The hookworm is a parasitic nematode worm that lives in the small intestine of its host, which may be a mammal such as a dog, cat, or human. Two species of hookworms commonly infect humans, Ancylostoma duodenale and Necator americanus. Necator americanus predominates in the Americas, Sub-Saharan Africa, Southeast Asia, China and Indonesia, while A. duodenale predominates in the Middle East, North Africa, India and (formerly) in southern Europe. Hookworms are thought to infect 800 million people worldwide. The A. braziliense and A. tubaeforme species infect cats, while A. caninum infects dogs. Uncinaria stenocephala infects both dogs and cats.
Hookworms are much smaller than the large roundworm, Ascaris lumbricoides, and the complications of tissue migration and mechanical obstruction so frequently observed with roundworm infestation are less frequent in hookworm infestation. The most significant risk of hookworm infection is anemia, secondary to loss of iron (and protein) in the gut. The worms suck blood voraciously and damage the mucosa. However, the blood loss in the stools is occult blood loss (not visibly apparent).
Ankylostomiasis, alternatively spelled anchylostomiasis and also called helminthiasis, "miners' anaemia", "tunnel disease", "brickmaker's anaemia" and "Egyptian chlorosis", is the disease caused by hookworms. It is caused when hookworms, present in large numbers, produce an iron deficiency anemia by voraciously sucking blood from the host's intestinal walls. The name is derived from Greek ancylo "crooked, bent" and stoma "mouth."
Hookworm is a leading cause of maternal and child morbidity in the developing countries of the tropics and subtropics. In susceptible children hookworms cause intellectual, cognitive and growth retardation, intrauterine growth retardation, prematurity, and low birth weight among newborns born to infected mothers. Hookworm infection is rarely fatal, but anemia can be significant in the heavily infected individual.
Italian physician Angelo Dubini was the modern-day discoverer of the worm in 1838 after an autopsy of a peasant woman. Dubini published details in 1843 and identified the species as A. duodenale. Working in the Egyptian medical system in 1852 German physician Theodor Bilharz, drawing upon the work of colleague Wilhelm Griesinger, found these worms during autopsies and went a step further in linking them to local endemic occurrences of chlorosis, which would probably be called iron deficiency anemia today.
A breakthrough came 25 years later following a diarrhea and anemia epidemic that took place among Italian workmen employed on the Gotthard Rail Tunnel. In an 1880 paper, physicians Camillo Bozzolo, Edoardo Perroncito, and Luigi Pagliani correctly hypothesized that hookworm was linked to the fact that workers had to defecate inside the 15 km tunnel, and that many wore worn-out shoes. In 1897, it was established that the skin was the principal avenue of infection and the biological life cycle of the hookworm was clarified. In 1899, American zoologist Charles Wardell Stiles brought this evidence to bear on health issues in the southeast United States, identifying "progressive pernicious anemia" seen in the southern United States was caused by A. duodenale and he also identified the other important hookworm species: U. Necator. Testing in the 1900s revealed very heavy infestations in schoolage children.
On October 26, 1909 the Rockefeller Sanitary Commission for the Eradication of Hookworm Disease was organized as a result of a gift of US$1 million from John D. Rockefeller, Sr. The five-year program was a remarkable success and a great contribution to United States public health, instilling public education, medication, field work and modern government health departments in eleven southern states. The hookworm exhibit was a prominent part of the 1910 Mississippi state fair. The program nearly eradicated hookworm and would flourish afterwards with new funding as the Rockefeller Foundation International Health Division.
In the 1920s, hookworm eradication reached the Caribbean and Latin America, where great mortality was reported among blacks in the West Indies towards the end of the 18th century, as well as through descriptions sent from Brazil and various other tropical and sub-tropical regions.
Early treatment was with thymol to kill the worms, followed by epsom salt to clear the body of the worms. Later on, tetrachloroethylene was the leading method. It was not until later in the mid-20th century when new organic drug compounds were developed.
The symptoms can be linked to inflammation in the gut stimulated by feeding hookworms, such as nausea, abdominal pain and intermittent diarrhea, and to progressive anemia in prolonged disease: capricious appetite, pica (or dirt-eating), obstinate constipation followed by diarrhea, palpitations, thready pulse, coldness of the skin, pallor of the mucous membranes, fatigue and weakness, shortness of breath and in cases running a fatal course, dysentery, hemorrhages and edema.
Blood tests in early infection often show a rise in numbers of eosinophils, a type of white blood cell that is preferentially stimulated by worm infections in tissues (large numbers of eosinophils are also present in the local inflammatory response). Falling blood hemoglobin levels will be seen in cases of prolonged infection with anemia.
In contrast to most intestinal helminthiases, where the heaviest parasitic loads tend to occur in children, hookworm prevalence and intensity can be higher among adult males. The explanation for this is that hookworm infection tends to be occupational, so that plantation workers, coalminers and other groups maintain a high prevalence of infection among themselves by contaminating their work environment. However, in most endemic areas, adult women are the most severely affected by anemia, mainly because they have much higher physiological needs for iron (menstruation, repeated pregnancy), but also because customarily they have access to much poorer food than the men.
An interesting consequence of this in the case of Ancylostoma duodenale infection is translactational transmission of infection: the skin-invasive larvae of this species do not all immediately pass through the lungs and on into the gut, but spread around the body via the circulation, to become dormant inside muscle fibers. In a pregnant woman, after childbirth some or all of these larvae are stimulated to re-enter the circulation (presumably by sudden hormonal changes), then to pass into the mammary glands, so that the newborn baby can receive a large dose of infective larvae through its mother's milk. This accounts for otherwise inexplicable cases of very heavy, even fatal, hookworm infections in children a month or so of age, in places such as China, India and northern Australia. (An identical phenomenon is much more commonly seen with Ancylostoma caninum infections in dogs, where the newborn pups can even die of hemorrhaging from their intestines caused by massive numbers of feeding hookworms. This also reflects the close evolutionary link between the human and canine parasites, which probably have a common ancestor dating back to when humans and dogs first started living closely together.)
