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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. However, the increase in pet travel and the spread of E. multilocularis in Europe may increase the risk of this parasite infecting humans in the UK.
Hydatid disease is one of the most geographically widespread zoonoses[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 2020.
- 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. Currently a dog coming into the UK must be given treatment for tapeworm and this must be recorded in a pet passport or third-country veterinary certificate. This may change if the UK leaves the EU[10, 11].
- 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.
- Infections are usually asymptomatic unless complications occur.
- 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 50-70 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 descibed in a few cases in the eye and viscera.
- 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 principally by ultrasound, supplemented by CT and/or MRI scans. Serological diagnosis is, unusually for parasitic infections, the basis for laboratory diagnosis. Specific antibody techniques are available. Cysts may be detected radiologically and differentiated from tumours and abscesses by use of biopsies and ultrasound-guided puncture.
In general, human disease is treated by surgical removal of the cyst with supplementary chemotherapy. Albendazole is the treatment of choice, with mebendazole second-line for those unable to tolerate it. Both drugs are unlicensed for this use in the UK. Further prescribing details may be obtained from the British National Formulary.
- Surgical removal may not prevent other cysts growing and causing further problems.
- 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. There are four options:
- The PAIR technique (percutaneous puncture of the cyst, aspiration, injection of chemicals, re-puncture).
- Anti-infective drug treatment.
- Watch and wait.
- In alveolar echinococcosis, more radical surgical excision is coupled with chemotherapy in operable cases but, if the cyst can only be partially resected, or is inoperable, long-term aggressive chemotherapy is employed, and improves survival.
- Liver transplantation has been performed in alveolar disease since the 1980s and the outcome has been good with five-year actuarial survival close to 70% and recurrence-free survival of 58%.
There are currently no effective drugs or vaccines to protect humans against the disease. However, cystic echinococcosis can be prevented if sufficient measures are taken to control the spread in domestic animals. Periodic de-worming of dogs, improved hygiene in the slaughtering of livestock, and public education campaigns have been found to lower and, in high-income countries, prevent transmission. The challenge will be to transfer these measures to resource-poor countries, and the World Health Organisation (WHO) is setting up pilot projects with a view to having a global strategy by 2020.
Vaccination of sheep with an E. granulosus recombinant antigen (EG95) offers encouraging prospects for prevention and control. Small-scale EG95 vaccine trials in sheep indicate high efficacy and safety with vaccinated lambs not becoming infected with E. granulosus.
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 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.
Further reading and references
; Incidental finding of cardiac hydatid cysts, report of two cases. BMC Med Imaging. 2018 Aug 1318(1):22. doi: 10.1186/s12880-018-0268-2.
; Medical treatment of cystic echinococcosis: systematic review and meta-analysis. BMC Infect Dis. 2018 Jul 518(1):306. doi: 10.1186/s12879-018-3201-y.
; Ultrasound and Cystic Echinococcosis. Ultrasound Int Open. 2018 Sep4(3):E70-E78. doi: 10.1055/a-0650-3807. Epub 2018 Oct 23.
; Echinococcus Multilocularis - A Disease Profile, Veterinary Practice, 2016
; World Health Organization, February 2018
; Worldwide epidemiology of liver hydatidosis including the Mediterranean area. World J Gastroenterol. 2012 Apr 718(13):1425-37. doi: 10.3748/wjg.v18.i13.1425.
; DPDx - Centers for Disease Control and Prevention
; Cystic Echinococcosis. J Clin Microbiol. 2016 Mar54(3):518-23. doi: 10.1128/JCM.02420-15. Epub 2015 Dec 16.
; New aspects of neotropical polycystic (Echinococcus vogeli) and unicystic (Echinococcus oligarthrus) echinococcosis. Clin Microbiol Rev. 2008 Apr21(2):380-401, table of contents. doi: 10.1128/CMR.00050-07.
; Hydatid disease - including symptoms, treatment and prevention, 2019
; Analysis of Lethality in Echinococcal Disease. Korean J Parasitol. 2017 Oct55(5):549-553. doi: 10.3347/kjp.2017.55.5.549. Epub 2017 Oct 31.
; Lesson of the month 2: Cough and right hypochondrial discomfort. Clin Med (Lond). 2015 Apr15(2):208-9. doi: 10.7861/clinmedicine.15-2-208.
; GOV.UK, 2019
; UK Government, 2017
; Multimodality imaging in diagnosis and management of alveolar echinococcosis: an update. Diagn Interv Radiol. 2016 May-Jun22(3):247-56. doi: 10.5152/dir.2015.15456.
; Orphanet, 2014
; Echinococcosis in Colombia - A Neglected Zoonosis?, 2015
; NICE Evidence Services (UK access only)
; Surgical approaches to hepatic hydatidosis ranging from partial cystectomy to liver transplantation. Ann Hepatobiliary Pancreat Surg. 2018 Aug22(3):208-215. doi: 10.14701/ahbps.2018.22.3.208. Epub 2018 Aug 31.
; National Public Health Service for Wales, 2004
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