PII - Cestodes 1

Page 1

The flatworms:

Cestodes: tapeworms

Trematodes: flukes

Cestode structure:

segmented body

hermaphrodite (don't have separate sexes, just need one worm inside a host for reporoduction

no alimentary canal (don't have a gut - different from other worms, absorb food from the surface)

tegument highly absorptive

Adult Cestode:

Scolex/head

Armed - suckers, rostellum (rows of hooks, hold on and don't get washed out)

Stobila or chain of segments or proglottids, neck where newest segment starts and bottom of worm is the oldest segments

- latches onto wall of intestine

a mature segment contains organs of reproduction (fertilisation within the segment)

a gravid segment contains eggs only

Taenia gravid segment:

segments continually budded from the neck region

Organs of sexual reproduction disappear as proglottid matures and moves down the chain, until uterus is full of eggs (gravid)

Taenia Egg:

radially striated eggshell

hexacanth (6-hooked) embryo or oncosphere

*very distinctive egg (can live in environment for number of years before it hatches)

Cestode life cycles:

indirect

final host (adult tapeworm) often SI

intermediate host - larval stage or metacestode (usually encysted)

*final host always contains the adult worm than undergoes asexual reproduction

Larval stages in IMH:

cyst contains the head(s) or scolex of the future tapeworm

ingested by final host, cyst digested and head latched onto wall of SI

starts to bud segments

Cyst names: describe the type of cyst

Cysticercus = single scolex

Hydatid = multiple scoleces

Taeniidae:

Final host: man/cat/dog SI

IMH: mammal - larval stages encysted

Scolex armed except from T.saginata

Adults are relatively non-pathogenic - do not have hooks

Taenia saginata:

final host - humans

5-15 metres

unarmed - no hooks, just suckers

IMH - cow (eating raw beef)

Life cycle: PPP = 2-3 months

eggs are ingested by susceptible bovine

onchopheres released in abomasum, penetrate SI wall

travels to skeletal muscle and encyst

Cysticecus bovis (12 weeks)

meat ingested by man

adults in SI in man

pass millions of eggs per day in gravid segments

- use hooks to penetrate the wall of the intestine

- once in skeletal muscle they encyst - 12 weeks for cyst to become infectious for the human (growing during this time)

* takes 2-3 months for tapeworm to mature and asexually reproduce

Eggs are long lived = live for a couple of years

T.saginata control:

meat inspection

> 25 cysts condemn

< 25 cysts chill infected carcasses (-10 for 10 days) - but if these cysts are distributed over the body = condemn

cooking meat (57 degrees)

restrict use of human sludge to cultivated fields, or no cattle grazing for at least 2 years

Taenia solium:

Final host: man (adult worm)

IMH: pig or man

life cycle similar to T.saginata = sister species, much more dangerous

*single cyst gives rise to a single tape worm

Man can also act as an IMH - accidental ingestion of eggs, auto-infection via reverse peristalsis

Pathology:

adult tapeworm in human - no direct pathology associated with adult in SI

in pig, no obvious pathology

cystic stage in humans always important

*cysts in CNS or eye - very common case of late onset epilepsy due to cysts in brain (very well studied in Mexico)

Diagnosis of T.solium: humans

proglottids (segments of tapeworm) not always apparent in faeces

poor sensitivity of egg detection

parasite Ag/DNA in stools detected by ELISA or PCR

Cysts = imaging (CAT, MRI) + serology (take blood) Western blot to detect Ab to cyst

*diagnosis of cystic stage relatively easy

Spread: close association between man and pig - poor meat inspection (people don't want to take their pigs to abattoir as if found to be infected it will be taken out of the food chain (home slaughter)

Control:

mass treatment to kill adult worms in humans (albendazole etc.) - expensive but don't have diagnostics to find who are infected or not

health education

mass treatment of pigs (oxfendazole) - kill cystic form in pig

meat inspection

pig coralling - control area they are travelling and what they are eating

vaccination of pigs

Problems:

cultural considerations

cost

long term sustainability of control

efficacy of vaccines (use Ag derived from egg (oncosphere)

UK Taenia species:

all transmitted between dogs (final host) and sheep (IMH) - common in sheep farming areas

*huge economic loss (sheep farming industry)