Soil transmitted helminths, Biology tutorial


Soil-transmitted helminth (simply, STH) infection is highly endemic in the tropical and subtropical regions of sub-Saharan Africa, Asia and Latin America, where up to 2 billion people encompass active infections. STH infection has remained basically neglected by the global health community because the people most influenced are among the most impoverished and due to the infection causes chronic ill health by insidious clinical presentations, instead of severe acute illness or high mortality.

Ascaris lumbricoides (large roundworm of man):

Infection having this roundworm is very common, with estimates of the annual incidence of infection being more than 1500 million cases, or around one quarter of the world's population. In addition to the species in man, Ascaris lumbricoides, a morphologically indistinguishable species Ascaris suum is found in the pig. The other related genera comprise Parascaris in equines and Toxascaris in a diversity of domesticated animals.

Life cycle:

Ascaris eggs are unsegmented when passed: beneath favorable conditions they need a period of around 2 or 3 weeks outside the host to build up to the infective phase. Excessive heat and dryness soon kill them; however they remain liable in moist soil for long periods. When completely embryonated eggs are wallowed, they hatch in the duodenum and then experience an extraordinary migration via the body before returning to settle down in the intestine and grow to the adulthood.


Female worms range from 20 to 35 cm in length, whereas males are seldom more than 30 cm long. The female worms might be as thick as a lead pencil; the males are definitely slimmer and might be differentiated through an incurved tail. Both sexes are creamy white, at times having a pinkish cast and the cuticle has fine circular striations.

Pathology and clinical symptoms:

The ingestion of small numbers of infective eggs at any one time perhaps gives mount to no recognizable symptoms, however larger numbers might provoke pneumonitis dining larval migration via the lung. This might take place from 4 days to 2 weeks after infection. Throughout this period sensitive persons might develop asthma attacks that can continue till the ultimate removal of the adult worms. A few adult worms in the bowel are doubtful to cause symptoms unless they migrate through the ampulla of Vater into the pancreas, bile ducts, gallbladder and liver or up the esophagus. A heavy infection is probable to cause bowel obstruction, particularly in children. In one series, three fourths of children with bowel obstruction presented having fever and generalized malaise.


Detection of eggs. A diagnosis might be made by means of finding larvae in the sputum or, more readily, in the gastric washing.


Compositing and a variety of schedules of storage of human excreta might give efficient techniques of ascariasis control by demolishing the parasite eggs before the night soil is employed.

The human hookworms:

The hookworms fit in to the Order Strongylida, a much large order and of great interest as it contain many significant pathogens of man and domesticated animals. This order is further categorized into three Super families, the Strongyloidea (that is, the hookworms in man) and two associated groups, the Super family 'Trichostrongyloidea', intestinal nematodes that are of veterinary significance in most of the domesticated animals (example: Haemonchus contortus in cattle and Nippostrongylus brasiliensis in rodents) and members of the Super family 'Metastrongyloidea' (that is, the lungworms, in domesticated animals).

In man there are two species able of causing the intestinal infections, Ancylostoma duodenale native to portions of Southern Europe, North Africa and Northern Asia portions of Western South America, and Necator americanus in Central and Southern Africa, Southern Asia, Australia and the Pacific Islands. These are much significant human pathogens. It has been anticipated that there are 1200 million cases of hookworm infection in man yearly, of which around 100 million of which are symptomatic infections by accompanying anaemia.


The adult parasites are small cylindrical worms, around 0.5-1.5mm long (Ancylostoma duodenale being slightly bigger than Necator americanus). The posterior end of the male worm is equipped by a characteristic copulatory bursa, employed to hold the female nematode in place throughout mating. The females themselves encompass a vulva positioned near the center of the body, slightly anterior in Necator and slightly posterior in the Ancylostoma. The anterior end of the parasites is made into a buccal capsule, absent in members of the other Strongylida super families, through which the different genera and species in the group might be differentiated. For illustration members of the genus Necator encompass capsules equipped with cutting plates on the ventral margins and in the capsule itself small dorsal teeth. In contrary members of the genus Ancylostoma encompass pairs of teeth on the ventral margin of the capsule. The number of teeth will differ between various species of Ancylostoma, however is generally between one and four pairs.

