150 
MAT,ARIA , 
tain families continued to be foci of anophe- 
line infection ( vivax ) for at least two suc¬ 
cessive autumns without the occurrence of 
fresh cases of malaria. Later they noted 
(1937) that children as often as not became 
carriers without a fresh attack, but adults 
usually were not good infectors without a 
recent history of acute symptoms. 
2. The effect of season and environment. 
In the community gametocyte carriers are 
most numerous during and after an epi¬ 
demic or at the height of the transmission 
season. A survey at these times will not 
give the same information as an interepi¬ 
demic survey, or one which includes febrile 
cases as one based on apparently healthy 
carriers. The picture also changes with 
the season for other and more mysterious 
reasons, due to the long term relapses of 
P. vivax in spring, the dominance of falci¬ 
parum in summer and fall, and the persis¬ 
tence of malariae into the winter. Seven 
patients infected simultaneously with P. 
falciparum and P. vivax by Boyd and 
Kitchen all had initial attacks character¬ 
istic of P. falciparum, but three relapsed 
in the following spring with P. vivax. 
Gill (1938a) has proposed the theory that 
there may be environmental causes (pos¬ 
sibly climatic or solar in origin) which op¬ 
erate periodically (in endemic regions) to 
produce waves of relapses which thus tend 
to occur simultaneously over great areas 
and stimulate gametocyte production on a 
large scale as the prelude, not the result, 
of epidemic outbreaks of the disease. "We 
know nothing about this except from rather 
ambiguous epidemiological evidence. 
3. The effect of immunity. A growing 
immunity represses gametocyte produc¬ 
tion ; it does not stimulate it. Gametocytes 
are found earlier in relapses, but they are 
more constant and abundant in primary 
attacks, and diminish in number with each 
recurrence of symptoms and parasites. In 
the tropics where immunity is more rapidly 
and permanently established in the first 
decade of life than in temperate climates, 
it is usual to find the gametocytes under 
more stringent control than the trophozo¬ 
ites. Thomson found in native children of 
Nyasaland that, while all forms of P. vivax 
and P. malariae had virtually disappeared 
from the circulation by the age of nine, all 
the children continued to show ring-forms 
of P. falciparum although no gametocytes 
of any kind were to be found. 
In a temperate climate with a very high 
transmission rate and a long summer, the 
effect of immunity on P. vivax infections is 
almost as striking as in the tropics, since 
the tolerance is more rapidly acquired and 
survives the winter pause more easily than 
in the case of P. falciparum. Thus in a 
village in Sardinia, Missiroli found three- 
quarters of the infants infected with P. 
vivax in the fall, but only one-third of the 
preschool group (2-5 years) and about a 
sixteenth of the older persons. P. vivax 
infections began to give way to P. falci¬ 
parum during school age. There were 
twenty times as many vivax gametocytes in 
the school children as in the adults, and 
twice as many P. falciparum gametocytes 
(Hackett 1937). 
4. The effect of diversity of species and 
strains of the parasite. Each species of 
plasmodium has its own characteristic out¬ 
put of gametocytes. In Egypt we found 
twice as many carriers in the P. falciparum 
cases as in the P. vivax cases, at all levels 
of intensity and in all seasons. A high 
inoculation rate will increase the P. falci¬ 
parum carriers at the expense of the other 
two species, while a low transmission favors 
P. vivax and P. malariae. In towns in 
Algiers after a whole year in which ex¬ 
traordinary climatic conditions prevented 
anopheline breeding, Parrot and Catanei 
found the following situation: 
Gametocyte carriers in P. falci¬ 
parum positives: 5 per cent 
Gametocyte carriers in P. vivax 
positives: 43 “ “ 
Gametocyte carriers in P. malariae 
positives: 64 “ “ 
Old cases of P. falciparum infection are 
the least likely and old cases of P. malariae 
infection the most likely to remain foci of 
anopheline infection. 
An important point in connection with 
a diversity of strains and species is that 
