Introduction
Human echinococcosis is a zoonosis caused by larval forms of Echinococcus tapeworms found in the small intestine of carnivores. Two species (E. granulosus and E. multilocularis) are of medical significance, causing cystic echinococcosis (CE) and alveolar echinococcosis (AE) in humans, respectively.
Alveolar echinococcosis is considered the most dangerous parasitic disease in Europe, while cystic echinococcosis is much milder. Due to the completely different growth behavior of the two diseases in humans, two different clinical pictures result. The appearance of AE resembles that of a malignancy, while the CE lesion appears benign due to the smooth-bordered pericyst.
According to WHO, cystic echinococcosis (CE) is the disease caused by the larval stage of Echinococcus granulosus (dog tapeworm). The actual tapeworm lives in the intestine of the definitive host (dog) and produces eggs that are excreted with the feces. The eggs are ingested by the intermediate/accidental host (human or animal). In the intestine, larvae (oncospheres) hatch from the eggs. These parasitic larvae penetrate the intestine and reach the liver or other organs via the portal vein system, where they transform into their second larval stage (metacestodes) and mature into cysts (hydatids). In the cyst, daughter cysts develop, in which protoscoleces (head structures of the future tapeworms) form. These protoscoleces can grow into adult tapeworms if larva-containing organs are consumed by a definitive host. Humans usually become infected by accidentally ingesting soil, water, or food contaminated with Echinococcus eggs from dog feces.
All age groups, including infants, children, and adolescents, are affected by cystic echinococcosis. The clinical course is highly variable and depends on the location of the cysts, their size, and the host reaction. The disease is characterized by slowly enlarging cysts (especially in the liver and lungs) that can remain asymptomatic for years and only become symptomatic due to their space-occupying effect, secondary bacterial infection of the cysts, cystobiliary or cystobronchial fistulas, or anaphylactic reactions after rupture. In most patients, only one organ is affected. Cysts are most commonly found in the liver (70%) and lungs (20%).
Epidemiology
Cystic echinococcosis (CE) is widespread worldwide. A particularly high prevalence is found in regions of South America, the Mediterranean coast, Eastern Europe, the Near and Middle East, East Africa, Central Asia, China, and Russia.
In Germany, the diagnosed cases almost exclusively involve patients from the aforementioned endemic areas.
In endemic areas, dogs are often fed with the viscera of slaughtered animals. Larva-containing offal causes adult tapeworms to develop in the main definitive host after consumption.
Infectiousness
The risk of infection is particularly high in hyperendemic areas where close contact with the main host (dog) occurs under poor hygienic conditions. There is no human-to-human transmission. Surgical material is not infectious.
Morphology
The cyst itself consists of the pericyst (periparasitic inflammatory reaction of the host tissue with the formation of a fibrous capsule) and the endocyst, the actual parasite's cyst component. The endocyst consists of an outer acellular and an inner germinative layer, where brood capsules with protoscoleces (head structures of the future adult worms), the so-called hooks, form.
Cyst Classification
Since 2003, there has been a standardized ultrasound classification according to WHO-IWGE (World Health Organization Informal Working Group on Echinococcosis), which is based on the division of cysts according to their sonographic morphology into active - transitional - inactive.
In this classification, 6 cyst stages are assigned to 3 clinical groups, with stage CL including an undifferentiated "cystic lesion" whose parasitic nature has not yet been definitively assessed.
Group 1: The active group includes unilocular cysts (CE1) or cysts with multiple daughter cysts (CE2).
Group 2: The transitional group includes cysts with a detached endocyst membrane ("water-lily" sign) (CE3a) and consolidated cysts with daughter cysts (CE3b). CE3a can solidify and become "inactive" or form daughter vesicles and transition to a CE2 stage.
Group 3: The "inactive" group shows involution with consolidation of the cyst content and increasing calcification and is considered avital in most cases.
In summary, CE1 and CE2 represent active cysts, CE3 is a transitional stage, with CE3b cysts considered biologically active. CE4 and CE5 are late inactive cyst stages.
Therapy
There is no standard therapy recommendation for the treatment of cystic echinococcosis. In principle, remissions without therapy are frequently observed and demonstrate the benign nature of this disease, as long as none of the risk constellations listed below are present.
The therapy of CE (cystic echinococcosis) depends on the cyst stage and is thus dependent on the morphology and vitality assessment. In principle, surgical removal is possible at all stages. However, there is a risk of overtreatment.
Due to the questionable inactivity, it is recommended to observe lesions in stages CE4 and CE5. In these stages, the lesion is monitored by imaging and serologically. If there is an increase in titer or size, surgical removal may be indicated.
In all other stages, a conservative therapy attempt with benzimidazoles albendazole or mebendazole for 3 to 6 months can be conducted for lesions up to about 5 cm, with a chance of healing in 60–70%.
For transhepatically puncturable cysts in stage CE1 or CE3a, which are smaller than 10 cm, puncture, aspiration, instillation, and reaspiration (so-called PAIR) is a promising and low-complication alternative to surgery, as the internal structure of these cysts is practically liquid. Here, the cyst is punctured and aspirated percutaneously or surgically (open or laparoscopically). Then, an anthelmintic or protoscolicidal substance is instilled, and this fluid is aspirated again after sufficient exposure time. The procedure is performed under albendazole protection to prevent secondary echinococcosis.
It should be noted that for cysts larger than 7-8 cm, the risk of a cystobiliary fistula increases (up to 80%). Since hypertonic NaCl (20%) or ethanol (95%), the two most commonly used protoscolicidal substances, can cause toxic cholangitis, a cystobiliary fistula must be definitively excluded by bilirubin determination in the cyst fluid and/or antegrade contrast imaging.
The surgical treatment is the first-choice therapy for
- Cysts > 10 cm
- Cysts in stage CE2 and CE3b with multiple daughter cysts, as the internal structure of the cysts is not suitable for PAIR.
- Cysts that are connected to the bile ducts.
- Cysts that compress organs or vessels.
- Cysts with the risk of rupture (traumatic or spontaneous) or already ruptured cysts.
- Cysts with a superficial location should not receive purely conservative therapy, as the wall of the cyst is thinned by the treatment, increasing the risk of cyst rupture, which can lead to secondary echinococcosis.
- Superinfected cysts when percutaneous treatment is not possible.
In the demonstrated case, there were three large echinococcal cysts of the liver in stage CE3b, one in the left and the other two, communicating with each other, in the right liver lobe. The endocystectomy aims to remove the entire endocyst with daughter cysts from the cyst cavity. In the vicinity of the cysts, massive adhesions to the surroundings are typically found.