SERUM CONCENTRATIONS OF THYROID-HORMONES IN CALVES WITH A TRANSIENT OR PERSISTENT INFECTION WITH BOVINE VIRAL DIARRHEA VIRUS

Calves persistently infected with bovine viral diarrhoea virus (BVDV) often appear small for their age and it is possible that the virus int erferes with their body metabolism by affecting the production of horm ones. In this study, the serum concentrations of thyroxine (T4) and tr iiodothyronine (T3) were measured in calves with transient or persiste nt BVDV infections. The mean (SD) concentrations of T3 and T4 were low er (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 a ge-matched control calves (2.39 [0.72] and 93.4 [22.1] nmol litre-l, r espectively). In each herd, the thyroid hormone levels were also lower in the persistently infected calves than in the controls, the differe nces 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 n ot among the controls, there was a significant correlation between hea rt 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 nos e-to-nose contact with a persistently infected calf. Their T3 and T4 l evels were decreased seven days (P<0.05 and not significant, respectiv ely) and nine days (P<0.001 and P<0.01, respectively) after the infect ion, when the clinical signs of the disease appeared.