Effect of cross-fostering piglets on within-litter transmission of porcine reproductive and respiratory syndrome virus under field conditions

Objective: This field study aimed to evaluate the effect of cross-fostering on the porcine reproductive and respiratory syndrome virus (PRRSV) transmission within litters. Materials and methods: A breed-towean farm was selected 15 weeks after a PRRSV outbreak with a L1C.5 strain. Litters included in the study had all piglets individually identified and sampled. Litters were classified as positive when at least one piglet tested positive (PRRSV-2 RNA by quantitative reverse transcriptase-polymerase chain reaction). Cross-fostering was carried out within 48 hours after birth to establish five study groups: one positive piglet moved to a negative litter (n = 15), one negative piglet moved to a positive litter (n = 3), one negative piglet moved to a negative litter (n = 13), and two control groups without cross-fostering (positive litters, n = 5; negative litters, n = 14). All piglets were resampled prior to weaning. The increase in new PRRSV-positive piglets within litters was compared across study groups. Results: Compared to negative litters without cross-fostering, there was no difference in the increase of new PRRSV-positive piglets for groups where negative piglets were moved to either negative or positive litters or the group where a positive piglet was introduced into a negative litter. However, positive litters that did not receive any piglets had a significantly higher count of new PRRSV-positive piglets. Implication: Under the conditions of this study, cross-fostering did not influence the increase in new cases of PRRSV-positive piglets.