Hydatid disease in man is caused mainly by infection by the larval stage of the dog tapeworm Echinococcus granulosus. It follows accidental ingestion of tapeworm eggs excreted in the faeces of infected dogs. Worldwide, four species of tapeworm are clinically important to man: E. granulosus, Echinococcus multilocularis, Echinococcus vogeli and Echinococcus oligarthrus, but only E. granulosus has been found in the UK.
Hydatid disease is one of the most geographically widespread zoonoses.[1, 2, 3]The natural hosts are canine predators, particularly domestic dogs and foxes (mainly the Arctic fox and the red fox). The condition is complicated and expensive to treat, and may require extensive surgery and prolonged drug therapy. The World Health Organization (WHO) aims for an effective disease control strategy by 2018.
- Cystic echinococcosis due to E. granulosus - this is also known as hydatid disease or hydatidosis and is the most common type by far
- Alveolar echinococcosis due to E. multilocularis: this type is uncommon but the most virulent.
- Polycystic echinococcosis due to E. vogeli.
- Unicystic echinococcosis caused by infection with E. oligarthrus.
Life cycle and spread
Echinococcus spp. require two mammalian hosts for completion of their life cycle. The animal hosts are mainly canid carnivores such as dogs, wolves and foxes. The tapeworm (a small one, about 5 mm long) lives in their gut and eggs are excreted with the stool to infect intermediate hosts. These mainly include herbivores such as sheep, goats. camels, pigs and rodents. The practice of feeding the viscera of slaughtered animals to dogs in endemic countries is responsible for the high incidence and spread of infection.
The egg hatches in the intermediate host and the larva invades the intestinal wall and is carried to the liver, lungs, brain and other organs, where it forms a hydatid cyst. In the polycystic form (E. vogeli) multiple cysts are the norm.Herbivores are then eaten by canid carnivores, where new adult tapeworms develop over about six weeks, and the cycle repeats.
Humans become infected by handling infected dogs (or other carnivore hosts) - most new infections occur in children. In humans the cysts persist and grow for many years and can become very large. Infection is considered primary when spread by ingestion, and secondary when larval tissue proliferates after spread from the primary site - usually after trauma to the cyst. In primary echinococcosis, larval cysts most often develop in a single organ. Man is a dead-end host for the organism - they do not play a role in the biological cycle and cysts ingested from sheep do not develop into adult tapeworms in humans.
Cysts can contain vast numbers of infectious scolices (tiny tapeworm heads produced by asexual reproduction). They are particularly found in the liver (70% of cases) but can be found in any organ. The cysts have a wall made from both host tissue (pericyst) and larval origin (endocyst) The cysts are fluid-filled and grow very slowly (about 1 cm in diameter every year). They are space-occupying but often otherwise inert unless disrupted. They may be discovered only at autopsy despite near lifelong presence.Contact of fluid from the cyst with the host's immune system can precipitate life-threatening anaphylaxis and other immunological complications. Incubation period prior to discovery or symptom development is long and can be up to fifty years. Other possible clinical features are varied and depend on:
- The size of cysts.
- The organs involved.
- Complications caused by, for example, the effects on structures adjacent to and within the organs, rupturing of cysts, infection and immunological reactions (asthma, membranous nephropathy, anaphylactic reactions).
E. granulosus is found throughout most of the world, particularly where sheep are raised. It is endemic in Asia, north Africa and the Americas and common throughout Europe (particularly Greece and Turkey) and Australasia. In the UK there are 'hotspots' in Wales (Powys, Monmouthshire and the southern slopes of the Brecon Beacons and the Black Mountains), a pocket of South Herefordshire on the Welsh borders, and the Western Isles of Scotland. Sheep infection rates in these areas are thought to be high. However, only around 10-20 cases are reported in the UK each year, and most of these are acquired abroad.
E. multilocularis occurs in the northern hemisphere, including central Europe and the northern parts of Europe, Asia, and North America.
E. vogeli and E. oligarthrus occur in Central and South America.
There is some concern that E. multilocularis, the most virulent form and a significant threat to human health, may spread to the UK from Europe. It is possible that infected dogs and foxes may spread this species and there is concern in relation to new regulations under the pet passport scheme which no longer require a tapeworm certificate.[7, 8, 9]
- Feeding dogs with raw offal.
- Allowing dogs to roam.
- Poor hygiene (both animal and personal).
