**Echinococcosis/Hydatidosis**

Antoni Soriano Arandes and Frederic Gómez Bertomeu

*University Hospital Joan XXIII of Tarragona Spain* 

#### **1. Introduction**

298 Current Topics in Tropical Medicine

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Echinococcosis is a zoonotic infection caused by adult or larval (metacestode) stages of cestodes belonging to the genus *Echinococcus* and the family *Taeniidae*. Life cycles imply two mammal hosts. Definitive hosts are carnivores containing adult forms in the gut. The infection is acquired by the intermediate hosts and humans after the ingestion of eggs from the feces of carnivorous definitive hosts, which harbour the adult egg-producing stage in the intestine. Eggs ingested by infected human develop into the infective metacestode stage causing various forms of Echinococcosis. The disease in humans and intermediate hosts is called Hydatidosis and is characterized according to the morphologic features of the larval stages: cystic echinococcosis (CE) caused by *Echinococcus granulosus* and related organisms, alveolar echinococcosis (AE) caused by *Echinococcus multilocularis*, and polycystic echinococcosis (PE) caused by *Echinococcus vogeli* or *Echinococcus oligarthrus*.

This disease is becoming an important public health problem in many parts of the world where dogs are used for cattle breeding. Control measures are unable to be implemented everywhere, and where control programs were initiated the success of them have been incomplete generating a re-emergence of the disease. This has also lead to the interruption of control measures excluding Echinococcosis from the list of notifiable diseases. As a consequence of this the incidence and prevalence of CE in Mediterranean countries in humans and animals are not known (Dakkak, 2010). Also, there are a number of factors that contribute to the increase of prevalence and to the spreading of CE in the Mediterranean Region. Cyprus is the only country where an eradication programme has been successfully implemented. However, CE has obtained important developments in the last decade, in the epidemiology, in the diagnosis of canine infection, in strain characterisation and in immune strategies against CE in animals. This scientific progress, together with effective health education programmes, will likely improve control programmes and reduce the time required to achieve significant decreases in prevalence or eradication. Thus, European authorities recognized, through the directive 2003/99/CE, Echinococcosis as a disease to be reported to the European Food Safety Authority (EFSA).

The "WHO/OIE Manual on Echinococcosis in Humans and Animals: a Public Health Problem of Global Concern", published in 2001, has been used as a fundamental issue for the redaction of this chapter.

#### **2. Echinococcosis**

The first part of this chapter is dedicated to explain the etiology to know the taxonomy and the life transmission cycles which perpetuate the agent in nature. The biology of the

Echinococcosis/Hydatidosis 301

months at lower ranges of temperatures (from +4ºC to +15ºC). However, they are very

Intermediate hosts are represented by a wide range of mammals which acquire the infection by the ingestion of eggs. The oncosphere is released from the keratinised embryophore in the stomach and small intestine. Bile activates the oncosphere which penetrates the wall due to the hook movements and secretions and arrives to the liver where some of them are retained (figure 2). All mammals (including man) in which metacestodes of *Echinococcus*  species develop after infection with eggs may be referred to as 'intermediate hosts'. However, man is an aberrant host because metacestode stages do not become fertile in this host or because does not interact in the transmission cycle. Once the oncosphere has reached its final location, it develops into the metacestode stage. Time of development is variable and it may take several months before protoscoleces are produced (fertile metacestode). There may be several thousand protoscoleces within a single cyst of *E. granulosus* or an aggregation of vesicles of *E. multilocularis.* Each single protoscolex is capable of developing into a sexually mature adult worm. Not all metacestodes produce protoscoleces (sterile

Fig. 2. Life-cycle of Echinococcus (WHO/OIE publication: Manual on echinococcosis in

Strain identification is possible for all four species of *Echinococcus* using morphological and biological features and/or molecular techniques, such as sequence comparison of a 366 bpfragment of the mitochondrial cytochrome oxidase subunit 1 DNA (CO1) and a 471 bpregion in the mitochondrial NADH dehydrogenase gene 1 (ND1), by analysis of a ribosomal (r) DNA fragment (1ST2) or by the random amplified polymorphic DNA-PCR (RAPD-PCR). Recent genetic studies have principally confirmed the concept of strain diversity within the

sensitive to desiccation and to high temperatures as 60-80ºC.

metacestode).

humans and animals)

causative agents of various forms of Echinococcosis is faced because it can help to understand the maintaining of these parasites in different geographic areas. Following to this part the epidemiology and clinical presentation forms are assessed. Finally, diagnosis, treatment and prevention are developed and emphasis is given to the identification of species and strains within the genus as an essential prerequisite to the establishment of local control programmes.

