Hookworm infection
Hookworm infection is an infection by a type of intestinal parasite known as a hookworm. Initially, itching and a rash may occur at the site of infection. Those only affected by a few worms may show no symptoms. Those infected by many worms may experience abdominal pain, diarrhea, weight loss, and tiredness. The mental and physical development of children may be affected. Anemia may result.
Two common hookworm infections in humans are ancylostomiasis and necatoriasis, caused by the species Ancylostoma duodenale and Necator americanus respectively. Hookworm eggs are deposited in the stools of infected people. If these end up in the environment, they can hatch into larvae, which can then penetrate the skin. One type can also be spread through contaminated food. Risk factors include walking barefoot in warm climates, where sanitation is poor. Diagnosis is by examination of a stool sample with a microscope.
The risk of infection can be reduced on an individual level by not walking barefoot in areas where the disease is common. At a population level, decreasing outdoor defecation, not using raw feces as fertilizer, and mass deworming are effective. Treatment is typically with the medications albendazole or mebendazole for one to three days. Iron supplements may be needed in those with anemia.
Hookworms infected about 428 million people in 2015. Heavy infections can occur in both children and adults, but are less common in adults. They are rarely fatal. Hookworm infection is a soil-transmitted helminthiasis and classified as a neglected tropical disease.
Signs and symptoms
No symptoms or signs are specific to hookworm infection, but they give rise to a combination of intestinal inflammation and progressive iron-deficiency anemia and protein deficiency. Coughing, chest pain, wheezing, and fever sometimes result from severe infection. 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.Larval invasion of the skin can produce a skin disease called cutaneous larva migrans also known as creeping eruption. The hosts of these worms are not human and the larvae can only penetrate the upper five layers of the skin, where they give rise to intense, local itching, usually on the foot or lower leg, known as ground itch. This infection is due to larvae from the A. braziliense hookworm. The larvae migrate in tortuous tunnels between the stratum basale and stratum corneum of the skin, causing serpiginous vesicular lesions. With the advancing movement of the larvae, the rear portions of the lesions become dry and crusty. The lesions are typically intensely itchy.
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.Cause
Hookworm infections in humans include ancylostomiasis and necatoriasis. Ancylostomiasis is caused by Ancylostoma duodenale, which is the more common type found in the Middle East, North Africa, India, and in southern Europe. Necatoriasis is caused by Necator americanus, the more common type in the Americas, sub-Saharan Africa, Southeast Asia, China, and Indonesia.Other animals such as birds, dogs, and cats may also be affected. A. tubaeforme infects cats, A. caninum infects dogs, and A. braziliense and Uncinaria stenocephala infect both cats and dogs. Some of these infections can be transmitted to humans.
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.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 possesses 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.
Life cycle
The hookworm thrives in warm soil where temperatures are over. They exist primarily in sandy or loamy soil and cannot live in clay or muck. Rainfall averages must be more than a year for them to survive. Only if these conditions exist can the eggs hatch. Infective larvae of N. americanus can survive at higher temperatures, whereas those of A. 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 can 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 to 5 years, though some adult worms have been recorded to live for 15 years or more. Ancylostoma adults are short-lived, surviving on average for only about 6 months. However, the infection can be prolonged because dormant larvae can be "recruited" sequentially from tissue "stores" over many years, to replace expired adult worms. This can give rise to seasonal fluctuations in infection prevalence and intensity.
File:CPS141privydigging.jpg|thumb|Civilian Public Service workers built and installed 2,065 outhouses for hookworm eradication in Mississippi and Florida from 1943 to 1947.
They mate inside the host, females laying from 10,000 to 30,000 eggs per day and some 15 to 56 million eggs during their adult lifetimes, which pass out in feces. Because 5 to 7 weeks are needed 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.
N. americanus and A. duodenale eggs can be found in warm, moist soil where they eventually hatch into first-stage larvae, or L1. L1, the feeding noninfective rhabditoform stage, will feed on soil microbes and eventually molt into second-stage larvae, L2, which is also in the rhabditoform stage. It will feed for about 7 days and then molt into the third-stage larvae, or L3. This is the filariform stage of the parasite, that is, the nonfeeding infective form of the larvae. The L3 larvae are extremely motile and 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 the skin, A. duodenale can infect both through penetration and orally. After the L3 larvae have successfully entered the host, they 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 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 release eggs, which are passed in the feces of the human host. These eggs hatch in the environment within several days and the cycle starts anew.
Pathophysiology
Hookworm infection is generally considered to be asymptomatic, but as Norman Stoll described in 1962, it is an extremely dangerous infection because its damage is "silent and insidious." An individual may experience general symptoms 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. 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 experience diarrhea and other gastrointestinal discomfort. However, the "silent and insidious" symptoms referred to by Stoll are related to chronic, heavy-intensity hookworm infections. Major morbidity associated with hookworm infection is caused by intestinal blood loss, iron deficiency anemia, and protein malnutrition. They result mainly from adult hookworms in the small intestine ingesting blood, rupturing erythrocytes, and degrading hemoglobin in the host. This long-term blood loss can manifest itself physically through facial and peripheral edema; eosinophilia and pica/geophagy caused by iron deficiency anemia are also experienced by some hookworm-infected patients. 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 have chronic hookworm infection can experience growth retardation as well as intellectual and cognitive impairments. 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 from one-third to half an inch long in the intestine chiefly through the labours of Theodor Bilharz and Griesinger in Egypt.
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/geophagy, 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. The worms suck blood and damage the mucosa. However, the blood loss in the stools is not visibly apparent.
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. 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 coworkers and other close 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. 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, 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.
Filariform larvae are the infective stage of the parasite: infection occurs when larvae in soil penetrate the skin, or when they are ingested through contaminated food and water following skin penetration.