Echinococcosis, or “hydatid disease,” is a tissue infection of humans caused by larvae of Echinococcus granulosus and E multilocularis. The former is a more common cause of human disease.
PARASITOLOGY AND LIFE CYCLE
The adult E granulosus inhabits the small bowel of dogs, wolves, and other canines, where it survives for a scant 12 months. The scolex possesses four sucking disks and a double row of hooklets. The entire strobila, however, measures only 5 mm in length, and contains but three proglottids: One immature, one mature, and one gravid. The latter segment splits open either before or after passage in the stool, releasing eggs that appear identical to those of T saginata and T solium. A number of mammals may serve as intermediates, including sheep, goats, camels, deer, caribou, moose, and—most importantly—humans. When one of these hosts ingests eggs, they hatch, releasing embryos that penetrate the intestinal mucosa and are then carried via the portal blood to the liver. There, many are trapped in the hepatic sinusoids. The rest traverse the liver and are carried to the lung, where they may lodge. A few pass through the pulmonary capillaries, enter the systemic circulation, and are carried to the brain, heart, bones, kidneys, and other organs. Many of the larvae are phagocytosed and destroyed by host immune cells. The survivors form a cyst wall composed of an external laminated cuticle and an internal germinal membrane. The cyst fills with fluid and slowly expands, reaching a diameter of 1 cm over the next 5 to 6 months (Figure 56–7). However, they may grow substantially larger in subsequent months and years, in some cases reaching diameters greater than 10 cm. In time, secondary “brood capsules” arise from the germinal layer and form within the original hydatid, or break through the cyst surface to form new “daughter cysts.” Within these brood capsules and daughter cysts, new protoscolices develop from the germinal lining. Degenerated protoscolices and germinal membranes fall to the bottom of the cyst to form hydatid “sand.” When hydatid-containing tissues of the intermediate host are ingested by a canine, scolices are released in the intestine where they develop into adult worms. The life cycle is illustrated in Figure 56–8.
Echinococcosis. A. Echinococcal cyst in lung with white lining membrane. B. Echinococcal cyst wall with lung parenchyma below and five scolices above. (Reproduced with permission from Connor DH, Chandler FW, Schwartz DQ, et al: Pathology of Infectious Diseases. Stamford, CT: Appleton & Lange, 1997.)
Life cycle of Echinococcus species. [Redrawn from Centers for Disease Control and Prevention (CDC).]
Adult in small intestine of canines
✺ Herbivores and humans serve as intermediate hosts
Larvae penetrate gut to reach portal or systemic circulation
✺ Cysts and daughter cysts develop in tissues
Cycle completed when canines ingest cysts
There are two major epidemiologic forms of E granulosus-induced echinococcosis: Pastoral and sylvatic. The more common pastoral form has its highest incidence in Australia, New Zealand, South and East Africa, the Middle East, Central Europe, and South America, where domestic herbivores such as sheep, cattle, and camels are raised by people in close contact with domestic dogs. Although approximately 200 human cases are reported each year in the United States, most were acquired elsewhere. Indigenous cases have been reported, however, particularly among Basque sheep farmers in western states and Native Americans in the southwest. Animal husbandry practices that permit dogs to feed on the raw viscera of slaughtered sheep perpetuate cycle. Transmission also depends on suboptimal hand hygiene, in that shepherds become infected while handling their dogs: Eggs are transferred from dog feces to their fur, where they are relocated onto their masters’ hands and later ingested. Sylvatic echinococcosis, in contrast, is found principally in Alaska and western Canada, where wolves act as the definitive hosts and moose or caribou are the intermediates. In two counties in California, a second sylvatic cycle involving deer and coyotes has been described. When hunters kill these wild deer and feed their offal to accompanying dogs, a pastoral cycle may be established.
✺ Hand-to-mouth infection of humans after dog contact
Pastoral infections maintained by allowing dogs to feed on sheep viscera
Sylvatic cycle in Alaska and western Canada
ECHINOCOCCOSIS: CLINICAL ASPECTS
The enlarging E granulosus cysts produce tissue damage by mechanical means. The clinical presentation depends on their number, location, and rate of growth. Typically, a latent period of 5 to 20 years occurs between acquisition of infection and subsequent diagnosis. Intervals as long as 75 years have been reported.
