Echinococcosis

Definition

Echinococcosis (Hydatid disease) refers to human infection by the immature (larval) form of tapeworm, Echinococcus. One of three forms of the Echinococcus spp., E. granulosus, lives on dogs and livestock, and infects humans through contact with these animals. Allergic reactions and damage to various organs from cyst formation are the most common forms of disease in humans.

Description

E. granulosus is found in many areas of Africa, China, South America, Australia, New Zealand, and Mediterranean and eastern Europe, as well as in parts of the western United States. The parasite lives in regions where dogs and livestock cohabitate. Direct exposure to infectious dogs, as well as parasitic eggs released into the environment during shedding, are both sources of human infection.

In humans, cysts containing the larvae develop after ingestion of eggs. Cysts form primarily in the lungs and liver. Cysts developing in the liver are responsible for about two-thirds of echinococcosis cases. Echinococcosis is a significant public health problem in many areas of the world, but control programs have decreased the rate of infection in some regions. In Kenya alone, the numbers of persons infected each year is as high as 220 per 100,000 population.

Causes and symptoms

After ingestion, the eggs develop into embryos within the intestines and then travel to the liver and lungs through major blood vessels. The embryos then begin to form cysts within the liver and lungs, causing damage as they enlarge over a period of five to 20 years. Cysts may become over 8 in (20.3 cm) or more in size and contain a huge amount of highly allergenic fluid. Studies show that while the liver is most often targeted, lungs, brain, heart, and bone can also be affected.

The major symptoms are due to compression damage, blockage of vessels and ducts (such as the bile ducts), and leakage of fluid from cysts. The following symptoms are frequent.

• Liver involvement causes pain and eventually jaundice or cholangitis due to blockage of bile ducts. Infection of cysts leads to abscesses in up to 20%.
• Lung cysts cause cough and chest pain.
• Bone cysts cause fractures and damage to bone tissue.
• Heart involvement leads to irregularities of heart beat and inflammation of the covering of the heart (pericardium).
• Allergic reactions occur from leakage of cyst fluid that contains antigens. Itching, fever, and rashes are frequent, and fatal allergic reactions (anaphylaxis ) have been reported. Eosinophils, which are blood cells involved in allergic reactions, are increased in many patients.

Diagnosis

X rays, computed tomography scans (CT scans), and ultrasound are very helpful in detecting cysts. Some cysts will develop characteristic hardening of organ tissues from calcium deposits (calcifications). Blood tests to detect antibodies are useful when positive, but up to 50% of patients have negative results. Examination of aspirated cyst fluid for parasites can be diagnostic, but carries the danger of a fatal allergic reaction. Treatment with anti-parasitic medications before aspiration is reported to decrease allergic complications and decrease the risk of spread during the procedure.

Treatment

Treatment depends on the size and location of cysts, as well as the symptoms they are producing. Surgical removal of cysts and/or surrounding tissue is the accepted method of treatment, but carries a risk of cyst rupture with spread or allergic reactions. Recent studies using medication alongside aspiration and drainage of cysts instead of surgery are very encouraging.

The medication albenzadole can be taken before or after surgery or alone without surgery. However, its effectiveness as a single treatment is still not known. Multiple courses of medication are often necessary, with cure rates of only about 30%. Response to treatment is best monitored by serial CT scans or similar x-ray studies.

Prevention

Good hand washing, treating infected dogs, and preventing dogs' access to slaughter houses discourage spread of the disease. Limiting the population of stray dogs has also been helpful.

Resources

OTHER

"Percutaneous Drainage Compared with Surgery for HepaticHydatid Cysts." New England Journal of Medicine Online. 〈http://content.nejm.org〉.

KEY TERMS

Allergenic— A substance capable of causing an allergic reaction.

Cholangitis— Infection or inflammation of the bile ducts; often causes abdominal pain, fever, and jaundice.

Computed tomography (CT) scan— A specialized x-ray procedure in which cross-sections of the area in question can be examined in detail.

Cyst— A protective sac that includes either fluid or the cell of an organism. The cyst enables many organisms to survive in the environment for long periods of time without need for food or water.

Embryo— The very beginning stages of development of an organism.

Jaundice— The yellow-greenish coloring of the skin and eyes due to the presence of bile pigments. The presence of jaundice is usually, but not always, a sign of liver disease.

Tapeworm— An intestinal parasite that attaches to the intestine or travels to other organs such as the liver and lungs.

Ultrasound— A noninvasive procedure based on changes in sound waves of a frequency that cannot be heard, but respond to changes in tissue composition.