Cestodes (Tapeworms)

Cestodes are tapeworms. There is a large variety but only those that are pathogenic to humans will be discussed here.

These cause "taeniasis".

Epidemiology

• Present worldwide
• Incidence higher in developing countries - 10% of population can be affected
• Pork tapeworm has a higher incidence

Morphology

T. saginata:

• Usually less than 5 m long but can grow up to 25 m; 12 mm broad
• Head, called scolex is pear shaped
• No hooks and no neck
• Four suckers in head
• Long flat body with several hundred segments called proglottids - hermaphroditic, egg producing sections
• Each proglottid 18x6 mm with branched uterus
• Eggs are round and yellow-brown in colour

• Variable size; can be up to 7m long - has a neck and long flat body
• Scolex is globular shape
• 4 suckers and hooks
• Proglottids are 5x10 mm and also have branched uteri

Life cycle

Poorly cooked meat is ingested by humans who are the only definitive hosts. The poorly cooked meat includes tapeworm larval cyst (cysticercus) which then release larvae. These attach to the small intestine by the scolex suckers. The worm then matures over 3-4 months during which the proglottids develop. The worm can survive for up to 25 years in humans during which time the gravid proglottids are released into the faeces.

The excreted eggs which are excreted in the faeces can survive on vegetation where they are then consumed by cattle or pigs. Once in these animals the eggs hatch and cysticerci are released. These pass into the animal circulation from the small intestine and reside in the muscle. Humans are then infected by eating raw meat containing the cysticerci.

Presentation

• Taeniasis:
  • This results from either T. saginata or T. solium and relates to the adult worm in the gut.
  • This depends on the load of the infectious agents.
  • Light infection may be asymptomatic and more heavier infection leads to epigastric pain, diarrhoea and vomiting.
  • These abdominal symptoms result from the tapeworm.
• Cysticercosis:
  T. solium can also lead to Cysticercosis where by larval cysts infiltrate the lung, liver, eye or brain. This results in inflammation leading to clinical features such as, blindness, neurology. In some countries e.g. Peru, Mexico neurocysticercosis accounts for 30% of seizures¹ making it an important cause of morbidity and mortality worldwide.

Investigations

• Recover eggs or proglottids in stool or perianal area
• Cysticercosis is confirmed by the presence of antibodies and imaging e.g. chest x-ray, CT brain.

Management

• Niclosamide or praziquantel (single dose) can be used (available on a named-patient basis) - satisfactory treatment requires expulsion of the scolex.
• Niclosamide is an alternative.
• There has been suggestion to use a purgative before and after to improve expulsion of the tapeworm.²

Prevention

• Inspect meat thoroughly.
• Adequate handling of food e.g. freezing or cooking.
• Cysticerci do not survive temperatures <10°C and > 50°C.
• WHO advocate the periodic treatment of tapeworms such as T. solium in endemic areas with albendazole. This can prevent neurocysticercosis at a later stage.¹

Results from eating raw or improperly cooked fresh water fish.

Epidemiology

Present worldwide especially in subarctic and temperate regions.

Morphology

• Longest tapeworm in humans - 3-10 m in length
• >3000 proglottids which are more broad than long
• Scolex is shaped as two almond leaves
• Eggs are 35-55 x 55-75 micrometers

Life cycle

Man and some animals are infected. The plerocercoid larvae result in infection in humans. The cycle begins by the ingestion of uncooked fish containing plerocercoid larvae which attach to the small intestine. In 3-5 weeks the worm matures to adult size. The adult worm releases eggs that are passed into the faeces. These eggs hatch in fresh water releasing ciliated coracidium. These are subsequently ingested by the water flea (cyclops) and release procercoid larva. The cyclops are then ingested by fresh water fish forming plerocercoid larva which when ingested leads to infection.

Presentation

Depends on the number of worms. Mild infection leads to:

• Abdominal discomfort
• Loss of appetite
• Loss of weight
• Malnutrition
• B12 deficiency may occur with heavier infections and may lead to anaemia and even subacute combined degeneration of the spinal cord.

