Serum concentrations of thyroid hormones in calves with a transient or persistent infection with bovine viral diarrhoea virus.

Calves persistently infected with bovine viral diarrhoea virus (BVDV) often appear small for their age and it is possible that the virus interferes with their body metabolism by affecting the production of hormones. In this study, the serum concentrations of thyroxine (T4) and triiodothyronine (T3) were measured in calves with transient or persistent BVDV infections. The mean (SD) concentrations of T3 and T4 were lower (P < 0.001) in the persistently infected calves (1.79 [0.67] and 69.2 [23.8] nmol litre-1, respectively) from three dairy herds than in age-matched control calves (2.39 [0.72] and 93.4 [22.1] nmol litre-1, respectively). In each herd, the thyroid hormone levels were also lower in the persistently infected calves than in the controls, the differences being significant (P < 0.05) with the exception of the T3 level in one herd. The girth of the infected calves over the heart was less (P < 0.001) than that of the controls, and, among the infected calves but not among the controls, there was a significant correlation between heart girth and the levels of T3 (r = 0.72, P < 0.001) and T4 (r = 0.60, P < 0.01). Six calves not previously exposed to BVDV were infected through nose-to-nose contact with a persistently infected calf. Their T3 and T4 levels were decreased seven days (P < 0.05 and not significant, respectively) and nine days (P < 0.001 and P < 0.01, respectively) after the infection, when the clinical signs of the disease appeared.