- Human echinococcosis is a parasitic disease caused by tapeworms of the genus Echinococcus.
- The two most important forms of the disease in humans are cystic echinococcosis (hydatidosis) and alveolar echinococcosis.
- Humans are infected through ingestion of parasite eggs in contaminated food, water or soil, or through direct contact with animal hosts.
- Echinococcosis is often expensive and complicated to treat, and may require extensive surgery and/or prolonged drug therapy.
- Prevention programmes involve deworming of dogs, improved slaughterhouse hygiene, and public education campaigns; vaccination of lambs is currently being evaluated as an additional intervention.
- WHO is working towards the validation of effective cystic echinococcosis control strategies by 2018.
Human echinococcosis is a zoonotic disease (a disease that is transmitted to humans from animals) that is caused by parasites, namely tapeworms of the genus Echinococcus. Echinococcosis occurs in four forms:
- cystic echinococcosis, also known as hydatid disease or hydatidosis, caused by infection with Echinococcus granulosus;
- alveolar echinococcosis, caused by infection with E. multilocularis;
- polycystic echinococcosis, caused by infection with E. vogeli;
- unicystic echinococcosis, caused by infection with E. oligarthrus.
The two most important forms, which are of medical and public health relevance in humans, are cystic echinococcosis and alveolar echinococcosis.
The life-cycle of Echinococcus granulosus occurs between domestic or wild carnivores, such as dogs, foxes, wolves, jackals, hyenas and cats (definitive hosts), and sheep, goats, cattle, pigs, yaks or other farm animals (intermediate hosts). Cystic echinococcosis is principally maintained in a dog–sheep–dog cycle. Humans are accidental intermediate hosts and become infected through the ingestion of soil, water or food (e.g. vegetables) contaminated with the parasite’s eggs shed in the faeces of the carnivores. Humans can also be infected by hand-to-mouth transfer of eggs after contact with the contaminated fur of a carnivore (most commonly, a dog).
Carnivores become infected when they ingest the organs of intermediate hosts that harbour the larval stages of the parasite (hydatids, or hydatid cysts). In the carnivore, the cysts develop into adult worms and live in the intestines where they produce eggs that are passed in the faeces, contaminating the ground. Intermediate hosts ingest eggs in the contaminated ground, the eggs develop into cysts, and the cycle continues.
Transmission of E. multilocularis to humans occurs through ingestion of soil, food or water contaminated with the parasite’s eggs shed in the faeces of foxes and other canids, including domestic dogs and, to a lesser extent, cats. Humans may also become infected by hand-to-mouth transfer of eggs after contact with the contaminated fur of foxes, dogs or cats. Intermediate hosts are small mammals (rodents and lagomorphs).
Signs and symptoms
Human infection with E. granulosus leads to the development of one or more hydatids located mainly in the liver and lungs, and less frequently in the bones, kidneys, spleen, muscles, central nervous system, and eyes.
The asymptomatic incubation period of the disease can last many years until hydatid cysts grow to an extent that triggers clinical signs. Non-specific signs include anorexia, weight loss and weakness. Other signs depend on the location of the hydatid(s) and the pressure exerted on the surrounding tissues.
Abdominal pain, nausea and vomiting are commonly seen when hydatids occur in 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. Clinical signs include weight loss, abdominal pain, general malaise and signs of hepatic failure.
Larval metastases may spread either to organs adjacent to the liver (e.g. the spleen) or distant locations (lungs, brain) following dissemination of the parasite via the blood and lymphatic system. If left untreated, alveolar echinococcosis is progressive and fatal.
Cystic echinococcosis is globally distributed and found in every continent except Antarctica. Alveolar echinococcosis is confined to the northern hemisphere, in particular to regions of China, the Russian Federation and countries in continental Europe and North America.
In endemic regions, human incidence rates for cystic echinococcosis can reach greater than 50 per 100 000 person-years, and prevalence levels as high as 5–10% may occur in parts of Argentina, Peru, east Africa, central Asia, and China. In livestock, the prevalence of cystic echinococcosis found in slaughterhouses in hyperendemic areas of South America varies from 20–95% of slaughtered animals. The highest prevalences are found in rural areas where older animals are slaughtered. Depending on the infected species involved, livestock production losses attributable to cystic echinococcosis stem from liver condemnation, reduction in carcass weight, decrease in hide value, decrease of milk production, and reduced fertility.