Investigations

Recover typical eggs or proglottids in stools.

Management

Praziquantel first line .

Prevention

• Freeze fish for 24 hours
• Thoroughly cook fish
• Pickling fish
• Preventing sewage contamination of fish reservoirs

This is a relatively small tapeworm (15-40 mm) and tends to infect children. The reservoir is rodents and transmission is oro-faecal. Thus cross infection and autoinfection is common in children.

Life cycle

The eggs are ingested and invade the small intestine where they mature into adult worms. These adults reside for several weeks.

Presentation

Light infection is associated with vague abdominal pain but enteritis can occur with heavier infections.

Investigations

Presence of eggs in faeces.

Management

Niclosamide or single dose of praziquantel are the drugs of choice.

Prevention

Good hygiene can effectively prevent spread.

Hymenolepis diminuta is the rat tapeworm. It is much longer than H. nana and primarily affects rats but very rarely it can be accidentally ingested by humans e.g. ingestion of insects that carry the parasite. Most interest in it relates to research. The presentation and management are similar to those of H. nana.

Echinococcus granulosus and E. multilocularis cause hydatid cysts. Dogs and other canids are the definitive hosts.

E. granulosus

Epidemiology

E. granulosus is common in Asia, Australia, East Africa, southern regions of Spain, South America and North America. In these areas 1-2 per 1000 population are affected. Incidence rates are higher in some rural areas.

Morphology

E. granulosus is the smallest of the tapeworms (3 to 9 mm long) and has only three proglottids.

Life cycle

The adult worms live in both domestic and wild carnivorous animals. Infected animals pass eggs in their faeces which are then ingested by grazing farm animals and humans. The eggs then localise in various organs resulting in a hydatid cyst which contains many larvae (called "hydatid sand"). Other animals may then consume the infected organs and the cysts then release proto-scolices. These pass into the small intestine leading to adult worms.

In humans the echinococcus eggs invade the small intestine and then enter the circulation. The cysts then locate and reside in organs including the liver, bone, lung and brain. Cysts are usually 1-7 cm but can be as big as 30 cm.

Presentation

Symptoms depend on the site where the cysts have located and are similar to a growing tumour. Examples include

• Large abdominal cysts lead to discomfort
• Liver cysts result in jaundice
• Lung cysts can lead to abscess formation
• Brain cysts can cause focal seizures and raised intracranial pressure
• Cyst content can lead to anaphylaxis

Investigations

• Eosinophilia
• Abnormal liver function tests
• Antibodies against hydatid fluid
• Imaging e.g. chest x ray, CT scan liver or abdomen, brain CT or MRI

Management

• Surgical removal of cyst or inactivation of cyst by injection of 10% formalin followed by resection
• Often complete resection of the cyst is impossible due to close proximity to major vessels⁴
• High doses of mebendazole maybe effective in some

Prevention

• Avoid contact with infected animals
• Eliminate the infection in domestic animals

E. multilocularis

This is similar to E. granulosus with similar morphology and life cycle. It tends to occur in parts of Asia, North America and Europe too. The intermediate host is rodents. The presentation is similar as E. granulosus but the cysts are multilocular. Again therapy involves surgery. E. multilocularis is resistant to praziquantel although high doses of albendazole may be effective. Prevention involves rodent control measures.

Sparganosis is rare and results from plerocercoid tapeworm larva of Spirometra spp. It has a tendency to affect the following:⁵

• Subcutaneous tissue
• Skeletal muscle
• Visceral organs
• Central nervous system
• Spinal cord

Epidemiology

Found worldwide - most common in East Asia
Infection results from:⁵

• Ingesting contaminated water or raw or inadequately cooked flesh of snakes or frogs
• Applying skin of infected animal to skin as a poultice

Presentation

Depends on which area of the body affected e.g. spinal involvement presents with weakness and paraesthesia

Investigations

• Eosinophilia may not be present if worm localised to an organ
• ELISA tests of serum or cerebrospinal fluids to detect antibodies to sparganosis

Management

• Treatment involves removal of the worm e.g. surgery
• Any surrounding inflammation may require corticosteroids