Posted by Dr. Steve Dudley on June 18 2009 13:54
Background
We have been testing the hypothesis that aerosols can transmit PRRSV over distances greater than 1 meter under controlled field conditions. In 2 attempts, we have not observed PRRSV transmission by aerosols originating from an infected finishing pig population to trailers of naïve pigs. Attempts to detect PRRSV from air samples collected in the infected animal airspace, from fan exhaust, or from air tubes using an all-glass impinger (AIG) have not been successful. Critical analysis of these data has identified many variables that may have influenced the outcome, including environmental variables (weather), and physical variables (tube size and configuration, ventilation system, and the potential lack of sensitivity of the AIG). There are also a series of biological variables/unknowns including the quantity of PRRSV contained in swine aerosols, the critical mass of animals required to generate infectious aerosols, the impact of concurrent pathogens (i.e. Mycoplasma hyopneumoniae) and the pattern of dispersion and rate of PRRSV inactivation over time within aerosols.
Recently, a great deal of emphasis has been placed on the ability to mechanically transmit PRRSV during cold weather. Resulting from this information, swine farm biosecurity has improved significantly and subsequently, the number of infections of naïve herds has decreased. However, farms continue to get infected with new strains of PRRSV, particularly in cold weather, and audits of the biosecurity process have eliminated all known non-porcine vectors, fomites, and the introduction of genetic material as sources of the new strain. Therefore, it was decided to re-evaluate aerosol transmission of PRRSV by creating a model that attempted to answer the central question whether PRRSV could be travel from "point A to point B" solely by air. The hypothesis was that PRRSV could travel across a wide range of distance via air currents following neutralization of the variables noted above. The components of the model were as follows:
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Dispersal of virus:
In order to create a "PRRSV aerosol", a household utensil called a cooking oil spritzer was employed. This utensil utilizes the injection of volumes of air into a set quantity of liquid to produce a spray solution made up of very fine particles. For the purpose of the study, 10 ml of PRRSV (MN 30-100 strain, total dose = 1 x10 6 virus particles) was placed into the 200-ml flask portion of the utensil. The flask was manually injected with air (50 strokes) until significant resistance was felt. This quantity of virus and air was considered to be an aliquot of aerosolized PRRSV. It was planned to disperse 3 aliquots of aerosolized virus into the open end of the dissemination device at minute 1, 10 and 30 during the air collection process.
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Dissemination of virus:
To create a means of disseminating virus across a wide range of distance, a viral dissemination pipe was constructed. It was planned to pull aerosolized virus down the length of a pipe. To power the model, a Granger split capacitor blower (1550 rpm, 1.43 amps, ¼ horsepower), capable of generating 470 cfm at 5 cm (2 inches) static pressure to 217 cfm at .3175 cm (1/8 inch) static pressure was obtained. To attach the pipe to the fan, a 10.16 cm (4-inch) PVC pipe-coupler was adhered to the intake port of the fan. Multiple sections of 3.33 m (10 foot) 10.16-cm PVC pipe were attached to the coupler to form a pipeline with a range of 1 meter to 166.6 m (500 feet) that could be easily modified through manually attachment-detachment of multiple pipe sections.
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Detection of virus:
To enhance the sensitivity of the detection methods, a portable air sampler (Spin Con 450, Camber Corp, Arlington, VA) was used. The Spin Con is an air centrifuge that employs periodic mixing of sterile water with the collected air to remove particulate matter, resulting in a 10 ml aliquot of liquid that can be tested. This device has the capability of collecting 450 liters of air per minute, and has been used by the US government to test high-risk facilities for the presence of Anthrax spores in ventilation systems. In contrast, the AIG collects 4 liters per minute. During collection, the intake port of the Spin Con was placed 5 cm from the exhaust port of the Granger fan and a 60-minute collection period was employed.
Design: Each replicate of the study tested the ability of the model to disperse, disseminate, and detect aerosolized virus over a specific distance. Distances evaluated in the study included 3.33 m (10 feet), 6.66 m (20 feet), 13.33 m (40 feet), 20 m (60 feet), 26.66 m (80 feet), 33.33 m (100 feet), 100 m (300 feet), 133.33 m (400) feet and 166.66 m (500 feet).
Each replicate also included the following set of controls:
- Virus control-Aerosolized virus collected directly from the dispersal utensil.
- Sham control-Aersols collected directly from the dispersal utensil (void of PRRSV).
- Protocol control-Application of a sham aerosol to each specific length of pipe prior to the use of aerosolized virus.
- Positive control-Aerosolized virus collected across a pipe length of 1 meter.
- Sanitation control-Following application of aerosolized virus to each specific length of pipe, the inner surface of the pipe was disinfected using a 10 % bleach solution. A cloth fastened to an extension pole that could be extended the length of the pipe applied the disinfectant. Following disinfecting, the interior of the pipe was rinsed with sterile water using a different cloth that was fastened to the other end of the pole. To test the efficacy of this method, the interior walls of the pipe were swabbed using sterile dacron swabs attached to the extension pole.
- Spin Con control-Following each collection, the interior of the Spin Con was automatically disinfected with a 10 % bleach solution and then rinsed with sterile water. Aliquots from each cycle were collected.
Testing methods: All samples were tested by TaqMan PCR and virus isolation.
Preliminary results:
On the sampling days, the environmental temperature was 3-4 degrees C (37-39 degrees F), and percent humidity ranged from 81-86%. Skies were overcast throughout the day with periodic snowfall observed. PRRSV RNA was detected by PCR in air samples collected over the following distances: 1 m, 3.33 m, 6.66m, 13.33 m, 20 m, 26.66 m, 33.33 m, 100 m (2/2 replicates) and 133.33 m (1/2 replicates). Testing of the 166.66 distance is pending. All virus controls and positive controls have been positive, while all other controls have remained negative. VI results are pending.
Preliminary conclusions: Using this model, PRRSV RNA can be transferred across a wide range of distances by air currents. This is the first report of PRRSV transmission by the airborne route that has evaluated distances of greater than 1 meter. Further testing is planned.