Hydatid Disease

Background

Hydatid disease in man is caused principally by infection with the larval stage of the dog tapeworm Echinococcus granulosus. It is an important zoonotic (acquired from animals) parasitic infection of humans, which follows accidental ingestion of tapeworm eggs excreted in the faeces of infected dogs.

Epidemiology

E. granulosus is distributed throughout most of the world, especially in areas where sheep are raised, and is endemic in Asia, North Africa, South and Central America, the USA, Canada, and the Mediterranean region. It is common throughout Europe, and 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 and most have arisen following exposure abroad. In many countries, hydatid disease is more prevalent in rural areas where there is close contact between man and dogs and the various domestic animals which act as intermediate vectors.

Worldwide distribution is shown below:

In the UK, the important intermediate hosts for E. granulosus are sheep; infection rates may be high in these animals, over 90% of cysts may be fertile. 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. Echinococcosis, or hydatid disease, is usually found in temperate and subtropical zones where sheep are raised. Most cases seen in the United States are imported, but parts of Alaska and Canada are endemic for the disease.

Method of Transmission

Sheep and other intermediate hosts such as cattle acquire hydatid disease by grazing on pastures contaminated with dog faeces containing eggs. Each egg hatches in the small intestine of the sheep where it penetrates the gut wall. This larval stage of the parasite (called an oncosphere) is carried via the bloodstream to target organs in other parts of the body (liver, lungs, brain, muscles, etc.) where they develop by expansion into a hydatid cyst (called a metacestode). This hydatid cyst is usually fluid filled, containing brood capsules in which are protoscolices produced by asexual reproduction.

Dogs are, in turn, infected by ingesting meat or viscera containing such cysts, for example by eating an infected rodent, or being fed infected sheep meat and viscera. The fertile cysts they ingest develop into adult tapeworms in the small intestine where they reproduce sexually. New reproductive segments (proglottids) of the worm are produced. After about a month, the animal host begins to excrete proglottids containing infective eggs in its faecal matter. E. granulosus lives in the dogs’ intestine for about a year but ceases to produce eggs after 6 - 10 months. Eliminated segments of the worm in dog faeces have been reported to migrate some distance from faecal matter over grass or garden vegetables before expelling eggs that subsequently adhere to the vegetation. Humans are intermediate hosts; they do not play a role in the biological cycle but may act as agents perpetuating the disease by feeding dogs infected meat and viscera.

Clinical Features

• Slowly enlarging echinococcal cysts generally remain asymptomatic until their expanding size or their space-occupying effect in an involved organ elicits symptoms.

• The liver and the lungs are the most common sites of these cysts. The liver is involved in about two-thirds of E. granulosus infections and in nearly all E. multilocularis infections. Since a period of years elapses before cysts enlarge sufficiently to cause symptoms, they may be discovered incidentally on a routine x-ray or ultrasound study.

• Patients with hepatic echinococcosis who are symptomatic most often present with abdominal pain or a palpable mass in the right upper quadrant. Compression of a bile duct or leakage of cyst fluid into the biliary tree may mimic recurrent cholelithiasis, and biliary obstruction can result in jaundice. Rupture of or episodic leakage from a hydatid cyst may produce fever, pruritus, urticaria, eosinophilia, or anaphylaxis. This ruptured hydatid cyst showing the pathognomonic "snake" sign.

• Pulmonary hydatid cysts may rupture into the bronchial tree or peritoneal cavity and produce cough, chest pain, or haemoptysis.
• Rupture of hydatid cysts may lead to multifocal dissemination of protoscolices, which can form additional cysts. Rupture can occur spontaneously or at surgery. Other presentations are due to the involvement of bone (invasion of the medullary cavity with slow bone erosion producing pathologic fractures)

• The central nervous system (space-occupying lesions), the heart (conduction defects, pericarditis), and the pelvis (pelvic mass). Below is an intracerebral cyst:

The larval forms of E. multilocularis characteristically present as a slowly growing hepatic tumor, with progressive destruction of the liver and extension into vital structures. Patients commonly complain of upper quadrant and epigastric pain, and obstructive jaundice may be apparent. The lesions may infiltrate adjoining organs (e.g., diaphragm, kidneys, or lungs) or may metastasize to the spleen, lungs, or brain.

Investigations

• Radiographic suspicion
• Examination of cyst fluid (hydatid sand) reveals the typical invaginated scolices. Diagnostic puncture is NOT recommended.

• Serology (via ELISA & Western Blot) is 80% to 100% sensitive and 88% to 96% specific for liver cyst infection but less sensitive for lung (50% to 56%) or other organ (25% to 56%) involvement. Negative serology does not exclude disease.

Treatment

• Cysts communicating with the biliary tree or branches should not have a cysticidal agent instilled because of the risk of postoperative sclerosing cholangitis.
• It may be more prudent to treat the patient perioperatively with an anthelmintic agent active against Echinococcus cysts (albendazole, mebendazole) to limit the risk of intraoperative dissemination of daughter cysts.
• Medical therapy for inoperative cysts with albendazole or mebendazole has provided improvement in most patients (55% to 79%) and cure in a smaller number (29%).
• The preferred agent is albendazole because of its greater absorption from the gastrointestinal tract and higher plasma levels. It is given for three or more cycles at a dose of 400 mg twice a day for 4 weeks followed by a 2-week rest without therapy. The alternative agent, mebendazole is poorly absorbed and must be taken at higher doses (50 to 70 mg/kg/day) for several months to achieve a therapeutic effect. The response to drug therapy depends on the cyst size and location.36,37 Unfortunately, bone cysts, which are frequently not amenable to surgery, respond less well to drug treatment than other cysts.
• The response to drug therapy is best monitored by:
• serial imaging studies
• cyst disappearance
• or shrinkage along with increasing cyst density.
• An intermediate intervention for inoperable cysts has been developed, known as the PAIR procedure (puncture, aspiration, injection, reaspiration). While the patient is receiving anthelmintics to reduce the risk of cyst dissemination, the hydatid cyst may be aspirated with a thin needle under CT guidance. Approximately 30% of the cyst fluid volume is removed. Detection of protoscolices in the cyst fluid allows confirmation of cyst viability. An equal volume of 95% ethanol or other scolicidal agent (e.g., 0.5% cetrimide) is then instilled into the cyst cavity and allowed to react for 30 minutes before removing the needle. Results indicate arrest or involution of cysts after treatment. Although this approach offers possible treatment for inoperable cysts, the risks remain incompletely defined.

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; infected organs must be condemned and destroyed.

Dog owners should practice good hygiene when handling their animals. In endemic areas, it particularly important to wash the hands after handling dogs, avoid contact with dog faeces, and to prevent dogs from soiling the immediate environment. Dogs should not be allowed to roam or to have access to raw sheep meat or viscera. All sheep carcasses should be disposed of properly and immediately. All dogs, especially those in rural endemic areas should be treated at 6 weekly intervals with a wormer containing Praziquantel. Vegetables, salads and fruit should be thoroughly washed before consumption.

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

AMH