#### **2.1 Etiology and life cycles**

*Echinococcus* presents certain unique characteristics that set it apart from the other major genus in the family, *Taenia.* The adult form is only few millimetres long, has no gut and all metabolic interchange takes place across the syncytial outer covering, the tegument. Scolex is the anterior part of the *Echinococcus* becoming an attachment organ with four muscular suckers and two rows of hooks. The body is the strobila and is segmented in a different number of reproductive units called proglottids. The adult worm is hermaphrodite with reproductive ducts opening at a common, lateral, genital pore, the position of which may vary depending on species and strain. The uterus dilates after fertilisation, eventually occupying most of the terminal segment when the eggs are fully developed. The eggs are ovoid, consisting of a hexacanth embryo surrounded by several envelopes and are morphologically indistinguishable to those of other tapeworms of the genus *Taenia*. The metacestode is the second larval stage and consists of a bladder with an outer acellular laminated layer and an inner nucleated germinal layer. Protoscoleces arise from the inner wall of the brood capsules (figure 1). The structure and development of the metacestode differs between the four species of *Echinococcus*.

Fig. 1. Representation of the metacestode of *Echinococcus granulosus* (WHO/OIE publication: *Manual on echinococcosis in humans and animals*)

The life cycle of *Echinococcus spp*. requires two mammalian hosts for its completion. Gravid proglottids containing eggs or free eggs are passed in the faeces of the definitive host, a carnivore. These eggs are ingested by an intermediate host, in which the metacestode stage and protoscoleces develop. The cycle is completed if such an intermediate host is eaten by a suitable carnivore. Eggs are highly resistant to the environmental factors being infective for

causative agents of various forms of Echinococcosis is faced because it can help to understand the maintaining of these parasites in different geographic areas. Following to this part the epidemiology and clinical presentation forms are assessed. Finally, diagnosis, treatment and prevention are developed and emphasis is given to the identification of species and strains within the genus as an essential prerequisite to the establishment of local

*Echinococcus* presents certain unique characteristics that set it apart from the other major genus in the family, *Taenia.* The adult form is only few millimetres long, has no gut and all metabolic interchange takes place across the syncytial outer covering, the tegument. Scolex is the anterior part of the *Echinococcus* becoming an attachment organ with four muscular suckers and two rows of hooks. The body is the strobila and is segmented in a different number of reproductive units called proglottids. The adult worm is hermaphrodite with reproductive ducts opening at a common, lateral, genital pore, the position of which may vary depending on species and strain. The uterus dilates after fertilisation, eventually occupying most of the terminal segment when the eggs are fully developed. The eggs are ovoid, consisting of a hexacanth embryo surrounded by several envelopes and are morphologically indistinguishable to those of other tapeworms of the genus *Taenia*. The metacestode is the second larval stage and consists of a bladder with an outer acellular laminated layer and an inner nucleated germinal layer. Protoscoleces arise from the inner wall of the brood capsules (figure 1). The structure and development of the metacestode

Fig. 1. Representation of the metacestode of *Echinococcus granulosus* (WHO/OIE publication:

The life cycle of *Echinococcus spp*. requires two mammalian hosts for its completion. Gravid proglottids containing eggs or free eggs are passed in the faeces of the definitive host, a carnivore. These eggs are ingested by an intermediate host, in which the metacestode stage and protoscoleces develop. The cycle is completed if such an intermediate host is eaten by a suitable carnivore. Eggs are highly resistant to the environmental factors being infective for

control programmes.

**2.1 Etiology and life cycles** 

differs between the four species of *Echinococcus*.

*Manual on echinococcosis in humans and animals*)

months at lower ranges of temperatures (from +4ºC to +15ºC). However, they are very sensitive to desiccation and to high temperatures as 60-80ºC.

Intermediate hosts are represented by a wide range of mammals which acquire the infection by the ingestion of eggs. The oncosphere is released from the keratinised embryophore in the stomach and small intestine. Bile activates the oncosphere which penetrates the wall due to the hook movements and secretions and arrives to the liver where some of them are retained (figure 2). All mammals (including man) in which metacestodes of *Echinococcus*  species develop after infection with eggs may be referred to as 'intermediate hosts'. However, man is an aberrant host because metacestode stages do not become fertile in this host or because does not interact in the transmission cycle. Once the oncosphere has reached its final location, it develops into the metacestode stage. Time of development is variable and it may take several months before protoscoleces are produced (fertile metacestode). There may be several thousand protoscoleces within a single cyst of *E. granulosus* or an aggregation of vesicles of *E. multilocularis.* Each single protoscolex is capable of developing into a sexually mature adult worm. Not all metacestodes produce protoscoleces (sterile metacestode).

