Echinococcosis (echinococcal cyst [EC], hydatidosis, hydatid disease) is a cyclozoonosis caused by the larval (metacestode) stages of cestodes (flat worms) belonging to the genus Echinococcus and the family Taeniidae. The biologic characteristics of the parasite enable its survival in nature. The disease exists in two forms: the larval stage (metacestode) and the adult stage (tenia). The parasites are perpetuated in life cycles with carnivores (dogs and wild canine) as definitive hosts. Humans are the accidental intermediate host (dead end) and animals (herbivores and omnivores) are both intermediate and definitive hosts.

The disease has been known since earliest times. It was known to Hippocrates, who speaks of “livers full of water.” The word echinococcus is of Greek origin and means “hedgehog berry.” Hydatid is also of Greek origin (hudatid, hudatis) and means a “watery vesicle.” The word hydatid also originates from the modern Latin word hydatis meaning a “drop of water.” Therefore, hydatid and cyst have the same meaning and the expression “hydatid cyst” (HC) is a pleonasm. The use of the term hydatid cyst in contemporary medicine is widespread and acceptable. The first North American case was observed in 1808 and published in 1822. The true nature of the disease was not known until the second half of the nineteenth century.

Echinococcosis is widespread and occurs in all continents, including circumpolar, temperate, subtropical, and tropical zones. It is not confined to sheep-raising communities. The varying prevalence and distribution is influenced by agricultural, educational, economical, medical, and cultural factors. In endemic areas, the annual incidence of cystic echinococcosis ranges from <1 to 200 per 100,000 inhabitants. Human cystic echinococcosis remains highly endemic in pastoral communities, particularly in regions of South America (Argentina, Uruguay, Chile), the Mediterranean littoral (Spain, France, Italy), Eastern Europe, the Near and Middle East (Turkey), East Africa (Maghreb countries), Central Asia, China, and Russia. Increasing migration, growing incidence of world trade and travel, high mobility of troops, and an increasing number of refugees make hydatidosis a global problem. In endemic areas hydatidosis remains a major health and socioeconomic problem. The estimated minimum global human burden of CE averages 285,000 disability-adjusted life years (DALYs) or an annual loss of U.S. $194,000,000. The mortality rate from cystic echinococcosis is about 2% to 4% but it may increase considerably if medical treatment and care are inadequate. Only infection with the metacestode is of clinical relevance.

Although echinococcosis is uncommon in the resident population of industrially developed countries, it is prevalent enough to be seen by most general surgeons in its simple or its complicated form. Because it is a surprise finding, even in tertiary centers, it is overtreated more often than not.

The treatment of hydatid disease has been the responsibility of surgeons for many years, and research into its treatment also seems to be the responsibility of surgeons. There is no other parasitic disease for which the primary treatment is surgical. Because there is no effective treatment of hydatid disease today, either surgical or medical, the best surgical technique is not known. Surgeons will be happy when a proper medical treatment is found and they are needed only for the complications.

A surgeon managing a patient with a HC should understand both the parasitology and pathology of the parasite and be fully aware of treatment limitations and possible complications. The dilemma of whether to perform tissue-sparing, conservative, or radical resectional surgery for hydatid disease is an artificial issue. The operative procedure should be tailored to every individual patient, taking into consideration many factors including the local expertise and facilities, prevalence of the disease, socioeconomic factors, and the age and condition of the patient.

This chapter presents the basics of this disease and it is intended to help the surgeon manage patients with HCs.

The previous recommendations of the World Health Organization Informal Working Group on Echinococcosis (WHO-IWGE) for the treatment of human echinococcosis were published in 2001. These recommendations have significantly influenced the way cystic echinococcosis was managed in all continents. Ten years later, after several expert meetings of the WHO-IWGE, a consensus was reached in 2009 about the management of cystic echinococcosis. The consensus has been reached on an image-based, stage-specific approach, which helps in choosing one of the following treatment options: (a) surgery, (b) percutaneous treatment (PT), (c) chemotherapy, and (d) watchful waiting. An image-based classification system differentiating the cyst stage has been reinforced to help in clinical decision making. Unfortunately, the panel of experts also concluded that the majority of published papers about the management of cystic echinococcosis were not consistent and well structured, so the quality of evidence and strength of recommendation is low. Since cystic echinococcosis is mostly treated in developing countries with limited resources, choosing the best treatment option must take into consideration the socioeconomic conditions, the available facilities, and the expertise of the attending surgeon. Nevertheless, surgery remains the most effective treatment. It is frequently technically demanding and risky and postoperative complications can be life threatening.

