Primary Bovine Viral Diarrhoea Virus Infection in Calves Following Direct Contact with a Persistently Viraemic Calf

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Abstract
Summary: Six calves, aged 24 to 58 days and not previously exposed to bovine viral diarrhoea virus (BVDV), were infected with this agent by nose‐to‐nose contact with a persistently BVDV viraemic calf. The study was conducted in two trials, using 3 calves in each. All 6 calves showed a peak interferon level in serum at 4 days post infection (dpi), and they seroconverted to BVDV at 16–21 dpi. The calves in trial 1 had diarrhoea for 2 or 3 days between 2 and 6 dpi and one calf again from 9 to 11 dpi. During the periods of fever, the calves were slightly depressed. Those in trial 2 were more depressed and their oral and nasal mucous membranes were reddened but they never had diarrhoea. In both trials, fever (up to 41.3°C) was a prominent symptom at 8 to 9 dpi and 2 calves showed a diphasic fever course. Respiratory affection was mild and no medical treatment was required. Haematological assessment demonstrated a transient but significant leukopenia and lymphopenia at 4 dpi (P < 0.01 and P < 0.05 respectively) and 11 dpi (P < 0.05 and P < 0.01 respectively). A significant decrease in thrombocyte count was seen at 4 dpi (P < 0.05, n = 3).This study has demonstrated that nose‐to‐nose contact is an effective way of transmitting BVDV from persistently infected to susceptible cattle.