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Mosquito disease


Mosquito diseaseThe malarial mosquitoes (Anopheles quadrimaculatus Say and others). The species of Anopheles are carriers of malaria. The adults (Fig. 1) are larger than those of the house mosquito and their wings are usually marked with light and dark spots. In alighting on an object, the body is tipped at quite an angle to the object on which it rests (Fig. 2) and these two differences will at once serve to distinguish the malarial mosquitoes from other species. Another distinction is in the length of the palpi of the female which in the house mosquito are short, while in the malarial mosquito they are as long as the beak and therefore quite noticeable.

Species of Anopheles are found from Canada to Mexico, east of the Rocky Mountains, Anopheles quadrimaculafus Say, the chief malaria carrier, being particularly abundant in the Southern states. Winter is passed as the adult, and the eggs are laid singly on the surface of water and hatch 2 or 3 days later. The larvae (Fig. 3) resemble those of the house mosquito but have a shorter respiratory tube and lie horizontally just below the surface instead of hanging head downward (Fig. 4). The larval period is about 2 weeks, followed by a pupal stage lasting 2 or 3 days. Accordingly, a new generation of these mosquitoes may appear about every 3 weeks.

Most species of Anopheles attack man chiefly during the twilight and early morning hours. The various species of Culex seek their food at night, though often beginning their work late in the afternoon.

The species of Anopheles must be regarded as at least potential carriers of malaria, but they vary greatly in their activity in this regard, some being evidently of little importance, as they do not feed habitually on man. Several of the species can carry not only the tertian but also the quartan and aestivoautumnal types of malaria.

In other parts of the world other species also act as carriers of these forms of the disease. The animals causing malaria belong to the Protozoa and those causing the disease in man are chiefly of three closely related kinds. With any of these the animal when introduced into the blood of man is a rather long and slender spindle with pointed ends. It now assumes an amoeboid form and attacks a red blood corpuscle, working into it, feeding on its haemoglobin contents and producing black granules. It feeds on the haemoglobin in the corpuscle until this has all been consumed and grows until it nearly fills the corpuscle. It now divides into many parts, each similar to the one which first entered the corpuscle, and these proceed to attack other corpuscles in a similar way. This breaking up of the animal into parts coincides with the "chill" of the disease and the interval of time between successive chills determines which type of malaria is present, a period of 2 days indicating the tertian type ; 3 days the quartan type, while a varying period indicates the aestivo-autumnal type. As the parasites increase in abundance and consume more of the corpuscles, the patient becomes anaemic and weaker.

Some of the products of division in the corpuscle, however, do not proceed to attack other corpuscles and increase in numbers but are of two different kinds which are the sexual stages. When these are taken into the stomach of an Anopheles which attacks a person having malaria, the two kinds fuse and the resulting animal penetrates the cells of the stomach wall of the mosquito and there remains, forming a cyst. Division of the animal here results finally in the production of cells like those which enter human blood, and these now escape into the body cavity of the mosquito and gradually gather in its salivary glands, whence they are expelled into the wounds caused by the feeding of the mosquito thereafter.

The time which must elapse after a mosquito has received the malarial parasites before it can transmit these to man varies but is usually at least 10 or 12 days and may in some cases be more than this.

The yellow-fever mosquitoes (Aedes aegypti L. and others). This mosquito is definitely known to be a carrier of yellow fever in the Western Hemisphere. It occurs in the tropical and subtropical regions and during warm weather may extend to the temperate regions but can survive there only while the temperature remains fairly high.

The adult of Aedes aegypti L. (Fig. 6) is a small mosquito with silvery lines along the back of the thorax and its legs are banded with white. It flies in the daytime and occurs mainly in towns and cities, being only rarely found in the country. Its eggs are laid singly or in small clusters on, or close to, water in houses or near by, it having apparently become a "wholly domesticated" species. The eggs hatch in 10 hr. to about 3 days, and the larvae hang downward from the surface. After a, week 10 days in this stage, they pupate for 2 or 3 days before the emergence or of the adult.

Feeding by the adult appears to be mainly during the warmer hours of sunny days though extending somewhat into the evening. Recent investigations have shown that besides Aedes aegypti a number of other species under laboratory conditions are capable of transmitting the germ or organism causing yellow fever, but whether they do this under natural conditions is not always proved. Certainly most, if not all, yellow fever in the New World is connected with the activities of Aedes aegypti. Apparently about 12 days is required after feeding on a yellow fever patient before the mosquito is able to transmit the organism causing the disease, but from that time on it can do this for well over a month.

Dengue and filariasis, two other important diseases of man in tropical and subtropical regions, are also known to be carried by mosquitoes.