Echinococcosis

Echinococcosis is a parasitic infection caused by the larval (metacestode) stage of Echinococcus tapeworm species. Humans are accidental intermediate hosts, infected by ingesting eggs shed in the faeces of infected definitive canine hosts (dogs, foxes and other canids).

There are two clinically distinct forms of major significance.

• Cystic echinococcosis is caused by Echinococcus granulosus. It produces slowly enlarging, generally well defined hydatid cysts, most commonly hepatic or pulmonary.
• Alveolar echinococcosis is caused by Echinococcus multilocularis. It produces a far more aggressive, infiltrative, tumour like growth pattern with a worse prognosis if untreated.

The distinction matters because the two species have markedly different natural history and management.

Pathophysiology and epidemiology

• Following ingestion of eggs, oncospheres are released within the small intestine and penetrate the intestinal wall to reach the liver via the portal circulation.
• The liver is the commonest site of larval cyst development for both species, though pulmonary and other organ involvement occurs, particularly in cystic disease. The oncospheres develop into the characteristic larval cyst structure.
• Cystic echinococcosis produces a well defined, typically unilocular cyst with a distinct laminated wall. It grows slowly over years and generally remains localised to the primarily affected organ.
• In cystic disease the life cycle is typically maintained between dogs (definitive host) and livestock, particularly sheep (intermediate host). Humans are an accidental, dead end intermediate host.
• Alveolar echinococcosis, in marked contrast, produces an infiltrative, poorly defined mass with local tissue invasion and, in advanced disease, metastatic spread. It resembles malignancy far more closely than the typically contained cystic disease.
• In alveolar disease the life cycle is typically maintained between foxes (definitive host) and small rodents (intermediate host). Human infection occurs incidentally through contamination of food or hands with fox faecal material.

• Cystic echinococcosis is frequently asymptomatic for years, given the typically slow growth rate.
• Symptoms develop as the cyst enlarges enough to cause mass effect: right upper quadrant discomfort or a palpable mass for hepatic cysts, cough and chest discomfort for pulmonary cysts.
• Less commonly, symptoms result from cyst complication. Rupture causes acute pain and, given the highly antigenic nature of cyst fluid, risk of anaphylaxis. Secondary bacterial infection produces an abscess like presentation. Biliary or bronchial communication causes specific related symptoms.
• Alveolar echinococcosis presents insidiously with symptoms mimicking hepatic malignancy, including right upper quadrant pain, weight loss and jaundice (from biliary involvement or compression).
• In advanced disease there are features of hepatic failure. Where metastatic spread has occurred (most commonly to the lungs and brain), symptoms refer to those specific sites.
• A relevant exposure history supports clinical suspicion: residence in or travel to an endemic region, and contact with dogs (particularly working or farm dogs with access to raw offal, a specific risk factor for cystic disease).
• For alveolar disease specifically, relevant exposure includes activities with potential fox faecal contamination (such as foraging for wild berries or mushrooms in endemic forested regions).

• Imaging (ultrasound, computed tomography or magnetic resonance imaging) is central to diagnosis.
• Cystic echinococcosis typically shows a well defined, often multiloculated cyst with characteristic internal daughter cysts and, in more mature cysts, a visible laminated membrane.
• Alveolar echinococcosis shows a poorly defined, infiltrative lesion with irregular margins and, frequently, characteristic scattered microcalcification. This can closely mimic hepatic malignancy on imaging alone.
• Serological testing supports diagnosis for both forms, with generally higher sensitivity for alveolar disease given its more invasive, immunologically stimulating growth pattern.
• Serology can be negative in a proportion of cystic disease, particularly with an intact, non leaking cyst wall limiting antigen exposure to the host immune system.
• Percutaneous biopsy or aspiration of a suspected hydatid cyst is generally avoided given the significant risk of cyst content leakage with resulting anaphylaxis or secondary peritoneal seeding.
• Diagnosis instead relies predominantly on characteristic imaging appearance, relevant epidemiological exposure history and supportive serology. Invasive sampling is reserved for genuine diagnostic uncertainty and performed only under specialist guidance with specific precautions.
• Where percutaneous aspiration is performed for cystic disease, it is usually part of a specific therapeutic technique rather than purely diagnostic sampling. It is undertaken with strict precautions and typically under cover of concurrent antiparasitic treatment, to reduce the risk and consequence of any content leakage.

• Cystic echinococcosis management is individualised according to cyst characteristics: size, number, stage of development, and the staging classification reflecting cyst activity and complexity.
• Watchful waiting suits small, inactive, asymptomatic cysts.
• Percutaneous treatment suits appropriately selected active, uncomplicated cysts. The PAIR technique comprises puncture, aspiration, injection of a scolicidal agent and re-aspiration, performed under antiparasitic drug cover.
• Surgical resection suits large, complicated or anatomically challenging cysts, or those not suitable for a percutaneous approach.
• Albendazole 400 mg orally twice daily is given as an adjunct to both percutaneous and surgical intervention, to reduce the risk of secondary spread from cyst content spillage during the procedure. It is typically started some days before and continued for a defined period afterward, following specific protocols for the intervention type.
• Albendazole may be used as sole medical therapy for small, uncomplicated cysts where intervention is not indicated or feasible, though cure rates with medical therapy alone are generally lower than with appropriately selected intervention.
• Alveolar echinococcosis requires radical surgical resection where feasible, given its invasive, malignancy like growth pattern, combined with prolonged albendazole therapy.
• Continue albendazole for a minimum of two years following apparently complete surgical resection. Where complete resection is not achievable because of residual or unresectable disease, continue it indefinitely, essentially as suppressive rather than curative therapy.
• This reflects the more aggressive natural history of alveolar disease and the correspondingly more intensive treatment required compared with cystic disease.
• Regular, long term imaging and serological follow up is essential after treatment of either form, given the potential for recurrence. It is particularly relevant and prolonged for alveolar disease, given its typically more protracted, slowly progressive natural history even with treatment.
• Prevention centres on interrupting the relevant life cycle. Deworm dogs in endemic regions (particularly relevant for cystic disease control, targeting the definitive host) and avoid feeding raw offal to dogs (breaking the livestock to dog transmission cycle).
• For alveolar disease prevention, practise hand hygiene and thorough washing of foraged wild foods in endemic regions with relevant fox populations.