The Flukes belong to the group of organisms known as the platyhelminths (PLAT-ee-HELL-minths) or "flatworms". They are so named because their bodies are flat in cross-section (unlike the nematodes, which are round). Most platyhelminths are hermaphroditic, meaning that they have both sexes in the one individual. Unlike other hermaphrodites like earthworms and snails, flatworms perform self-fertilisation, an advantage when you may be the only flatworm in an animal. Platyhelminths feed by absorbing nutrients across their cuticles or skins - they usually have no digestive tract to speak of. Flatworms have completely different metabolisms to nematodes, so common, over the counter worming tablets directed against nematodes like threadworm will have no effect against these worms, and treatment frequently requires prescription medication.

Fluke Life-Cycles The long and complex life-cycle of the fluke can be made easier to understand through the use of a nonsense mnemonic : Every - Egg Mirror - Miracidium (free-living in water) Spotted - Sporocyst (in snail) Red - Redia (in snail) Certainly - Cercaria (free-living in water/snail) Met - Metacercaria (in 2nd intermediate host) Approval - Adult

The flukes have flat, leaf-shaped, unsegmented bodies. They normally have a pair of suckers - an oral and a ventral one - which help the fluke hang on to the part of the body in which they live. Flukes have complex life-cycles, usually involving two or more intermediate hosts. The first intermediate host is usually a snail, which is infected by the miracidium a free-swimming stage which hatches out of the egg. The second intermediate "host" may be a range of small animals or even plants. The definitive host usually contracts the infection by ingesting the last intermediate host containing the larval fluke.

Intestinal Flukes

Tissue Flukes

The human liver plays host to two major flukes, both of which live in the bile ducts. Fasciola hepatica (FASS-ee-OH-la hep-AT-ick-ah), the large liver fluke of sheep, can find its way into our bodies through eating the metacercaria stage encysted on waterplants such as watercress, bamboo shoots or water chestnuts. The flukes grow to around 30mm x 13mm and may end up blocking the bile ducts. This can lead to jaundice and other liver complications. A similar (although, as the name suggests, larger) species, Fasciola gigantica has a similar life-cycle to Fasciola hepatica.

The Chinese Liver Fluke, Clonorchis sinensis (clon-ORK-iss sy-NEN-siss) is a smaller fluke which also lives in the bile ducts. Complications are similar to that described for Fasciola hepatica. Humans contract Clonorchis by eating the metacercaria in undercooked freshwater fish. The eggs of both Fasciola hepatica and Clonorchis sinensis are laid into the bile ducts and are eventually passed out with the faeces. The bile salts stain them a charateristic golden brown colour.

The Lung Fluke, Paragonimus westermani (PAH-rah-GON-ih-muss WEST-er-man-eye) is a fluke which settles in the lungs and pleural cavities of its hosts. It is around the size of your small fingernail (approximately 10-12mm around), and its presence causes inflammation of the respiratory passages. The eggs of Paragonimus may be found in the sputum, or they may be swallowed and passed out with the faeces. Humans contract Paragonimus by ingesting the metacercaria encysted in crustaceans - the Asian delicacy of "drunken prawn" (where the prawn is not cooked but soaked in rice wine) is an effective way of catching this parasite in endemic areas. Besides Southeast Asia, other strains of Paragonimus are found in Central and South America (eg. P. mexicana).

The schistosomes (SHISS-tow-soams) are flukes which break some cardinal fluke rules. Firstly, schistosomes are not hermaphroditic - they have two sexes, with the male being larger than the female (normally in parasites, the female is larger because eggs take up more room than sperm). At first glance, the schistosomes may appear to be nematodes - they have a long, apparently cylindrical body unlike any other fluke. However, on closer examination, it can be seen that this "round" body is in fact the flat fluke's body curled up like a tube of paper. At any rate, the prominent oral and ventral suckers show the schistosome's true heritage.

The reason for this body shape is that the schistosome must live in the blood vessels of its host - a wide flat body is not good for moving through cylindrical tubes. The groove created by the fold in the male schistosome provides a place for the female worm to sit during mating (see figure). Once mature, schistosomes stay attached and mating for life.

