Tapeworms : The Cestodes

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Tapeworms : The Cestodes

 

The tapeworms belong to another branch of the platyhelminths known as

the cestodes. Like the flukes, they are flat in cross-section, they

are entirely parasitic and are hermaphroditic. However, the tapeworms

are highly specialised parasites. Many of their organ systems have

disappeared in their millions of years of parasitic existence, and

there is no free-living organism which even closely resembles them.

The common ancestor which the tapeworms had with the other

platyhelminths has been lost to time.

 

Tapeworms consist of an anchoring organ or scolex which attaches them

to the intestinal wall (adult tapeworms are invariably intestinal).

The scolex may be armed with suckers, hooks, both or neither to help

it hold fast. Growing out behind the scolex are the segments or

proglottids - repeating organs which are complete reproductive

organisms in themselves (they have both male and female sexual organs

and self fertilise). These continue to grow out from the scolex in a

chain, maturing as they go, until the last segments break off and are

passed out with the faeces.

 

Tapeworm life-cycles tend to follow a set pattern which depends on the

presence of a predator-prey relationship.

 

Tapeworm Lifecycle

 

Tapeworm Myths

 

Being the biggest and among the most well known of the parasites,

tapeworms have a certain mythology built up around them. One method

for curing tapeworms is still told to me by university students as the

way " Grandma and Grandad " use to treat worms : stick a biscuit (or a

lump of meat) up the anus each day for a week. Then on the 8th day,

stop doing it. When the tapeworm comes out looking for its biscuit,

grab it an pull it out. Naturally this is a joke - tapeworms wouldn't

recognise a biscuit and lack the gullet to digest one anyway.

 

Another legend which regularly makes the rounds is " The Hollywood

Tapeworm Diet " , whereby young starlets and supermodels are allegedly

intentionally infecting themselves with large tapeworms in an effort

to lose weight. While this comes under the category of " anything can

happen in Hollywood " , Jan Brundvand, Urban Folklorist extraordinaire

had classified it as a bona fide urban legend. Another variation

mentions the doctor who purposefully infects himself with a tapeworm

to stop himself from getting food poisoning while travelling through

India. Plenty of the locals have tapeworms and food poisoning

concurrently to make this tale reliable

 

As for the dietary benefits of tapeworms, large parasites like the

beef tapeworm and Ascaris certainly do compete for us for nutrients.

However, like any method of starvation, they tend to rob us of

micronutrients (like vitamins) before they get to the stuff we don't

really need. Signs of infection with large tapeworms (especially with

the broad fish tapeworm, Diphyllobothrium latum) include conditions

associated with vitamin B12 deficiency (eg. megaloblastic anaemia).

Just to make matters worse, one of the symptoms of have a large lump

of tapeworm protein in one's guts is to generate an immune response

and the resultant ascites - a collection of fluid in the abdomen

resulting in a pot belly - not exactly the look that the dieters are

looking for.

 

The adult tapeworms live in the predator (definitive host), while the

prey plays host to the intermediate stages, most commonly called cysts

or cysticerci (SISS-ih-SIR-KIE). Eggs laid by the adult pass out in

the faeces of the definitive host and are eaten by the intermediate

host. These develop into the intermediate stage or cyst, which

generates the adult when eaten by the definitive host. A tapeworm may

pass through several intermediate hosts before it finds the right

definitive host.

 

Adult tapeworms live reasonably at peace with their hosts. They do not

feed off us, but rather rob us of our digested food - tapeworms lack a

digestive tract and absorb nutrients directly across the skin or

cuticle. Problems arise when the tapeworm becomes too large and starts

blocking the bowel or robbing us of vital nutrients - the large

tapeworms may cause deficiencies of vitamins such as B12 if left for

too long,

 

Things may get a bit more serious if we play host to the intermediate

stage of the parasite. Tapeworm cysts may grow to a reasonable size

and, because they are found deep in the tissue, they may start taking

up valuable space. If the cyst settles in the brain, eye or liver,

normal function of these organs may be severely restricted. The host's

body may also play a role - allergic reactions to the presence of the

parasite tissues may end up exacerbating the problem, and

calcification of large cysts may lead to a worse scenario than the

live parasite.

 

 

 

 

 

 

 

 

 

 

 

Intestinal Tapeworms

 

The intestinal tapeworms are classified here as those which reach the

adult stage in the human intestine. Note that some tapeworms (eg.

Taenia solium) may use humans as both intermediate and definitive host.

