A calf persistently infected with bovine virus diarrhoea virus (BVDV) was super-infected with a heterologous BVDV strain, C874, which contained non-cytopathogenic and cytopathogenic viruses. High titres of cytopathogenic BVDV were recovered in the three to four weeks after the challenge. Thereafter low titres of cytopathogenic virus were recovered repeatedly from the blood and the nose, with the titres in nasal secretions increasing in the four weeks before the onset of clinical signs. Neutralising antibodies against the challenge cytopathic virus (C874cp) were first detected 21 days after the super-infection, but these antibodies failed to neutralise the persisting noncytopathogenic and cytopathogenic viruses isolated from the animal during the course of the infection. Serum collected from 105 days after the super-infection neutralised the cytopathogenic viruses isolated on day 105 and postmortem. These data indicate that unaltered wild-type C874cp was not directly responsible for the late-onset mucosal disease.
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