36
THE ESSEX NATURALIST
recognised species of malarial parasite pathogenic to man. In the
way just described, the sexual forms (gametocytes) are taken
into the stomach of a mosquito from a person suffering with the
disease. Here, within about twenty minutes, the male and female
fuse and the fertilised female escapes being digested by penetrat-
ing the stomach wall of the mosquito and becoming encysted
under the wall's outer membrane. The contents of such a cyst
multiply, and eventually a large number of sporozoites are re-
leased into the body cavity. From here, being mobile, they reach
the salivary glands. The infected mosquito when taking another
blood meal injects these sporozoites and they enter the blood
stream where eventually young parasites are formed. There is
now no question of male and female forms, for multiplication
within the human host is non-sexual and takes place within the
red blood cells. Each parasite divides into many merozoites; the
membrane of the cell gives way and the merozoites are poured
into the blood stream. Some are destroyed by the white cells, others
enter fresh red cells and start again the non-sexual cycle. This
process of intra-cell development naturally takes some time, and
this varies with the species of parasite. The significance of this
will be appreciated later. The sexual forms (gametocytes),
mentioned above, in the human body are formed from the
ordinary parasites of the non-sexual cycle. Certain of these be-
come sexually differentiated—instead of proceeding in the usual
way in intra-cell development, they change within their host cells
into special forms of males and females whose sole function is to
carry on that sexual cycle in the mosquito into which they are
later taken.
It is most important to note that the development of the
parasite in the insect host is determined largely by temperature.
If the atmospheric temperature is below 60° F., the parasite can-
not develop at all. At 60° F., growth is very slow and the cycle
from blood meal to gland infection takes about thirty days. If,
however, the temperature is 70° F., the cycle is completed in
fifteen days, and at 80° F., ten days. At 85° F. and over, the
insect is adversely affected and seldom lives for more than a week.
It is obvious that in England the seasons of the year when the
atmospheric temperature is suitable for development in the insect
host are very limited, seldom exceeding three months.
As for the disease itself: the incubation period varies with
the type of disease, but for our indigenous type (benign tertian)
it is of the order of fourteen days. In many cases, however, the
first clinical attack is delayed, and takes place in the early Spring
before mosquitoes are prevalent, presumably the result of infec-
tion the previous Autumn.
It takes the form of an intermittent fever; that is, a fever
characterised by regular recurrences every day, or every two, or
three days (called quotidian, tertian and quartan fever respec-
tively). The recurrences usually begin about mid-day, often with
severe chills and shivering, succeeded by high temperature reach-