Present Status on the Taxonomy and Morphology of <em>Echinococcus</em> Granulosus: A Review

Review Article

Austin J Vet Sci & Anim Husb. 2015; 2(2): 1013.

Present Status on the Taxonomy and Morphology of Echinococcus Granulosus: A Review

Rahman WA¹*, Elmajdoub LE², Noor SAM² and Wajidi MF³

¹School of Food Science and Technology, University Malaysia Terengganu, Malaysia

²School of Biological Sciences, University Sains Malaysia, Malaysia

³School of Distance Education, University Sains Malaysia, Malaysia

*Corresponding author: Wahab A Rahman, School of Food Science and Technology, University Malaysia Terengganu, 21300 Kuala Terengganu, Malaysia

Received: May 09, 2015; Accepted: September 04, 2015; Published: September 09, 2015

Abstract

Echinococcus granulosus has been researched extensively and previously discussed in detail as it is the species that is most widely distributed throughout the world. It is a cyclozoonotic infection which has important global implication as it is found in many animals and humans and involves a large group of countries. The high incidence and wide distribution of this parasitic infection that man shares with animals make the disease one of the most serious zoo noses in many parts of the world. Epidemiological studies indicate that the sheep strain of E. granulosus is the leading agent of cystic echinococcosis in humans. The common intermediate host of E. granulosus is many domesticated mammals such as sheep, cattle, pigs, goats and camels. Sheep, which harbor the most fertile hydatid cysts, are the most important intermediate host and represent the most important source of infection to dogs through the feeding of infected offal, giving rise to high prevalence rates as shown in many countries of the world.

Keywords: Taxonomy; Morphology; Echinococcus granulosus

Introduction

Echinococcosis or hydatid disease, caused by the larvae of the closely related species Echinococcus granulosus and E. multilocularis is the most serious tapeworm infection occurring in man and domesticated animals. This is not because it is particularly common but because of the lack of an effective chemotherapeutic agent and the difficulty of surgery often results in a poor prognosis. The two species have been clearly differentiated and the presence of strains (regarded by some as subspecies, and of each, adapted to various hosts) had been established as early as the 1950s’, but until today there is still much confusion in various aspects of the disease, especially epidemiology. The present review is an attempt to re-visit some important nonmolecular aspects of the disease.

History of Hydatid Disease

Hydatidosis is a disease with an extremely long history as it has been known since the time of Hippocrates, and it is caused by the tapeworm Echinococcus granulosus. According to [1] Echinococcus granulosus has been known since ancient times, for example, Aretaeus and Galen were familiar with hydatid cysts. The generic name Echinococcus is from the Greek words “echinos”, which means hedgehog and “kokkos” which means berry, while the species term granulosus is from the Latin word “granulum” which means little grain. The term hydatid is from the Greek word “hydatis” which means a drop of water [1,2].

At present Echinococcus granulosus, the causative agent of hydatidosis is almost ubiquitous. It was Redi in 1684, Hartmann in 1685, and Tyson in 1691 who first suspected their animal and the bladder were really worms. In 1766, Pallas first mentioned the similarity of hydatids in man and other mammals and Goeze in 1782 first studied the protoscoleces of the metacestoda and recognized their relationship to those of taenial origin and differentiated the hydatid cyst from the cysticercus. In 1695, Hartman first observed the adult worms in the dog’s intestine, and later Rudolphi in 1808 studied details of the adult worms. Then Von Siebold in 1852 followed by Haubner, Leuckart, Kuchenmeister and Nettleship fed protoscoleces of cysts on domestic animals to dogs and observed the development of the adult worms in the intestines of the hosts. Later Naunyn, 1863 in Germany, Krabbe, 1863 in Iceland, and Thomas, 1885 in Australia obtained adults worms in dogs from protoscoleces of human origin [2].

Taxonomy of Echinococcus Granulosus

According to Yamaguti, Braithwaite [3,4] and Thompson (1986), the tapeworms whose larval stage causes hydatid cysts belong to the following:

Phylum: Platyhelminthes.

Superclass: Eucestoda.

Class: Cestoidea.

Subclass: Cestoda.

Order: Cyclophyllidea Ben; Braun, 1900.

Family: Taeniidae Ludwig, 1886.

Genus: Echinococcus Rud, 1801.

The genus Echinococcus was established for the larval cestoda which was earlier designated Taenia visceralis socialis granulosus by Goeze in 1782 obtained from the liver of sheep. In 1786, Batsch adapted the binary system of nomenclature of Linnaeus and gave the name Hydatigene granulosus to the taxon described by Goeze [5]. In 1963, Rausch and Nelson reviewed the species of Echinococcus and concluded that the status of E. felidis Ortlepp, 1937 and E. patagonicus Szidat, 1960 was uncertain, but that both were possibly co-specific with E. granulosus. In addition, E. cameroni Ortlepp, 1934; E. intermedius Lopez-Neyra and Soler, 1943; E. longimanubrius Cameron, 1926; E. lycaontis Ortlepp, 1934; E. minimus Cameron, 1926, and E. Ortlepp Lopez-Neyra and Soler 1943 were considered synonyms of E. granulosus Rausch and Nelson 1963.

Strains of Echinococcus Granulosus

Echinococcus granulosus has been divided into several strains according to the host [6]. Echinococcus granulosus has been researched extensively and discussed in detail as it is the species that is most widely distributed throughout the world. For example, it has six strains which have been identified as G1 genotype (common sheep strain), G2 genotype (Tasmanian sheep strain), G4 genotype (horse strain), G3, G5 genotypes (cattle strain), G6 genotype (camel strain), G7 genotype (pig strain) [7,8]. In addition, Haag et al. [9] recorded five potential hybrid genotypes between cattle and sheep strains found only in Southern Brazil.

Distribution of Echinococcus Granulosus

According to Belding, Beck and Davies, Beaver et al., Schantz et al., Shambesh [1, 10-12] E. granulosus has a worldwide distribution. Indeed as Abdou, Matossian et al., Polydorou [13-15] point out, hydatidosis is a cyclozoonotic infection which has important global implication as it is found in many animals and humans and involves a large group of countries. For example, hydatid disease is considered endemic in humans and animals in North Africa, while in the rural areas of Libya, Tunisia, Algeria and Morocco, it is classified as hyper endemic. In addition, it is prevalent in countries in Latin America, the Middle and Near East, the Mediterranean littoral, a number of African countries and several European ones. The high incidence and wide distribution of this parasitic infection that man shares with animals make hydatidosis one of the most serious zoonoses not only in Egypt, but also in the Middle East, Cyprus, Iraq, Jordan, Kuwait, Lebanon, Iran and Syria.

Epidemiological studies indicate that the sheep strain of E. granulosus is the leading agent of cystic echinococcosis in humans. It has been extensively studied in a number of different areas and is now present in Asia, Africa, South and Central America and the Mediterranean region and also common in parts of Europe, especially the United Kingdom and Australia [11,12]. Those regions with the most extensive and intensive E. granulosus infections are sheep and cattle-raising countries, including Tanzania and the southern half of South America and Paraguay [16]. In addition, human infection occurs frequently throughout northern, southern and eastern Europe, Siberia, Turkestan and Mongolia as illustrated in (Figure 1).