We have studied the behavior of microfilariae of a strain of Brugia malayi infectious to man (transferred to a rhesus monkey) in relation to a general hypothesis about the causation of microfilarial periodicity. Briefly, it was postulated that the daytime disappearance of the microfilariae of Wuchereria bancrofti (and of other filariae) is due to their accumulation in the small arterioles of the lungs, this accumulation being brought about by reflexes in the microfilariae stimulated by the abrupt increase in oxygen tension that takes place in the pulmonary capillaries; during nighttime this increase of oxygen tension is smaller, and the microfilariae pass through the lungs back into the general circulation.
In the present work we found that if the host breathes oxygen at night (which produces an increase in oxygen tension in the pulmonary capillaries), the microfilariae of B. malayi accumulate in the lungs, and conversely if the host is subjected to a reduced oxygen pressure by day they are liberated from the lungs into the circulating blood; in these ways their normal 24-hour migrations are reversed. Breathing carbon dioxide does not affect their behavior.
When the body temperature is lowered (by day), microfilariae are liberated in small numbers from the lungs, and conversely when the temperature is raised (by night) some of them accumulate there. Intravenous injection of serotonin causes an extensive but transient liberation from the lungs; acetylcholine has a similar but much weaker effect.
These responses of the microfilariae accord with the hypothesis described above.