There are many species of hookworms that infect mammals. The most important, at least from the human standpoint, are the human hookworms, Ancylostoma duodenale and Necator americanus, which infect an estimated 15-20% of the world's population.
Hookworms average about 10mm in length and live in the small intestine of the host. The hookworm life cycle begins when the adult males and females mate, and the female produces eggs that are passed in the feces. Depending on the species, female hookworms can produce 10,000-25,000 eggs per day. These eggs can then contaminate the soil and, in warm moist conditions, they will hatch and develop to the stage where they can become infective. About two days after passage the hookworm egg hatches, and the juvenile worm (or larva) develops into an infective stage in about five days. The larvae live in the surface of the soil. These larvae infect humans by burrowing through the skin (for example, through bare feet), and migrating through the blood system to the lungs. From here they pass to the throat, where they are swallowed, and move down to the intestine. The adult worms feed in the intestine by attaching themselves to the lining with sharp "teeth". Here they produce eggs, which are then passed out with the feces.
Juveniles (larvae) of the dog and cat hookworms (A. caninum and A. braziliense, respectively) can infect humans, but the juvenile worms will not mature into adult worms. Rather, the juveniles remain in the skin where they continue to migrate for weeks (or even months in some instances) until killed by the host's inflammatory response. This causes local intense itching and results in a condition known as "cutaneous" or "dermal larval migrans" or "creeping eruption." Hence the importance of not allowing dogs and cats to defecate indiscriminately.
Causes and Development
Individuals become infected, usually by walking bare footed across contaminated soil. Soil contamination can be extremely heavy, particularly in those cultures which utilise human excreta as fertilizer. The eggs are susceptible to urine, however, and it has been found that if fecal material is mixed with urine the eggs can be killed.
There are three pathological phases to hookworm infections:
- Cutaneous or invasive phase: When larvae initially penetrate the skin they can cause irritation and itching and if they fail to locate a blood capillary they can wander through the skin causing a condition known as cutaneous larval migrans, leaving a track visualized under the skin by the presence of the host inflammatory reaction.
- Pulmonary phase: This occurs during the period when the larvae are bursting out of the capillaries in the lungs into the alveolar spaces. This causes local haemorrhaging at the site. This is rarely symptomatic, except when there is a heavy infection, when it can result in pneumonitis, and can also cause a cough and a sore throat.
- Intestinal phase: Adult worms are usually restricted to the anterior third of the small intestine, but where infections are very heavy they can occupy the whole length of the small intestine. The worms clamp onto the mucosa abrading the surface and sucking the blood. Proteolytic enzymes from the dorsal pharyngeal gland are released into the buccal cavity which aid in digestion.
Signs and Symptoms
The most common effect from hookworm
infections is the varying degrees of anemia
, depending on the level of the infection and the nutritional state of the patient. Patients also can suffer from protein deficiency which manifests as dry hair, skin and edema
, but protein loss can have far-reaching effects including reduced immunocompetence and reduced ability to produce gama globulin (Antibody
The infection is particularly severe in children, and the development of a pot belly, as a result of the edema, is a common symptom of the infection. The protein deficiency also can result in significantly delayed physical development in children. In its most progressive state can result in death.
Diagnosis and Tests
The presence of hookworms
can be demonstrated by finding the characteristic eggs in the feces; the eggs can not, however, be differentiated to species.
Treatment and Prevention
The drug of choice for the treatment of hookworm
disease is mebendazole which is effective against both species, and in addition, will remove the intestinal worm Ascaris also, if present. The drug is very efficient, requiring only a single dose and is inexpensive – the "perfect drug". However, treatment requires more than giving the anthelmintic
, the patient should also receive dietary supplements to improve their general level of health, in particular iron
supplementation is very important.
Topical treatment with thiabendazole ointment is very effective in controlling dermal larval migrans.
The mouthparts of hookworms
are modified into cutting plates. Attachment of hookworms to the host's small intestine
causes hemorrhages, and the hookworms feed on the host's blood. Hookworm
disease can have devastating effects on humans, particularly children, due to the loss of excessive amounts of blood. Heavy hookworm infections can account for as much as 200ml of blood loss per day.