Echinococcus spp.(Pathogen – Tissue Cestode) (Cystic Disease)

Organism:
Currently, four species are recognized within the genus Echinococcus: E. granulosus (which causes cystic disease), E. multilocularis (which causes alveolar disease), E. vogeli (which causes polycystic disease), and E. oligarthrus (which causes polycystic disease).  The discussion will focus on E. granulosus. See comments on other forms below.

Life Cycle:
The adult worms of E. granulosus are very small (3 to 6 mm long) and consist of a scolex, neck, and only a single proglottid at each stage of development (immature, mature, and gravid). There may be several hundred worms in the intestine of the canine host (usually the dog). The worms may survive in the host for up to 20 months, and each gravid proglottid contains few eggs compared with some of the other tapeworms that are much larger. After the gravid proglottids and eggs are passed in the feces, they may be swallowed by an intermediate host, including humans. These eggs will hatch in the duodenum. The released oncospheres will penetrate the intestine and be carried via the bloodstream, where they will be filtered out in the various organs. The most common site in humans is the liver (60 to 70% of cases). Usually by the fifth month, the wall of the hydatid cyst has become differentiated into an outer friable, laminated, nonnucleated layer and an inner nucleated germinal layer. Various daughter cysts (brood capsules) bud off from the inner germinal layer and may remain attached or float free in the interior of the fluid‑filled cyst. The individual scolices bud off from the inner wall of the daughter cysts; these scolices and free daughter cysts are called hydatid sand. Each scolex will normally invaginate to protect the hooklets. Although not every cyst will produce daughter cysts and/or scolices, this general tissue organization is called a unilocular cyst, in which the cyst contents are held within a single limiting cyst wall.

Acquired:
Infection in humans is acquired through accidental ingestion of eggs from the tapeworm in the dog’s intestine.

Epidemiology:
The percentage of infected hosts varies throughout the world, but human infection is still much less common than infection of any of the reservoir hosts. The risk of infection depends on the association between humans and dogs. Those at high risk include populations where dogs are used to herd sheep and are also intimate members of the family, often having unrestricted access to the house and family members.  Cystic echinococcosis has been recorded in 21 of China’s 31 provinces, autonomous regions, and municipalities (approximately 87% of the territory).  Hydatid disease caused by E. granulosus is a zoonosis of major public health concern throughout Latin America, particularly in the Andean and South Cone regions.  Cystic echinococcosis is also widely found throughout the region comprising Arab North Africa and the Middle East.  In endemic areas around the world, the practice of giving raw viscera of slaughtered livestock to the dogs enhances transmission; however, in areas where this practice has been curtailed, prevalence figures have decreased.

Clinical Features:
Hydatid disease in humans is potentially dangerous; however, size and organ location will greatly influence the outcome. The majority of hydatid cysts occur in the liver, with symptoms that may include chronic abdominal discomfort, occasionally with a palpable or visible abdominal mass. If cysts are in a vital area or bone (osseous), even relatively small cysts can cause severe damage. Some unilocular cysts may remain undetected for many years, until they become large enough to crowd other organs. Cysts in the lungs are usually asymptomatic until there is cough, shortness of breath, or chest pain. 
During the life of the cyst, there may be small fluid leaks into the systemic circulation that sensitize the patient. Later on, if the cyst should burst or there is a large fluid leak, serious allergic sequelae, including anaphylactic reactions, may occur.  Release of cyst tissue may lead to abscess formation, emboli, and/or the development of additional young cysts at secondary sites. 

Clinical Specimen:
Radiologic studies:  Hydatid cysts should be considered in patients with abdominal masses with no clearly defined diagnosis. Although eosinophilia is present in 20 to 25% of cases, it is merely suggestive. Many asymptomatic cysts are first discovered after radiologic studies.
Serum:  Serologic tests are also very helpful, the standard test being the enzyme‑linked immunoelectrotransfer blot test.

