Echinococcal infection
Essentials of Diagnosis
- Radiographic finding of a cyst.
- Positive echinococcal serology.
- Aspiration of a cyst revealing hydatid sand or hooks.
- Typical histological appearance of the cyst wall.
General Considerations
The normal life cycle of Echinococcus species does not involve humans. Human disease occurs when humans become accidental intermediate hosts for the parasite, and tissue invasion is followed by cyst formation (hydatid cysts). The definitive hosts for echinococcal species are canines (usually dogs), in whom the adult worms live. There may be several hundred worms within a host, and the worms are small, usually 3-6 mm long.
The scolex attaches to the dog intestine, and a single proglottid is attached to each scolex. As the proglottids mature and become gravid, containing several eggs, they detach and are passed in the stool. The eggs are resistant to desiccation and may remain viable for weeks. The eggs are ingested by intermediate hosts, commonly sheep and cattle but occasionally humans. The eggs hatch in the duodenum, and the larval forms penetrate the intestinal mucosa and disseminate through the bloodstream to distant sites, most commonly the liver. Within tissues, the larva develops a surrounding cyst composed of an outer layer and an inner, fluid-filled layer. Within 1 year, cysts may reach 5-10 cm in diameter.
Within the inner layer, daughter cysts form and may detach and float in the fluid, and daughter scolices may develop. Ingestion of meat containing hydatid cysts leads to infection of definitive hosts, and adult worms develop. Infections with Echinococcus granulosus cause the formation of unilocular cysts. By contrast, infections with Echinococcus multilocularis are associated with multilocular cysts. E multilocularis also differs from E granulosus in that definitive hosts include foxes, wolves, cats, and dogs, and intermediate hosts include small rodents. Endemic areas for E granulosus include Africa, the Middle East, southern Europe, Latin America, and the southwestern United States. For E multilocularis, forested areas of Europe, Asia, and North America are endemic.
Clinical Findings
Signs and Symptoms
In humans, the presentation of infection depends on where the hydatid cyst forms. The principal locations for cysts in humans include the liver (60%), lung (20%), muscle (4%), kidney (4%), spleen (3%), soft tissues (3%), brain (3%), bone (2%), and other sites (1%). In the liver, cysts may be diagnosed incidentally or may present as pain or a visible mass. Pulmonary cysts are usually asymptomatic, but if sufficiently large, they may cause cough, dyspnoea, or pleuritic pain.
Although rare, hydatid cysts in the brain are potentially the most serious and may cause obstructive hydrocephalus with ataxia and dementia, or a mass effect with seizures, headache, or focal neurological deficits. Bony hydatid cysts most commonly involve vertebrae and present with bone pain. Other presentations include soft tissue swelling and bone pain, or pathological fractures secondary to cysts weakening cortical bone. In about 20% of infected patients, cysts are multiple, so all patients in whom the diagnosis is suspected require a thorough evaluation.
| Site | Approximate frequency in humans | Typical manifestations |
|---|---|---|
| Liver | ≈ 60% | Incidental finding, right upper quadrant pain, palpable mass, signs of biliary or portal involvement in advanced disease |
| Lung | ≈ 20% | Often asymptomatic; cough, dyspnoea, pleuritic chest pain with larger cysts or mass effect |
| Muscle / soft tissue | ≈ 6-7% combined | Localised swelling or mass, sometimes discomfort; may be detected on imaging or incidentally |
| Kidney / spleen | Each ≈ 3-4% | Flank or abdominal pain, mass effect, or incidental finding on imaging |
| Brain | ≈ 3% | Headache, seizures, focal neurological deficits, signs of raised intracranial pressure or obstructive hydrocephalus |
| Bone | ≈ 2% | Persistent bone pain, pathological fractures, vertebral involvement with possible neurological compromise |
| Other sites | ≈ 1% | Organ-specific symptoms depending on location; may mimic neoplastic lesions |
Laboratory Findings
Eosinophilia may be present in about 25% of patients, but it is a non-specific marker. Serological testing is available using a variety of techniques, and if positive, the results provide supportive evidence of echinococcal infection. However, a negative serological test result does not rule out hydatid cyst disease. More recent serological techniques may help differentiate E granulosus infection from E multilocularis infection. Cross-reaction between cysticercosis and hydatid cyst disease has been noted with some serological assays. Another potential diagnostic procedure is cyst aspiration, although it poses some risk of anaphylaxis (see complications). This test is potentially useful for diagnosing E granulosus hydatid cysts. A small volume of fluid is removed and examined microscopically for the presence of hydatid sand (daughter cysts and scolices). If a cyst is old, sand may not be present; in that case, a centrifuged specimen should be examined for the presence of hooks.
