The two common hookworms infection of man, Ancylostoma duodenale and Necator americanus, attach themselves to the small bowel by their buccal capsules and suck blood, thereby causing chronic blood loss. Depending on their number and the iron stores of the host, a variable degree of anemia often results: A distinction is made between hookworm infection when the load- is too light to produce symptoms and hookworm disease.
Pathogenic Chain And Etiology of hookworms Infection
The human hookworms measure about 1 cm. in length, the female being slightly longer than the male, which is recognizable by an expanded posterior end, the copulating bursa. A. duodenale is larger than Necator americanus, and the two species are differentiated most easily by the fact that A. duodenale has two pairs of teeth in its buccal capsule, whereas N. americanus has a pair of cutting plates only. The eggs of the two species are indistinguishable and are passed in the feces. In warm moist soil the larva hatches within 48 hours. The rhabditiform larva has a free-living cycle in the soil, feeding on bacteria, during which time it molts twice. The filariform larvae resulting from the second molt are the infective stage for man and can survive several months in favorable conditions.
They tend to migrate up grass stems or gain any elevation up to 3 feet, and on coming into contact with the skin of man, quickly penetrate it, enter the bloodstream, and are transported to the lungs. Like Strongyloides, there they leave the vascular system and emerge into the alveoli, migrate up the bronchi and trachea, and down the esophagus to reach the small intestine where maturity is attained. Eggs appear in the stool five or more weeks after invasion, and the adults live one to nine years.
Epidemiology of hookworms.
Over 400 million people have hookworm infections, but in the majority the worm load is small. Ancylostoma duodenale is the Old World hookworm, being prevalent in Europe, North Africa, and the Middle and Far East. Necator americanus is found more in the New World and tropical Africa. During the past 30 years both parasites have become widely distributed, and rigid geographic demarcations are not possible.
The survival of larvae is favored by a damp sandy soil, high in humus content at a temperature of 24 to 32° C. Promiscuous defecation and the absence of shoes are the chief factors responsible for infections. Such conditions occur in mines as well as on the surface of the ground. Urban people tend to have less hookworm than agricultural rural workers at locations where night soil is often used as fertilizer. Infection can be acquired by ingesting or handling contaminated vegetables. Coffee, banana, sugar cane, rice, and sweet potato fields are ideal for the growth and development of larvae. Often one locality is used as a communal latrine in the area, and people reinfect themselves by visiting these sites again and again.
The distinction between hookworm infection and hookworm disease is important. Methods of estimating the intensity of the infection show that in endemic areas most patients have few worms and no significant anemia. Those who have hookworm anemia have more worms, and these heavy infections could be the result of repeated exposure or a failure of immunity on the part of the host. With canine hookworm, small repeated infections give almost complete immunity. Antibodies have been demonstrated in sera of infected patients.
Pathology and Pathogenesis.
Inflammatory cell infiltration is seen at the site of penetration of the hookworm larvae, and in the lungs small hemorrhages occur with eosinophilic and leukocytic infiltration. An eosinophilia is present during the invasive phase. The main feature of the established disease is the active sucking of blood by the worms. They create a negative pressure in the buccal capsule and suck in a piece of mucosa which acts as both an anchorage and a source of blood. It is still not clear what hookworms abstract from the blood. Vital preparations in vitro show red cells being vigorously expelled from the posterior end. Ancylostoma sucks between 0.16 and 0.34 ml. of blood per day and Necator, 0.03 to 0.05. The development of anemia depends upon three factors: the iron content of the diet, the state of the iron reserves, and the intensity and duration of infection.
When iron intake is high, a heavy worm load is needed to produce a significant anemia. Up to 60 per cent of the iron from the hemoglobin extracted by the hookworm is reabsorbed in the intestine. At open operation small punctate hemorrhagic spots are encountered at the site of attachment. In addition to anemia, hypoalbuminemia occurs owing to a combination of blood loss and a low rate of albumin synthesis, possibly associated with anoxia affecting liver cell function. Malabsorption is said to occur in a few cases with partial villous atrophy and chronic inflammatory cell infiltration of the lamina propria.
Clinical Manifestations of hookworms Infection.