- Regular close contact with dogs.
The cysts of E. granulosus may take many years to produce clinical symptoms, and indeed may never do so. In the UK only E. granulosus occurs and this leads to cystic echinococcosis rather than the complications of alveolar echinococcosis.
In theory, echinococcosis can involve any organ. However, in practice, the liver is the most common organ affected, followed by the lungs. These account for 90% of cases.
- In one Italian study around half of all infections were asymptomatic.
- Patients of any ages may be affected, including small children.
- Many affected patients have a single cyst.
- Symptoms can be produced by mass effect or complications of loss of cyst integrity.
- Symptoms therefore depend on locality and size of cysts, speed of growth and complications of rupture.
- Pressure symptoms can take a long time to become evident, except when they involve the brain or eyes.
- Most cysts causing symptoms are larger than 5 cm in diameter. Symptoms can include vague pains, cough, low-grade pyrexia and abdominal fullness. Later, as the mass presses on surrounding organs, symptoms become more specific.
- In the abdomen, where there is less restriction on growth through pressure from other organs, cysts may grow to several litres.
- The liver is the most commonly affected organ.
- In the liver, symptoms of obstructive jaundice and abdominal pain can develop. Pressure of the cyst on the biliary tract can cause biliary colic, jaundice, and urticaria. Vomiting of hydatid membranes (hydatid emesia) and passage of membranes in the stools (hydatid enterica) occur rarely.
- Involvement of the lungs may result in chronic cough, dyspnoea, pleuritic chest pain or haemoptysis. Expectoration of cyst membranes and fluid may be observed with intrabronchial rupture.
- Secondary complications may occur as a result of infection of the cyst or leakage of the cyst.
- Minor leaks can cause increased pain, flushing and urticaria.
- Major rupture can result in severe anaphylactic reaction, which may be fatal if not treated quickly.
- Rupture into the biliary tree can lead to obstruction by daughter cysts, resulting in cholangitis.
- Rupture into bronchi can cause expectoration of cyst fluid.
- Infection of the cyst results in a range of symptoms from mild fever to full-blown sepsis.
- Partially or totally calcified cysts are not uncommon.
- Most cysts are 1 cm-15 cm in size but cysts containing up to 48 litres of fluid have been reported.
- This is due to infection by the metacestode stage of E. multilocularis.
- It is characterised by tumour-like infiltrative and destructive growth with the potential to cause serious disease with a high fatality rate amongst untreated or inadequately treated individuals.
- The liver is the primary site of infection in 99% of cases, and the symptoms may closely mimic those of cirrhosis or carcinoma.
- There is typically a 5- to 15-year asymptomatic incubation period followed by a chronic course. Peak age at diagnosis is 50-65 years.
- The clinical picture is one of progressive liver dysfunction leading to liver failure. This can occur over weeks, months or years.
- Symptoms are primarily of cholestatic jaundice (about a third) and epigastric pain (about a third), fatigue, weight loss, hepatosplenomegaly and altered LFTs.
- Distant metastases are possible, and involvement of other organs (for example, in the lung, brain, and bone) occurs in as many as 13% of the patients.
- Disease in the lung causes cough, haemoptysis, dyspnoea and pyrexia.
- Disease in the brain causes raised intracranial pressure and can cause epilepsy.
- Disease in the vertebrae can lead to compression of the spinal cord, causing paraplegia. In the long bones it may cause fractures and deformity.
- Polycystic echinococcosis is the rarest type of echinococcosis, and may be caused by E. vogeli and E. oligarthrus, which are confined to Latin America.
- E. oligarthrus is only rarely found in humans, cases having only ever been desrcibed in a few cases in the eye and viscera.[5, 10]
- Only around a hundred cases of E. vogeli were recorded prior to 2000 but this is now believed to be the 'tip of the iceberg'.
- The metacestode has a polycystic structure.
- Again it most commonly affects the liver but infection in other organs (the lung, spleen, pancreas, omentum, stomach, mesentery, diaphragm, pericardium, intercostal muscles) is more common, as are multiple sites of infection.
- Presentation and symptoms are as for cystic echinococcosis but with a greater likelihood of multiple cysts.
Diagnosis is made by a combination of clinical, imaging, serological and molecular techniques. For example:
- Ultrasound for abdominal cysts with fine-needle aspiration.
- CXR or CT scan for those in the lung.