Laboratory Diagnosis:
Radiologic studies:  Hydatid cysts should be considered in patients with abdominal masses with no clearly defined diagnosis. Although eosinophilia is present in 20 to 25% of cases, it is merely suggestive. Many asymptomatic cysts are first discovered after radiologic studies. The cyst will usually have a well‑defined margin with occasional fluid level markings. These studies can also be helpful in diagnosing osseous involvement. Scans may also demonstrate a space‑occupying lesion, particularly in the liver.  Once the cyst is discovered and surgical removal is selected as the approach, some of the cyst fluid can be aspirated and submitted for microscopic examination in order to detect the presence of hydatid sand, thus confirming the diagnosis. This procedure is definitely not without risk due to possible fluid and/or tissue leakage or dissemination. Cyst aspiration is usually performed at the time of surgery.  Hydatid sand may not always be present. Also, if the cyst is old, the daughter cysts and/or scolices may have disintegrated, so only the hooklets are left. These may be difficult to find and identify if there is debris within the cyst.  A drop of centrifuged fluid can be placed on a slide; another slide can be placed on top of the drop. The two slides can be rubbed back and forth over the fluid. The grating of the hooks on the glass may be felt and heard (hydatid sand sounds like glass grating on sand grains). If the individual scolices are intact, then routine microscopic examination of the centrifuged fluid as a wet mount will confirm the diagnosis. If the cyst is sterile (no daughter cysts or scolices), the diagnosis could be confirmed histologically
Serum:  Serologic tests are available, including an enzyme‑linked immunoelectrotransfer blot test which apparently offers greater sensitivity and specificity than do the enzyme‑linked immunosorbent assay (ELISA) and arc‑5 double‑diffusion assay (DD5); when the tests were run simultaneously, the greatest number of cases was detected by using a combination of the enzyme‑linked immunotransfer blot (EITB) and DD5 tests. Newer EIA procedures appear to provide greater than 90% sensitivity and specificity when compared to the EIBT.

Organism Description:
Hydatid cyst: The cyst will usually have a well‑defined margin with occasional fluid level markings. These studies can also be helpful in diagnosing osseous involvement. Scans may also demonstrate a space‑occupying lesion, particularly in the liver.

Hydatid cysts on x-ray and scans

Laboratory Report:
Echinococcus granulosus. Hydatid sand and/or hooklets present OR serology results indicated (with interpretation)

Treatment:
A number of options are available for treatment of cystic echinococcosis, including surgery, puncture aspiration injection reaspiration (PAIR), and chemotherapy.  Surgery is generally considered the treatment of choice for a complete cure, although this approach is limited to unilocular cysts in operable body sites.  In cases where multiple cysts are present in several different sites or in patients with a high surgical risk, PAIR and/or chemotherapy are considered appropriate options.
During the last few years, experience with the benzimidazole derivatives mebendazole and albendazole suggests that under certain circumstances, chemotherapy may replace surgery.  This approach is recommended for patients with inoperable cystic disease and those with multiple cysts in several body sites; however, chemotherapy for bone disease is less effective.  Although relapses after chemotherapy have been reported, patients are usually sensitive to retreatment.  In general, side effects are mild.
Garcia, L.S. 2007.  Diagnostic Medical Parasitology, 5th ed., ASM Press, Washington, D.C.

Control:
Improved control of adult dog tapeworm prophylaxis, elimination of feeding sheep entrails to dogs.
 

Echinococcus multilocularis (Alveolar Hydatid Disease)
For many years, pathologists recognized the difference between the unilocular type of human hydatid cyst and the alveolar form. Some workers believed that the alveolar form was a variant species, while others thought that it might arise from an early unilocular cyst. Investigations in Alaska and Germany served to define the characteristics of alveolar hydatid disease caused by Echinococcus multilocularis. Alveolar hydatid disease is the most lethal of helminthic diseases, with radical surgery still being the only curative therapy. However, resection has been possible in only 25 to 57% of patients. Most cases are diagnosed in rural residents, suggesting direct or indirect contact with fecal material from infected foxes or dogs. The spread of infections and increasing numbers of documented cases in the former USSR, China, Japan, and Alaska are causing concern among public health personnel in these regions.  There is some variation among isolates from North America and Eurasia; however, there appear to be no actual genetic strain differences
Although the alveolar form of hydatid disease has been found in other tissues, the liver is the most common site. The disease may resemble a slowly growing carcinoma and may present symptoms of intrahepatic portal hypertension.  In humans there are several stages of the infection; they have been described as:  initial, progressive, advanced, stability, and abortive course).

Echinococcus multilocularis in tissue and in scans

Echinococcus oligarthrus and Echinococcus vogeli (Polycystic Hydatid Disease)
E. oligarthrus and E. vogeli are the only indigenous species known in the neotropical area of the New World. The E. oligarthrus life cycle involves wild felids such as pumas, jaguars, and wild cats as definitive hosts, while the intermediate hosts harboring the larval forms (hydatid cysts) include the paca, agouti, spiny rats, and opossums. Further studies to determine which other wild animals might be involved in the life cycle are under way. The definitive hosts of E. vogeli include the bush dog, while the intermediate hosts are the same as for E. oligarthrus. 

Multiple cysts in tissue; protoscolex hooklets; chest x-ray showing polycystic structures