Echinococcus granulosus. Hymenolepis nana

At a glance

1. Echinococcus granulosus is a tapeworm of 2 to 7 mm with three proglottids, in the small intestine of dogs. Sheep, other livestock and people swallow its eggs and grow its larva, the hydatid cyst, in the liver and lungs.
2. The hydatid cyst is walled by the host's fibrous capsule, then the parasite's laminated and germinal layers; the germinal layer buds brood capsules full of protoscolices, and daughter cysts float in the fluid.
3. Hydatid disease is silent for years, then acts like a slowly growing benign tumour. A ruptured or spilled cyst can cause anaphylaxis and new cysts. Ultrasound diagnoses it; surgery, PAIR or albendazole treat it, by the cyst's stage.
4. Hymenolepis nana, the dwarf tapeworm, 15 to 40 mm long, is the commonest tapeworm of people, mostly children. Its eggs are infective as soon as they are passed, and spread from person to person.
5. It can complete its life cycle in one person, and its eggs can hatch without leaving the gut, so worms build up. Praziquantel treats it.

01Echinococcus granulosus

Echinococcus granulosus, the dog tapeworm, is one of the smallest tapeworms and one of the most dangerous. The adult lives harmlessly in the small intestine of dogs and other canids, its definitive hosts. Its larva, the hydatid cyst, grows in the organs of its intermediate hosts, sheep most of all, but also goats, cattle, pigs, camels and horses; and of people, who are infected in the same way.

The adult

The adult is only 2 to 7 mm long:

• Scolex: armed, like Taenia solium's, with four suckers and a rostellum ringed by hooks, 25 to 50 in the genus.
• Neck: short, making the proglottids.
• Proglottids: usually just three, one immature, one mature and one gravid; the last, gravid proglottid is the largest, longer than wide, its uterus packed with eggs. It breaks off and leaves in the dog's faeces.

The egg is a taeniid egg, round, 30 to 35 µm, radially striated, with a six-hooked oncosphere inside: it cannot be told from the eggs of Taenia. It is infective as soon as it is passed.

The hydatid cyst

Swallowed by an intermediate host, the egg hatches in the small intestine, and the oncosphere crosses the gut wall and is carried in the blood, mostly to the liver and the lungs. There it grows, slowly, over years, into a fluid-filled cyst that can reach a large size: a big cyst can hold more than a litre of fluid and millions of protoscolices. Its wall, from the outside in:

Inside:

• Brood capsules, small vesicles budded from the germinal layer, each holding several protoscolices, the scolices of future adults, turned in on themselves.
• Daughter cysts, smaller cysts in the fluid, each with its own laminated and germinal layers.
• Hydatid sand, loose brood capsules, protoscolices and hooklets settled at the bottom.

Unlike the cysticercus of Taenia, with its one scolex, one hydatid cyst makes thousands of future worms: the larva multiplies by budding.

02The life cycle

1. The dog. The adult lives in the small intestine; the gravid proglottids release eggs, which leave in the faeces, infective at once.
2. The sheep, the intermediate host. A sheep grazing soiled pasture swallows the eggs; the oncospheres travel to its liver and lungs and grow into hydatid cysts.
3. Back to the dog. A dog fed the offal of slaughtered sheep, or scavenging carcasses, eats organs with cysts. The protoscolices turn out, attach to its gut wall and become adults in 32 to 80 days.

People are aberrant intermediate hosts. They swallow the eggs after handling a dog, whose fur can carry them, or in food or water soiled with its faeces, and grow the same cysts. But no dog eats them, so for the parasite a person is a dead end. Many infections are caught in childhood.

03Hydatid disease

Cystic echinococcosis, or hydatid disease, is found on every continent but Antarctica, mostly where dogs herd sheep: around the Mediterranean, in the Middle East, Central Asia and China, East Africa, and South America, where up to 5 to 10% of people are infected in parts of Argentina and Peru. More than a million people have it at any one time.

The cysts are silent for years, until they grow large enough to press on what is round them; the symptoms are those of a slowly growing benign tumour. Where they lie:

Less often they grow in the bones, the spleen or the muscles.

A cyst can rupture, into the bile ducts, the peritoneum, the pleura or a bronchus, by itself or after an injury. Its fluid is highly antigenic: rupture can cause fever, urticaria and anaphylaxis, and the protoscolices it spills can grow into new cysts elsewhere, secondary echinococcosis. A lung cyst that bursts into a bronchus may be coughed up and cure itself, or become infected into an abscess.

Diagnosis

• Ultrasound is the imaging of choice, with CT or MRI. Daughter cysts and hydatid sand inside a cyst are diagnostic; a simple cyst can be hard to tell from a benign cyst, an abscess or a tumour. The WHO staging of the ultrasound picture, from CE1 to CE5, classes a cyst as active, transitional or inactive, and guides treatment.
• An X-ray may show the cyst wall calcified, as about 40% become.
• Serology is positive in about 85%; eosinophilia is found in about 30%.
• Hydatid sand in fluid drawn from a cyst proves it.

