here are some comments from Harold Townson (Liverpool School of Tropical Medicine) as posted on the parasitology newsgroup...
1) A nocturnally periodic W.bancrofti (as found in East Africa for example) becomes diurnally periodic if the individual sleeps by day and is active by night.
2) Blood oxygen plays its part. Hawking suggested c. 30 years ago, based on experiments with subjects breathing pure oxygen at night, that when the difference between venous and arterial blood oxygen tensions is under 53mmHg, mff will leave the lung for the general circulation and hence appear in the peripheal blood. When the difference is greater, they accumulate in the lungs.
3) Periodicity can vary for the same parasite in different hosts. Thus, B.malayi has three forms (at least). In humans, one is nocturnally periodic and another nocturnally sub-periodic. The third is diurnally sub-periodic. In Malaysia both type 1 and 2 occur, in the Philppines type 1 and 3. Type 1 has a wide distribution from Kerela eastwards. That which is nocturnally periodic in humans (ie type 1 above), becomes subperiodic when passaged into cats. On the otherhand the sub-periodic form is sub-periodic in both cat and human but is nocturnally periodic in two species of monkey.
4) Much of the relevant work was carried out many years ago but many questions remain unanswered. Denham and McGreevy reviewed some of this in Advances in Parasitology vol 15, (1977) - a good source of the earlier references.
Harold Townson
Liverpool School of Tropical Medicine