Manure management practices recycle nutrients in animal manures for crop production. Harmful microbes and viruses in the manure are generally reduced in the soil environment over time. The soil properties influencing how long animal virus persistence are poorly understood and may be specific even down to the type of microbe present. Recently, porcine epidemic diarrhea virus (PEDV), which causes nearly 100% mortality in newborn piglets, has become a serious challenge for swine production. An important concern is whether PEDV in manure applied to nearby farmland may be a source for herd reinfection. How long will PEDV persist in the soil and still be infectious? Are some soils better suited to reduce PEDV risk?
What did we do?
A laboratory study was conducted to mimic a standard manure application practice (manure slurry application into soil) to determine if it reduced the potential for PEDV reinfection. In our study, we tested a range of soil types spiked with PEDV-positive manure slurry and evaluated how PEDV detection and potential infectious risk was affected by soil type. Quantitative PCR and live swine bioassays were used to enumerate PEDV and to determine whether manure and soil samples contained infectious PEDV (Stevens et al., 2018).
What have we learned?
Manure Slurry/Soil Incubations. PEDV genomes declined at different rates depending upon the type of soil tested (Figure 1). While PEDV declined rapidly and was not detected by PCR in Soil #1, #2, and #5 in just 24 hours, PEDV genomes in Soil #6 and #7 decreased more slowly the other soils. Soils #3 and #4 displayed an intermediate rate of decline and reached our detection limit at 48 hours. Soil is an important factor on PEDV persistence.
Swine Bioassay. Several of the samples tested positive for infections PEDV (Table 2) even when PCR indicated no virus was present; PCR molecular detection of the virus did not produce a complete picture of PEDV survival. For instance, the PCR method indicated no virus in soil #1 or #2 at 24 hours, yet the soil-manure mixture caused disease in a swine bioassay test—the gold standard test for infectious PEDV.
|Table 2. Outcome of Swine Bioassay|
|Manure-slurry Soil Composite|
†Animals inoculated by oral gavage of 10 mL of phosphate buffer-diluted sample. A porcine epidemic diarrhea virus positive (Pos) or negative (Neg) score is based on fecal swab molecular diagnostic test (reverse transcriptase quantitative polymerase chain reaction).
Are there any soil environmental factors that can help predict whether/how long infectious PEDV lasts in soils? Anything that would damage or disrupt the membrane or proteins on the outside of PEDV would render the virus non-infectious. Theoretically moist soils with lots of active bacteria would release enzymes to chew up PEDV proteins or alkaline (high pH) soils may denature PEDV proteins and damage membranes to inactivate PEDV. On the other hand, soils where manure rapidly dries would help preserve PEDV. None of these hypotheses could explain the PCR or swine bioassay results. Only one factor seemed related to PEDV persistence—high soil phosphorous seemed to protect the virus. No single factor seemed to destroy the virus.
Additional studies are underway determining where PEDV is found within three production sites and the surrounding environment immediately after an outbreak of PEDV. The sites will be monitored over 18 months to signs of PEDV re-emergence.
Corresponding author: Dan Miller, Research Microbiologist, USDA Agriculture Research Service; email: Dan.firstname.lastname@example.org
Other authors: Erin Stevens (Department of Animal Science, University of Nebraska – Lincoln); Amy Schmidt (Department of Biological Systems Engineering, University of Nebraska – Lincoln); Sarah Vitosh-Sillman and J. Dustin Loy (School of Veterinary Medicine and Biomedical Sciences, University of Nebraska – Lincoln).
Stevens EE, Miller DN, Brittenham BA, Vitosh-Sillman SJ, Brodersen BW, Jin VL, et al. Alkaline stabilization of manure slurry inactivates porcine epidemic diarrhea virus. Journal of Swine Health and Production. 2018;26(2):95-100.
Funding for this research was provided by the National Pork Board and USDA Agriculture Research Service operational funds. USDA is an equal opportunity provider and employer.
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