Imaging
On radiography, cysts appear either as unilocular cysts with an air-fluid level (E granulosus) or as multiloculated cysts with little or no fluid (E multilocularis).
Differential Diagnosis
Because hydatid cysts usually present as liver pain or a mass, a lung mass with irritative obstructive symptoms, or seizures with focal neurological symptoms, the primary differential diagnosis is to exclude primary or metastatic malignancy in each of these organs. In a unilocular hydatid cyst, the radiographic appearance is often sufficient to exclude malignancy. By contrast, multilocular cysts caused by E multilocularis are slow growing, often with little or no fluid, and frequently have central necrosis, which is suggestive of malignancy. Therefore, biopsy and histological examination are necessary to definitively differentiate these cysts from malignancy.
Complications
On occasion, hydatid cysts may leak fluid into the host's systemic circulation, causing sensitisation. Subsequent fluid leakage may then induce an allergic response or even anaphylaxis. In addition, release of cyst tissue may be associated with embolisation and the development of additional cysts at distant sites. Cysts may also become secondarily infected, producing abscesses. Mechanical complications of the cyst are also possible, most commonly leading to portal hypertension, ascites, and portosystemic shunting.
Treatment
Treatment for hydatid cysts caused by E granulosus infection combines surgical and pharmacological interventions. Solitary unilocular cysts at operable sites are generally treated with surgical excision, percutaneous drainage, or both. Extreme care is required to avoid spillage of cyst contents and subsequent seeding to other sites. One approach is to remove a portion of the cyst fluid and instil a cysticidal agent such as 95% ethanol before removing the entire cyst. Some authorities also recommend pre- and postoperative therapy with either albendazole or mebendazole.
Recent experience with albendazole suggests it is a promising alternative. Mebendazole is an alternative therapy but may be less effective than albendazole. Complicated or multiloculated cysts (caused by E multilocularis) require surgery, often in association with albendazole as above. An experimental approach to inoperable cysts involves oral therapy with albendazole or mebendazole, combined with percutaneous aspiration and instillation of 95% ethanol. Although promising, this approach has not been validated in large trials, and treatment should be guided by specialised expertise.
Prognosis
The prognosis of hydatid disease is variable. With early diagnosis and treatment of simple unilocular cysts, the outlook is excellent; by contrast, advanced multilocular disease at multiple sites with advanced portal hypertension is potentially lethal. Medical therapy of inoperable E granulosus cysts is associated with cure in 30% of cases and improvement in 50%. Inoperable E multilocularis infection is associated with a 10-year mortality rate in 90% of cases. In such cases, some authorities recommend indefinite treatment with albendazole or mebendazole.
Prevention & Control
In areas endemic for hydatid disease, transmission commonly occurs incidentally during activities such as camping and picking berries. Education is the best prevention strategy for transmission in these settings. Routine screening of household pets and appropriate treatment of animals found to carry Echinococcus spp. is another important control measure. Pet owners should be educated on good hygiene practices to prevent accidental inoculation with eggs from dog stools. Finally, carcasses of infected hosts must be disposed of in ways that prevent transmission to canines.