Serological diagnosis is, unusually for parasitic infections, the basis for laboratory diagnosis. Western blot assay for antigens is highly specific. Indirect haemagglutination tests and enzyme-linked immunosorbent assays are widely used.
- Serological diagnosis can be difficult from brain and eye cyst fluid samples.
- Young children produce minimal serological reactions.
- No standard sensitive and specific test exists for cystic echinococcosis antibody detection.
In general, human disease is treated by surgical removal of the cyst with supplementary chemotherapy (mebendazole or albendazole). Surgical removal may not prevent other cysts growing and causing further problems.
- Treatment differs for cystic echinococcosis and alveolar echinococcosis.
- Polycystic echinococcosis is treated as for cystic echinococcosis.
- Mebendazole and albendazole are well tolerated and show definite efficacy against cystic echinococcosis.Concomitant use of praziquantel is sometimes advised.
- Surgery remains the mainstay of treatment. Less invasive methods and combinations with chemotherapy are being developed.
- Long-term follow-up is recommended.
- In cystic echinococcosis, risks versus benefits, indications, and contra-indications for each individual must be considered before deciding on type and timing of surgery.
- In alveolar echinococcosis, more radical surgical excision is coupled with chemotherapy (mebendazole or albendazole) in operable cases but, if the cyst can only be partially resected, or is inoperable, long-term aggressive chemotherapy is employed, and improves survival.
- In Europe, liver transplantation has been carried out in around 40 patients with inoperable alveolar echinococcosis and chronic liver failure.Studies suggest that the risk of post-transplant recurrence is relatively high.
This is indicated as an adjunct to surgery in patients with inoperable lung or liver cysts (due to the site of cysts or patients being too ill for surgery), for patients with cysts in two or more organs, and for peritoneal cysts.
Two drugs are available, both from the benzimidazole group: albendazole and mebendazole.
- Albendazole appears to have better absorption and better clinical results than mebendazole. It is taken together with a fat-containing meal twice-daily (manufacturer's recommendation, 10-15 mg/kg/day). The drug is licensed for cyclical treatment only (ie 28 days of treatment followed by 14 days of interruption), but trials using continuous long-term treatment do not show any adverse effects.
- Mebendazole is taken with a fat-containing meal three times daily (recommended daily dosage 40-50 mg/kg/day). If used alone, chemotherapy is usually required for months or years.
- Early pregnancy.
- Bone marrow suppression.
- Chronic hepatic disease.
- Large cysts with the risk of rupture.
- Inactive or calcified cysts.
- A relative contra-indication is bone cysts because of the significantly decreased response.
Most frequent adverse effects of benzimidazoles:
- Gastrointestinal disturbances.
- Reversible alopecia.
- Elevation of serum transaminases.
- Neurological symptoms.
- Carbamazepine (may decrease efficacy).
- Dexamethasone and cimetidine (may increase toxicity).
- FBC and LFTs should be performed every two weeks.
- The patient should be advised to report adverse effects.
- Drug level monitoring is ideal, but few laboratories provide this service.
- Imaging of the cyst is required to follow up morphological resolution.
Newer drugs such as praziquantel are being evaluated. Trials suggest that a combination of praziquantel with albendazole is more effective than albendazole alone.
- One study of 1,000 patients showed 30% cure rate (as measured by disappearance of cyst), 3-50% had decrease in size of cyst, and 20-40% had no change.
- The younger the patient, the better the outcome.
Puncture, Aspiration, Injection, Re-aspiration (PAIR) is an effective and safe treatment.
Indications for PAIR:
- Cysts not suitable for conventional surgery.
- Multiple cysts in segments I, II, and III of the liver (as described in the Couinaud liver classification).
- Cysts larger than 5 cm in diameter and either Gharbi type I or II, or type III if not a honeycomb (multilocular) cyst - see 'Gharbi classification', below.
- Patients refusing conventional surgery.
- Relapse after surgery or chemotherapy.
- Type I - purely cystic.
- Type II- purely cystic plus hydatid sand.
- Type III - membrane undulating in the cystic cavity.
- Type IV - peripheral or diffuse distribution of coarse echoes in a complex and heterogeneous mass.
Aspiration and re-aspiration are repeated until the aspirate is clear, and the cyst is then filled with isotonic sodium chloride.
- Cover with a benzimidazole is imperative (four days prior to, and one to three months after, the procedure).