The eggs are bluntly rounded, thin shelled and are nearly indistinguishable among the different species, measuring around 60 by 40 µm and the eggs of Ancylostoma being slightly bigger than those of Necator. 

Life cycle:

Humans are infected by hookworm when third-phase filariform larvae in soil. They penetrate via the skin, specifically into areas like unprotected feet. Once infected, the filariform larvae migrate to the blood circulation. They break out of the pulmonary blood vessels into alveoli and then crawl up the trachea and is swallowed by saliva to re- enter the intestinal tract. They join themselves to the mucous membrane of the small intestine to mature into adults.

Pathology and clinical symptoms:

At times some itchy papules at the site of larval penetration, ground itch take place. Patients having chronic infections might experience vague mild GI symptom s, slight anemia due to the loss of blood and weight loss or weakness. In heavy infections, microcytic hypochromic anemia is generated.


Recovery of the eggs in the samples of stool.


Appropriate sanitation practices, correct fecal disposal and personal protection by covering the exposed feet.

Trichuris trichiura (human whipworm):

The very first written record of Trichuris trichiura was prepared by Morgani, an Italian scientist, who recognized the presence of the parasite in a case of worms residing in the colon in the year 1740. Precise Morphological explanation and figures were first recorded in the year 1761 by Roedere, a German physicist. Soon after morphology and visual representation of the worms, Trichuris trichiura was specified taxonomy (throughout the 18th century).

This is the third most general round worm of humans. It is distributed globally, with infections more frequent in the regions having tropical weather and poor hygiene practices and among children. 


Adult worms are generally 3 to 5 cm long, with females being bigger than males as is typical of nematodes. The thin, clear majority of the body (that is, the anterior, whip-like end) is the oesophagus and it is the end that the worm threads to the mucosa of the colon. The widened, pinkish gray area of the body is the posterior and it is the end which includes the parasite's intestines and reproductive organs. Trichuris trichiura comprises of feature football-shaped eggs, which are around 50-54µm long and have polar plugs (as well termed as refractile prominences) at each end.

Life cycle and transmission:

Humans can become infected by the parasite due to ingestion of infective eggs through mouth contact with hands or food contaminated with egg-carrying soil. Though, there have as well been rare reported cases of transmission of Trichuris trichiura through sexual contact. Some main outbreaks have been traced to the contaminated vegetables (that is, due to the presumed soil contamination).

Unembryonated eggs (that is, unsegmented) are passed in the faeces of a prior host to the soil. In the soil, these eggs build up into a 2-cell phase (that is, segmented egg) and then into an advanced cleavage phase. Once at this phase, the eggs embryonated and then become infective, a process which takes place in around 15 to 30 days). Subsequently the infective eggs are ingested by way of soil-contaminated hands or food and hatch within the small intestine, releasing larvae to the gastrointestinal tract. These larvae burrow into a villus and build up into adults (over 2 to 3 days). They then migrate to the cecum and ascending colon where they thread their anterior part (whip-like end) into the tissue mucosa and reside permanently for their year-long life span. Around 60 to 70 days after infection, female adults start to discharge Unembryonated eggs (that is, oviposit) into the cecum at a rate of 3,000 to 20,000 eggs per day, linking the life-cycle to the beginning.

Signs and symptoms:

Light infestations are mostly asymptomatic (have no symptoms). Heavier infestations, particularly in small children, can present gastrointestinal problems comprising abdominal pain and distention, bloody or mucous-filled diarrhoea and tenesmus.

Growth retardation, weight loss, nutritional deficiencies and anaemia (due to the long-standing blood loss) are as well feature of infection and these symptoms are more common and severe in the children.

Control and Prevention:

Enhanced facilities for faeces disposal have reduced the incidence of whipworm. Hand-washing prior to food handling and avoiding ingestion of soil by thorough washing of food which might have been contaminated by egg-containing soil are other preventive measures. Mass Drug Administration (preventative chemotherapy) has had a positive result on the disease burden of trichuriasis in East and West Africa, mainly among children, who are at highest risk for infection. Enhancement of Sewage and Sanitation systems and also enhanced facilities for faeces disposal has helped to limit defecation to the soil and contain potentially infectious faeces from bodily contact.

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