17th June 2024

Cystic helminth disease and key differentials

Hydatid disease

Cystic echinococcosis, or hydatid disease, is caused by ingestion of the eggs of Echinococcus granulosus, a cestode (tapeworm). This can lead to cystic disease of a variety of organs, mainly the liver and lungs. Hydatid cysts can be asymptomatic for years and detected on incidental imaging. Right upper quadrant pain may occur due to cyst growth. Rupture of lung cysts may cause pulmonary manifestations such as a pneumothorax. A variety of stages can be observed using different imaging modalities, with a WHO classification system existing for ultrasound diagnosis. The water lily sign is caused by the detachment of the endocyst membrane, which then floats within the cystic fluid. Serology can also be used to help with the diagnosis. Due to the slow growth of cysts caused by E. granulosus, a watchful waiting approach can be taken to see if the cyst growth has ceased. Active treatment strategies may involve chemotherapy with albendazole, with or without more invasive management. PAIR – Puncture, Aspiration, Injection (of a scolicidal agent such as hypertonic saline), Reaspiration – is a radiologically-guided technique that can be used to avoid formal surgery. Surgery can then be reserved for more complicated cases or calcified cysts. Alveolar echinococcosis is a rarer disease caused by E. multilocularis, which involves rapid destruction and invasion of tissue, requiring more complex management.

It is important to be able to differentiate hydatid liver cysts from amoebic liver abscesses caused by the protozoa Entamoeba histolytica. Occurring worldwide, there is a strong association between poor sanitation and hygiene and amoebiasis. Amoebiasis is generally a self-limiting gastrointestinal infection, causing abdominal pain and loose stools with blood or mucus. The most common extra-intestinal manifestation is amoebic liver abscess. Abscesses are usually isolated in nature, though multiple can be present. Right upper quadrant pain and fever are the predominant symptoms. In terms of diagnosis, stool microscopy is not brilliant as it is difficult to differentiate E. histolytica from E. dispar, a non-pathogenic species. Stool antigen tests may be of use. Serology is often preferred, though it can give a false negative in early disease and a false positive if the patient has been previously infected. Amoebic liver cysts are similar in appearance to pyogenic liver cysts on imaging. Medical therapy with metronidazole is usually first line for amoebiasis and amoebic liver cysts. Aspiration and drainage are reserved for special circumstances. These include diagnostic uncertainty, failure of medical therapy, clinical or radiological evidence of impending rupture, and left-sided liver cysts (which may rupture into the pericardium). Fluid obtained from amoebic liver cysts is normally thick and brown, as opposed to the thin, clear fluid found in hydatid cysts.

Neurocysticercosis

Another tapeworm cyst to be aware of is that of Taenia solium. Once again acquired via the faecal-oral route, cysticercosis is a result of larval migration to tissues and organs including muscles and the brain, with the latter termed neurocysticercosis. Consumption of infected meat simply leads to intestinal tapeworm infection. Neurocysticercosis is a leading cause of seizures in the tropics and East Asia. As this is a pig tapeworm, cases are rarer in areas where pigs are not reared for religious reasons. Diagnosis of neurocysticercosis is usually based on imaging and clinical history. Treatment includes anti-epileptics and, if anti-helminth medication is to be used, steroids can prove useful in preventing morbidity and mortality from the ensuing inflammatory response.