Hydatid Cysts

Updated: Sep 12, 2016
  • Author: Imad S Dandan, MD; Chief Editor: BS Anand, MD  more...
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Overview

Background

Hydatid disease is a parasitic infestation by a tapeworm of the genus Echinococcus. It is not endemic in the United States, but the change in the immigration patterns and the marked increase in transcontinental transportation over the past 4 decades have caused a rise in the profile of this previously unusual disease throughout North America. This has led to the necessity for physicians to be more aware of its clinical features, diagnosis, and management.

Patient education is aimed at teaching the population at risk about the disease, its methods of transmission, its hosts, and the methods of prevention.

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Pathophysiology

Human echinococcosis is a zoonotic infection caused by the tapeworm of the genus Echinococcus. Of the 4 known species of Echinococcus, 3 are of medical importance in humans. These are Echinococcus granulosus, causing cystic echinococcosis (CE); Echinococcus multilocularis, causing alveolar echinococcosis (AE); and Echinococcus vogeli. E granulosus is the most common of the three. E multilocularis is rare but is the most virulent, and E vogeli is the most rare.

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Etiology

Echinococcosis is caused by larval cestodes of the phylum Platyhelminthes (tapeworms).

Their life cycle involves only 2 hosts, one definitive and the other intermediate. Humans act as an accidental intermediate host. The life cycle has 3 developmental stages, (1) the adult tapeworm in the definitive host, (2) eggs in the environment, and (3) the metacestode in the intermediate host. Metacestodes are ingested by the definitive host. The metacestodes mature into the tapeworm in the definitive host and, in turn, release eggs into the environment. The intermediate host ingests the eggs, which hatch into metacestodes, which infest the liver, lungs, muscles, and other organs of the intermediate host. [1]

Two biological forms of E granulosus have been recognized (depending on the geographic location and type of the intermediate host), (1) the northern type and (2) the European type, as follows:

  • The northern type is maintained in the tundra by a predator-prey relationship between the wolf and large deer, but dogs and coyotes can also become infested. Humans become infested in areas where reindeer are domesticated.
  • Intermediate hosts for the European type include camels, pigs, sheep, cattle, goats, horses, and many other animals. The definitive host for the European biotype is overwhelmingly the dog, but it also occurs in foxes, hyenas, and jackals. This is the most common biotype. The adult stage of  E multilocularis occurs mainly in foxes and rarely in wolves, coyotes, lynxes, cats, and black bears.

The intermediate hosts for E multilocularis are 8 families of rodents, including mice, rats, hamsters, gerbils, and squirrels.

E vogeli is a neotropical species maintained in the bush dog and the paca. It can easily infect other mammals that are exposed to its feces. It is the most rare form of the echinococci.

Exposure to food and water contaminated by the feces of an infected definitive host or poor hygiene in areas of infestation can lead to echinococcosis.

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Epidemiology

United States data

Despite an increase in its occurrence, echinococcosis remains a very rare disease (< 1 case per 1 million inhabitants) in the continental United States. Northern Alaska has endemic areas of E granulosus, but the frequency of infection remains low (< 1 case per 100,000 inhabitants).

International data

Echinococcosis is also unusual in northern Europe. The endemic areas are the Mediterranean countries, the Middle East, the southern part of South America, Iceland, Australia, New Zealand, and southern parts of Africa; the latter 5 are intensive endemic areas. Central Asia, particularly China, is also an endemic area. [2, 3] The incidence of CE in endemic areas ranges from 1-220 cases per 100,000 inhabitants, while the incidence of AE ranges from 0.03-1.2 cases per 100,000 inhabitants, making it a much more rare form of echinococcosis. An epidemiological study of cyctic echinococcosis in Romania emphasized the need for improved preventive measures. [4] Infestation with E vogeli is the most rare form of echinococcosis and is reported mainly in the southern parts of South America.

Sex-, age-, and race-related demographics

No sexual predilection is recognized for hydatid cysts.

Because of the restricted geographic distribution of the echinococcal worms, persons of certain races are affected more commonly than others; however, the parasite has the capability of infecting persons of all races equally.

The cysts grow slowly, and a cyst is rarely diagnosed during childhood or adolescence unless the brain is affected. CE is a disease of younger adults, with an average age at diagnosis of 30-40 years, whereas AE is a disease of older adults, with an average age at diagnosis of older than 50 years.

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Prognosis

Prognosis mainly depends on the type of infestation (ie, whether it is CE or AE). In CE, the prognosis is generally good, and complete cure is possible with total surgical excision without spillage. Spillage occurs in 2-25% of cases (depends on location and surgeon's experience), and the operative mortality rate varies from 0.5-4% for the same reasons.

In AE, the prognosis is much worse. Cure is only possible with early detection and complete surgical excision. In patients in whom the latter is not possible, the addition of long-term chemotherapy has decreased the 10-year mortality rates from 94% to 10%.

Mortality/morbidity

Morbidity is usually secondary to free rupture of the echinococcal cyst (with or without anaphylaxis), infection of the cyst, or dysfunction of affected organs. Examples of dysfunction of affected organs are biliary obstruction, cirrhosis, bronchial obstruction, renal outflow obstruction, increased intracranial pressure secondary to a mass, and hydrocephalus secondary to cerebrospinal fluid outflow obstruction.

In CE, mortality is secondary to anaphylaxis, systemic complications of the cysts (eg, sepsis, cirrhosis, respiratory failure), or operative complications.

In clinical cases of AE, the mortality rate is 50-60%. This figure reaches 100% for untreated or poorly treated AE. Sudden death has been reported with AE in asymptomatic patients (autopsy diagnosis).

Complications

All the usual complications related to the surgical procedure and anesthesia apply.

Complications related to the parasite include the following:

  • Recurrence
  • Metastasis
  • Infection
  • Spillage and seeding (secondary echinococcosis) - Allergic reaction or anaphylactic shock

Those related to the medical treatment include the following:

  • Hepatotoxicity
  • Anemia
  • Thrombocytopenia
  • Alopecia
  • Embryotoxicity
  • Teratogenicity
  • Spillage and seeding (secondary echinococcosis)

Complications related to PAIR include the following:

  • Hemorrhage
  • Mechanical damage to other tissues
  • Infections
  • Allergic reaction or anaphylactic shock
  • Persistence of daughter cysts
  • Sudden intracystic decompression leading to biliary fistulas

Those related to scolicidal agents includes chemical sclerosing cholangitis.

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