure (i.e., setbacks) and limiting irrigation to times of low wind speed would also benefit public health.