THE PARASITIC MODE OF NUTRITION

We have already seen how green plants build up their food by photosynthesis, and how some fungi feed saprophytically upon dead ready-made food. Animals may feed entirely upon vegetable food or upon flesh food. Those that find their food dead are called carrion feeders ; such are the carrion crow, the vulture and the hyena. Others, notably birds of prey and the cat and dog families, hunt and kill their food ; these are the carnivores. Some animals are omnivorous and will eat plant and animal food. These include man and all the pig family.

An organism may obtain nourishment by inhabiting or attaching itself to another, conferring no benefit upon it but not necessarily harming it, although in the great majority of cases some damage does result to the host, rendering it unhealthy or weak. This is in contrast to the phenomenon of symbiosis, e.g. Hydra and Zoochlorella where the two different species may be of mutual benefit. An organism absorbing nourishment from living protoplasm is a parasite.

The parasitic habit is apparently such an easy way of surviving bad conditions that we find examples of it in most groups of animals and many groups of plants. The influence of this habit is most forcibly seen when it is pointed out that all diseases, except those due to faulty environment, injury and faulty development, are due to parasites. Silicosis is a disease common among miners and rock-workers, due to inhaling silica in dust—a case of faulty environment. ’Shock ’is often a result of poisons absorbed by the body from an injured part— the poisons being produced by the damaged tissues. Diabetes is caused by faulty development—part of the pancreas failing to function properly. Moreover, diseases due to parasites are more widespread and more serious in their effects and may cause early death.

Fungal Diseases

All the fungi which are not saprophytic are parasites, many of them of great economic importance, for the majority of plant diseases are caused by them—such are ’rusts ’ and mildews, which ruin crops and render plant food unpalatable.

Potato ’blight ’ and the rust of wheat^cause losses of thousands of pounds ’ worth annually of these valuable food crops.

Certain species attack animals. Ringworm is due to a fungus inhabiting the living roots of human hair. Salmon disease is caused by the hyphaj of Saprolegnia clogging the gills of the fish. Bees and house-flies also suffer from fungal diseases.

Viruses

These cause diseases such as leaf mosaic and leaf roll in potato and tomato plants, and measles and smallpox in man.

They are so small as to be invisible under the ordinary microscope, but have been viewed by means of the electron microscope. Though not regarded as living organisms, they possess the power of reproduction within the cells of their ’hosts. ’

Bacterial Diseases

Animal Parasites. — The animal parasites are of two kinds, those which inhabit the host and those which attach themselves to the outside of it.

The ENDOPARASITES are most commonly found in the intestine, since the mouth is the most ready means of entry. So easy is the parasitic mode of life that all the great invertebrate groups include parasitic species, but the most important are the pathogenic bacteria and certain protozoa, flukes, such as the liver fluke in sheep, flatworms like the tapeworm in the intestine of man and ’threadworms ’like Ascaris also in the human intestine. They all absorb nourishment over their whole surface, or possess a very simple gut, since their food is in a partly or easily digested state. Being plentifully supplied with food, they multiply rapidly. The older segments of the tapeworm are almost entirely bags of eggs, large numbers of them being essential to ensure that some at least reach a new host, since the eggs do not in this case develop unless they pass out of the original host and are ingested by another. To prevent the whole animal from being passed along with the food in the intestine, the head end, from which the segments are budded off, attaches A itself by hooks and suckers to the gut wall.

The host may develop an abnormal appetite, having to nourish a guest which is relatively enormous compared with most parasites, as well as its own body. The tapeworm entirely lacks an alimentary canal, although related flatworms possess a simple one. Being flat and ribbon-like, it is able to absorb sufficient nourishment through its surface from the digested food in which it lies.

Ascaris has a simple canal. The muscular pharynx ingests tiny solid particles as well as dissolved foods from the contents of the intestine of the host. Digestion and absorption occur in the midgut, whilst egestion is effected by the muscular rectum.

Endoparasites excrete waste substances into the surrounding environment, and these are generally poisonous, being termed toxins. Generally it is not the presence of such parasites themselves which causes trouble but their waste products which produce the ill-effects., which causes malaria, the toxins become generally distributed and all parts can be affected.

Gambia fever is due to toxins produced by Trypanosoma gambiense living in the blood plasma. Should this parasite make its way into the fluid of the brain and spinal cord, sleeping sickness results and generally proves fatal. This illustrates how the effect of the toxins varies according to the tissues which are invaded. Both examples quoted above belong to the Protozoa. Another protozoan parasite is Entamoeba dysenteric, which is very much like Amoeba, and causes dysentery ; infection is due to drinking contaminated water.

The ECTOPARASITES upon land animals consist largely of insects. Aquatic ectoparasites include many Crustacea, which lie beyond the scope of this article, as well as the Cyclostomes among vertebrates which attach themselves by their toothed suctorial mouths to larger fish, rasping off pieces of flesh by the toothed tongue.

The insect ectoparasites show adaptation to their mode of life. Bugs and lice remain upon the animal host relying upon contact for transmission from one host to another. They are flat, and therefore not readily brushed off, some species being removed with difficulty, and in nearly all cases are witigless, unlike the Diptera which have wings, and remain attached to the host only whilst feeding. The Aphaniptera are also wingless, and are more or less permanently attached, but able to jump from one host to another, and are flattened from side to side for wandering amongst hair.

The plant lice can do much damage to tender crops, but the other insect parasites, although most objectionable and annoying, do not themselves have serious effects—the danger with them comes from the fact that they can most effectively infect the host with a dangerous endoparasite when they pierce the skin and inject saliva to prevent the clotting of blood, for such parasites inhabit the salivary glands of the insect. Plague is due to Bacillus pestis, spread by the rat-flea ; lice have been proved to transmit typhus and other fevers ; the tsetse fly transmits sleeping sickness, due to Trypanosome gam-biense; the female mosquito, Anopheles, transmits malaria due to Plasmodium sp. Plant lice also transmit certain plant viruses.

CLEANLINESS

It is convenient here to consider the necessity for both internal and external physical cleanliness. In temperate civilized countries it is difficult to realize the number and variety of pests parasitic upon the human being. The ceremonial washing of hands and feet in subtropical and tropical Eastern countries has the definite advantage of removing troublesome ectoparasites from the skin before they have time to do much damage or ’settle in. ’

The outer layer of the skin is constantly being renewed, and if the dead cells are not removed they harbour and provide food for parasites which produce substances that attack the living cells, making the skin tender and affording an entry to microscopic endoparasites. This also is true of particles of food between the teeth : the acids produced by germs lodging there attack the teeth, and thus they decay. Moreover, these poisons getting into the alimentary canal can seriously interfere with general health. The need for removing waste material from the canal periodically is obvious.

Many of the scourges of the Middle Ages have disappeared, or are now easily controlled, as we now know them to be due to parasites and not to ’acts of God, ’ and can take measures against them accordingly. The above also shows the importance of a knowledge of parasitism, of the habits of organisms, e.g. the breeding grounds of insects which are ectoparasites and which carry pathogenic endoparasites, and of cleanliness of hair, skin, teeth and the alimentary canal if we are to be free from much which would make life less worth living.

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