Treatment

The choice depends on the cyst's stage, size and place, and on what the hospital can do:

• Surgery, removing the cyst whole where possible, for large or complicated cysts: ruptured, with daughter cysts, pressing on vital structures, communicating with the bile ducts, or larger than 10 cm.
• PAIR: under ultrasound, puncture, aspiration of the fluid, injection of a substance that kills the protoscolices, and re-aspiration.
• Albendazole: alone, for small cysts and cysts that cannot be operated on; and around every operation or PAIR, from a week before to at least 4 weeks after, to prevent new cysts from spilled protoscolices. Long courses need blood counts and liver tests.
• Watch and wait for inactive cysts (CE4 and CE5) that cause no symptoms: calcified or degenerated cysts can be left alone.

Echinococcus multilocularis, a tapeworm of foxes whose larvae grow in small rodents, causes alveolar echinococcosis in people: not one large cyst but a spongy mass in the liver that invades like a cancer and spreads. It is found in the northern hemisphere, in central Europe, Russia and China, and is fatal if untreated: it needs radical surgery and years of albendazole.

Prevention

• Deworm dogs with praziquantel at least 4 times a year, and keep them from eating offal: destroy infected organs at slaughter.
• Wash hands after handling dogs.
• Vaccinate lambs with the EG95 vaccine, registered in China and Argentina.

04Hymenolepis nana

Hymenolepis nana, the dwarf tapeworm, is the commonest tapeworm of people, found worldwide, and most of all in children and in crowded institutions where hygiene is poor. Rodents carry it too.

The adult and the egg

• The adult is 15 to 40 mm long and about 1 mm wide, and lives in the ileum.
• Scolex: four suckers and a rostellum that can be pulled in, armed with a single crown of 20 to 30 hooks. The neck is long and slender.
• Proglottids: many, much wider than long, each overlapping the next. A mature proglottid has just three testes, with the ovary between them, and all the genital pores open on the same side. The gravid proglottids break up in the gut and release their eggs.
• The egg is oval, 30 to 50 µm, with two membranes. From a knob at each pole of the inner one, 4 to 8 polar filaments spread into the space between them; the oncosphere inside has six hooks. The eggs are infective as soon as they are passed, but do not survive more than about 10 days outside.

The life cycle

Unlike any other tapeworm in this course, H. nana needs no intermediate host:

1. Eggs are swallowed, from soiled hands, food or water, from person to person.
2. In the small intestine the oncosphere hatches and burrows into a villus, where it grows into a cysticercoid, a small larva with no tail.
3. After 5 to 6 days the villus breaks, the cysticercoid returns to the lumen, turns out its scolex and grows into an adult in the ileum.
4. The adult lives only 4 to 6 weeks. Its eggs leave in the stool, or hatch in the gut without leaving it: internal autoinfection, which lets worms build up in great numbers and the infection last for years.

A grain beetle (Tribolium, Tenebrio) or a flea can serve as an optional intermediate host: it eats the eggs, and a tailed cysticercoid grows inside it. A person who swallows the insect by accident is infected too.

Hymenolepiasis

• Symptoms: usually none. Heavy infections cause crampy abdominal pain, diarrhoea, loss of appetite, weakness, headache and anal itching, and can be mistaken for pinworm infection.
• Diagnosis: the eggs, with their polar filaments, in the stool.
• Treatment: a single dose of praziquantel; nitazoxanide or niclosamide are alternatives. A stool sample a month later confirms the cure.
• Prevention: handwashing and hygiene, above all in families and institutions with children, and keeping food away from rodents, beetles and fleas.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1From the outside in, what are the layers of a hydatid cyst's wall?

Germinal layer, laminated layer, host's capsuleThe host's fibrous capsule, the laminated layer, the germinal layerLaminated layer, host's capsule, germinal layerTegument, muscle layer, parenchyma

2Why is a person a dead end for Echinococcus granulosus?

The eggs cannot hatch in the human gutThe cyst can only become an adult worm if a dog eats it, and dogs do not eat people's organsPeople carry the adult worm, which lays no eggsThe cyst dies within a few weeks

3Why must the contents of a hydatid cyst never be spilled during surgery or aspiration?

The fluid is acid and burns the tissueThe fluid can cause anaphylaxis, and spilled protoscolices can grow into new cystsThe protoscolices become adult worms in the abdomenThe cyst wall bleeds heavily

4What makes the life cycle of Hymenolepis nana unusual among tapeworms?

It needs two intermediate hostsIt can complete its whole cycle in one host, the larva growing in a villus of the same gutIts eggs must mature in waterIt lives only in dogs

5Adult Hymenolepis nana live only 4 to 6 weeks. How can an infection last for years?

The adults divide in twoInternal autoinfection, eggs hatching in the gut without leaving it, keeps making new wormsThe eggs survive months in the soilThe cysticercoids lie dormant in the liver