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The objective of this research was to quantify the likelihood of PRRSV transmission via aerosol as a function of exposure dose. This data is critical to understanding of airborne transmission of PRRSV between pigs, within herds, and between herds.
Materials and methods
The study used PRRSV isolate MN-184 (kindly provided by Dr. Scott Dee, UMN). All procedures were approved by the Iowa State University IACUC committee (#9-07-6429-S). All pigs were confirmed PRRSV negative prior to commencement of the experiment and were housed in HEPA-filtered isolation units throughout the experiment to avoid inadvertent transmission of pathogens.
The study was conducted in 10 replicates, 10 pigs per replicate, with pigs randomly assigned to treatment. One negative control pig and one positive control pig were included in each replicate. The negative control pig was used to validate biosecurity procedures, i.e., was present in the laboratory during the aerosol exposure treatments, was housed in the same room as pigs exposed to PRRSV aerosols, and was monitored throughout the observation period. The positive control pig was inoculated intramuscularly with 10 ml of fluid from the last air sample in each run to prove that infectious, airborne PRRSV was in the air to which pigs were exposed throughout the experiment.
To conduct the experiment PRRSV MN-184 was aerosolized into a dynamic aerosol toroid. Pigs to be exposed to the PRRSV aerosol were anesthetized, fitted with a canine surgical mask attached to a pediatric spirometer. Each pig respired 10 liters of virus aerosol. Air samples were collected before and after each pig were used to estimate the exposure dose.
Serum samples collected 5 and 10 days post-exposure (DPE) were tested for the presence of PRRSV to determine whether exposure resulted in infection. The dose-response curve for exposure to airborne PRRSV was derived from the proportion of pigs infected by dose.
Results and discussion
Three replicates were disqualified due to failure to meet quality criteria; therefore, the infectious dose 50 (ID50) estimate was based on 7 replicates. Initial analysis showed that the infective dose 50 (ID50) of MN-184 under the parameters of this study (pig body size and age, exposure dose and time) was <1 x 101 TCID50. Under comparable conditions, this ID50 estimate is much lower than a previous estimate based on PRRSV isolate VR-2332 (Hermann et al., 2009). Thus, the data suggest that (1) the ID50 for airborne PRRSV MN-184 is extremely low and (2) the ID50 varies among isolates.
Materials and methods
The study used PRRSV isolate MN-184 (kindly provided by Dr. Scott Dee, UMN). All procedures were approved by the Iowa State University IACUC committee (#9-07-6429-S). All pigs were confirmed PRRSV negative prior to commencement of the experiment and were housed in HEPA-filtered isolation units throughout the experiment to avoid inadvertent transmission of pathogens.
The study was conducted in 10 replicates, 10 pigs per replicate, with pigs randomly assigned to treatment. One negative control pig and one positive control pig were included in each replicate. The negative control pig was used to validate biosecurity procedures, i.e., was present in the laboratory during the aerosol exposure treatments, was housed in the same room as pigs exposed to PRRSV aerosols, and was monitored throughout the observation period. The positive control pig was inoculated intramuscularly with 10 ml of fluid from the last air sample in each run to prove that infectious, airborne PRRSV was in the air to which pigs were exposed throughout the experiment.
To conduct the experiment PRRSV MN-184 was aerosolized into a dynamic aerosol toroid. Pigs to be exposed to the PRRSV aerosol were anesthetized, fitted with a canine surgical mask attached to a pediatric spirometer. Each pig respired 10 liters of virus aerosol. Air samples were collected before and after each pig were used to estimate the exposure dose.
Serum samples collected 5 and 10 days post-exposure (DPE) were tested for the presence of PRRSV to determine whether exposure resulted in infection. The dose-response curve for exposure to airborne PRRSV was derived from the proportion of pigs infected by dose.
Results and discussion
Three replicates were disqualified due to failure to meet quality criteria; therefore, the infectious dose 50 (ID50) estimate was based on 7 replicates. Initial analysis showed that the infective dose 50 (ID50) of MN-184 under the parameters of this study (pig body size and age, exposure dose and time) was <1 x 101 TCID50. Under comparable conditions, this ID50 estimate is much lower than a previous estimate based on PRRSV isolate VR-2332 (Hermann et al., 2009). Thus, the data suggest that (1) the ID50 for airborne PRRSV MN-184 is extremely low and (2) the ID50 varies among isolates.
T.D. Cutler, A. Kittawornrat, S.J. Hoff, C. Wang, J. J Zimmerman. Median infectious dose (ID50) of PRRSV isolate MN-184 for young pigs via aerosol exposure. 2010 AASV Annual Meeting: Implementing Knowledge; 362.
