Flukes : The Trematodes

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Flukes : The Trematodes

 

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

The principal intestinal fluke infecting humans is Fasciolopsis buski

(FASS-ee-oh-LOP-siss BOOH-skee. Distributed throughout east Asia,

Fasciolopsis buski is found mainly in both humans and pigs, although

dogs are also known to be infected. It is a large fluke, growing to a

length of 75mm x 8-20mm, and up to 3mm thick. As such it is similar to

the Liver Fluke Fasciola hepatica, although it may be differentiated

from this species by the shorter and less distinct cephalic cone (or

" nose " at the anterior end of the fluke) in Fasciolopsis buski. The

life-cycle of Fasciolopsis buski closely matches that of Fasciola

hepatica, in that the infectious metacercaria stage is usually found

on aquatic plants such as water chestnuts, lotus roots and water

bamboo. The adult flukes live in the small intestine (duodenum and

jejunum) and cause localised inflammation. The outward signs of

infection are usually the abdominal symptoms (discomfort, nausea,

diarrhoea) associated with the inflammation of the gut wall caused by

the presence of the parasite. Large numbers of flukes may obstruct the

flow of materials in the intestine. Infected individuals may also

suffer an immune sensitisation caused by the foreign proteins present

in the parasite, which presents as fluid build up (oedema) in the

face, abdomen and lower limbs.

 

Images of Fasciolopsis buski are contained in Wormlearn

 

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).

Images of the following Tissue Flukes are contained in WormLearn :

 

Fasciola hepatica

Fasciola gigantica

Clonorchis sinensis

Paragonimus westermani

 

Schistosomes : The Blood Flukes

 

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

 

Zoonotic Flukes

 

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

Images of the following Zoonotic Flukes are contained in WormLearn :

 

Echinostoma sp.

Heterophyes heterophyes

 

This page Copyright 1997 Dr Peter Darben.

Last Updated 27.7.2003