This chapter is based on these new concepts.

The genetic and phenotypic variations of E. granulosus have been extensively studied. The dog is the most common definitive host and the majority of cases in humans are caused by the sheep strain. The adult tenia lives adhered to the small intestine of the dog (definitive host). It measures from 3 to 6 mm in length, and has three or four proglottids, of which the last one is larger and gravid. The gravid proglottid containing between 200 and 800 ova is shed approximately every 15 days and the ova are expelled in the feces. When the microscopic eggs are ingested by an intermediate host the life cycle is perpetuated. The intermediate hosts are Bovidae.

There are three known forms of echinococcosis in humans: (a) cystic echinococcosis (hydatid disease) caused by E. granulosus, (b) alveolar echinococcosis (alveolar hydatid disease) caused by E. multilocularis, and (c) polycystic echinococcosis caused by E. vogeli or E. oligarthus (?). Although E. granulosus and E. multilocularis occur simultaneously in large endemic areas, mixed infections of cystic echinococcosis are extremely rare. This chapter deals with cystic echinococcosis (hydatid disease) caused by E. granulosus only (Fig. 1).

Humans are an accidental, intermediate host and become infected when they accidentally ingest eggs of the tapeworm. This can happen directly, by contact with dogs, or indirectly by food, water, and contaminated objects. Direct contact during childhood is an important route. The eggs hatch in the duodenum, and the released oncosphere penetrates the mucosa and reaches a blood vessel. The bloodstream can carry the oncosphere to any part of the body, but it most frequently settles in the liver and lungs. Once settled, the parasite develops its larval stage, the HC, the clinical presentation of E. granulosus. The cyst is a chronic, well-localized, and adapted space-occupying affliction that is not affected by the functional status of the host. The protoscolex can multiply asexually ad infinitum in the intermediate host as long as the host is alive. Because the parasite is always confined within the HC, a vital hydatid cyst has the tendency to slowly enlarge and interact with the host. Following a symptom-free period of varying length, depending on the site and number of cysts, life-threatening complications arise. The disease, benign in nature, can cause devastating damage to the liver and death to the patient.

This cycle with two hosts, one definitive and the other intermediate, is the sexual cycle and the resulting disease in humans or animals is “primary echinococcosis.” There is an asexual, “minor” cycle in which the new HCs develop from any vital element of the larval stage (e.g., protoscoleces, daughter cysts) of the parasite in the same intermediate host. This can occur subsequent to invasive and operative procedures, or following spontaneous or trauma-induced cyst rupture. This disease in humans is “secondary echinococcosis.”

The cellular response of host tissue to the presence of the larva is an attempt to encase the parasite in fibrous tissue. The parasite reacts by forming around itself a spherical enclosure of inert protective chitinous material. If the parasite survives host defense mechanisms, in approximately 5 to 7 days after infestation, the larva (metacestode) develops into a 6- to 7-mm vesicula. This is the endocyst and it is made up of two distinct layers: (a) the inner germinative layer (live parasite, germinal membrane, proligerous layer) and (b) a protective outer laminated acellular layer (cuticle or laminated layer). The laminated membrane never grows thicker than 2 to 5 mm, regardless of cyst size. After 21 days, the cyst is visible to the naked eye. In 3 months it can measure 3 to 5 cm in diameter. Compression and interaction of the endocyst with the adjacent host’s tissue produces a fibrous layer called the ectocyst or pericyst. By 5 months the pericyst is formed and a typical mature HC is present.

