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microfilaria - periodicity

Prof H. Townson htownson at liverpool.ac.uk
Fri Sep 22 08:02:55 EST 1995

Re: Eberhard Kniehl's query.

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) As Mark Sidall says, 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

Hope that helps

Harold Townson
Harold Townson			| Liverpool School of Tropical Medicine
htownson at liverpool.ac.uk	| Pembroke Place
0151-708-9393 voice		| Liverpool L3 5QA
0151-708-8733 fax		| UK

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