Preliminary characterization of the transmission of porcine reproductive and respiratory syndrome virus
Howard T. Hill
The objective of this dissertation is to provide a more complete characterization of the transmission of porcine reproductive and respiratory syndrome virus (PRRSV). Three papers are presented within the dissertation. The first paper focuses on modes of transmission between infected and susceptible animals. Portals of exit and duration of shedding of virus from infected animals are investigated in the second paper. The final paper concentrates on the occurrence and duration of infection in swine;In the first study, five trials were conducted to study transmission of virus to pigs placed in different degrees of contact with inoculated pigs. The study demonstrated that although direct contact is probably the most efficient mode of transmission it was not required for transmission to occur. Transmission across distances of 102 cm were demonstrated. The failure of transmission to occur in pigs separated by relatively short distances questioned the role of aerosols in PRRSV transmission;The second paper reports the results of 2 trials in which weekly samples were collected from inoculated and control pigs. While the pigs were anesthetized, serum, saliva, conjunctival swab, urine by cystocentesis, and feces were collected. Following anesthesia, the endotracheal tube was rinsed in saline and the rinse retained. All samples were assayed for PRRSV. Virus was isolated through day 14 post inoculation (PI) from urine, day 21 PI from serum, day 35 PI from endotracheal tube rinse, and day 42 PI from saliva. No virus was recovered from conjunctival swabs or fecal samples. Recovery of PRRSV from saliva has not been reported previously. Virus-contaminated saliva, especially when considered in the context of social dominance behavior among pigs, probably plays an important role in PRRSV transmission;In the third study, serum samples were collected from 4 inoculated pigs every 2 to 3 days until day 42 PI and then approximately every 14 days until day 213 PI. Oropharyngeal samples were collected at the time of serum collection on days 56 to 213 PI. Viremia continued up to 23 days. Persistent infection with PRRSV was demonstrated by isolation of virus from oropharyngeal samples for up to 157 days after challenge.