Alterations in plasma luteinising hormone and testosterone concentrations and responses to injection of gonadotrophin-releasing hormone in sheep infected with Trypanosoma congolense

The influence of experimental Trypanosoma congolense infection on plasma luteinising hormone (LH) and testosterone and the pituitary-testicular responsiveness to gonadotrophin-releasing hormone (GnRH) during the acute (Day 22) and chronic (Day 62) phases of infection were examined in nine uninfected control and ten infected rams. Blood samples were collected twice a week starting 4 weeks prior to infection and three times a week during the infection period which lasted 79 days, and at 20 min for 1 h before and 3 h after injection of GnRH (20 mg iv). Plasma testosterone levels started declining after the onset of parasitaemia within the first week of infection and mean testosterone concentrations in infected rams were significantly lower (P < 0.01) than in control rams. No significant difference in circulating LH concentration was observed between control and infected rams after infection, although the mean plasma LH concentration after week 8 in the latter appeared to be lower. LH response to GnRH on Day 22 was higher (P < 0.05) in infected rams and on Day 62 was similar in the control and infected rams. Testosterone responses after GnRH on both occasions were not significantly different although pre-injection means were lower in the infected rams. The evidence from this study suggests that the pituitary responsiveness to GnRH was not impaired by a T. congolense infection in rams and the pituitary and testicular endocrine functions are affected differently in trypanosomosis.