A first-of-its-kind field study was recently completed to estimate the risk of acute gastrointestinal illness from airborne pathogens during manure irrigation and to identify the other variables, such as distance and weather conditions that affect airborne pathogen transport. It coincided with a larger effort, known as the Manure Irrigation Workgroup, to explore benefits, concerns and remaining questions associated with manure irrigation.
In an April webinar explaining the findings of the work group, Dr. Mark Borchardt, research microbiologist, USDA Agricultural Research Service, shared how the study was conducted and its results.
During irrigation, manure takes on two different forms – droplets and aerosols. The droplets are heavy and fall to the ground, while the aerosols become airborne. In the air, they become dispersed and diluted. Some will undergo inactivation due to solar irradiance, high temperatures and low relative humidity. The remaining microbes have the potential to reach people through inhalation, landing on their skin, on food they may consume or a vector object, such as a pet. For this study, the researchers focused on direct inhalation.
The large, multidisciplinary study used three approach methods – field data, modeling and risk assessment.
Field data
Borchardt said measurements were collected from 25 field trials on active dairy farms that used a traveling gun, center pivot or tanker with a splash plate to irrigate manure. Due to certain circumstances, only 21 trials were entered into the data set.
Each trial had a portable weather station that continuously collected data. The irrigation events took place during a wide range of weather conditions, with temperatures logged from 42°F to 88°F, relative humidity from 28 percent to more than 80 percent, wind speeds from 2 mph to 13 mph, wind gusts from 4 mph to 21 mph and solar irradiance from darkness to a bright blue Wisconsin June day, he said.
Microbe concentrations were measured at multiple distances from the point of manure application. They were measured with qPCR to get a genetic signal and culture methods to see visible bacteria.
One particular set of culture samples that Borchardt shared was taken on May 22, 2014, from a traveling gun manure irrigation event. Wind speed was 11 mph, temperature at 68°F, 50 percent relative humidity and solar irradiance at 530 watts per square meter. When the samples were placed on culture plates, gram-negative bacteria was absent for the sample collected upwind. There were high concentrations at the point of application, but only two colonies appeared at sample distances of 100 and 350 feet away from the application site. By 500 and 670 feet, no colonies appeared on the culture plates.
“Remember, we have deposition, dispersion (dilution) and inactivation, so we’re left with lower concentrations,” Borchardt said. “Nonetheless, this doesn’t tell us anything about health risk.”
Modeling
Statistical modeling was then used to predict air concentrations for risk assessment and relate air concentrations to weather conditions and microbe concentrations in manure.
“The most important variables for airborne concentrations of microorganisms during manure irrigation are the distance, wind speed and initial pathogen concentrations in the manure. These were the three most important variables,” Borchardt said.
He continued, “Surprising to us, sunshine didn’t turn out to be that important and relative humidity wasn’t all that important either.”
Risk assessment
There are two ways to assess the health risk for infectious disease. The first is to look at groups of people, which wasn’t a viable option here. The other is to use quantitative microbial risk assessment, which relies on dose-response models to estimate the dose of the pathogens and the probability of illness.
Using the quantitative assessment, Borchardt said inputs included pathogen prevalence, distance, age, inhalation rate and time spent outdoors.
He reported it was rare to find pathogens in the manure samples from the three farms in the field study. If a pathogen was present, it was campylobacter. They did not find any salmonella or E. coli.
Therefore, two pathogen surrogates were used to conduct the assessment. These microbes are friendly and typically found in the gastrointestinal tract of the cow. Bovine bacteroides is relatively resistant to inactivation in the environment and provided a worst-case scenario. Gram-negative bacteria was the other surrogate. The ratios established for each were related to the amount of pathogens found in stored manure as cited in established literature.
They looked at four different scenarios reflecting a different level of conservatism to protecting public health. The least conservative view was using the typical prevalence of the level of pathogens with the gram-negative strain. The next was a little more conservative with a typical prevalence, but with the more environmentally resistant bovine bacteroides strain. Then they looked at 100 percent prevalence (saying all dairies have the pathogen) for each type of pathogen surrogate.
In the U.S., there are two acceptable levels of risk for acute gastrointestinal illness. The first is set for drinking water at 1 in 10,000 people per year. The second is for recreational water exposures (e.g., beaches), which is at 32 out of 1,000 swimmers per event. “We used those two because it’s the only benchmark we have available to us,” Borchardt said.
Looking at the median of the risk of distribution – the point at which 50 percent of the risk estimates are above and 50 percent below, which is typical in a quantitative microbial risk assessment – all four scenarios were somewhere between the acceptable risk levels for drinking water and the acceptable risk levels for recreational water in the U.S.
They also studied the 75th percentile of the risk distribution where the risk levels are higher. For the most part, the scenarios were within the acceptable range, with the exception of the most conservative scenario – figuring all dairies have environmentally resistant bacteria – where the risk estimate for salmonella falls below acceptable drinking water levels when at a distance of 500 feet or less from the irrigated manure.
Various risk estimates were done, because it will be up to policy makers to determine how conservative the policies need to be towards protecting public health, whether it is the median level of risk, the 75th percentile or any other level of risk.
By using actual field data, state-of-the-art statistical modeling and risk assessment methods, this unique study was utilized by the Manure Irrigation Workgroup as it made its recommendations for the irrigation of manure from dairy farms.
In short, Borchardt found that illness risk is on the order of 1 in 100,000 to 1 in 100 per irrigation event at 500 feet downwind from the application. That risk level is dependent upon pathogen type and pathogen prevalence. Risk can also be impacted by downwind distance and the number of irrigation events. PD
Read the related article, “Public outcry prompts work group to address 5 concerns for manure irrigation.”
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Karen Lee
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