Detection of Foot-and-mouth Disease by Reverse Transcription Polymerase Chain Reaction and Virus Isolation in Contact Sheep Without Clinical Signs of Foot-and-mouth Disease

Two non-vaccinated sheep were experimentally infected with FMDV and one day later 4 other sheep were brought in contact. Although the contact sheep showed no clinical signs, serology indicated that all sheep became infected. Various secretion samples, taken over a period of at least one month, and various tissue samples were examined for the presence of FMDV by RT-PCR and by virus isolation. FMDV was most often found in saliva (mouth swabs), followed by nasal secretion and sera. Faecal material, wool and milk were less suitable. The period of detection with the highest frequency of positive isolations was between 2 to 4 days pi for the infected sheep and between 5 to 10 days pc for the contact animals. It was established that in subclinically infected sheep, with a very low amount of virus present, FMD viral RNA could be detected by a sensitive RT-PCR-ELISA although virus isolation and standard RT-PCR remained negative. Moreover there was some evidence of active spreading of FMDV from the contact sheep to two sentinel pigs. This indicates that serologically positive contact sheep without clinical signs may be considered as a danger for the transmission of FMDV.