Effect of challenge dose and route on porcine reproductive and respiratory syndrome virus (PRRSV) infection in young swine.

Porcine reproductive and respiratory syndrome virus (PRRSV) is perceived to be highly infectious because of the rapid spread of the virus through populations of domestic swine throughout the world. However, no information has been published on the minimum infectious dose of PRRSV and the effect of challenge dose on clinical response. In this experiment, ten groups of pigs (n = 3 per group) were inoculated with one of five different quantities (10(1)-10(5) fluorescent foci units per millilitre) of PRRSV (isolate ISU-P) by either intramuscular or intranasal routes. Clinical signs and body temperature were monitored for 21 days. Serum was collected periodically throughout the study period to monitor the presence of virus in serum and the early immune response of pigs. A 2-mL inoculum containing 10(1) fluorescent foci units of virus per millilitre was found sufficient to achieve infection by either route. Time to onset of clinical signs was highly associated with challenge dose (P < 0.01), regardless of route of exposure. However, no dose- or route-dependent differences in the severity of clinical manifestation were observed. No significant differences in the time of onset or degree of humoural immune response to PRRSV infection were observed between different treatment groups. However, intramuscular exposure appeared to induce a more uniform antibody response compared to intranasal exposure. These results confirmed that PRRSV is highly infectious; a fact that should be taken into consideration when designing strategies for the prevention and control of PRRSV.