The 24-Hour Periodicity of Microfilariae: Biological Mechanisms Responsible for Its Production and Control

An explanation is presented which covers most of the experimental data about the mechanisms by which the periodicity of microfilariae is maintained: During the night-time (with Wuchereria bancrofti and similar filariae) the microfilariae are evenly distributed throughout the blood and they are thus available for ingestion and transmission by mosquitoes. During the day-time they accumulate in the small vessels of the lungs, and hence they are few in the peripheral blood; this phase is probably adapted to allow the microfilariae to enjoy favourable physiological conditions in the lungs. The accumulation is due to an active reflex by the microfilariae themselves; and it probably depends on a sideways migration through the precapillary network of arterioles. The factor in the lungs which holds up the passage of the microfilariae so that they accumulate there (in preference to the capillaries of other organs), is the great increase in oxygen tension, which may be termed the ‘oxygen barrier’. The 24 h cycle of the microfilariae is orientated to the 24 h cycle of the host; and some rhythmic change in the host acts as a cue to the microfilariae. Each microfilaria has a weak endogenous circadian rhythm of its own, but the rhythms of the individual microfilariae are dominated by that of the host, so that all the different individuals do approximately the same thing at the same time, and they do it at the right time (i.e. right for transmission). Different species of microfilariae respond differently to the same stimuli, and they depend on different arrangements for the maintenance of their rhythms. Three main groups of periodic microfilariae may be recognized. (а) W. bancrofti, Brugia malayi, etc. These depend upon the absolute size of the venousarterial (VA) difference in oxygen tension (‘oxygen barrier’) which is lower by night (e.g. 40 mmHg) than it is by day (e.g. 55 mmHg) and so the microfilariae pass through the lungs by night but accumulate there by day. If at night the patient is caused to breathe oxygen, the arterial oxygen rises; or if he is caused to take vigorous muscular exercise, the venous oxygen tension falls; in both cases the VA difference becomes greater and the microfilariae accumulate in the lungs. (b) Loa loa of man, Edesonfilaria malayensis of monkeys in Thailand and Monnigofilaria setariosa of East African mongooses. In this group the sensitivity of the microfilariae to the oxygen barrier is greatly increased or decreased by the 24 h changes