The transfer of pathogenic bacteria from individual hog operations to other nearby hog operations or human dwellings is a concern of the swine industry yet very little information on pathogen transfer distances is available in the literature. In this study, on-site and downwind viable particle sampling was conducted at a 6,000-head swine finishing operation to determine the influence of meteorological conditions and distance from the buildings on the concentration and dispersion of pathogens and total bacteria emitted from the buildings. Airborne particles (aerosols) were collected by impaction onto blood and MacConkey agar plates using single-stage viable particle samplers. Additional size-fractionated aerosol collections were made within the buildings and at the exhaust fan using six-stage viable particle samplers. A total of 796 blood agar and 209 MacConkey plates were sampled on five days during November and December. Associated atmospheric conditions monitored during each sample day included air temperature, humidity, wind speed and direction, and solar radiation. The experimental data was compared to predictions from atmospheric dispersion models.
The mean concentration of bacteria in the exhaust air was 190,000 cfu/m3 (5,400 cfu/ft3). Results showed elevated (above background levels) atmospheric concentrations of total aerobic bacteria up to 300 to 2,000 ft downwind of the buildings under near-neutral to unstable atmospheric conditions. However, no pathogenic species were detected in any downwind samples. While 25% of incubated MacConkey agar plates showed bacterial colonies, only two agar plates (1%) collected at one of the pit fans had 2 or 3 colonies that tested positive for E. coli. No Streptococcus suis or Salmonella were detected in any of the samples.
The dispersion of viable aerosols in the near-field environment of the buildings was predicted using the point-source, line-source and multi-source Eulerian atmospheric dispersion models. As expected, the multi-point source model was found to be the best predictor of viable aerosol dispersion downwind of the hog operations. Two distinct sources of viable aerosols were evident in the results of the downwind samples. One source was apparently the viable particles emitted from the exhaust fans of the finishing buildings. The other source apparently consisted of viable particles that re-entrained from the soil surface.