HOOKWORM, Anatomy and Life His tory. The hookworms or ankylostomes are true roundworms (Nematoda, q.v.) and belong to the order of the strongyles. They are char acterized by a well-developed buccal capsule armed with teeth or cutting plates, and in the male a fan-shap i ed bursa around the posterior end of the body. The anterior end is curved dorsad so that the mouth opens toward the wall of the intestine though the worm, when attached, lies parallel to the lining of the canal.
The hookworms are all dangerous blood sucking parasites, and give rise to profound and lasting anemias in the host. Many species are known to occur in the higher mammals; two are found in man, often called the Old W World and the New World hookworm. The former was discovered in 1843, though the disease to which it gives rise was recorded in ancient writings and the effects of its presence are recognizable in many regions and races. It is designated scientifically Ancylostoona* duodenale.
The New World hookworm, Necator americanrs, was discovered by Stiles in 1902 in the South and has proved to be the most common form in North, Central and South America.
Later recognized by Looss among the parasites from pigmies in forests of 'Central Aftica, it is now thought to have been imported into the western continent by the slave trade. The negro seems to be tolerant of its presence but in the white to which it has been abundantly transferred in the regions where the two races are in close contact, it evokes' a most serious anemia which results in marked physical and mental deterioration of the white host. Stile, regards the present condition of the "poor white trash" of the South to be due largely to its attacks. The two hookworms are so much with which they are associated and might be confused (Fig. 6). Development proceeds within the eggshell slowly, the rate varying with the external temperature, until the embryo takes form (Fig. 7) and later a minute larva (Fig. 5, a) is hatched out. This passes its existence free in mud or feces until, after the second molt, the second stage of development lies within the larval skin of the first and in a cyst so that this worm is often spoken of as the encysted stage (Fig. 5, b). In this form known as the infective stage it seeks out water and there awaits an opportunity for gaining en trance into a new host. Eggs or younger
larvm do not cause infection even when intro duced into man by chance.
alike in appearance that careful microscopical examination is demanded for their differentia tion. Their effects on the host are in general similar though those due to Necator are more intense and persistent.
The adult Ancylostoma duodenale (Fig. 1) measures in the male 8 to 11 mm. in length by 0.4 to 0.5 in breadth, and in the female 10 to 18 mm. long by 0.5 to 0.6 wide. The most significant region is the anterior end (Fig. 2, A).' On looking down into the oral capsule one notes at the anterior ventral margin two pairs of large uncinate teeth turning inwards, and deeper within the capsule two short lateral cutting plates. In Necator the corresponding region (Fig. 3) shows that the, ventral teeth are replaced by broad cutting plates and the deepei lateral plates are lancet shaped. These differences facilitate a diagnosis when the micro scope is employed.
The bursa of the male Ancylostonta duo denale (Fig. 4, A), is also easily distinguished under suitable circumstances from that of the male Necator americanus (Fig. 4, B), as appears from the figures.
The life history of the hookworm is strik ingly complex. The eggs are thin shelled and when evacuated in feces are ordinarily in the four-celled stage, i.e., they have started on de velopment but little and hence are readily dis tinguishable from those of other nematodes The distribution of human wastes over the surface of the ground as is prevalent in many regions gives the chance for the young larva: on hatching out to be dispersed generally in the mud and water of the soil. In those regions where the temperature does not go low enough to kill these larva the top layer thus becomes loaded with them in the infective stage. By the contamination of food and water supplies they may gain access directly to the human alimentary system, and such direct infection does take place though doubted by many after the indirect method of infection had been dis covered. It seems likely, however, that the in direct method is the more common as it is surely the more difficult to combat.