University of Minnesota – “Incidence of PRRS transmission during gestation in sow herds chronically infected with PRRS virus”

Summary:
In the field, swine practitioners are using the incidence of PRRSV positive litters at birth as an estimation of PRRSV activity in the sow herd. However, in some cases, the low virulence of the strain and the low incidence of infection make it difficult to detect. Questions to be answered are: how many litters and pigs within litters should be sampled to detect infection? What is the effect of pooling samples on the sensitivity to detect PRRSV positive individuals by PCR technique? What is the effect of dose of a mildly virulent strain of PRRS on incidence? And, can samples be collected by filter paper disks? A filter paper disk protocol to collect blood samples for detection of PRRSV by PCR has been described in Europe, but little is known about the ability to detect U.S. strains and the effect on test sensitivity when these disks are pooled. Therefore, the overall goal of this experiment is to study the dynamics of infection in pregnant gilts, concentrating on the litter as the outcome. This experiment will provide important evidence related to the number and characteristics of litters and piglets to be sampled. The trial also will provide the opportunity to validate sampling protocols necessary for accurately monitoring these herds.

Objective:
This study will attempt to determine the incidence of traNSPlacental PRRSV infection and the proportion of infected piglets within infected litters following infection of the dam with a low dose of a low virulence isolate. Additionally, attributes of individual piglets and litters that make them more likely to be detected as PRRSV positive by PCR will be identified. A new filter disk technique for PRRSV PCR analysis will be evaluated and used to determine the amount of pig-to-pig PRRSV transmission that occurs within 24 hours of birth.

Relevance:
One of the priorities for the PRRS Initiative is the design and implementation of regional eradication protocols. In cases of regional eradication, the management of endemically infected herds plays a very important role. Sow herds represent the source of most PRRS virus that is propagated downstream in nurseries and finishing barns, therefore if PRRS can be stopped in the sow herd, the chances for controlling and eradicating PRRS from the nursery and finishing are improved. Understanding how to evaluate these herds in terms of how many and which piglets should be sampled per litter, the effect of pooling on the sensitivity to detect positive individuals and the use of the filter paper disk protocol to sample piglets would assist the decision-making process regarding the selection of control and/or eradication strategy.