 

Taenia saginata : The Beef Tapeworm

 

Taenia saginata (tay-EE-nee-ah SAJ-in-ARE-tah) is a giant among the

human parasites. A complete specimen may grow to 8m in length - nearly

the length of the digestive tract of an adult human. The worms attach

high up in the small intestine and grow downwards. The segments of

this worm may reach 1.5x1cm. With such a large body, these worms are

prime candidates for causing nutritional deficiencies in the host. As

the name suggests, the intermediate host for this worm is the cow, and

the cysticerci may be observed as 0.5cm diameter fluid filled bladders

in the muscle of these animals. Meat inspectors will look for these

cysticerci in the cattle slaughtered at abattoirs. The gravid (ie.

full of eggs) proglottids passed in the faeces are the diagnostic

stage of the worm, and these may be differentiated from those of

Taenia solium (SO-lee-um) by counting the lateral branches of the

darkly coloured uterus (T. saginata has approximately 20, T. solium

has 9-12). Taenia saginata is endemic in Australia, although modern

methods of meat processing mean that it is rarely seen now. Aside from

nutritional problems, the presence of this tapeworm generally causes

mild to moderate abdominal symptoms (nausea, pain, etc).

 

There is a probably apocryphal tale of a false pregnancy in a Brisbane

hospital caused by a beef tapeworm - the presence of the worm stopped

the patient from menstruating, caused the collection of fluid in the

abdomen (ascites) and the nausea was associated with morning sickness.

As nice as the story is, it is probably a parasitologist's version of

an urban legend (see Tapeworm Myths box).

 

Taenia solium : The Pork Tapeworm

Taenia solium (tay-EE-nee-ah SO-lee-um) is a close relative of Taenia

saginata, although the intermediate host of this parasite is the pig.

Humans are infected by eating the cysticercus in undercooked pork. T.

solium is slightly smaller than T. saginata (3-4m long), but is

considered more dangerous. Unlike T. saginata, humans are susceptible

to developing the cysticercus of T. solium if the eggs are ingested.

Therefore, if someone harbours a pork tapeworm, they pose a risk to

themselves and others around them of developing cysticercosis. These

cycticerci may lodge in the brain, eye or muscle, causing serious

problems. Furthermore, if the body kills the parasites, calcium salts

are laid down in their place, creating tiny pebbles in the soft tissue.

 

Images of Taenia saginata and Taenia solium are contained in Wormlearn

 

 

Diphyllobothrium latum - The Broad Fish Tapeworm

 

Diphyllobothrium latum (DIE-fill-oh-BOH-three-um LAR-tum) belongs to a

simple group of tapeworms known as the pseudophyllideans

(SUE-doe-fill-ID-ee-uns - see also Spargana). These tapeworms, while

still featuring the basic tapeworm body plan of holdfast and segments,

share a number of features in common with the flukes. Pseudophyllidean

eggs are similar in appearance to fluke eggs and the organism that

hatches out (the coracidium) is similar to the miracidium in flukes.

Pseudophyllidean tapeworms also need more than one intermediate host

to complete their life-cycles (normally a waterflea and some small

vertebrate). The infective stage to the definitive host is known as

the plerocercoid (PLAIR-oh-SIR-coyd) larva.

 

In Diphyllobothrium the plerocercoid is found in large marine fish. If

the flesh of these fish containing the parasite is served raw or

undercooked, the adult tapeworm may develop in humans.

Diphyllobothrium is large tapeworm and infection with this parasite is

associated with vitamin B12 deficiency.

 

Images of Diphyllobothrium latum are contained in Wormlearn.

 

 

Hymenolepis nana and Hymenolepis diminuta

 

Hymenolepis nana (HI-men-oh-LEAP-iss NAH-nah) and Hymenolepis diminuta

(DIE-min-YOU-tah) are tapeworms normally found in the mouse and the

rat, respectively. The intermediate host for these tapeworms are grain

beetles, so humans can become infected should they accidentally

swallow the beetles while eating grain products. H. nana is a minute

tapeworm, less than 0.5mm wide and less than a centimetre long. H.

diminuta is slightly larger (4-5cm in length). H. nana has the unusual

characteristic of being able to complete its life-cycle without the

aid of the intermediate host - the intermediate stage is capable of

developing within the tissues of the definitive host. This makes H.

nana the only tapeworm in which an adult will grow after the

definitive host ingests the eggs. Infection with these tapeworms

normally causes mild to moderate intestinal symptoms. H. nana is one

of the more commonly identified tapeworms in certain regions of Australia.

 

Images of Hymenolepis nana and Hymenolepis diminuta are contained in

Wormlearn.

 

 

Dipylidium caninum - The Dog Tapeworm

 

One of the more common parasites of domestic dogs is the tapeworm

Dipylidium caninum (DIE-pill-ID-ee-um kay-NINE-um). This parasite

grows to around 10-15cm, with individual segments resembling cucumber

seeds in size and shape. The intermediate host for dog tapeworms are

the dog and cat fleas Ctenocephalides canis (TEN-oh-KEFF-ah-LIE-dees

KAY-niss) and Ctenocephalides felis (FEEL-iss). The eggs passed out in

the dog's faeces are eaten by the larval stage of the flea, and the

immature tapeworm stays with the flea through its metamorphosis to the

blood-sucking adult. When the flea bites, the dog may bite at the flea

and swallow the larval tapeworm. In heavily infested dogs, their

mouths may harbour many of the parasites in the bodies of fleas

removed during grooming.

 

The segments of Dipylidium are capable of limited movement, and if

this happens in the dog's rectum or anus, it causes intense itching.

