The periodicity of microfilariae VIII. Further observations on Wuchereria bancrofti

Abstract

1) Stimulation of sympathetic nerves by the administration of isoprenaline at night usually caused a fall in the number of microfilariae in the blood of patients infected with Wuchereria bancrofti. (Stimulation of parasympathetic nerves by acetylcholine etc., has no effect upon the microfilariae of W. bancrofti). 2) Administration of hydrocortisone in the evening had no effect upon the microfilarial cycle, and it was concluded that there was no evidence to link the microfilarial cycle with the circadian cycle of the adrenal glands. 3) Administration of oxygen in the early morning is as effective in causing accumulation of microfilariae in the lungs as it is in the evening; the duration of this effect varies according to the phase of the microfilarial cycle at which it is applied. 4) Hypoxia produced artificially by causing a volunteer to breathe an atmosphere deficient in oxygen during the daytime produced only a small liberation of microfilariae from the lungs. During the prolonged hypoxia associated with chronic heart failure, however, the microfilarial count during the daytime was greatly raised and in two of four patients there was also a lag by 4–12 hours in the timing of the microfilarial cycle. 5) The microfilarial count was not affected by a general anaesthetic. Attempts to demonstrate an electrophoretic charge upon microfilaria collected at night were unsuccessful. Microfilariae could not be found in the sputum. 6) It has previously been shown that the circadian cycle of the microfilariae of W. bancrofti is due to their active accumulation in the lungs during the day and their passive liberation therefrom during the night. The hypothesis is now put forward that this is brought about by two factors: (i) a “fixative force” which fixes the microfilariae in the pulmonary capillaries, and (ii) a “switch” mechanism which switches this force on by day and off by night. Both are situated in the microfilaria and not in the host. The “fixative force” is increased by raising the oxygen pressure in the pulmonary capillary. The “switch” mechanism has an inherent circadian rhythm, but this is influenced by the circadian rhythms of the host so that the microfilarial rhythms are synchronized with that of the host (and with one another). In the case of microfilariae with different periodic rhythms, e.g., Loa loa, or Dirofilaria immitis, the two factors would still be present but their responses to stimuli would be different.