Effect of African trypanosomiasis on plasma cortisol and thyroxine concentration in goats

Changes in plasma cortisol and thyroxine (T4) levels were measured weekly in female goats experimentally infected with Trypanosoma congolense. Values for plasma cortisol (range 10 to 25 nmol litre-1) and T4 (range 65 to 120 nmol litre-1) were within normal ranges in all goats before infection and in control animals throughout the 24 weeks of study. Cortisol/T4 ratios of 0.23 to 0.15 (or 1:4 to 1:7) were obtained. In the infected goats a significant increase in cortisol and decline in T4 were simultaneously observed within one week of the onset of parasitaemia and fever. A peak cortisol/T4 ratio of 2.0 (2:1) was obtained four weeks after infection when cortisol levels rose to 59.0 +/- 8.9 nmol litre-1 and T4 declined to 29.4 +/- 2.2 nmol litre-1. Thereafter the mean levels fluctuated but remained high (over 30 nmol litre-1) for cortisol and low (under 50 nmol litre-1) for T4 up to 18 weeks after infection. Both hormones tended to return to normal levels towards the end of the study. The changes in mean cortisol levels showed a significant inverse correlation with changes in T4 (r = -0.57, P less than 0.001, n = 26). It is suggested that in trypanosomiasis, hypothalamic stress causes increases in plasma cortisol levels and at the same time suppresses the activity of the thyroid gland.