Economic Importance.— Many of the nematodes parasitic in man and animals are of con siderable medical and veterinary importance. Their ill effects, generally inappreciable in animals living in a state of nature, tend to become more marked in the artificial conditions under which man and his domestic animals live. Over-crowding and insanitary conditions are the chief causes of heavy infestation with such parasites, and their incidence in man is greatest in tropical and subtropical countries.
The parasites themselves may cause lesions of the tissues in various ways, or their presence may lead to secondary invasion by pathogenic organisms of other kinds. Sometimes the presence of the worms may merely cause mechanical irritation, and so lead to inflammatory conditions. But in many cases they appear to secrete toxic substances which react detrimentally upon the host, and nervous and other disturbances, apart from local effects, may result.
Perhaps the most important of the forms occurring in man are the hookworms (Ancylostoma and Neca tor), which are probably responsible, in some countries, for more disease, both physical and mental, than any other single cause. Their ravages are due in part to the ulceration of the intestine caused by their blood-sucking and tissue-destroying habits, but probably in part also to toxic proper ties. "Hookworm campaigns," having as one of their principal objects the education of the natives in the use of proper sanitary precautions, have been attended with con siderable success in many parts of the world.
Filariasis, or infestation with worms of the Filariid group, is another condition which may be of considerable importance. As has been mentioned, these worms, whose larvae circulate in the blood, and several species of which occur in man, are transmitted by mosquitoes and other blood sucking flies. There is reason for believing
that at least one of these species is con cerned in the causation of elephantiasis, a terrible and disfiguring disease of wide occurrence in hot countries. Others cause tumours of the skin and other parts of the body.
Ascariasis, or infestation with Ascarids, has assumed a new importance during recent years, since it has been realized that the migrating larvae, on their passage through the lungs, are capable of inducing a form of pneumonia in their hosts when present in sufficient numbers. This is a frequent cause of death in young pigs, and deserves consideration in connection with human disease. Certain respiratory diseases in domestic animals, more especially "husk" in sheep and calves, are due to various species of lungworms (Metastrongylidae) ; while "gapes," a dis ease annually responsible for the death of large numbers of young poultry, is due to the invasion of the trachea and bronchii by a nematode (Syngamus). Parasitic gastritis and enteritis in cattle, sheep and horses are commonly due to nematodes, mostly belonging to the order Strongyloidea. The stomach-worm (Haemonchus) of sheep and cattle is one of the best known, but several others seem to be also, at times, responsible for losses.
Lastly, mention should be made of certain "free-living" forms, mainly belonging to the genera Heterodera and Anguillulina (Tylenchus), which are important as agricultural pests. These attack the roots, stems, leaves or ears of cultivated plants of various kinds, and may cause great deterioration or even destruc tion of the crops. (H. A. B.)