THE RÔLE OF INSECTS IN SPREADING DISEASE.
Insects are now known to be important agents, (_a_) as carriers and
(_b_) as intermediate hosts of disease-agents.
The common =domestic fly= (_Musca domestica_) is the unwelcome
companion of man in nearly every country. The eggs are usually laid
and the larvæ undergo their development in excrement, but the female
sometimes selects meal, bread, or fruit for the purpose. In practice,
however, one of the best means of diminishing the number of domestic
flies is to insist on the daily removal of all manure, especially horse
manure, and to sprinkle the manure receptacle in the interval with
lime. The fly may obviously be the means of conveying infected material
from place to place. Anthrax has been ascribed to this cause. Nuttall
has proved experimentally that flies are able to carry the infection
of plague, and that they die of the disease. The presence of enormous
numbers of flies in cholera times has been noted. Experimentally,
flies caught in cholera wards have been found to harbour the cholera
spirillum. It is probable that they play a serious rôle in spreading
the infection of cholera. Hence all infectious dejecta (stools and
urine) should be covered until finally disposed of, and food should
be protected against flies. The same remarks apply for enteric fever.
Flies fed with pure cultures of the bacillus of enteric fever pass
these bacilli in their dejecta in a still virulent condition. In camps,
especially in connection with large armies, there is the strongest
reason for believing that flies carry infection from latrines to food.
Flies have been known to feed on the expectoration of consumptive
patients and it is possible therefore that they may thus infect food.
The =bed bug= (_Cimex lectularius_) has been stated to be capable of
conveying by its bite the infection of plague and other diseases from
an infected to a healthy person; but Nuttall’s experimental results
were entirely negative.
=Fleas= (_pulex_) probably do not play any part in spreading anthrax.
Experimentally, anthrax bacilli die off rapidly in fleas. In India,
persons who had handled rats dead of plague frequently acquired the
disease. This was explained by Simond on the supposition that the fleas
abandoned the dead rat for the human subject. The rats which appeared
to have caused plague in man were stated to have died but a short time
before; and the handling on the day after their death of rats dead of
plague was stated to be safe because the rats’ fleas had then deserted
the dead rat. It is assumed that the flea injects the poison of plague
under the skin. On the contrary it is to be remembered that the fleas
infesting rats and mice belong to a different family from that which
attacks man. Whether this is a usual means of conveying plague may
therefore be regarded as still doubtful. That rats convey plague to
man is certain; whether fleas act as an intermediary remains somewhat
uncertain.
The =Mosquito= family (_Culicidæ_) has been found to be an important
if not the sole means of spreading certain serious diseases to man.
To this family belong all true gnats or mosquitoes; but the only two
genera which have been proved to be able to cause disease are _Culex_
and _Anopheles_. The culex may usually be distinguished by the fact
that when alive and at rest its head is below the level of the thorax
and abdomen, thus giving the insect a hump-backed appearance, while
the body of the anopheles under the same circumstances is all in a
line.[10] The =anopheles= is more slender and its head smaller than
that of the culex. The anopheles usually confines its blood-sucking
operations to the evening and night. During the day it remains in dark
corners. It lays its eggs usually in a natural pool or pond on the
ground, on the surface of the water. In about two days a minute larva
is hatched out. This grows rapidly, assumes the pupa form, from which
the perfect insect emerges. The female insect alone is blood-sucking.
In about 20 days after birth, it lays from 150 to 200 eggs. Its
relation to malaria may be gathered from the following historical
sketch. In 1880 Laveran found in the red blood corpuscles of malarious
patients minute bodies which he regarded as not bacteria, but a very
low form of animal life, possessing amœboid movements. These grew at
the expense of the blood corpuscles, deposited a dark pigment, and
often assumed the appearance of a “rosace,” a rounded body with little
spherules at its circumference. Golgi in 1889 observed differences
between the rosaces of tertian and quartan fever, and found that the
periods of occurrences of the fever corresponded with the times of
maturation of the rosaces. It was concluded therefore, that the rosaces
caused the fever by shedding their sporules into the blood. These
sporules when thus shed were found to attach themselves to, and grow
in, other red blood corpuscles. It is now known that there are three
species of the parasite, in one of which the parasites are crescentic
in shape. The examination of a drop of blood from a patient now enables
a doctor to recognise which of these three forms of malaria he is
dealing with.
Laveran observed that certain forms of the parasite presented
“flagella,” _i.e._ filaments exhibiting very active movements. Manson
having observed that flagella were not found in blood first drawn,
but only appeared after a little time had elapsed, conceived the idea
that the function of these must be that of spores. Having previously
observed that a microscopic worm, filaria, is drawn with the blood into
the stomach of a kind of mosquito (page 278), and finds in the latter a
secondary host, he concluded that a similar cycle of events might occur
in malaria. Ross tested this theory, and by causing mosquitoes bred
in bottles from the larva to bite persons affected with the crescent
form of malaria, after repeated unsuccessful attempts, was eventually
able to find in comparatively rare mosquitoes which had thus bitten a
malarious patient, small rounded bodies embedded in the wall of the
stomach. These were watched and found to present appearances identical
with those of the parasite of malaria. Similar pigmented bodies were
subsequently found in other mosquitoes.
The malarial parasite belongs to the Protozoa, of which it is one of
the smallest members. Man is its _intermediate_ host, and the anopheles
its _definitive_ or final host. In the red blood corpuscle of man it is
a unicellar organism, from 1 µ to 8 µ in diameter. It has two methods
of reproduction, endogenous by spore formation and exogenous or sexual.
The former occurs in man; the latter in the mosquito. Without the
latter, the parasite being unable to pass from man to man, would die
with its host. In endogenous multiplication spores are formed which
separate from the original parasite and gain access to other red blood
corpuscles. The large pigmented spheres and the crescent bodies require
to enter the stomach of the anopheles to attain full development. In
the anopheles the crescents become spherical, flagella are shot out,
having a length of 4 to 5 times the diameter of a red blood corpuscle.
These represent the male element, while other spheres without flagella
are the females. By the fusion of these two a fertilised cell is
produced (_the travelling vermicule_), which now assumes the shape of
a spear-head and is actively mobile. The travelling vermicule pierces
the stomach wall of the mosquito and develops into a _zygote_. If
an infected mosquito is examined on a succession of days under the
microscope, the following stages can be traced. The zygote consists of
pigmented spheres 7 to 8 µ in diameter, lying in the muscular fibres
of the mosquito. These grow, and become surrounded by a capsule.
Smaller spheres form and subdivide, bud-like processes develop on
their surfaces; these gradually become sickle-shaped and protrude into
the body cavity. They increase in size. until they attain dimensions
of from 40, to 60 µ. Eventually they rupture, and the sickle-shaped
bodies (_sporozooites_) escape and are carried in the body fluid of
the mosquito to its salivary glands. These sporozooites are about 14 µ
long, and human infection is caused by them. They have been traced as
far as the end of the proboscis of the mosquito. (See also page 307).