Hookworm therapy to treat a number of these diseases is currently in the trial stage at the University. Some people, including Dr. David Pritchard of the University of Nottingham, and Jasper Lawrence of Autoimmune Therapies, have independently infected themselves with hookworms and reported positive results.
See the image for the biological life cycle of the hookworms where it thrives in warm earth where temperatures are over 18°C. They exist primarily in sandy or loamy soil and cannot live in clay or muck. Rainfall averages must be more than 1000 mm (40 inches) a year. Only if these conditions exist can the eggs hatch. Infective larvae of Necator americanus can survive at higher temperatures, whereas those of Ancylostoma duodenale are better adapted to cooler climates. Generally, they live for only a few weeks at most under natural conditions, and die almost immediately on exposure to direct sunlight or desiccation.
Infection of the host is by the larvae, not the eggs. While A. duodenale can be ingested, the usual method of infection is through the skin; this is commonly caused by walking barefoot through areas contaminated with fecal matter. The larvae are able to penetrate the skin of the foot, and once inside the body, they migrate through the vascular system to the lungs, and from there up the trachea, and are swallowed. They then pass down the esophagus and enter the digestive system, finishing their journey in the intestine, where the larvae mature into adult worms.
Once in the host gut, Necator tends to cause a prolonged infection, generally 1–5 years (many die within a year or two of infecting), though some adult worms have been recorded to live for 15 years or more. On the other hand, Ancylostoma adults are short lived, surviving on average for only about 6 months. However, infection can be prolonged because dormant larvae can be "recruited" sequentially from tissue "stores" (see Pathology, above) over many years, to replace expired adult worms. This can give rise to seasonal fluctuations in infection prevalence and intensity (apart from normal seasonal variations in transmission).
They mate inside the host, females laying up to 30,000 eggs per day, which pass out in feces. Because it takes 5–7 weeks for adult worms to mature, mate and produce eggs, in the early stages of very heavy infection, acute symptoms might occur without any eggs being detected in the patient's feces. This can make diagnosis very difficult.
Moxidectin has been released in the United States as part of Advantage Multi (imidacloprid + moxidectin) Topical Solution for dogs and cats. It utilizes moxidectin for control and prevention of roundworms, hookworms, heartworms, and whipworms.
In the late 1800s and early 1900s, many Mississippians were plagued by hookworms. They did not have indoor plumbing or proper sanitation facilities. As a result, hookworms, spread by fecal contamination of the environment, were very prevalent (as well as other diseases caused by lack of sanitation).
Diagnosis depends on finding characteristic worm eggs on microscopic examination of the stools, although this is not possible in early infection. As the eggs of both Ancylostoma and Necator (and most other hookworm species) are indistinguishable, to identify the genus, they must be cultured in the lab to allow larvae to hatch out. If the fecal sample is left for a day or more under tropical conditions, the larvae will have hatched out, so eggs might no longer be evident. In such a case, it is essential to distinguish hookworms from Strongyloides larvae, as infection with the latter has more serious implications and requires different management. The larvae of the two hookworm species can also be distinguished microscopically, although this would not be done routinely, but usually for research purposes. Adult worms are rarely seen (except via endoscopy, surgery or autopsy), but if found, would allow definitive identification of the species.
Albendazole is effective both in the intestinal stage and during the stage the parasite is still migrating under the skin.
In case of anemia, iron supplementation can cause relief symptoms of iron deficiency anemia. However, as red blood cell levels are restored, shortage of other essentials such as folic acid or vitamin B12 may develop, so this might also be supplemented.
|Genus and Species||Necator americanus||Ancylostoma duodenale|
|Common Name||New world hookworm, American murderer||Old world hookworm|
|Etiologic Agent of:||Necatoriasis, Uncinariasis||Ancylostomiasis, Wakana disease|
|Infective stage||Filariform larva||Filariform larva|
|Portal of Entry||Usually via skin penetration rather than ingestion||Usually via ingestion rather than skin penetration|
|Mode of Transmission||Skin > Mouth||Mouth > Skin|
|Habitat||Small Intestine (jejunum, ileum)||Small Intestine (duodenum, jejunum)|
|Pathogenic Stage||L3 Larva||L3 Larva|
|Maturation time in host (days)||49-56||53|
|Mode of Attachment||Oral attachment to mucosa by sucking||Same|
|Mode of Nutrition||Sucking and Ingesting of blood||Same|
|Pathogenesis||Larva – ground / dew itch, creeping eruption Adult – IDA Microcytic, Hypochromic Anemia||Same|
|Laboratory diagnosis||Concentration methods and Direct Fecal Smear||Same|
|Treatment||Albendazole, Mebendazole, or Pyrantel Pamoate||Same|
|Length of adult hookworm (mm)||5-9 for males; 9-11 for females||8-11 for males; 10-13 for females|
|Shape||Head curved opposite to curvature of body, giving a hooked appearance to anterior end||Head continuous in same direction as the body|
|Egg output per female worm per day||5,000-10,000||10,000-25,000|
|Blood loss per worm per day (ml)||0.03||0.15-0.23|
|Temperature at which 90% of eggs hatch (C)||20-35||15-35|
|Diagnostic Feature - Adult||Semi-lunar cutting plate; Bipartite dorsal ray||Male – Tripartite dorsal ray|
|Diagnostic Feature - Egg||In Morula||Same|