Circadian rhythms of Trypanosoma congolense in laboratory rodents

Abstract
There is a circadian rhythm in the numbers of Trypanosoma congolense appearing in the peripheral blood of rats and mice. The maximum occurs about 22.00 and the minimum about 12.00 hours. The amplitude varies with different strains and hosts, ranging up to 70% of the mean between maximum and minimum. This rhythm was demonstrated in four different strains of T. congolense. It was more marked in rats than in mice and was also present in a guinea-pig. There is also a minor rhythm in the percentages of dividing forms, with a maximum about 06.00, a minimum about 22.00 and an amplitude of about 26%. Nevertheless, the absolute numbers of dividing forms follow the curve for total numbers with a maximum about 22.00, a minimum about 12.00, and an amplitude of about 45%. Accordingly the rhythm in total numbers is due to alterations in the distribution between peripheral blood and deep tissues, and not to rhythm in the multiplication of trypanosomes. Since the maximum of the rhythm in rats and mice occurs when the hosts are most active (i.e. at night) it seems probable that the maximum in cattle would also occur during the phase of activity, i.e. in the daytime, which is the period when tsetse flies suck blood. In that case, the rhythm would facilitate transmission by the insect vector. A laboratory strain of T. vivax in rats and another in mice were examined. The strain in rats probably showed a rhythm with a maximum at 06.00. No rhythms were found in a strain of T. brucei in rats or in another strain in mice. No rhythms was found in an English strain of T. lewisi.

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