> To: parasite at net.bio.net>> From: mes at zoo.toronto.edu (Mark Siddall)
>> Subject: Re: microfilaria - periodicity
> My understanding is that microfilariae do not carry pocket watches
> and that in
> fact, this has more to do with the worms' responding to human
> physiological changes. I seem to recall something about oxygen
> tension in the capillary beds of the lungs... but I'm sure someone
> else out
> there has more insight.
Please excuse for my poor English!
1. In reply to Mark Siddal (Gratiano asked the same question to Sir
Patrick Manson a century ago :"Do microfilariae carry watches?") ,
microfilariae _do_ have a watch, or at least a biorhythm of their own!
Old experiments dealing with prisoners or volunteers whose biorhythm had
been 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 (_Dirofilaria immitis_,
sub-periodic) from a donor dog to a recipient dog whose rhythm had been
inverted : during several days, the microfilariae kept the rhythm they
had in the donor.
In general, 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 have the opportunity to continue
their life cycle in the vector mosquito host.
3. Periodicity occurs when microfilariae migrate at the same time
between the skin and the lungs , 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 during daytime; 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 example, 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 individual hosts, mainly those 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, on the contrary 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 "timing coaptation" between parasite and its vector 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, periodicity(3).
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).
(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"
Laboratoire d'Informatique Appliquee
Centre Orstom d'Ile-de-France
32, avenue Henri-Varagnat
93143 BONDY Cedex
tel.: (1) 48 02 59 76/48 02 55 00
fax : (1) 48 47 30 88
e-mail: pichon at orstom.fr