The site of skin penetration by the larvae is associated with pruritus and the development of an erythematous papular eruption (ground itch) which may la.st several days or even longer, depending on the host’s immune response. Within a week after penetration the patient may have a transient asthmatic attack, but this is not as commonly seen as in invasive ascariasis.
Established hookworm disease is associated with general symptoms of anemia, weakness, fatigue, dyspnea, palpitation, and mental and physical retardation. Pica may be noted. On examination the skin and mucus membrane are pale. Peripheral edema is due possibly to a variety of factors, namely, hypoalbuminemia, a rise of capillary venous pressure, and tissue anoxia. In severe cases there is evidence of congestive cardiac failure. The pulse is rapid with a high pulse pressure. On auscultation of the enlarged heart, a third heart sound and an ejection type of systolic murmur are commonly heard; regurgitant systolic and even diastolic murmurs may occur and disappear when the anemia is corrected. For this reason it is very unwise to diagnose a valvular lesion clinically in such anemic patients until that anemia has been corrected. The hemoglobin may be very low (2 grams per 100 ml.) and the patient still ambulant. The patient may complain of upper abdominal pain, and radiological changes suggestive of a duodenitis have been reported.
Diagnosis and Differential Diagnosis of hookworms Infection.
Stool microscopy, either by a direct smear or by a concentration technique, reveals hookworm ova. Quantitation of the egg excretion enables the physician to decide whether the patient has a significant worm load. This is done by diluting a known volume of stool and counting a sample (Stoll technique). Hookworm ova are 60 to 70/u, long by 35 to 40/x wide and have a characteristic morphology with a clear shell, and developing embryo inside. They must be distinguished, however, from both Trichostrongylus and Ternidens deminutus’, both of which have larger eggs. Test tube cultivation of ova and differentiation of the resultant larvae are of value in doubtful cases. A skin test using an extract of Necator americanus larvae of standard nitrogen content has proved useful in screening a hookworm-infected population. Fluorescent antibody, complement-fixation, and hemagglutination tests have been developed but have little clinical application.
The anemia of hookworm disease is a classic iron deficiency anemia with a low hemoglobin, mean corpuscular hemoglobin concentration and serum iron, and a high iron-binding capacity. On the film the red cells are microcytic and hypochromic. Folic acid deficiency may sometimes be superimposed on macrocytosis. The serum albumin is low, and liver function tests may be abnormal. In the edematous patient there may be confusion with kwashiorkor, wet beriberi, or the nephrotic syndrome. The anemia has to be differentiated from other iron deficiency anemias.
Treatment. Tetrachloroethylene in a dose of 0.1 mg. per kilogram of body weight is effective (maximum of 5 ml. in a single dose). It is usually dispensed in 1-ml. gelatin capsules or in a liquid that should be kept in the refrigerator in a dark bottle. It is given in the morning on an empty stomach; all food should be withheld for six hour ; and any fatty foods withheld for the rest of the day
Repeated treatments may be necessary.
Vertigo, vomiting, and dizziness may follow therapy. Bephenium hydroxynaphthoate (Alcopar) is more expensive but is being increasingly used. The standard dose for an adult is 5 grams containing 2.5 grams of bephenium base; it is taken as granules in a glass of water on an empty stomach. Three daily doses may achieve total eradication. Loose stools, nausea, and vomiting have followed treatment. Both drugs will remove a large number of worms at the first dose, but to get rid of the last 5 to 10 per cent is often very difficult. In many areas total eradication may not! be desirable or necessary. In general, tetrachloroethylene is said to be more effective in Necator americanus infections. If Ascaris are also present, Alcopar is preferred, or both Ascaris and hookworm will respond to thiabendazole. Tetrachloroethylene irritates Ascaris and may cause migration.
The anemia responds well to ferrous sulfate, 200 or 400 mg. three times daily. Occasionally the hemoglobin is so low, or there is an associated acute infection, that the patient presents in extremis owing to heart failure. Pregnancy may also precipitate acute heart failure. Intraperitoneal blood transfusion or exchange transfusions have been life-saving. The prognosis is good in most cases. It is wise to delay treating the hookworm infection until the hemoglobin is above 50 per cent after oral iron. Without treatment of the worm infection, the anemia will relapse when iron therapy is discontinued. Prevention involves the sanitary disposal of human excreta and the prevention of soil pollution. The wearing of shoes cuts down the opportunities for infection.