Another substantial deviation from the normal fluke way of doing things is that the schistosome life-cycle does not feature a metacercaria stage. Instead, the cercariae emerge from the snail in water and penetrate the skin of the definitive host, finding their way to the blood stream in the process.

Schistosomes generally live in the veins draining the abdominal organs - the mesenteric veins and the portal vein. They attach themselves to the walls of the blood vessels by means of their suckers and, after mating, lay their eggs into the blood stream. The eggs pass down the blood vessels until they become too large to proceed. Most schistosome eggs have a spine or knob which shears through the wall of the blood vessel like a can-opener (see figure). Aided by muscle movement from the host, the egg gradually works its way through the tissue until it passes out with the faeces or urine.

Bilharzia and Condoms In some parts of Egypt, infection with S. haematobium was so high that by the time people reached puberty, everyone was showing signs of infection. In fact the appearance of blood in the urine of boys was so common that it was regarded as natural - a male version of menstruation. Carvings and paintings from ancient Egypt show what appear to be male figures wearing condoms. While this could be artistic license, these figures never appear without a hieroglyph which bears a striking resemblance to a S. hamematobium egg. Schistosome eggs are among the largest of the parasitic worm eggs (up to 150µm long), but one still needs a fairly strong magnifying glass to see them in that amount of detail. Were the ancient Egyptians aware of this little parasite, and were the condoms a means of stopping blood dripping out and staining their white robes ? You be the Judge !

There are three major species of schistosome infecting humans : Schistosoma mansoni (SHISS-tow-SOAM-ah MAN-son-eye), Schistosoma haematobium (heem-ah-TOW-bee-um) and Schistosoma japonicum (jap-ON-ick-um). S. mansoni and S. haematobium are found in Africa and together cause the disease known as Bilharzia. S. mansoni lives in the veins draining the intestines, so infection with this species results in intestinal upset and produces eggs in the faeces. S. haematobium lives in the veins draining the bladder and causes cystitis and blood in the urine, with eggs passed out with the urine as well. S. japonicum is found in Asia, and lives in the veins draining the intestines, resulting in similar presentation to S. mansoni, although somewhat more acute in presentation.

Schistosomiasis is difficult to diagnose, due to the fact that the female worms produce very few eggs, and that the eggs are not concentrated by routine faecal concentration techniques. Because the worms are found deep in the tissues, drugs used to treat them must be capable of reaching them, and frequently have nasty side effects on the human host. In rare cases, the schistosome may lodge in sensitive spots - in a recent case, schistosomiasis caused paraplegia after the worms lodged in the spine.

Schistosomes and Swimmers' Itch Have you ever been swimming in freshwater and felt and incredible itching all over your skin ? If so, it's likely that you've come into contact with zoonotic schistosomes. Although Australia is free of the human schistosomes, our native fauna have their own species. In standing bodies of freshwater such as ponds or dams the waterfowl that live around the area may harbour their own species of schistosome. These worms have similar life-cycles in birds as the human species, except when the cercaria try to penetrate our skin, they stop halfway through. Our body mounts an inflammatory response and that causes the "swimmers' itch". In the ocean, bird schistosomes are believed to be the source of "sea lice", that stinging sensation you may feel washing over you as the swell passes.

Luckily, human schistosomes are not endemic to Australia (although . . .). When travelling abroad (particularly to Africa and Asia) one should heed the warnings and avoid swimming in bodies of freshwater. Every now and then a football team on a tour ignores this advice, and comes back with something to keep the local parasitologists talking.


Images of the following Schistosomes are contained in WormLearn :

Schistosoma mansoni
Schistosoma haematobium
Schistosoma japonicum

Echinostoma ilocanum is a small (6.5mm x 1-2mm), elongate fluke characterised by a row of spines around the oral sucker. Normally a parasite of dogs, little is known about the pathology of Echinostoma in humans, although, since it resides in the intestine, abdominal symptoms appear to be the most common. It is contracted by eating raw or undercooked snails containing the metacercaria stage. Echinostoma is mostly found through East Asia.

Heterophyes heterophyes is a minute (1-2mm in length) fluke which lives in the small intesting of its definitive host (normally dogs, cats and other fish eating mammals). Its eggs are similar to a host of related species, so the true epidemiology of this worm is unknown, although it is mostly recorded throughout coastal regions in Asia and Northern Africa