- The technique can be performed on liver, bone, and kidney cysts but should not be performed on lung and brain cysts.
- Transhepatic puncture is recommended for superficially located cysts.
- Early pregnancy.
- Lung cysts.
- Inaccessible cysts.
- Superficially located cysts (risk of spillage).
- Type II honeycomb cysts.
- Type IV cysts, and cysts communicating with the biliary tree (risk of sclerosing cholangitis from the scolicidal agent).
- Spillage of cyst contents.
- Anaphylactic reactions.
- Sclerosing cholangitis (chemical).
- Biliary fistulas.
- One meta-analysis demonstrated greater clinical and parasitological efficacy in patients treated with PAIR plus chemotherapy than those undergoing radical surgery such as cystectomy or partial organ resection
- The PAIR group also had lower rates of morbidity, mortality and disease recurrence, and shorter hospital stays,
- Large liver cysts with numerous daughter cysts.
- Liver cysts with communication to biliary tree or pressing on surrounding structures.
- Superficially located single liver cysts.
- Likely to rupture infected cysts.
- Cysts in the lungs, brain, kidneys, eyes, and bones.
- Medically unsuitable for surgery.
- Multiple cysts in multiple organs.
- Inaccessible cysts.
- Cysts that are inactive, calcified or very small.
Choice of technique - must be individualised for each patient. Options include:
- Radical surgery (total pericystectomy or partial affected organ resection, if possible).
- Conservative surgery (open cystectomy).
- Simple tube drainage for infected and communicating cysts.
- The more radical the procedure, the lower the risk of relapses but the higher the risk of complications.
- Protection of surrounding tissue with cetrimide-soaked pads.
- Suction evacuation of the cyst.
- Sterilisation of the cyst cavity by injection of a scolicidal agent.
- Avoiding spillage which might cause seeding and secondary infestation.
- Hepatic cysts: evidence sought for bile duct communication - if present, it should be sutured.
- Concomitant treatment with benzimidazoles reduces risk of secondary echinococcosis (start four days pre-operatively, and continue for one month).
Treatment with benzimidazoles is indicated for:
- Two years peri-operatively for patients considered for radical resection, in order to combat undetected infected tissue.
- Long-term therapy (three to ten years) for patients who have partial resection.
- Inoperable conditions.
- Liver transplants.
Contra-indications: if chemotherapy is the only available option, the risks outweigh the benefits for all but early pregnancy and severe leukopenia.
Drug choices and monitoring: these are as for cystic echinococcosis, but the duration of treatment is different (see above).
Outcome: a significant increase in 10-year survival rates exists in patients receiving chemotherapy compared with patients who are not (85-90% versus 10%, respectively).
Indications: this is useful when radical resective surgery is not possible - for example:
- Vena cava thrombosis.
- Portal vein thrombosis.
- Necrotic collections.
- Bleeding oesophageal varices.
Examples of minimally invasive procedures done under ultrasound or CT guidance include:
- Sclerosis of oesophageal varices.
- Contra-indicated if post-interventional chemotherapy is not possible, as the risk of spreading the parasite is high.
- Indications: resectable liver lesion as assessed by imaging.
- Contra-indications: inoperable lesions, extensive lesions, lesions involving other organs.
- The only curative procedure is radical surgery with complete excision of the lesion.
- Total hepatectomy with transplantation is warranted in some cases if no extra hepatic disease is present.
- Re-emergence of the parasite in the transplanted liver, and distant metastasis, may occur under immunosuppression.
- Partial resections of unresectable masses decrease the parasite load and may aid chemotherapy.
There are currently no effective drugs or vaccines to protect humans against the disease.
There are no specific signs of hydatid disease in farm animals. Hydatid infection in food animals is, in nearly all cases, confined to the lungs and the liver. A summary of preventative measures is outlined in the box below.
- Dog owners should practise good hygiene when handling their animals.
- It is important to wash the hands after handling dogs.
- Avoid contact with dog faeces.
- Prevent dogs from soiling the immediate environment.
- Prevent dogs from roaming or having access to raw sheep meat or viscera.
- All sheep carcasses should be disposed of correctly and immediately.
- All dogs, especially those in rural endemic areas should be treated at six-weekly intervals with a wormer containing praziquantel.
- Vegetables, salads and fruit should be thoroughly washed before consumption.
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