✺ Disease caused by mechanical effects of cysts after many years
The majority of cysts are found in the liver, and/or in the lung. One-fifth of all patients show involvement of multiple sites. Many patients are asymptomatic when the lesion is discovered on routine imaging or physical examination. Occasionally, the patient may present with hemoptysis, pain in the right upper quadrant of the abdomen, or a tender hepatic mass. Significant morbidity is uncommon, and death rare. However, hydatid cysts may reach enormous size (Figure 56–7B). They may eventually rupture, inducing fever, pruritus, urticaria, and—at times—anaphylactic shock. Germinal tissue or brood capsules may also spread to other areas, leading to dissemination of the infection. Rupture of pulmonary lesions also induces cough, chest pain, and hemoptysis. Liver cysts may break through the diaphragm or rupture into the bile duct or peritoneal cavity. Most patients who develop symptoms, however, present with a tender, palpable hepatic mass. Intrabiliary extrusion of calcified cysts may mimic the signs of acute cholecystitis; complete obstruction results in jaundice. Bone cysts produce pathologic fractures. Lesions in the CNS may manifest with blindness or seizures. Cardiac lesions have been associated with conduction disturbances, ventricular rupture, and embolic metastases. It has been suggested that circulating antigen–antibody complexes may deposit in the kidney, initiating membranous glomerulonephritis.
✺ Many patients asymptomatic
Cysts may attain large size
✺ Rupture leads to hypersensitivity manifestations and dissemination
In E granulosus-infected patients, chest X-rays may demonstrate pulmonary lesions as slightly irregular, round masses of uniform density, often devoid of calcification. In contrast, more than one-half of hepatic lesions display a smooth, calcific rim. CT, ultrasonography, and MRI may reveal either a simple fluid-filled cyst or daughter cysts with hydatid sand. Endoscopic retrograde cholangiography has been valuable for determining cyst location and possible communication with the biliary tree. Because of the potential for an allergic reaction or spread of infection, diagnostic aspiration may be contraindicated. Nevertheless, in select cases, ultrasonically guided percutaneous drainage, followed by the introduction of hypertonic saline to kill protoscolices and germinal layer, has proved safe and useful both diagnostically and therapeutically (see PAIR later). In patients with ruptured pulmonary cysts, scolices may be demonstrated in the sputum.
✺ Radiologic and scanning appearance characteristic
✺ Routine cyst puncture may be perilous
In some cases, confirmation of the diagnosis before drainage requires serologic testing. Unfortunately, current options are not totally satisfactory. Indirect hemagglutination and latex agglutination tests are positive in 90% of patients with hepatic lesions and 60% of those with pulmonary hydatid cysts. Polymerase chain reaction assay has been shown to be capable of detecting picogram quantities of Echinococcus genomic DNA in fine-needle biopsy material from patients with suspected echinococcosis.
Serologic diagnosis important, but needs improved sensitivity
For years, the only definitive therapy available was surgical extirpation. Patients with pulmonary hydatid cysts of the sylvatic type and small calcified hepatic lesions underwent surgery only when they became symptomatic or the cysts increased dramatically in size over time. However, for uncomplicated lesions, Puncture, Aspiration, Infusion of scolicide, and Reaspiration (PAIR) can be used in lieu of surgery. The scolicide of choice is probably hypertonic saline, although other chemicals have been used. If performed properly, this technique is safer and better tolerated than open surgery. Presently, it is recommended that high-dose albendazole be administered before and for several weeks (or years in the case of E multilocularis infection) after surgery and/or aspiration. Infected dogs should be dewormed, and infected carcasses and offal burned or buried. Hands should be carefully washed after contact with potentially infected dogs.
✺ Treatment may include PAIR with concomitant albendazole
A shepherd is shocked to learn that he has hydatid disease, because he is a lifelong vegetarian who has never consumed undercooked animal organ meat. How do you explain this?
Think ➪ Apply 56-2. Presumably, he acquired hydatid disease from the feces of his sheepdog, which in turn was infected while eating livestock offal filled with cysts. Dogs get worms by eating cysts; humans get cysts by consuming eggs in dog stool.
Echinococcus multilocularis is found primarily in subarctic and arctic regions in North America, Europe, and Asia. The adult worms are found in the gut of foxes and, to a lesser extent, coyotes. Their larval forms find harborage in the tissues of mice and voles, the rodent prey of canines. Domestic dogs may acquire adult tapeworms by killing and ingesting these larva-infected sylvatic rodents. Humans are infected with larval forms through the ingestion of eggs passed in the feces of their domestic dogs or ingestion of egg-contaminated vegetation. Unlike the larval forms of E granulosus, those of E multilocularis bud externally, producing proliferative, multilocular cysts that slowly but progressively invade and destroy the affected organs and adjacent tissues.
Foxes main definitive hosts
Larvae bud externally; produce multilocular cysts
The clinical course in humans is characterized by epigastric pain, obstructive jaundice, and, less frequently, metastasis to the lung and brain, thus closely mimicking liver cancer. As with E granulosus, medical treatment of E multilocularis often fails to achieve cure. Patients with multilocular infection may require surgical management. The prognosis is grim if not diagnosed early.