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microfilaria -periodicity

Gaston PICHON pichon at orstom.fr
Mon Aug 5 08:32:30 EST 1996

Please excuse for my poor English!

1. In reply to Mark Siddal (Gratiano asked the same question to Sir 
Patrick Manson a century ago) , microfilariae _do_ have a watch, or at 
least a biorhythm of their own! Old experiments dealing with prisoners 
or volunteers whose biorhythm was reversed showed that the parasite 
biorhythm follows the host biorhythm, but with a time-lag of more than a 
week. There was also an experiment of transfusion of microfilariae from 
a donor dog to a recipient dog whose rhythm  was inverted : during 
several days, the microfilariae kept the rhythm they had in the donor. 
In many cases, the host's rhythms do not produce the parasitic rhythm, 
but only give timing cues to the parasite.

2. As Hawking proposed: a parasite biorhythm is the way to control a 
conflict, between its short-term needs and its long-term needs. The 
oxygen beeing lethal for the microfilariae, they actively stop in the 
precapillar arterioles of the lungs (where oxygen pressure is the 
lowest) during 12 hours, then let thewselves passively float in the 
general blood circulation where they can continue their life cycle in 
the vector mosquito host. 

3. Periodicity occurs when microfilariae migrate from the skin to the 
lungs and reversely at the same time, like migration of birds, which 
implies a) that the microfilariae have an autonomous rhythm  b) that 
they are synchronous. A question is: why the parasites should be 
synchronous? In fact, they have not to. The Polynesian strain of _W. 
bancrofti_, which is sub-periodic (it is present at night, but less 
frequent than in day; the _Aedes_ vectors bite only at daytime) shows a 
decreasing, density dependent, relative amplitude. Subperiodicity is 
caused not by weakening of parasites response to host stimuli, but to 
diversity of rhythms. The most numerous are the adult parasites, the 
greatest is the diversity of biorhythms and the smallest is their global  
relative amplitude (1). The parasite _Mansonella_ozzardi_ is an extreme 
exemple, that I called crypto-periodicity: if all the hosts are studied 
alltogether, we conclude to a (globally) non-periodic parasite. But if 
one looks at the hosts with small parasite density, one finds two hosts 
with a highly significant periodicity, both beeing out of phase (peak 
hour respectively at 2 a.m. and 18 p.m.) (3).

4. Thus, if biorythm can be considered as an adaptation to the need to 
survive both at short and long term, periodicity, in terms of biorythms 
synchronism, is only the result of selection s.s. by vectors having a 
more or less narrow nycthemeral activity, which have eliminated the less 
frequently met parasite biorhythms. The adaptation of this synchronism 
is dubious: high parasite burdens beeing deleterious for vectors (and 
thus for parasites), it is not advantageous for transmission that "the 
maximum of parasites meet the maximum of vectors". A simulation model 
shows that non-periodicity woud yield greater transmission than 
periodicity: transmission is ensured in spite of, and not due to, 

5. Comparative study of periodicity gives some cues on the speciation of 
the genus Wuchereria (4) and the ethnogenesis of the Polynesian people, 
and on their possible contacts with Ocean Indian (Nicobar Islands) (5).

Gaston Pichon

(1) G.Pichon, R.Thirel, M.Chebret, 1979- Nouvelle approche de la 	
périodicite chez la filaire _Wuchereria bancrofti var. pacifica_. 	
Cah.ORSTOM, ser. Ent.med.Parasit., 17 (2), 89-105
(2) G. Pichon, 1981- Migration des microfilaires et des peuples 	
oceaniens...Contribution a la prehistoire du Pacifique. 	Ann.Parasit. 
Hum.Comp., 56 (1), 107-120
(3) G. Pichon, C. Mullon, 1990- Le synchronisme microfilaires-	vecteurs 
est-il une adaptation? VII International Congress of 	Parasitology, 
Paris,1990, Bull.Soc.Fr.Parasit.,8 (2), 819.
(4)	O. Bain, A.S. Dissanaike, J.H.Cross, C. Harinasuta, S. Sucharit, 	
1985- Morphologie de _Wuchereria bancrofti_ adulte et sub-	adulte. 
Ann.Parasit. Hum.Comp., 66 (5), 613-630
(5) G.Pichon, 1983-Crypto-periodicity in _Mansonella ozzardi_ . 	
Trans.Roy.Soc.Trop.Med.Hyg., 77(3), 331-333

UR 41 "Epidemiologie des Maladies a Vecteurs" 
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e-mail: pichon at orstom.fr

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