Manure Management and PRRS Outbreaks: A 12-Fold Higher Risk in the 15 Days Post-Handling

This article explores the alarming link between swine manure management practices — specifically pit/lagoon agitation and emptying — and the occurrence of Porcine Reproductive and Respiratory Syndrome (PRRS) outbreaks.

The study, conducted between July 2019 and June 2020 and collecting data from 150 affected sow farms, demonstrates a definitive temporal relationship between these practices and the probability of reporting a PRRS outbreak. Authors Carles Vilalta, Julián Montoya, and César Corzo published this information in a scientific article available in Spanish.

    

The data is clear: the risk of a PRRS outbreak is 12 times greater during the first 15 days after emptying manure pits and lagoons compared to the rest of the year. This risk remains consistent across various conditions, including different seasons, types of manure storage (pit vs. lagoon), geographical location (high vs. low pig density), herd size, and the farm's PRRSV status at the time of the outbreak.

In a significant finding from the Morrison Swine Health Monitoring Project (MSHMP) at the University of Minnesota (UMN), 40% of sow farms experienced a PRRS outbreak within one month after agitating manure pits or lagoons. The primary months for pit emptying in the studied U.S. farms were October, April, and November.

  

Producers and professionals must implement strict biosecurity measures during the period immediately following manure handling—especially the first 15 days—to mitigate the high transmission risk. The concentrated movement of tractors and application equipment during these peak handling periods increases the infection pressure due to the aerosolization of manure (both inside and outside the farm) and the contamination of nearby fields and roads from constant vehicle traffic.

While the virus has low survival in manure (typically short-lived and temperature-dependent, between one to eight days in suspension ), the act of agitation and application appears to be the critical trigger. This highlights the need to re-evaluate operational biosecurity protocols during the crucial window of manure application.

    

Despite the strong temporal correlation, the exact mechanisms explaining the surge in outbreaks remain unknown. Viability studies show PRRSV survival in manure is short—only recoverable for the first day after adding the virus in one study, and generally up to eight days in another, depending on temperature.

Furthermore, even when the virus is detected in manure, the viral load is typically low (Montoya et al., 2021, reported a mean Ct value of 37.56 in positive samples, indicating a low viral load). The current hypotheses for the outbreaks focus on the "neighborhood effect"—where nearby farms share a higher transmission risk due to simultaneous manure application activity and the resulting viral aerosolization and field/road contamination. This raises a key question about the infectivity of low-titer aerosols created during handling.

The data also confirm the cyclical and seasonal nature of PRRS in the U.S., with the epidemic phase peaking between January and February, and a second, less evident peak occurring in late spring/early summer, both periods coinciding with manure agitation and application.

    

Given the known low survival and low viral load of PRRSV in stored manure, is the primary risk truly from aerosolization of residual virus during agitation, or is the "neighborhood effect" of shared vehicles and personnel moving between farms during concentrated application periods the main, preventable biosecurity failure?