The diagnostics and treatment of tropical diseases

by E. R. Stitt

As a practical point, the anaerobic conditions in the intestines seem to prevent development of the hookworm ova or at any rate the absence of the oxygen, so necessary for the segmentations preliminary to the formation of the embryo, prevents it. Therefore hookworm ova in freshly passed faeces never show other than commencing segmentation while development of the larvae of Strongyloides takes place in the intestines, so that in freshly passed faeces we find, generally, actively moving larvae or at least eggs containing fully developed embryos. Hookworm ova very rarely show more than 4 segments or exceptionally 8 segments in the freshly passed egg.

In the presence of oxygen these ova rapidly develop into larvae, particularly at a temperature of about 27°C. Beyond 37°C. and below 14°C. development does not seem to take place.

The rhabditiform larvae grow rapidly and by the 3rd day are about 300 microns long and undergo a primary moulting. By the 5th day the bulb-like swellings disappear and the larva becomes possessed of a straight oesophagus, thereby becoming a strongyloid larva. It then undergoes a 2nd ecdysis or moulting, but instead of casting off this old covering, it retains it as a protecting sheath. The full grown larva is about 550 by 24 microns. At this time it ceases to take food but can move actively in its sheath so that it can crawl up blades of grass or vertical sides of mines. They can live in this state for months, when moisture and shade are present, but are rapidly killed by drying. Before becoming encysted hookworm larvae are readily destroyed by the action of the sun or chemicals or even by dilution of the faeces, especially with urine. Cort and others have found it a common occurrence for mature larvae to lose their sheaths while living in the soil, and to continue their lives in the unsheathed state. The proportion losing their sheaths varies from 52 to 98%.

This is the infecting stage in which the larvae bore their way into the skin, which is the usual method of infection, or, occasionally, by entering the mouth on vegetables or otherwise.

Looss thought that they entered the skin by way of the hair follicles but the idea now is that they can bore into any part of the skin. It only requires a few minutes for the larvae to enter the skin. From the subcutaneous tissues they effect an entrance into lymphatics or veins, go to the right heart, thence to lungs. From the alveolar capillaries they pass into the pulmonary alveoli, thence up the bronchi and trachea, to pass out of the larynx and then down the oesophagus to the stomach. The larva loses its protecting sheath in the stomach and in a few days develops a provisional buccal capsule.

By the end of the 2nd week, after another ecdysis, the larvae have grown to be about 2 mm. long and 130 microns broad and in about 4 weeks become adults, usually in the jejunum, where, after fertilization of the females by the males, the giving off of eggs begins. The adults attach themselves to the mucosa of the intestine, feeding on the deeper structures of the mucosa, or on the tissues of the submucosa. Sambon believes that the larvae can work their way into the jejunum without going there by way of the trachea and oesophagus.

By providing an exit to the trachea, Fülleborn demonstrated that in dogs, infected with the dog hookworm, great numbers of larvae poured out of the trachea. In other dogs he stitched the oesophagus to the skin and noted larvae coming out of these openings. In these dogs, with the ordinary channel obstructed, infection did occur with, however, only a few worms, thus showing the truth of Sambon’s views but at the same time demonstrating the unimportance of such a route of infection.

The mouth cavity of the embryo is about as deep as the diameter of the embryo at the posterior end of the mouth cavity, that of Strongyloides is only about one-half as deep as the diameter. The genital anlage of Strongyloides is much larger than that of Ancylostoma.

Epidemiology.—When faeces containing hookworm eggs are deposited where conditions of moisture, warmth and shade exist, they develop into the infecting stage, which is the nonfeeding but motile larva inside the cuticle formed by the second moulting. While eggs and younger larvae are killed rapidly, the encysted larvae withstand drying for considerable periods.