The Life Cycles of Parasites

THE LIFE CYCLES OF PARASITES The parasitic mode of life is by no means uniform and unevent ful. It is, at least, as full of events and perilous adventures as the life of any free-living organism.

The main types of life histories of parasites can be illustrated easily by a few examples of the species attacking man. They show all the variety of means by which the infective stages of the parasites are liberated from the infected host and dissemi nated to invade fresh healthy hosts.

Protozoa.

Plasmodium, the cause of malaria (q.v.) in man, is a minute unicellular parasite (see PROTOZOA) living in, and feeding upon the red blood corpuscles. The parasite grows inside the corpuscle, elaborates a special black pigment, and multiplies forming a rosette of several young forms. Then the red blood corpuscle breaks and liberates the young stages which imme diately invade fresh blood corpuscles and undergo the same development. The multiplication of the parasites and the invasion of fresh corpuscles occurs at regular intervals, and provokes the characteristic attacks of fever. After a period of ordinary multi plication, some of the parasites develop into forms which will not multiply in the blood of man, but only undergo development when they are taken up with the blood by a mosquito (Anopheles). The main features of this development are the fertilization of the female element (macrogamete) by the male element (microga mete), and the formation of a cyst which will form an immense number of small elongated bodies known as sporozoites. The latter will ultimately invade various organs of the mosquito, especially its salivary glands ; and will be inoculated into a healthy man when he is bitten by the infected mosquito. The mosquito can transmit the disease only ro to 12 days of ter it becomes infected. Trypanosomes (T. gambiense and T. rhodesiense) are unicellular mobile organisms living and multiplying in the blood plasma and lymphatic glands of man and ultimately causing the much dreaded African sleeping sickness (q.v.). The parasites are transmitted from man to man by the bites of tse-tse flies (Glossina palpalis and others). The transmission is either mechanical, tak

ing place within 24 hours of the infective meal, or cyclical, infec tion then taking place after about 18 days when the parasites have accomplished their cycle of development within the tse-tse fly. Flukes.—Schistosomes, which cause Bilharziasis (q.v.) in man, differ from other flukes by having the sexes separate, the female living in a tube formed by the body of the male. The coupled worms live in the venous system surrounding the intes tinal (Schistosoma mansoni) or urinary (S. haematobium) tracts of man. When fertilized, the female leaves the male, migrates into the small blood vessels which it distends, and there lays spined eggs. The eggs rupture the vessels, appear in the sur rounding tissues and reach the lumen of the organs, to be expelled with the urine or faeces of the host. On reaching fresh water, the eggs burst, and liberate microscopic, ciliated organisms (miracidia) which within 24 hours have to find a fresh-water snail, which is the secondary host, otherwise they perish. They penetrate into the snail and become transformed into shapeless sacs (sporocysts), which by external budding form daughter sporocysts. The latter develop a number of small organisms (cercaria) composed of an elliptical body and a forked tail. When completely developed they leave the body of the snail, swim actively in water, and within 48 hours must find their primary host, man, otherwise they die. Man is infected through the skin of any portion of the body which comes in contact with water containing living cercaria. The latter penetrate the skin, the blood vessels, and finally the veins of the intestinal or urinary tracts.

Other Worms.

The hookworms (Ankylostoma duodenale and Necator americanus) live attached by their widely open mouths to the wall of the intestine of man. The females lay numerous eggs, which are expelled with the excrement. Development takes place outside the host. Small larvae hatch from the eggs, and become infective. Infection of man takes place through any portion of the skin which comes in contact with this larval stage.