a handbook of the mosquitoes of north america - Systematic Catalog ...

48 MOSQUITOES OF NORTH AMERICA
DISEASES TRANSMITTED BY MOSQUITOES
In North America, at least four important diseases are trans-
mitted by mosquitoes and only by mosquitoes. These are ma-
laria, yellow fever, dengue and filariasis. Each of these will
be discussed but briefly.
Malaria
Malaria is caused by a minute Protozoan which invades the
red blood corpuscles (Fig. 16). There are known to be
three distinct species of malarial parasites and each produces
a distinct type of disease. The parasites are known as Plas-
medium zizlax, causative agent of tertian malaria ; PZasnzod’iu~~z
mnlariae, causative agent of quartan malaria ; and Plasmodium
falciparzrna, the agent of malignant tertian, sub-tertian, perni-
cious or aestivo-autumnal malaria. The common and most
prevalent type of malaria in North America is the tertian.
Pernicious or aestivo-autumnal malaria occurs in the states
bordering on the Gulf of Mexico. According to Hoffman
(1916) the prevalence of the various types of malaria in the
southern states is about 6.5 per cent for tertian, 13 per cent
for quartan and 22 per cent for aestivo-autumnal. In order
to understand the essential rble played by Anopheline mosqui-
toes in the distribution of malaria, a very brief outline of the
life-cycle of Plasmodiwz viz~aax is here presented.
Plasmodium zizlax, in man, lives and multiplies asexually
in the red blood corpuscles (Fig. 16). This is called the asex-
ual cycle. As the organisms grow (Nos. 1 to 4), they cause
the red cells to become enlarged and they absorb the cell con-
tents. At the end of about forty hours, the trophozoite is
mature and is now called a schizont (No. 5). Each schi-
zont then divides into a number, 15 to 24, of merozoites and
these are discharged into the blood stream by the rupturing of
the cell (Nos. 6 and 7). Along with the merozoites, are lib-
erated the wastes, pigments, and probably a toxin. Each mero-
zoite now attacks a new blood cell and, in about forty hours,
becomes a schizont, dividing, and the merozoites are liberated