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Strongyloides stercoralis is the threadworm parasite of man. There are about 38 species of threadworms found in sheep, swine, goat, ox, deer, camel, rabbit, primates, dogs, and cats. Some species are parasites of birds, reptiles and amphibians. Worms indistinguishable from S. stercoralis have been recovered from dogs and cats.
Parasitic infection with nematodes is typically more of a nuisance disease than a serious medical condition, but this nematode has the unique ability to complete its replication cycle entirely within a human host -- and this ability can lead to so-called "hyperinfection" in immunocompromised persons.
These worms have a heterogenetic life cycle which consists of - A parasitic generation (homogenic life cycle)
- A free-living generation (heterogonic life cycle).
The heterogonic life cycle is advantageous to the parasite as it affords the opportunity for the parasite to exist and reproduce for one or more generations in the absence of a host. Also the number of infective juveniles may be increased by reproduction of the free-living females and males.
Parasitic females anchor themselves with their mouths to the mucosa of the small intestine or burrow their anterior ends into the submucosa. Reproduction in the host is by parthenogenetic females which lay several dozen eggs each day. Eggs are released into the lumen of the gut or the submucosa where they hatch and juveniles pass into the lumen. These first-stage juveniles are 300-380µm long and are usually passed with the feces. Juveniles develop either to free-living adults or to infective filariform juveniles. Third stage juveniles are the infective stage. They are 490-630µm long. This is a resting stage which does not develop further until it penetrates through skin or is ingested. Following skin penetration they are carried by the blood to the lungs, where they exit into the alveoli, travel up the trachea, are swallowed, and mature in the small intestine. If ingested, migration through the lungs is not necessary.
The free-living adults can produce successive generations of free-living adults. Both free-living and parasitic females can produce juveniles that will become filariform, infective juveniles and juveniles that will mature into free-living adults. Autoinfection is also possible if juveniles have time to molt twice during passage down the digestive tract. In these cases they penetrate the lower gut mucosa or perianal skin, migrate in the typical way, and mature. Both free-living and parasitic females can produce juveniles that will become filariform, infective juveniles and juveniles that will mature into free-living adults. Autoinfection is also possible if juveniles have time to molt twice during passage down the digestive tract. In these cases they penetrate the lower gut mucosa or perianal skin, migrate in the typical way, and mature.
Parthenogenetic females reach a length of about 1/10th of an inch (2.0 to 2.5mm). The buccal capsule of both sexes is small, and they possess a long, cylindrical esophagus that lacks a posterior bulb. The vulva is in the posterior third of the body; the uteri are divergent and contain only a few eggs at a time. The free-living adults both have a rhabditiform esophagus. The male is up to 0.9mm long and 40-50µm wide.
Incidence
The threadworm of man occurs from about 35° north latitude to 30° south latitude. Generally the distribution is limited to warm moist areas because such climates are favorable to the survival of the juvenile stages. In the United States, strongyloidiasis is relatively uncommon. Endemic concentrations exist in rural areas of the southeastern United States and Appalachia, with prevalence rates close to 4%. Infections acquired in the United States, while not usually associated with larva currens, are not clinically silent; the infected individuals usually have a chronic relapsing illness of mild to moderate severity. Among veterans of the US military forces who served in Southeast Asia, the prevalence of larva currens in those with confirmed strongyloidiasis is high, with studies showing a range of 30-90%.
Strongyloidiasis is endemic in tropical and subtropical countries. The worldwide prevalence is approximately 35 million cases, and rates are as high as 40% in certain regions.
Signs & Symptoms
- Dermatitis is produced by migration of the infective juveniles through the skin (cutaneous infection).
- The mild to severe symptom of pneumonia can occur during migration to air-sacs of lungs. (Cases of reproduction in the air-sacs have been observed but they are relatively rare).
- Inflammation of the intestinal mucosa.
- Diarrhea accompanied by emaciation and exhaustion.
Diagnosis & Tests
Diagnosis is based on finding juveniles in freshly passed stools, by a direct smear in cases of heavy infection or following concentration by Baermann isolation or zinc flotation with centrifugation. Feces which have been in contact with soil my be contaminated with soil-dwelling rhabditids.
Treatment & Prevention
In massive infections death may result unless therapeutic measures are taken. Prevention involves mainly sanitation involving proper disposal of human wastes. Infected persons and animals may be treated with appropriate anthelmintics. (Vermox or Pyrantyl Pamoate; Thiabendazole or Cambendazole).
