Intestinal Nematodes

There are five major species of intestinal nematodes found in humans. Because these worms spend a certain amount of time in the soil, they are sometimes known as geohelminths.

Ascaris lumbricoides (ASS-car-iss or ass-CAR-iss lum-brick-OI-dees) is the largest nematode by mass which infects humans. Females of this species can grow to 40cm long and be as thick as a pencil. When alive, the worms are a pale pink colour with a bright red "speed stripe" down each side of their bodies.

Ascaris Facts : Female Ascaris may lay up to 300,000 eggs per day, and up to 8,000,000 eggs in her lifespan WHO estimates that roughly one quarter of the world's population is infected by Ascaris The eggs of Ascaris are so chemically resistant, that the embryo's inside can stand immersion in concentrated bleach or formalin. About the only thing that can kill Ascaris eggs is heat - 82°C for 5 seconds should do the trick The eggs of Ascaris are coated with a jelly-like substance raised up into lumps and bumps. These acts as little suction cups, making the eggs stick to just about everything.

Ascaris live in the small intestine of the host, feeding on intestinal contents. After mating, the female can lay a phenomenal number of eggs (see side bar), which are passed out with the faeces. These eggs are incredibly thick shelled and chemically and physically resistant and must lie in the soil at around 28°C for 20 days until they become infective. We become infected by accidentally eating the infective eggs.

Unlike hookworm, Ascaris do not actually feed on us. However, their size can cause problems in itself. If the worm burden is too high, the worms may block the intestine entirely. Worse still, Ascaris has a tendency to wander places it shouldn't. If the conditions start to become unfavourable (eg. the host gets a fever, undergoes anaesthetic or takes worming tablets, the worms will try to get away. If they wander down the intestine, they may be passed with the stool (giving one and awful fright the next time one goes to the toilet), they may wander back up through the stomach and be vomited up (there have been cases of the worms suffocating infants by climbing up behind the nasal passages), or they may move sideways, crawling up and blocking the bile ducts or even perforating the bowel. This is why treatment for this worm must be undertaken with medical supervision.

Ascaris is found all around the world, although it is more common in the tropics and in places with poor hygiene. Pigs have an almost identical species called Ascaris suum, although it is still not known whether the human species can infect pigs and vice versa. Ascaris may be cleared up using common worming treatments, although treatment should be monitored by a doctor.

Enterobius vermicularis (ent-er-ROE-bee-uss ver-MICK-you-LAR-iss) is though to be the most common intestinal worm in the world. It is said that everyone in the world has either had threadworm infection, has it now, or will have it in the future. The range of Enterobius stretches from the tropics to the Arctic Circle and across all socio-economic groups. Threadworm is more common in children.

Threadworms are small white worms, just under 1cm long - as the name suggests, they resemble small pieces of white cotton thread. The worms live in the upper parts of the large intestine and in the appendix, feeding on intestinal contents. When the females have mated, instead of laying their eggs into the faeces and abandoning them to the whims of fate, they migrate down the colon, out of the rectum and lay their eggs on the perianal skin. This causes the most common symptom associated with threadworm infection (itchy bum). Naturally, if one scratches the area, the eggs are transferred to the hands and then swallowed the next time the fingers are placed in the mouth, thus starting a new infection.

Enterobius Facts : Enterobius eggs are triangular in cross-section and flat along one side, making them look like tiny taco shells Enterobius is more common in temperate climates than in tropical climates. One of the reasons put forward for this is that folks in cooler climates wear tighter underwear. Enterobius is more common in Caucasians than African Americans, although since it doesn't produce an immune response (which might explain a genetic difference), no-one knows why

The symptoms of threadworm infection are usually mild. Only 25% of people show any sign of the worms at all. As well as the itchiness, afflicted people may experience nausea, intestinal upset and disturbed sleep patterns. Rarely, the female worms get lost while laying their eggs and they may wander places they shouldn't. If a worm migrates up the female genital tract, complications may arise as the body launches an attack against it. Threadworms have also been found in the liver, lungs and ears, although it's not at all clear how they got there. Threadworms have only killed one person on record, a man in 1947 who was immunocompromised. There has been some speculation as to whether threadworms are a cause of appendicitis, although it is difficult to be sure, as the worms may also be found in healthy appendices.

Threadworms may be easily treated using common worming treatments, although the eggs may survive in the environment for a while, posing a risk for reinfection. Because threadworm eggs are not laid into the intestine, normal faecal screening will not diagnose an infection. Therefore a specialised test must be performed to confirm diagnosis : a piece of clear sticky tape is pressed against the perianal skin and then placed on a microscope slide for examination. If eggs were present on the perianal skin, they should be visible on the sticky tape. This is not a commonly performed procedure - most doctors diagnose the condition on symptoms alone.

