Hydatid Disease

The term Hydatid disease describes infection with the larval stage of the cestode (or tapeworm) Echinococcus spp. There are 4 known species of which 3 are of medical importance to humans:

• Echinococcus granulosus, causing cystic echinococcosis (CE) - most common of the three
• Echinococcus multilocularis, causing alveolar echinococcosis (AE) - rare but is the most virulent
• Echinococcus vogeli, causing polycystic echinococcosis¹ - very rare

Transmission is from eggs found in faeces of dogs and accidentally swallowed, usually by children. Larvae develop over many years to form fluid-filled cysts in various organs, particularly the liver. Cysts can grow to considerable size and contain a large amount of fluid and vast numbers of infectious scolices.¹

• The life cycle of the Echinococcus helps to explain how the disease is transmitted and develops:^2,1
  • The Echinoccoccus adult tapeworms are about 5mm long and live in the small intestines of dogs (and other canid species such as coyotes and wolves).
  • The cyst stages occur in intermediate hosts, typically sheep (but also cattle, horses, camels, pigs and goats to name a few).
  • Tapeworm eggs are passed in the faeces of infected dogs and then are ingested by grazing sheep. These eggs hatch into embryos in the intestine and then penetrate the intestine to be carried by the blood to major organs such as liver and lungs. This larval stage of the parasite is called an oncosphere.
  • The larval stage spreads via the bloodstream to target organs in other parts of the body (liver, lungs, brain, muscles for example) where they develop by expansion into a hydatid or echinococcal cyst (called a metacestode).
  • Within the larval echinococcal cysts protosolices (multiple tiny tapeworm heads) are produced by asexual reproduction.
  • To complete the cycle these are then ingested by dogs feeding on the viscera of the infected intermediate host and about 6 weeks later adult egg producing tapeworms develop in the dog intestines.
  
  
  The practice of feeding the viscera of slaughtered animals to dogs in endemic countries is responsible for the high incidence and spread of infection. Humans are effectively intermediate hosts and become infected by handling infected dogs (or other carnivore hosts).
  
  
• Hydatid disease or echinococcosis can be either primary (spread by ingestion) or secondary (larval tissue proliferates after spread from the primary site, usually from trauma). In primary echinococcosis larval cysts develop in a single organ in most cases (about 80% of cases). About 70% of cases involve the liver.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). 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).

It is found in sheep farming areas mainly in developing countries. There are endemic areas in every continent ranging from South America, Africa, parts of Europe (notably Greece and Turkey), Australasia, Russia, China and the Middle East.

• E. granulosus, which causes cystic echinococcosis (CE), is the only species of the tapeworm found in the UK.¹
• 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.²
• E. multilocularis and E. vogeli have not been found in the UK.¹
• In the UK there are well-documented ‘hotspots’ of infection in Wales and the Western Isles of Scotland.
• Approximately 10-20 human cases are reported in the UK each year (most have arisen following exposure abroad).
• Current evidence suggests that the main areas for Hydatid disease in Wales are Powys, Monmouthshire and farms on the southern slopes of the Brecon Beacons and the Black Mountains. A pocket of disease is also present in the part of South Herefordshire adjacent to South Powys.¹

Risk factors

It is very important that the risk particularly in endemic areas is properly understood and appropriate measures taken. Risk factors include:

• 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.
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.

Cystic echinococcosis

• Symptoms can be produced by mass effect or complications of the cyst.
• Pressure symptoms can take a long time to become evident, except when they involve brain or eyes. Most cysts causing symptoms are larger than 5 cm 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 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 result in a range of symptoms from mild fever to full-blown sepsis.

Alveolar echinococcosis

• The liver is the primary site of infection, and the symptoms may closely mimic those of cirrhosis or carcinoma.
• The clinical picture is one of progressive liver dysfunction leading to liver failure. This can occur over weeks, months or years.
• Distant metastases are possible, and involvement of other organs (for example in lung, brain, bone) can occur in as many as 13% of the patients.
• Disease in the liver produces hepatomegaly, occasionally obstructive jaundice, mild upper gastrointestinal symptoms.
• Disease in the lung causes cough, haemoptysis, dyspnoea and pyrexia.
• Disease in brain causes raised intracranial pressure and can cause epilepsy.
• Disease in vertebrae can lead to compression of the spinal cord causing paraplegia. In the long bones it may cause fractures and deformity.

• Liver abscess
• Pyonephrosis
• Epidermoid cyst
• Simple cysts
• Neoplasia

Diagnosis is made by a combination of clinical, imaging, serological and molecular techniques. For example:

• Ultrasound for abdominal cysts with fine-needle aspiration.³
• Chest X-ray or CT 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 hemagglutination tests and enzyme-linked immunosorbent assays are widely used. However:
  • 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 echinococcus antibody detection.

See Management of Hydatid Disease and further reading list.
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.

There are currently no effective drugs or vaccines to protect humans against the disease.¹

The infected organs of animals must be condemned and destroyed. However 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 preventive measures is outlined in the box below.