LOOKING on the seamy side of life, much is disclosed that is harsh, cruel, and ugly. Nature, in the widest sense of the term, is not beautiful in all her moods, and one of the most unpleasant phases is found in a study of parasitism.
Among living things, at different evolutionary stages individuals are found preying one upon another. One organism attacks another at a vulnerable spot, either weakening its prey or, in extreme cases, bringing about its death.
Animals are parasitic on other animals and on plants.
Plants are parasitic on other plants and on animals.
From our point of view there is something to be said to the credit of certain parasites. The Ichneumon Fly is responsible for limiting the number of Cabbage White Butterflies in the world, and thus preventing, to some extent, the depredations of a predatory horde of caterpillars .
In most cases of parasitism it is, however, again from our point of view, useless to look for any beneficent aim in parasitic attacks.
Man, himself, is the host of many animal parasites.
The terrible sleeping sickness of West and Central Africa is caused by a protozoon, Trypanosoma gambiense, introduced into the blood of man by the bite of one of the Tsetse flies .
From the blood-stream the parasites pass into the lymph. They swim in this by means of a long flagellum and reach the fluid in the hollow spinal cord . In the brain each artery is surrounded by lymph, whose work it is to protect the extremely delicate brain tissue from even such a slight shock as the pulsing of an artery.
To these small lymph canals the flagellate protozoa make their way. They cause drowsiness that cannot be overcome, a wasting away of the body, and, ultimately, death.
Malarial fever results from inoculation by mosquitoes. As these flies pass their larval and pupal stages in water, one means of stamping out many parasitic tropical diseases is to drain marsh lands and so bring about their death. The larva of the flies come up to the surface of the water to collect supplies of atmospheric air. If the thinnest possible film of oil is allowed to spread over the surface of the water, their breathing is interfered with. This is another means of extermination.
Flatworms and Threadworms are human parasites in all parts of the world.
The Tapeworm is one of the commonest. The adult worm may be many feet in length . Its embryo stages are frequently swallowed by pigs in their, often un- hygienic, food. They are the cause of measly pork. In, some parts of Europe, where pork is eaten in a semi-raw state, diseases caused by the Tapeworm are rife among the human population. The measly condition of the pork cannot be detected by the unpractised eye. The safeguard is very thorough cooking. This advice applies to all meat, not to pork alone.
Another very common parasite in man is the Human Roundworm, Ascaris lumbricoides. &earls These worms are most unpleasantly . white. They have a very muscular body wall and move with abrupt twists .
The male is usually about 7 inches long. The female reaches a length of 17 inches and lays, on an average, 1,50o eggs daily. This enormous power of reproduction is characteristic of para- sites and is their counter-stroke to the long odds that are against their finding their particular host.
The Roundworm has neither respiratory nor vascular systems. Its alimentary canal is of the narrowest because, as it lives in the small intestine of man, its three-lipped mouth takes in solid or liquid food practically digested.
The eggs are expelled with the human faces. The conditions for their development are a certain amount of moisture, a temperature above 600 F., and an area free from putrefaction. This last condition explains their expulsion from the host. The bacterial flora make their development impossible in the large intestine of man.
The tiny worms are swallowed in, or on, food. They pass to the small intestine and enter the capillaries of the villi . Carried by blood and lymph they reach the lungs. Because of their size they cause congestion of the lung-capillaries. Hamorrhage follows, and they are driven into the alveoli. Now, on their roundabout journey, they travel up the bronchi and trachea and get to the pharynx. They are now swallowed a second time and, passing down cesophagus, stomach, and duodenum, once more reach the coils of the intestine. During this circuitous journey the worms have increased in size from .28 mm. to 2 mm.
Ascaris is universal. The safeguard against the parasite is just the exercise of reasonable care, especially, perhaps, care in the matter of drinking water. If this is polluted, certainly the Threadworm, in some stage, will be in it.
Cleanliness is the great weapon of defence against most parasitic attacks. Cleanliness, that is, in every direction : in yards and gardens (rubbish heaps are bad) – in the care of dustbins as well as in bodily cleanliness and matters of the house – cleanliness, too, in the treatment of raw foods, whether these be animal or vegetable.
In some cases an individual’ may be utterly unaware that he is the host of a parasitic worm. In other cases the para- site may throw his whole constitution completely out of gear. As he mustparasites as well as himself, he is, at times, voraciously hungry. At others he is sick, and the mere sight of food is distasteful. He may suffer from diarrhcea and anamia. The parasitic action produces toxins in the blood which the anti-toxins do their best to overcome . Temporary bronchitis is caused by the passage of the parasite in the lungs, and the whole bodily disturbance leads to various nervous diseases.
The Green Fly (Aphis) is parasitic upon plants . All the characteristics of the insect are concerned with the production of the greatest possible number of descendants to prey upon our summer plants.
The female, throughout the summer, is parthenogenetic – that is, the eggs which she produces in tens of thousands develop without fertilisation. Not only this, but the young are hatched within her body – these are all female and are themselves capable of reproducing within two or three days.
Towards autumn both males and females are produced. As a result fertilised eggs are laid in the autumn, and, the following spring, give rise to females which carry on the traditions of the race.
It must be admitted that the behaviour of the Green Fly is only an exaggeration of a world-wide law. Following up the matter to a logical conclusion, it is obvious that, strictly speaking, all animals are by their nature parasitic – they are utterly and entirely dependent upon the plants that provide their food.