Vol. 17, No. 19
THE JOHNS HOPKINS MICROBIOLOGY NEWSLETTER
Monday, May 11, 1998
A. Provided by Marguerite-Hawkins, Epidemiology and Disease Control Program, Division of Outbreak Investigation, Maryland Department of Health and Mental Hygiene.
From May 1, 1998 through May 7, 1998, 7 outbreaks have been reported to the Maryland Department of Health and Mental Hygiene. They are as follows: 5 food-related gastroenteritis, 1 presumed viral gastroenteritis and 1 scabies
B. The Johns Hopkins Hospital. Information provided by Dr. Edward Weir, Dept. of Pathology.
Patient Clinical History: A 60 year old woman from Saudi Arabia presented for evaluation of a palpable abdominal mass in the right upper quadrant. Abdominal films revealed a 7.5 x 4.0 cm solid and cystic lesion with focal calcifications in the right hepatic lobe. Ultrasound-guided biopsy demonstrated multiple calcified scolices. After the procedure, the patient experienced right upper quadrant pain, SOB, itching, nausea and vomiting. She remained hemodynamically stable. The patient was admitted for treatment of anaphylaxis and observation. Serology for Echinococcus was positive. After an adequate course of albendazole, the patient underwent surgical excision of a unilocular cyst. Histologic examination of the surgical specimen was consistent with a hydatid cyst of Echinococcus granulosus.
Genus Echinococcus: Echinococcus is a tapeworm of the order Cyclophyllidea and is morphologically characterized by a scolex with four suckers and a double row of 30-36 hooklets. There are four documented species, two of which are associated with hydatid disease in man, E. granulosus and E. multilocularis. All species of the genus Echinococcus are adapted to hosts that have an obligatory predator-prey relationship. The percentage of infected hosts varies throughout the world. Areas of endemicity of growing concern are the sheep- and cattle-herding regions of Argentina, Uruguay, Brazil, Iran, New Zealand, and Australia. The definitive host is usually the domestic dog, the wolf, and occasionally the fox and other canines. There may be several hundred worms in the small intestine of the canine host. After eggs are passed in the feces, they may be ingested by an intermediate host such as sheep, cattle, camels, moose, caribou, and humans. The eggs will hatch in the duodenum, penetrate the bowel wall, and gain access to the bloodstream. Many of the larvae are phagocytosed and destroyed in various organs. 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. Daughter cysts (brood capsules) bud off from the inner germinal membrane and float free in the cyst fluid. Individual scolices bud from the inner layer of the daughter cysts; these scolices and associated free daughter cysts are called hydatid sand. Each scolex will normally invaginate to protect the hooklets. Although not every cyst produces daughter cysts, this general organization is called a unilocular hydatid cyst and is typical of E. granulosus infection. In contrast, the cyst produced by E. multilocularis is multilocular (alveolar) which lacks a limiting membrane and whose larval forms in tissue are usually sterile, that is, without scolices.
Clinical Disease: Hydatid disease in humans is potentially dangerous; however, the size and organ location of the cyst greatly influence prognosis. The majority of hydatid cysts occur in the liver (70%) where they may cause abdominal discomfort and a palpable or visible mass. Cysts occuring in the lung (20%) are usually asymptomatic but may produce shorten of breath and a cough as they progressively enlarge. Hydatid cysts have also been reported to cause tissue damage by mechanical means at other sites such as kidney, brain, spleen, bone, and soft tissues. During the life of the cyst, there may be small fluid leaks into the systemic circulation that sensitize the patient. Later, if the cyst should burst or if there is a large fluid leak, serious allergic sequelae, including anaphylactic reactions may occur. Additional complications of a ruptured cyst may include abscess formation, arterial embolization, and dissemination to distant sites.
Diagnosis: Echinococcal cysts should be considered in patients with abdominal masses with no clearly defined diagnosis. Although eosinophilia is present in 20 to 25% of patients, it is merely suggestive. Many asymptomatic cysts are first discovered after radiologic studies. The cyst will usually have a well-defined margin and occasional fluid level markings. Infection suspected on imaging studies may be confirmed with serologic testing. Specific ELISA, enzyme immunoassay precedures, and hydatid antigen immunobinding assays (available at the CDC) appear to provide approximately 90% sensitivity and specificity for liver cysts, but lesser sensitivity for lesions in the lung and other tissues. Many assays are currently being studied to improve specificity for diagnosis. Imaging studies remain more sensitive than serodiagnosis, and a characteristic scan in the face of negative serology should still suggest the diagnosis of echinococcosis.
Treatment: Surgery is generally considered the treatment of choice, although this approach is limited to unilocular cysts in operable body sites. Installation of a cysticidal agent (hypertonic saline, iodophor, or ethanol) has been used intraoperatively to kill the germinal layer and daughter cysts. It may be necessary to use perioperative antihelmintics (albendazole, mebendazole) to limit the risk of intraoperative dissemination. Medical therapy for inoperable cysts has provided improvement in some and a cure in a minority of patients. Serum IgE may be a useful marker of therapeutic success in patients with pretreatment specific IgE antibodies. Multilocular cyst disease caused by E. multilocularis is more aggressive with invasion of the tissue in a tumor-like fashion. Complete surgical excision is extremely difficult since the cyst does not have a limiting capsule. The overall prognosis is usually grave with a E. multilocularis infection, however, spontaneous cyst death has been reported. Prevention of hydatid disease includes worming infected dogs, burning infected carcasses, and careful hand-washing after contact with potentially infected dogs.
References
1. Garcia LS, et al. Diagnostic Medical Parasitology. 3rd edition. Washington DC: ASM Press, 1997.
2. Shantz PM, etal. Echinococcosis (hydatidosis). In Warren KS, eds. Tropical and Geographical Medicine. 2nd edition. New York: McGraw-Hill, 1990: 505.
3. Force L, et al. Evaluation of eight serologic tests in the diagnosis of human echinococcosis and follow-up. Clin Infect Dis. 1992, 15:473-80.