The pericyst is an integral part of both the liver and the parasite, and it is difficult to remove it from the liver. The parasite (endocyst) is separated from the pericyst (ectocyst) by a small capillary space and it can easily be detached from it (Figs. 2 and 3). The pericyst is largely avascular but there are spaces within it that enclose blood vessels and small bile ducts. The structure of the pericyst seems to be more complex than previously described. According to Chinese authors it is the result of two different formative processes and it consists of two different layers. The inner layer of the pericyst (the frank “exocyst”) is the result of an inflammatory and an unusual granulomatous reaction aimed at walling of the parasite and resorbing the released antigen. The granulomatous layer increases in size with growth of the parasite; it frequently contains osteopontin (regulates macrophage accumulation and calcium deposition) and resembles the granulomatous reaction to other infective agents (e.g., TBC). The outer layer of the pericyst is called the “adventitial” layer and it is composed of compressed Glissonian biliovascular elements and tributaries of the hepatic veins. Separating the “ectocyst” from the “adventitial” layer is the basis of “subadventitial cystectomy” pioneered by Chinese authors. There is no cleavage plane between the liver parenchyma and the “adventitia” and it cannot be readily separated. The pericyst can be 1 to 1.3 cm thick and it is not present in pulmonary and brain ECs. Vessels and ducts in the “adventitia” remain patent for a long time even in large cysts (hypervascular rim on computerized tomography [CT] scan). Calcification impedes the passage of nutrients and oxygen into the cyst.

A liver HC is typically unilocular. A developing cyst usually survives, with a steady
increase in size of about 1 to 1.5 mm in diameter per month, depending on the site and adjacent structures. The hydatid fluid in a vital cyst is under pressure (30 to 70 cm H₂O), and the volume of a human HC can be many liters.

The germinal membrane is of microscopic dimensions and is responsible for the production of the crystal-clear hydatid fluid, the ectocyst, brood capsules, scoleces, and the daughter cysts. The host and parasite in a HC are never physically connected. This means that a HC always separates from, or can be readily lifted out from within, a surgically exposed pericystic cavity (Fig. 3). When managing HCs, it must be assumed that any amount of hydatid fluid, no matter how clear, contains potentially infective protoscoleces.

Daughter cyst formation is considered a defense reaction and it is known as the process of endogenic vesiculation. Daughter cysts are true replicas of the mother cyst but without a pericyst. Daughter cysts produce multivesicular cysts that are more frequent in adults. The presence of daughter cysts is a problem for chemotherapy, protoscolicide activity, and the standard PAIR procedure (puncture–aspiration–injection–reaspiration). Ectogenic vesiculation of E. granulosus is infrequent. It occurs when there is a small rupture or defect in the laminated membrane and the germinal layer passes through and creates a “satellite” HC (Figs. 4 and 5).

Hydatid fluid is antigenic. This antigenicity is rarely of great clinical significance and the real incidence of allergic reactions in hydatidosis is not known. The incidence appears to be lower than previously described in literature. Allergic reactions range from skin rash to a frank anaphylactic reaction, usually following sudden rupture of the cyst. The antigenicity of hydatid fluid is the basis of serodiagnostics.

HCs in humans are generally long-standing and large, although not all grow to enormous proportions. Host defense mechanisms, injury, and the functional status of the pericyst are factors that determine cyst vitality, growth, and development. When a HC dies its wall can become calcified. Cyst vitality should not be estimated according to the calcification pattern. Dead cysts are generally no longer a threat to their host, but the issue of how to manage patients should take into consideration many factors.

The clinical features of liver hydatid disease depend on the site, size, number, vitality, and stage of development of the cyst. Simple, uncomplicated liver HCs are usually asymptomatic or present with nonspecific symptoms. Complicated liver HCs cause specific symptoms. If a patient has multiple HCs in the liver, the cysts are different in size, and any one of them can be the cause of symptoms. Most frequently it is the complicated cyst that causes the symptoms and signs of the disease.

I base my experience of this topic on 1,168 consecutive patients operated for
liver hydatidosis at the First Surgical Clinic, University of Belgrade School of Medicine, during January 1963 to December 2009. Dominant symptoms and signs of the patients are presented in Tables 1 and 2. Only one in three symptomatic patients had moderate to severe pain. The majority of the patients (79.88%), had uncomplicated cysts.

Symptomless, unsuspected HCs can be detected during routine examination or at autopsy. Clinical latency, judging by the size of the HCs at operation, is an outstanding feature of the disease. Adults contracting the disease can rapidly develop severe clinical manifestations. Children in endemic areas with large cysts can progress to “hydatid cachexia” (Tables 1 and 2).

The parasite survives because of a complex interaction with the host through the pericyst. It is not known why some cysts occasionally stop developing and growing. Involution and death of the parasite are probably the result of malfunction of the pericyst or some other unknown noxious factor. Involution of the parasite is protracted, can probably last for years, and is accompanied with gross structural changes. The final phase is partial or total calcification of the pericyst.