Trichuris trichiura (try-CUE-riss try-kee-YOU-rah) is a medium sized worm (up to 3-4cm in length) which lives in the colon. The name whipworm is well-deserved - the first two-thirds of the worm is thin and hair-like, which the last one third is fat (like the handle of a whip). This shape suits the worm well, as its head end needs to be narrow so it can easily burrow through the intestinal tissue. The fat tail end (which hangs out into the intestine) ensures that the worm can still produce many eggs.

After mating, female Trichuris lay distinctive barrel-shaped eggs which are passed out with the faeces. The eggs must lie for 20 days at 28-30°C in the soil in order to become infective. Humans contract the infection by accidentally eating the eggs.

Trichuris Facts : Trichuris eggs have been found in the fossilised faeces (coprolites) of stone age humans from 10,000 years ago. The eggs were not infective, but they were still recognisable.

Because Trichuris actually feeds upon our tissues, the symptoms that these worms cause are usually more severe. The burrowing of the worms causes a severe irritation which, in severe cases, can lead to rectal prolapse (protrusion of the rectum out through the anus). Normally, symptoms include diarrhoea, abdominal pain and nausea, and in severe cases, normal functioning of the intestine may be interfered with, leading to vitamin deficiencies and malnutrition.

Trichuris is reasonably resistant to the commonly available drugs, so to treatment whipworm infection, multiple dosages are often required. Whipworm is found throughout the world, mostly confined to the tropical regions.

Human Hookworm

Humans are infected by two species of hookworm : Ancylostoma duodenale (AN-sill-OSS-toe-mah dew-oh-den-AH-lee) and Necator americanus (neck-AY-tor am-er-ick-AH-nuss). Both have similar habits and life-cycles, although there is some difference in their geographic range and susceptibility to certain worming treatments.

The hookworms are so called because their heads are bent towards their undersides, forming a slight hook. This allows the worm to lie flat along the intestine and stops it from being knocked off by passing faecal material. Adult hookworms live in the small intestine, grabbing a hunk of the intestinal mucosa in their large mouths and pumping blood and tissue through their bodies. It has been shown that one adult hookworm can drain 0.1mL of blood from its host per day. In a relatively mild infection load of 1000 worms, this can mean the loss of 100mL of blood each day. Aside from the normal nausea, diarrhoea and abdominal discomfort caused by the irritation the worms cause, people with chronic infections can also suffer from iron deficiency anaemia due to the loss of blood.

Hookworm Facts : The so-called "poor white trash" of the Southern United States were much maligned because they were said to be lazy and wouldn't work. The reasons why they seemed lazy was because they were found to be suffering from chronic hookworm infection which made them anaemic. Studies showed that miners in certain parts of India were the only people in their area who were free of hookworm. It turned out that the reason for this was that cockroaches living in the mines were eating their faeces and kill the hookworm eggs before they could hatch and infect the miners.

After mating, female hookworms lay their eggs into the intestine to be passed out with the faeces. The eggs are thin shelled and, under the right conditions, hatch within 48 hours. The larval hookworms live for a time in the soil until they are infective. Then they climb to the top of a blade of grass and wait for a host to walk by. If the larval worms contact the skin of the host, they immediately burrow in, entering the bloodstream. They travel with the blood to the lungs where they mature for a little longer, and then erupt out of the lungs and climb up the trachea. Here, they are swallowed down into the intestine to mature. This sort of life-cycle is called tracheal migration.

Hookworms are a little fussy as to the type of soil they need to mature in. If it is too dry or too moist the larval hookworms will not survive. Therefore hookworms are limited in geographic range to the tropics and sub-tropics. They can be treated with normal worming treatments, although Necator may require multiple doses.

Strongyloides stercoralis

Strongyloides stercoralis (STRON-jih-LOY-dees STIR-cor-AH-liss) is also known as threadworm in some American texts, although we'll name it Strongyloides to save confusion. Very little was known about this worm until recently. It was first described in French soldiers returning from Indo-China late last century, but the incidence was so rare as to not warrant any further attention. However, with the increasing awareness of immunosuppression, Strongyloides is becoming more important.

Strongyloides is an opportunistic pathogen, which does not normally maintain an infection for long in humans. In fact, this worm can live quite happily in the soil without ever coming into contact with a human host. Adult females (the males do not infect humans) live in the small intestine, feeding on cells and intestinal contents. They produce eggs by parthenogenesis which hatch soon after laying. The larvae are passed out with the faeces (therefore the larva, not the egg, is the diagnostic stage) and go into the soil. From here they can either form infective larvae, which infect humans by the route as hookworms, or they can mature in the soil and undergo a number of generations as free-living nematodes, before forming the infective larvae. The males are only found in the soil.Strongyloides does not normally cause problems in immunocompetent hosts. However, if the host's immune system is depressed for any reason, the nematodes made produce what is known as a hyperinfection syndrome, where they virtually infect every part of the body. This is a particular danger for patients in the last stages of HIV infection.