Afflicted dogs may be observed to " scoot " or drag their backsides

along the ground to relive the itching. This, of course helps to crush

the segments and release the eggs into the environment.

 

If humans swallow infected fleas, they too may become infected. This

is easier than it sounds. Allowing the dog to lick your mouth may

transfer the larval tapeworms to your mouth, while crushing the fleas

between your fingernails spreads the parasites across your hands.

Children are particularly prone to infection with Dipylidium. Like

other flatworms, this tapeworm is not affected by routine dog worming

treatments (ie. those directed against the intestinal nematodes like

roundworm, whipworm or hookworm). Instead, dogs should be routinely

wormed with a broad spectrum anthelmintic - one which has been shown

to be effective against tapeworms.

 

Images of Dipylidium caninum are contained in Wormlearn.

 

 

Tissue Tapeworms

 

The tissue tapeworms are classified here as those tapeworms which do

not reach an adult stage in the human host. Instead, they infect

humans which their intermediate or cyst stage. Such infections are

frequently more serious than those caused by the adult tapeworm, as

they involve more intimate connection with the host's tissue.

Echinococcus granulosus - The Hydatid Tapeworm

 

A dangerous tapeworm found in dogs is the hydatid tapeworm

Echinococcus granulosus (ee-KINE-oh-COCK-uss GRAN-you-LOW-suss). This

tapeworm normally cycles between carnivores such as dogs, wolves or

foxes (as definitive hosts) and herbivores such as sheep (as

intermediate hosts. The adult tapeworms are small - rarely more than

three segments long and cause minor symptoms in the dog. However, upon

eating the eggs from the dog faeces, the intermediate host develops

the hydatid cyst. The cysticerci of tapeworms such as Taenia are

rarely larger than 0.5cm in diameter, and one cycticercus upon

ingestion generates one adult tapeworm. However, in the case of

hydatid disease, the cysts may grow to the size of football and

generate many parasites. From the inside surface of the fluid filled

cyst grow hundred of tiny protoscolices (pro-toe-SCO-liss-ees) which

bud off and fall to the bottom of the cysts, forming what is known as

hydatid sand. Each one of these protoscolices is capable of generating

a new adult tapeworm upon ingestion by a dog.

 

A Sudden death due to a Hydatid Cyst

 

The Journal of Forensic Sciences (Kok, A.N. (1993) A sudden death due

to a hydatid cyst. JFSCA 38, 978-980) features a report on a young

Turkish teenager who collapsed and died suddenly after being tackled

in a football game. An autopsy revealed that he had a single

golfball-sized hydatid cyst in his liver which was ruptured during the

tackle. In response to the fluid from the cyst moving into his

tissues, his body mounted a massive allergic reaction which killed him

in minutes

 

If a human should ingest eggs from the dog's faeces, the hydatid cysts

will grow inside them. The cysts are typically found in the liver and

lungs, but may also be found in the brain. As they grow larger, they

gradually replace the tissue in which they grow. Worse still, should a

cyst rupture, the release of foreign parasite proteins into the body

can trigger a massive and sometimes fatal allergic reaction (see box).

The only means of treating hydatid cysts is through surgical removal,

and the surgeons must be very careful not to rupture the cyst during

removal, due to the risk of allergic reaction and the fact that

protoscolices may be capable of starting " daughter " cysts.

 

Naturally, it is easier to prevent hydatid infection than to cure it.

Sheep farming areas normally have intensive anti-hydatid programs,

regularly worming dogs and ensuring farm dogs are not fed uncooked

offal (the New Zealand Government went so far as to supply farmers

with guaranteed " hydatid-free " dog biscuits to feed their sheep dogs).

 

There are a number of variations on the life-cycle here in Australia,

with hydatids cycling between dingos and kangaroos and, just recently

reported, between feral pigs and pig hunting dogs.

 

Images of Echinococcus granulosus are contained in Wormlearn.

 

 

Taenia solium - The Pork Tapeworm (see reference in Intestinal Tapeworms)

 

Spargana

 

Closely related to Diphyllobothrium is Spirometra (SPY-roe-MET-rah).

The definitive hosts of this tapeworm are small carnivores such as

cats and dogs, while the intermediate hosts are waterfleas and small

cold blooded vertebrates such as frogs and snakes. Should a human

accidentally ingest the infected waterfleas, or the plerocercoid larva

in undercooked snake or frog flesh, the worm will not mature in its

new host, but remain at that stage. This is known as the sparganum

(spar-GARN-um). Spargana are long flat unsegmented white ribbons

around 5cm long that creep about through the flesh by peristaltic

movements of their bodies. They cause localised inflammation and

irritation wherever they go, and may end up in sensitive areas such as

the eye or brain. They can be best treated by surgical removal. Feral

pigs from tropical parts of Australia frequently show spargana in

their flesh.

 

Images of spargana are contained in Wormlearn.

 

 

This page Copyright 1997 Dr Peter Darben.

Last Updated 27.7.2003