Echinococcosis, a parasitic disease that occurs in two main forms in humans

Kolkata: Echinococcosis is a parasitic disease that occurs in two main forms in humans: cystic echinococcosis (also known as hydatidosis) and alveolar echinococcosis, caused by the tapeworms Echinococcus granulosus and Echinococcus multilocularis, respectively.

Dogs, foxes and other carnivores harbour the adult worms in their intestine and evacuate the parasite eggs in their faeces. If the eggs are ingested by humans, they develop into larvae in several organs, mainly the liver and lungs.

Both cystic and alveolar echinococcosis are characterized by asymptomatic incubation periods that can last many years until the parasite larvae evolve and trigger clinical signs.

Both diseases can cause serious morbidity and death.

A number of herbivorous and omnivorous animals act as intermediate hosts of Echinococcus by ingesting parasite eggs in contaminated soil and developing parasitic larval stages in their viscera.

Carnivores are definitive hosts of the parasite; they are infected through the consumption of viscera of intermediate hosts that harbour the parasite and also through scavenging infected carcases. Humans are accidental intermediate hosts and are unable to transmit the disease.

Transmission of cystic echinococcosis is principally maintained in a dog–sheep–dog cycle, although several other domestic animals may be involved including goats, swine, horses, cattle, camels and yaks.

Transmission of alveolar echinococcosis usually occurs in a wildlife cycle among foxes, other carnivores and small mammals (mostly rodents). Domesticated dogs and cats can also be infected.

For both diseases, humans become infected through the ingestion of soil, water or food (e.g. green vegetables, berries) contaminated with the parasites’ eggs shed in the faeces of the carnivores, and also by hand-to-mouth transfer of eggs after contact with the contaminated fur of a carnivore, most commonly a dog.

Cystic echinococcosis is characterized by an asymptomatic incubation period that can last many years until the parasite cysts evolve and trigger clinical signs, depending on the location and size of the cysts and the pressure exerted on the surrounding tissues.

In cystic echinococcosis, the larval stages of the parasite develop as one or more cysts mainly in the liver and lungs, and less frequently in the bones, kidneys, spleen, muscles, central nervous system and eyes.

Abdominal pain, nausea and vomiting commonly occur when cysts invade the liver. If the lung is affected, clinical signs include chronic cough, chest pain and shortness of breath.

Alveolar echinococcosis is characterized by an asymptomatic incubation period of 5–15 years and the slow development of a primary tumour-like lesion which is usually located in the liver.

Lesions may also involve other organs such as the spleen, lungs and brain following dissemination of the parasite via the blood and lymphatic system. Clinical signs include weight loss, abdominal pain, general malaise and signs of hepatic failure. If left untreated, alveolar echinococcosis is progressive and fatal.

Both cystic echinococcosis and alveolar echinococcosis can be expensive and complicated to treat, sometimes requiring extensive surgery and/or prolonged drug therapy.

Human cystic echinococcosis is diagnosed with imaging tools such as ultrasound or computed tomography, and its laboratory confirmation relies on serological tests. Four options exist for the treatment of abdominal cystic echinococcosis: (i) percutaneous treatment of the hydatid cysts with the PAIR (Puncture, Aspiration, Injection, Re-aspiration) technique; (ii) surgery; (iii) anti-parasitic drug treatment; and (iv) expectant management (“watch and wait”).

The choice must primarily be based on the ultrasound images of the cyst, following a stage-specific approach, and also on the medical infrastructure and human resources available to treat patients.

Diagnosis of alveolar echinococcosis is based on clinical findings and epidemiological data, imaging techniques, histopathology and/or nucleic acid detection, and serology. Early diagnosis and radical (tumour-like) surgery followed by anti-infective prophylaxis remain the key elements.

If the lesion is confined, radical surgery offers cure. Unfortunately, in many patients the disease is diagnosed at an advanced stage, and palliative surgery, if carried out without or with incomplete anti-parasitic treatment, frequently results in relapses.(UNI)