Nematodes have/are

They are everywhere! About 80 000 species of Nematode have been described so far, but some authorities estimate that there may be as many as 500 000 to 100 million species in all! They live in all environments and can parasitize nearly all animals and plants. In fact there is scarcely an animal alive that does not harbour a population of parasitic nematodes at some stage in its life, and that does include you! Around 50 different species are known parasites of humans (see below for more details). Luckily most of the time they are so harmless that we don't even know they are there. Parasitic species may be up to nine metres long, but most species are less than five centimetres long. The longest known nematode is found in the placenta of sperm whales. To give you an idea of just how ubiquitous they are, you could expect to find around 10 million in one square yard of soil. One apple left on the ground to decompose had 90 000 individual nematodes living on and in it. One fig had 50 000 individuals belonging to over 8 species on and in it. In 1914 Nathan Cobb famously wrote "If all the matter in the universe except nematodes were swept away, our world would still be recognizable, its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes."

The nematode body pattern

Compared to other phyla the external structure of the Nematoda is very uniform. They all have slender, elongated bodies with tapered ends. Some have specialised mouthparts with hooks and stylets, and the only other projecting parts are concerned with reproduction. The cuticle can have as many as nine layers, with cross fibres forming a spiral network. The body is a high pressure pseudocoel within a cuticle covered body wall with only longitudinal muscles. Locomotion is basically wriggling or thrashing with the longitudinal muscles on one side contracting, while the other side expands, deforming the body into S-shaped curves. The cuticle prohibits radial expansion, so serves the purpose of circular muscles. Males are usually smaller than females and the posterior is curled into a hook. This can be easily seen in the drawing below right. This shows a male and female Ascaris sp. in the gut of a pig.

Above right is a cross-section of a female Ascaris sp., and on the right is a male and female ascaris inside a pig. The muscle arm extends to either the ventral or dorsal nerve cord. The pseudocoel functions as a hydrostatic skeleton. The cuticle is non-cellular and secreted by the epidermis, and is mainly collagen laid down in three criss-crossing layers. There are no circular muscles, and it is the body movement which moves food from the mouth to the pharynx, intestine, rectum and anus.

Common parasitic nematodes of humans

Hookworm, Ancylostoma duodenale, and Necator americanus. Juveniles in the soil can burrow into human skin. The eggs are usually eaten along with food. However, it appears that hookworm infections in humans are not always totally bad news! They have been shown to lessen the allergic effects of pollen in hay fever sufferers (The Lancet, vol 308, p686), and in Ethiopia ascaris infections reduced the severity of asthma. (The Lancet, vol 358, p 1493) . Nector americanus females are up to 11 mm long, and males 9 mm long. Heavy infections can result in anemia.

Intestinal roundworm, Ascaris lumbricoides. Eggs are eaten with contaminated food. Infection is common in some rural areas of the US with up to 60% of the population infected. Common in southern USA and other areas of the world. It is said that as much as one sixth of the world's human population is infected by Ascaris sp. alone.

Ascaris life cycle. Ascaris sp. adults can range from 2 - 30 cm long, with the male slightly smaller. The female can lay 200 000 eggs a day, and these have thick resistant shells. The eggs pass out of the host in the faeces. If the egg is swallowed by another host, the host's digestive juices dissolve the thick egg shell and liberate the young worm. The young worm then burrows through the intestinal wall into the veins, and is carried to the heart and lungs. It then breaks into the alveoli, climbs the bronchial tubes, and up into the trachea. A heavy infestation can cause pneumonia at this stage. Finally the juvenile worm is swallowed, passes down into the stomach and returns to the alimentary canal again where it matures and spends the rest of its life. From ingestion of eggs to maturity takes around two months. A small number of worms in an otherwise healthy human won't do too much harm, but they can cause an allergic reaction. Large numbers can cause blockages in the intestine, perforations, anaemia and lack of energy; severe infestations can result in mental retardation. Wandering worms can be passed with faeces, can be coughed up, and can even enter the middle ear.

Usually the first indication of infection in areas where hygiene and sanitation are good, and the infected person is healthy is finding a worm in the toilet as you are about to flush. Many people ignore this thinking the worm is an earthworm that somehow made its way up through pipes. It is too much of a shock to them to believe that it came out of their bodies. Ascaris infections are easily cleared up with modern medicine if treated promptly. The most common cause of infection in the affluent world is eating contaminated food while on exotic vacations, and eating the eggs in uncooked vegetables. In children infection commonly occurs when they put contaminated fingers or toys into their mouths.

Pinworm, Enterobius vermicularis. The female pinworm, which is up ti 12 mm long, creeps out of the human anus and lays her eggs around the margin of the anus at night. This action causes itching and scratching by the human. The eggs are caught under the fingernails, and easily spread to other humans or back to the original host. Eggs can also be inhaled with dust or licked off fingers which have touched dusty surfaces. The most common worm parasite in the modern affluent areas of the world. 30% of US children are estimated to be infected, however infection causes little harm in a healthy human. In Taiwan studies have shown that those infected with pinworm are less likely than uninfected members of the general population to have hay fever (Clinical Experimental Allergy, vol 32, p1029).

Trichina worm, Trichinella spiralis. Infection occurs when eating infected meat, and can result in the potentially lethal trichinosis. Just over 2% of the U. S. population is infected.

Whipworm, Trichuris thrichiura. Infection occurs when eating contaminated food, or through unhygienic habits. This parasite tends to be found in areas where Ascaris sp. is also common. Adults live in the intestine, but the larva burrow through the gut wall into the muscles causing cysts which can cause great pain and even death.

Filarial worms, Wucheria bancrofti and Brugia malayi etc. Infection comes from infected female mosquitoes taking a blood meal. Repeated and long exposure can lead to elephantiasis. Another filarial worm carried by the blackfly causes onchocerciasis, river blindness.

Loa-loa, the African eye worm lives in the subcutaneous tissues of man. The worm migrates and if it passes across the cornea of the eye it can be seen - hence its common name. Some believe that this is origin of lovers gaze into each other's eyes as an infected partner would not make a good mate.

Guinea worm, Dracunulus medinensis, occurs mainly in Asia and Africa. In humans infection occurs from drinking water containing Cyclops which have been infected by the worm. The worm larvae in the cyclops hatch out and penetrate the human intestine. Eggs are passed out of the human when a gravid female migrates to the skin and causes an ulcer. When this ulcer comes into contact with water the free-swimming larvae are released. The larvae swim until they either die or are eaten by a cyclops. A mature female is 1 mm in diameter but can reach 120 cm in length.

Above is Dorylamius stagnalis, a freshwater nematode. It is found in mud, and among plant roots at the bottom of ponds. Its mouth has a stylets for piercing, and it grows to 5 - 8 mm long.

Below is a piece of a false-killer whale's gut which has a heavy infestation of Bolbosoma capitatum, the thorny-headed worm. There have been cases where infestations reach up to 600 worms per square metre of gut.

One nematode, Caenorhabditis elegans, has become very well known as it was the very first multicellular organism to have its full genome sequenced. It is unusual in that it has a fixed number of cells (959) in its body.