Cystic echinococcosis

Abstract

Cystic echinococcosis (CE)\cystic hydatid disease is one of the most widespread and important global helminth zoonosis. The parasite Echinococcus granulosus, continues to be a cause and consequence of poverty through its impact on public health and animal production. Human behaviour is largely responsible for the perpetuation of the parasite which flourishes in low socio-economic regions of the world where there are poor veterinary and medical facilities and where home slaughter of livestock is performed and dogs go untreated. The parasite is maintained in a wide spectrum of intermediate hosts, including sheep, goats, camels, cattle, pigs and equines. A number of wild intermediate hosts occur, including cervids in the northern part of the North American continent and Eurasia, marsupials in Australia and wild herbivores in East and southern Africa. The application of a range of molecular techniques to the characterization of the parasite has confirmed the existence of mostly host-adapted strains and genotypes of the parasite. Several new species have been proposed which include: the sheep strain (E. granulosus, sensu stricto, G1, G2, G3), the horse strain (E. equinus, G4), the cattle strain (E. ortleppi, G5), the camel (G6) and pig (G7) strain (E.intermedius, G6, G7, G9), the cervid strain (E. canadensis, G8, G10) and the lion strain (E. felidis). All strains except the horse and lion strain are known to be infective to humans. The ubiquitous domestic dog serves as the most important definitive host for the transmission of the parasite throughout its wide geographical range. Lions, hyenas and jackals also serve as definitive hosts in the wildlife cycles in Africa, wolves, foxes and jackals in the northern hemisphere and dingoes in Australia.
A wide range of diagnostic techniques, including necropsy, arecoline purgation, coproantigen ELISA and DNA based tests are available for detecting E. granulosus infection in the definitive host. The technique used will depend on the prevailing circumstances. Unfortunately this plethora of tests is not available for intermediate animal hosts where diagnosis at post mortem still remains the most reliable option. In humans, imaging techniques including ultrasound, nuclear magnetic resonance (NMR) or computer aided tomography (CAT-scan provide not only a method of diagnosis but also reveal important clinical information on the location, condition, number and size of the hydatid cysts in man. The properties of ultrasound (little patient preparation time, no ionizing radiation, portable equipment, cost and wide application in medicine) has made it the most widely used diagnostic technique and is the only imaging technique for screening of populations in rural areas, where the disease is most common. With the increasing use of ultrasound for the diagnosis and screening of CE a classification system has been developed which can be used to assess the likely development of a cyst and hence guide the clinician in treatment options for the patient. The present international classification system is based on the evaluation of the WHO- Informal Working Group on Echinococcosis. This group also updated the recommendations for diagnosis and treatment of CE in 2009. As a result of a multidisciplinary approach treatment relies on surgery and/or percutaneous interventions, especially “Puncture, Aspiration, Injection, Re-aspiration”(PAIR) and/or antiparasitic treatment with albendazole (and alternatively mebendazole). The treatment protocol is based on cyst type and number, organ location and presence/absence of complications. A proper follow-up of the patients should assess the efficacy of the treatment, detect treatment complications, and timely disclose recurrences, which are not unusual, and are a major complication of the disease.
CE is largely a preventable disease. Elimination programmes have focused on frequent periodic treatments of dogs with anthelmintics and the control of slaughter of domestic livestock. This has resulted in the elimination of the parasite from a number of island nations, including, New Zealand, Tasmania, Iceland and southern Cyprus. In many regions elimination or even control remains a problem as the parasite is endemic over vast areas of low income countries where there may be limited resources for control. In some areas, such as former communist administered countries, the parasite is resurgent. New tools are becoming available to control the parasite, including a highly effective vaccine in sheep which prevents the infection in sheep and breaks the transmission cycle. In addition cost effective methods are being developed which may be appropriate in low income countries where financial resources are not available for intensive control programmes that have been successful in high income countries.