Hookworm
From Wikipedia, the free encyclopedia
| Necator americanus andAncylostoma duodenale | |
|---|---|
 | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Nematoda |
| Class: | Secernentea |
| Order: | Strongiloidae |
| Family: | Ancylostomatidae |
| Genus: | Necator/Ancylostoma |
| Species | |
N. americanus
A. duodenale | |
The hookworm is a parasitic nematode 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. A. duodenale predominates in the Middle East, North Africa, Indiaand (formerly) in southern Europe, while N. americanus predominates in the Americas, Sub-Saharan Africa, Southeast Asia, China, and Indonesia. Hookworms are thought to infect more than 600 million people worldwide. The A. braziliense and A. tubaeforme species infect cats, while A. caninum infects dogs. Uncinaria stenocephala infects both dogs and cats. Ancylostoma caninum has infected humans, with 150 cases reported in Brisbane, Australia from 1998-1992.
Hookworms are much smaller than the larger 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 not visibly apparent.
Ancylostomiasis, also known by several other names, is the disease caused when A. duodenale hookworms, present in large numbers, produce an iron deficiency anemia by sucking blood from the host's intestinal walls.
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. In developed countries, hookworm infection is rarely fatal, but anemia can be significant in a heavily infected individual.
[edit]
There are no specific symptoms or signs of hookworm infection. As mentioned above, they give rise to a combination of intestinal inflammation and progressive iron/protein-deficiency anaemia. Larval invasion of the skin might give rise to intense, local itching, usually on the foot or lower leg, which can be followed by lesions that look like insect bites, can blister ("ground itch"), and last for a week or more.Signs and symptoms
Animal hookworm larvae on penetrating humans may produce a creeping eruption called cutaneous larva migrans. The larvae migrate in tortuous tunnels in between stratum basale and stratum corneum of the skin, causing serpigenous vesicular lesions. With advancing movement of the larvae, the rear portions of the lesions become dry and crusty. The lesions are typically intensely pruritic.
Coughing, chest pain, wheezing, and fever will sometimes be experienced by people who have been exposed to very large numbers of larvae. Epigastric pains, indigestion, nausea, vomiting,constipation, and diarrhea can occur early or in later stages as well, although gastrointestinal symptoms tend to improve with time. Signs of advanced severe infection are those of anemia and protein deficiency, including emaciation, cardiac failure and abdominal distension with ascites.
[edit]Pathophysiology
[edit]Morphology
A. duodenale worms are grayish white or pinkish with the head slightly bent in relation to the rest of the body. This bend forms a definitive hook shape at the anterior end for which hookworms are named. They possess well developed mouths with two pairs of teeth. While males measure approximately one centimeter by 0.5 millimeter, the females are often longer and stouter. Additionally, males can be distinguished from females based on the presence of a prominent posterior copulatory bursa.[1]
N. americanus is very similar in morphology to A. duodenale. N. americanus is generally smaller than A. duodenale with males usually 5 to 9 mm long and females about 1 cm long. Whereas A. duodenale possess two pairs of teeth, N. americanus possesses a pair of cutting plates in the buccal capsule. Additionally, the hook shape is much more defined in Necator than in Ancylostoma.[1]
[edit]Pathology
Hookworm infection is generally considered to be asymptomatic, but as Norman Stoll described in 1962, hookworm is an extremely dangerous infection because its damage is “silent and insidious.”[2]There are general symptoms that an individual may experience soon after infection. Ground-itch, which is an allergic reaction at the site of parasitic penetration and entry, is common in patients infected with N. americanus.[1] Additionally, cough and pneumonitis may result as the larvae begin to break into the alveoli and travel up the trachea. Then once the larvae reach the small intestine of the host and begin to mature, the infected individual will suffer from diarrhea and other gastrointestinal discomfort.[1] However, the “silent and insidious” symptoms referred to by Stoll are related to chronic, heavy-intensity hookworm infections. Major morbidity associated with hookworm is caused by intestinal blood loss, iron deficiency anemia, and protein malnutrition.[3] They result mainly from adult hookworms in the small intestine ingesting blood, rupturing erythrocytes, and degrading hemoglobin in the host.[4] This long-term blood loss can manifest itself physically through facial and peripheral edema; eosinophilia and pica caused by iron deficiency anemia are also experienced by some hookworm-infected patients.[1] Recently, more attention has been given to other important outcomes of hookworm infection that play a large role in public health. It is now widely accepted that children who suffer from chronic hookworm infection can suffer from growth retardation as well as intellectual and cognitive impairments.[4] Additionally, recent research has focused on the potential of adverse maternal-fetal outcomes when the mother is infected with hookworm during pregnancy.
The disease was linked to nematode worms (Ankylostoma duodenalis) from one-third to half an inch long in the intestine chiefly through the labours of Theodor Bilharz and Griesinger in Egypt (1854).
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.
[edit]Life cycle
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 orloamy 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.[4][5]
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 and some 18 to 54 million eggs during their lifetime, 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.
- Summary of biological life cycle
N. americanus and A. duodenale eggs can be found in warm, moist soil where they will eventually hatch into first stage larvae, or L1. L1, the feeding non-infective rhabditoform stage, will feed on soil microbes and eventually molt into second stage larvae, L2. L2, which is also in the rhabditoform stage, will feed for approximately 7 days and then molt into the third stage larvae, or L3. L3 is the filariform stage of the parasite, that is, the non-feeding infective form of the larvae. The L3 larvae are extremely motile and will seek higher ground to increase their chances of penetrating the skin of a human host. The L3 larvae can survive up to 2 weeks without finding a host. While N. americanus larvae only infect through penetration of skin, A. duodenale can infect both through penetration as well as orally. After the L3 larvae have successfully entered the host, the larvae then travel through the subcutaneous venules and lymphatic vessels of the human host. Eventually, the L3 larvae enter the lungs through the pulmonary capillaries and break out into the alveoli. They will then travel up the trachea to be coughed and swallowed by the host. After being swallowed, the L3 larvae are then found in the small intestine where they molt into the L4, or adult worm stage. The entire process from skin penetration to adult development takes about 5–9 weeks. The female adult worms will release eggs (N. Americanus about 9,000-10,000 eggs/day and A. duodenale 25,000-30,000 eggs/day) which are passed in the feces of the human host. These eggs will hatch in the environment within several days and the cycle with start anew.[3][4][6]
- Incubation period
The incubation period can vary between a few weeks to many months and is largely dependent on the number of Hookworm parasites an individual is infected with.[7]
[edit]Diagnosis
Diagnosis depends on finding characteristic worm eggs on microscopic examination of the stools, although this is not possible in early infection. The eggs are oval or elliptical, measuring 60 µm by 40 µm, colourless, not bile stained and with a thin transparent hyaline shell membrane. When released by the worm in the intestine, the egg contains an unsegmented ovum. During its passage down the intestine, the ovum develops and thus the eggs passed in feces have a segmented ovum, usually with 4 to 8 blastomeres. 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. Classification can be performed based on the length of the buccal cavity, the space between the oral opening and the esophagus: hookworm rhabditoform larvae have long buccal cavities whereas Strongyloides rhabditoform larvae have short buccal cavities.[1]
Recent research has focused on the development of DNA-based tools for diagnosis of infection, specific identification of hookworm, and analysis of genetic variability within hookworm populations.[8] Because hookworm eggs are often indistinguishable from other parasitic eggs, PCR assays could serve as a molecular approach for accurate diagnosis of hookworm in the feces.[8][9]
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