Fig. 2. Life-cycle of Echinococcus (WHO/OIE publication: Manual on echinococcosis in humans and animals)

Strain identification is possible for all four species of *Echinococcus* using morphological and biological features and/or molecular techniques, such as sequence comparison of a 366 bpfragment of the mitochondrial cytochrome oxidase subunit 1 DNA (CO1) and a 471 bpregion in the mitochondrial NADH dehydrogenase gene 1 (ND1), by analysis of a ribosomal (r) DNA fragment (1ST2) or by the random amplified polymorphic DNA-PCR (RAPD-PCR). Recent genetic studies have principally confirmed the concept of strain diversity within the

Echinococcosis/Hydatidosis 303

The causative agent of CE is the metacestode of *Echinococcus granulosus*, becoming a cystic structure filled with a clear fluid. Most of the cysts grow slowly in size and become surrounded by host tissue (pericyst) encompassing the endocyst of metacestode origin. The endocyst consists of the outer laminated layer and the inner cellular germinal layer, which may form brood capsules and protoscoleces. The minimum time required for the development of protoscoleces in cysts in humans is not exactly known, but it is expected to be 10 months or longer after infection (Pawlowski, 1997). Fertile (with protoscoleces) and sterile (without protoscoleces) cysts may coexist in the same patient. Frequently, smaller daughter cysts are formed within a larger mother cyst. If these smaller cysts are growing in close proximity to each other forming clusters the appearance of "polycystic" needs to be

The initial phase of primary infection is always asymptomatic remaining as this for many years or permanently. However, the infection may become symptomatic when cysts press adjacent tissues or induce other pathological events. When symptoms appear suddenly a spontaneous or traumatic cyst rupture has to be suspected. Spontaneous cure is possible but improbable, and is due to the collapse and resolution of cysts or due to the cyst rupture into the bile duct or the bronchial tree. The fatality rate is highly dependent on the severity of the

The age of the symptomatic infected patients can vary from below 1 year of age to over 75 years old. In a study from Madrid (Spain), over 1,473 patients admitted to a children's hospital, 2%were <1 year old, 21% between 1 and 4 years and 77% between 5 and 14 years (Utrilla et al., 1991). Many patients (about 40% up to 80%) with CE have a single organ

Clinical symptoms of CE are variable and depend on the organ involved, the size of the cysts, the interaction between the expanding cysts and the adjacent organ structures, and the complications related to the cyst rupture and bacterial infection. CE involving the liver can remain asymptomatic for more than ten years (Frider et al., 1999). Liver and lungs are the two more frequent organ sites involved. Complications affecting the biliary tract are the most common and include the cystic rupture into bile ducts. Other complications are

Ultrasonography (US) is used for the diagnosis of the cystic structure and portable units are suitable to take into account in field situations. Immunodiagnostic tests for detecting specific antibodies are commonly used for the aetiological confirmation of the findings of imaging

Protoscoleces or hooks of *E. granulosus* are found in aspirated fluid samples. This technique is not performed frequently because the material can only be available after a surgical intervention, therapeutic puncture (PAIR) or diagnostic puncture. Direct diagnosis can also

bacterial infection of the cyst, intraperitoneal rupture, and lung involvement.

b. Confirmation by imaging and identification of suspicious cyst structures c. Confirmation by detection of specific antibodies with immunodiagnostic tests

Diagnosis of CE is done through different steps as follows:

e. Material obtained by biopsy puncture or surgery is examined.

d. If doubt diagnostic puncture may be considered

**2.2.1 Cystic echinococcosis (CE)** 

distinguished from AE or PE.

infection and on facilities for treatment.

involved and harbour a solitary cyst. **2.2.1.1 Clinical presentation of CE** 

**2.2.1.2 Diagnosis of CE** 

examinations.