Ultrasonography is the imaging technique of choice for the diagnosis of both cystic echinococcosis and alveolar echinococcosis. This technique is usually complemented or validated by computed tomography (CT) and/or magnetic resonance imaging (MRI) scans.
Sometimes, cysts can be incidentally discovered by radiography. Specific antibodies are detected by different serological tests and can support diagnosis. Biopsies and ultrasound-guided punctures may also be performed for differential diagnosis of cysts from tumours and abscesses.
Both cystic echinococcosis and alveolar echinococcosis are often expensive and complicated to treat, sometimes requiring extensive surgery and/or prolonged drug therapy.
Four options exist for the treatment of cystic echinococcosis:
- percutaneous treatment of the hydatid cysts with the PAIR (Puncture, Aspiration, Injection, Re-aspiration) technique;
- anti-infective drug treatment;
- ‘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.
For alveolar echinococcosis, early diagnosis and radical (tumour-like) surgery followed by anti-infective prophylaxis with albendazole 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-infective treatment, frequently results in relapses.
Health and economic burden
Both cystic echinococcosis and alveolar echinococcosis represent a substantial disease burden. Worldwide, there may be in excess of one million people living with these diseases at any one time. Many of these people will be experiencing severe clinical syndromes which are life-threatening if left untreated. Even with treatment, people often face reduced quality of life.
For cystic echinococcosis, there is an average 2.2% postoperative death rate for surgical patients and about 6.5% of cases relapsing after intervention that require prolonged recovery time. Present estimates suggest that cystic echinococcosis results in the loss of at least one million DALYs1 annually and possibly up to three million.
Annual costs associated with cystic echinococcosis are estimated to be 3 billion US dollars for treating cases and losses to the livestock industry.
Alveolar echinococcosis results in the loss of about 650 000 DALYs annually, with most of the disease burden concentrated in western China.
Prevention and control
Cystic echinococcosis is a preventable disease as it involves domestic animal species as definitive and intermediate hosts. Periodic deworming of dogs, improved hygiene in the slaughtering of livestock (including proper destruction of infected offal), and public education campaigns have been found to lower and, in high income countries, prevent transmission and alleviate the burden of human disease.
Vaccination of sheep with an E. granulosus recombinant antigen (EG95) offers encouraging prospects for prevention and control. Small-scale EG95 vaccine trials in sheep indicate high efficacy and safety with vaccinated lambs not becoming infected with E. granulosus.
A programme combining vaccination of lambs, deworming of dogs and culling of older sheep could lead to elimination of cystic echinococcosis disease in humans in less than 10 years.
Alveolar echinococcosis prevention and control is more complex as the cycle involves wild animal species as both definitive and intermediate hosts. Regular deworming of domestic carnivores that have access to wild rodents should help to reduce the risk of infection in humans.
Culling of foxes and unowned free-roaming dogs is applicable but appears to be highly inefficient. Deworming of wild and stray definitive hosts with anthelminthic baits resulted in significant reductions in alveolar echinococcosis prevalence in European and Japanese studies. Sustainability and cost–benefit effectiveness of such campaigns are however controversial.
WHO is helping to identify countries to develop and implement pilot projects leading to the validation of effective cystic echinococcosis control strategies by 2018.
After 2018, scaling-up interventions in selected countries for the control and elimination of cystic echinococcosis as a public health problem using the validated strategy will be a priority.
The cost of implementing cystic echinococcosis pilot projects from 2013 to 2017, in order to obtain validated echinococcosis strategies in three countries by 2018, is estimated at about 10 million US dollars.
WHO also supports capacity building through training courses targeting medical and paramedical personnel focused on the clinical management of cystic echinococcosis in rural areas of affected countries.
1 One DALY (disability-adjusted life year) can be thought of as one lost year of "healthy" life. The sum of these DALYs across the population, or the burden of disease, can be thought of as a measurement of the gap between current health status and an ideal health situation where the entire population lives to an advanced age, free of disease and disability.