The role of the defective interfering particle DI9c in mucosal disease in cattle

Mucosal disease occurs in cattle persistently infected with a noncytopathogenic strain of bovine viral diarrhoea virus (BVDVnc) following in utero infection. The disease can be initiated by superinfection with a cytopathogenic biotype (BVDVc) of the virus with antigenic "homology" to the persisting virus. A BVDVc isolated from a clinical case of mucosal disease has been discovered to consist of a defective interfering particle, DI9, and an associated BVDVnc helper virus. A defective virus corresponding to DI9 was recently recovered from an infectious cDNA clone and was named DI9c. To evaluate the role of DI9 in the pathogenesis of mucosal disease a two-part experimental study was carried out which included clinical, haematological, pathological and virological investigations. Eight of nine calves persistently infected with BVDVnc were experimentally inoculated with DI9c. The defective virus was propagated in cells preinfected with the same strain of virus used to persistently infect the calves in utero. The calves were euthanased on days 4, 7, 14, 21, 28, 40, 40 or 87 post inoculation. None of the inoculated animals developed classical mucosal disease, neither clinically nor pathologically. DI9c was not found in serum, nasal swab or tissue samples from the calves by observing cytopathogenic effect and/or using a polymerase chain reaction after reverse transcription (RT-PCR) of viral RNA. DI9c did not replicate to a detectable extent in these assays, and its participation in the pathogenesis of mucosal disease could not be proven.