a. Clinical suspect or screening

species *E. granulosus*, previously based on morphological and biological features. Several molecular techniques are now available which would quite easily allow the identification of certain *E. granulosus* strains using genetic markers. To prepare the identification of the strain using molecular techniques protoscoleces are collected from *E. granulosus* cysts being washed several times in physiological saline solution and preserved in 70% ethanol. The material needs to be examined by an experienced laboratory. The rDNA ITS1 (internal transcribed spacer) region has been shown to be a potentially very useful genetic marker for distinguishing strains and species of *Echinococcus* and small quantities of *Echinococcus*  material can be characterised using a PCR-RFLP 'fingerprinting' technique (Bowles & McManus, 1993). Other method is the single strand conformation polymorphism (SSCP) which is technically simple and has high resolution capacity under optimised conditions. The utility of SSCP has been demonstrated for the categorisation of different *Echinococcus* genotypes (Gasser et al., 1998). The different strains of *Echinococcus* have an epidemiological significance for the prognosis in infected patients. Epidemiological studies have evidenced that the sylvatic strain of *E. granulosus* in northern North America is causing a benign infection with low pathogenicity, and affecting the lungs. Also, these epidemiological observations have been demonstrated in the People's Republic of China. In contrast, in parts of Kenya and Libya, it has been suggested that there are local virulent strains of *E. granulosus* (Thompson, 1995). Isoenzyme and molecular studies have confirmed that sheep strain is infective to humans (Bowles & McManus, 1993). Developmental differences between species and strains of *Echinococcus* are likely to be a limiting factor in control programmes which employ regular adult cestocidal treatment of definitive hosts for breaking the cycle of transmission (Thompson, 1995).

#### **2.2 Clinical forms of Echinococcosis**

The metacestodes of all four recognised *Echinococcus* species can infect humans and cause various forms of echinococcosis (Table 1). Among these forms cystic and alveolar echinococcosis are of special medical importance.


Table 1. Forms of Echinococcosis

Primary echinococcosis is established when metacestodes develop in various sites of the human body from oncospheres liberated from ingested eggs of *Echinococcus* spp. In CE liver and lung are the most frequently affected organs.

Secondary echinococcosis occurs when metacestode material spreads from primary site to adjacent or distant organs and proliferates. Regarding CE this form occurs after release of viable parasite material during invasive treatment procedures.

species *E. granulosus*, previously based on morphological and biological features. Several molecular techniques are now available which would quite easily allow the identification of certain *E. granulosus* strains using genetic markers. To prepare the identification of the strain using molecular techniques protoscoleces are collected from *E. granulosus* cysts being washed several times in physiological saline solution and preserved in 70% ethanol. The material needs to be examined by an experienced laboratory. The rDNA ITS1 (internal transcribed spacer) region has been shown to be a potentially very useful genetic marker for distinguishing strains and species of *Echinococcus* and small quantities of *Echinococcus*  material can be characterised using a PCR-RFLP 'fingerprinting' technique (Bowles & McManus, 1993). Other method is the single strand conformation polymorphism (SSCP) which is technically simple and has high resolution capacity under optimised conditions. The utility of SSCP has been demonstrated for the categorisation of different *Echinococcus* genotypes (Gasser et al., 1998). The different strains of *Echinococcus* have an epidemiological significance for the prognosis in infected patients. Epidemiological studies have evidenced that the sylvatic strain of *E. granulosus* in northern North America is causing a benign infection with low pathogenicity, and affecting the lungs. Also, these epidemiological observations have been demonstrated in the People's Republic of China. In contrast, in parts of Kenya and Libya, it has been suggested that there are local virulent strains of *E. granulosus* (Thompson, 1995). Isoenzyme and molecular studies have confirmed that sheep strain is infective to humans (Bowles & McManus, 1993). Developmental differences between species and strains of *Echinococcus* are likely to be a limiting factor in control programmes which employ regular adult cestocidal treatment of definitive hosts for

The metacestodes of all four recognised *Echinococcus* species can infect humans and cause various forms of echinococcosis (Table 1). Among these forms cystic and alveolar

> Hydatid disease, Hydatidosis, *E. granulosus* echinococcosis

Alveolar hydatid disease, *E. multilocularis* echinococcosis

*oligarthrus E. oligarthrus* echinococcosis

**Echinococcosis Causative agent Disease synonyms** 

**Echinoccosis** *Echinococcus vogeli E.vogeli* echinococcosis

Primary echinococcosis is established when metacestodes develop in various sites of the human body from oncospheres liberated from ingested eggs of *Echinococcus* spp. In CE liver

Secondary echinococcosis occurs when metacestode material spreads from primary site to adjacent or distant organs and proliferates. Regarding CE this form occurs after release of

*Echinococcus granulosus* 

*Echinococcus multilocularis* 

*Echinococcus* 

breaking the cycle of transmission (Thompson, 1995).

echinococcosis are of special medical importance.

and lung are the most frequently affected organs.

viable parasite material during invasive treatment procedures.

**2.2 Clinical forms of Echinococcosis** 

**Forms of** 

**Cystic Echinococcosis** 

**Alveolaar Echinococcosis** 

**Polycystic** 

**Polycystic Echinococcosis** 

Table 1. Forms of Echinococcosis
