The effect of l-thyroxine on the anaemia response in Trypanosoma congolense infected rabbits

Abstract

The development of anaemia is a major pathological manifestation in chronic trypanosomosis. The anaemia in African trypanosomosis coincides with a marked decrease in plasma concentration of both thyroxine (T 4) and 3,5,3′ triidothyronine (T 3). To evaluate the effect of trypanosome-induced hypothyroidism on the development of anaemia, sexually mature white New Zealand rabbits were used. Three groups were set up, each of ten rabbits: one group was infected with Trypanosoma congolense; the second group was infected but given replacement doses of thyroxine (treated); the third group was not infected. Small volumes of blood were collected for the determination of parasitaemia and packed cell volume (PCV). The concentrations of T 3 and T 4 were measured in plasma by radioimmunoassay. The decrease in PCV correlated closely ( y= −0.38 x + 15.2; r=0.82, P=0.001) with the intensity and duration of parasitaemia. The critical PCV value was 0.15 11 −1 with a peak parasitaemia of approximately 5×10 6 trypanosomes ml −1 of blood. There was a significant correlation between the plasma T 3 and PCV ( y=0.049 x+0.57; r=0.66, P=0.020). There was also a good positive correlation between T 4 and PCV ( y=14.5+3.03; r=0.95, P<0.001) in the infected untreated group. The PCV levels were significantly different among the three groups of animals ( P<0.05). The infected-treated animals sustained longer periods of infection than the infected and untreated ones. The sustained physiological level of bioactive thyroid hormones T 3 and T 4 significantly arrested the decline in PCV as the disease progressed. The hormonal treatment thus enhanced the survival of the animals infected with Trypanosoma congolense.