Complications
Almost all of the fatal cases of helminthic infection in the United States are caused by this autoinfecting nematode, Strongyloides.
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Conditions that suggest Strongyloides stercoralis Infection:
Risk factors for Strongyloides stercoralis Infection:
KEY | | Strong or generally accepted link |  | | Proven definite or direct link |
GLOSSARY
Anterior
In the front or forward part of the organ or toward the head of the body.
Buccal
Pertaining to, located near, the cheek.
Chronic (Chronicity)
Usually referring to chronic illness: Illness extending over a long period of time.
Dermatitis
A general term used to refer to eruptions or rashes on the skin.
Diarrhea
Excessive discharge of contents of bowel.
Endemic
Used to refer to a disease that constantly occurs in any particular geographical region.
Esophagus (Esophageal)
Commonly called the "food pipe", it is a narrow muscular tube, about nine and a half inches long, that begins below the tongue and ends at the stomach. It consists of an outer layer of fibrous tissue, a middle layer containing smoother muscle, and an inner membrane, which contains numerous tiny glands. It has muscular sphincters at both its upper and lower ends. The upper sphincter relaxes to allow passage of swallowed food that is then propelled down the esophagus into the stomach by the wave-like peristaltic contractions of the esophageal muscles. There is no protective mucosal layer, so problems can arise when digestive acids reflux into the esophagus from the stomach.
Helminth (Helminthic, Helminths)
The general term given to an intestinal parasite.
Lumen
Space in the interior of a tubular structure.
Lung (Lungs, Pulmonary)
Organ of the body, located in the chest cavity which is designed to bring oxygen from the air into the blood stream, while also expelling carbon dioxide and other waste gases out of the body. Pulmonary: Related to the lungs.
Millimeter (Millimeters, mm)
A metric unit of length equaling one thousandth of a meter, or one tenth of a centimeter. There are 25.4 millimeters in one inch.
Mucous Membranes (Mucosa, Mucous Membrane, Mucus Membranes)
The membranes, such as the mouth, nose, anus, and vagina, that line the cavities and canals of the body which communicate with the air.
Parasite (Parasites, Parasitic, Parasitical)
An organism living in or on another organism.
Pinworms (Enterobiasis, Enterobious vermicularis, Oxyuriasis, Pinworm, Pinworm Infection, Seatworm, Threadworm)
Small intestinal parasitic worms that infect mainly children but which are also common in families with children. Pinworms are about 1/2 an inch (1.25cm) in length, white, thin and pointed at both ends. The most common symptom is anal itching, as the worms come out at night to lay their eggs.
Pneumonia (Pneumonitis)
Inflammation, usually infectious, of the lungs. Unless the result of only moderate chemical or smoke irritation, it is a potentially life-threatening condition. There are so many defenses against an infection this deep in the body that the very presence of pneumonia signals a pathogen of great virulence or impaired or exhausted immunity, or all three. Pneumonitis: Inflammation of the lungs, from whatever cause. It may be concurrent with pneumonia or pleurisy.
Small Intestine (Small Bowel)
The small intestine lies between the stomach and the large intestine. It is about 6 meters (20 feet) long and its primary function is to digest (break down) food and absorb nutrients (vitamins, minerals, proteins, carbohydrates, and fats). The small intestine makes up more than 70% of the length and 90% of the surface area of the gastrointestinal (GI) tract.
Strongyloides stercoralis (Larva Currens, Strongyloidiasis)
A roundworm that has the capacity to replicate itself in humans. These parasites are found in hot, humid areas and usually produce no symptoms although there may be mild skin and abdominal symptoms in few cases; hives (urticaria) is seen usually in the buttocks and wrists; there may me peptic ulcer-like pain aggravated by food intake.
Trachea
The cartilage tube that brings air from the larynx to the two bronchi that enter the lungs. It is lined with mucous membranes and ciliated epithelia.
Tropics (Tropical)
The region of the earth's surface lying between 23°27 North of the equator and 23°27 South of the equator.
Zinc
An essential trace mineral. The functions of zinc are enzymatic. There are over 70 metalloenzymes known to require zinc for their functions. The main biochemicals in which zinc has been found to be necessary include: enzymes and enzymatic function, protein synthesis and carbohydrate metabolism. Zinc is a constituent of insulin and male reproductive fluid. Zinc is necessary for the proper metabolism of alcohol, to get rid of the lactic acid that builds up in working muscles and to transfer it to the lungs. Zinc is involved in the health of the immune system, assists vitamin A utilization and is involved in the formation of bone and teeth.
Last updated: Sep 28, 2008
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