Manure contains microorganisms that are naturally present in the gut of an animal, some of which are pathogenic. Pathogens in manure (bacteria, viruses, parasites, or fungi) can vary in concentration and type. Some pathogens in manure are zoonotic, meaning they can be transmitted between animals and humans. When these pathogens are transferred to humans, they can cause illness.

Some common pathogens found in manure that pose risk to humans include Salmonella spp., E. coli O157:H7, Campylobacter spp., Listeria monocytogenes, and Giardia spp.

When manure is applied to cropland, pathogen loss to surface and ground waters as well as potential contamination of growing food pose risks to humans if they come in contact with the pathogens. Risk can be reduced with proper handling to reduce exposure routes and manure processing techniques to inactivate pathogens before land application.

Manure pathogens can be inactivated in a variety of ways including composting, anaerobic digestion, advanced treatment, pelleting, drying, and pyrolysis. Most of these methods use heat to inactivate pathogens, but there are other conditions and processes that can reduce the pathogen concentration as well (e.g. physical separation, ultraviolet light (UV), chemicals).

While it is not common to raise the temperature to standards for pasteurization (145 degrees F for 30 minutes or 161 degrees F for 15 seconds), there are some places in the world that use this technique, although it is expensive.

Processing techniques like composting and anaerobic digestion also raise the temperature of manure. Composting may not result in pathogen inactivation rates as high as pasteurization but can significantly reduce pathogen concentrations reducing risk of transport to humans or animals.

Many standards recommend compost systems reach 131 degrees F for 3 to 15 days depending upon the compost method used to significantly reduce pathogen concentrations. Time and temperature recommendations for anaerobic digestion include a 15 day residence time for temperatures from 95 to 131 degrees F.

Manure solid liquid separation systems may not necessarily inactivate pathogens, but they commonly separate a greater fraction of the pathogens into the liquid stream, decreasing the concentration in the solid stream as compared to the unseparated slurry. The solid stream could then be used in places where there may be higher risk for loss or transport to humans or animals.

For all processes, it is critical to maintain specific operating conditions and record the information so the process can be effective and documented. It is also important to limit pathogen regrowth following a manure processing system so risk of illness does not then increase.

Handling manure properly can also reduce risk. Storing manure can result in reductions of pathogens where many studies have shown limited survival after three to six months of storage. However, it should be noted that some pathogens can survive much longer during manure storage so while this may reduce some pathogens, there is still risk.

In addition, manure is commonly added to storages throughout the storage period so you will have shortened time for manure added closer to emptying the storage.

Pathogens have variable survival rates following land application based on the pathogens in the manure and the conditions of the field. Once it reaches the field, there are not a lot of management options to further reduce risk.

When applied to crops, recommended best management practice is to avoid direct contact with growing crops and maximize the duration from manure application to harvest, but specific care should be taken if using on food for human consumption.

There are many practices that can be integrated in all steps of manure management to reduce the risk associated with pathogens. Decisions should focus on how and where the manure will be used and the practices determined based to minimize risk.

If you are interested in more information on pathogen management in manure systems, please attend the 2019 Midwest Manure Summit where USDA ARS research microbiologists will provide more detailed information on pathogen management and risk reduction.

Larson is an Associate Professor and Extension Specialist with the University of Wisconsin-Madison

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