MALARIA EXPEDITION TO NIGERIA 
57 
Periodicity 
In the case of F. nocturna Manson and others have been able on several 
occasions to demonstrate a 'periodicity' in the life of the blood embryo. Manson 1 
thus describes the phenomenon: — ' If under ordinary conditions of health and habit, 
the blood of a patient be examined during the day, the parasite is rarely seen, or, if 
it be seen only one or two specimens at most are encountered in a slide. It would be 
found, however, that as evening approaches, commencing about five or six o'clock, 
the filariae begin to enter the peripheral circulation in gradually increasing numbers. 
The swarm goes on increasing until about midnight, at which time it is no unusual 
thing to find as many as three hundred, or even six hundred, in every drop of 
blood After midnight the numbers begin gradually to decrease ; 
by eight or nine o'clock in the morning the filariae have disappeared from the 
peripheral blood for the day. This diurnal periodicity is, under normal conditions, 
maintained with the utmost regularity for years. Should, however, as Mackenzie 
has shown, a filarial patient be made to sleep during the day and remain awake at 
night, the periodicity is reversed ; that is to say, the parasites come into the blood 
during the day and disappear from it during the night. It cannot be the sleeping 
state, as some have conjectured, that brings about this periodicity ; for the ingress of 
the filariae into the peripheral blood commences three or four hours before the usual 
time for sleep, and the egress several hours before sleep is concluded, and this egress 
is not complete until several hours after the usual time of waking 
A recent opportunity has enabled me to ascertain that, during their diurnal temporary 
absence from the cutaneous circulation, the filariae retire principally to the larger 
arteries and to the lungs, where, during the day they may be found in enormous 
numbers.' v 
To illustrate this phenomenon of periodicity we give the following table 
re-constructed from data given by Manson. 2 
1. Manson, Tropical Diseases. London, 1900. P. 489. 
2. Manson, The Filaria Sanguinis Hominis. London, 1883. 
H 
