Genetic variants within the genus Echinococcus identified by mitochondrial DNA sequencing.

Publisher Summary This chapter focuses on the public health importance of taeniid cestode zoonoses especially cystic echinococcosis, alveolar echinococcosis and T. solium cysticercosis. The chapter addresses the problem of neurocysticercosis and epilepsy in developing countries particularly Latin America and its public health consequences in the USA and elsewhere. E. multilocularis , which is the cause of one of the most pathogenic of all human parasitic infections, appears to be spreading in parts of western Europe, USA, and Japan. The remarkable development in the past 20 years or so of high resolution imaging technologies, such as computerized tomography (CT) scan and ultrasound have enabled precision detection of taeniid larval cystic infection in humans. Coupled with improvements in immunodiagnostic test sensitivity and antigen specificity, diagnosis of cestode zoonoses is now more accurate than ever before. Although further developments in immunodiagnosis are required, laboratory tests have provided improved capability for screening populations in both epidemiological and community studies. Parallel and recent development of genus specific coproantigen tests for human taeniasis and canine echinococcosis have also provided major new tools for epidemiological and surveillance programs.

Detection, screening and community epidemiology of taeniid cestode zoonoses: cystic echinococcosis, alveolar echinococcosis and neurocysticercosis.

Publisher Summary This chapter discusses the nature, extent, and significance of variation particularly, below the species level, in the causative agent of hydatid disease, Echinococcus. An application of current taxonomic procedures to the classification of a group of organisms in Echinococcus ensures an important role for the explanatory and predictive value of taxonomic determinations in studies on the epidemiology and control of hydatid disease. Despite increasing access to highly discriminatory differential criteria and a more flexible taxonomic approach, problems of nomenclature are likely to remain. Many genetically distinct populations within the recognized species of Echinococcus cannot be adequately allocated to a formal taxonomic category. However, in parasitic organisms the recognition of variation may be of great medical and agricultural significance and it is for this reason that the term “strain” has been widely used to describe intraspecific variants in Echinococcus and other parasites. It is essential that strains are indentified and characterized in each endemic area if effective control programs are to be instituted. Future research should aim to develop even more reliable and practically applicable differential criteria for this purpose. Biochemical and molecular dissection of the parasite in a quest toward this end may also unravel the functional nature of strain variation and provide direction for the development of chemotherapeutic agents.

The nature, extent and significance of variation within the genus Echinococcus.

Molecular variation in Echinococcus

The taxonomy ofEchinococcus is reviewed and consideration given to speciation within the genus. The morphological and other biological characteristics of all species and subspecies so far described are given, and evidence for the existence of other intraspecific variants is presented. A total of 16 species have been described but only four have well-defined characteristics and are generally accepted as being taxonomically valid;E. granulosus, E. multilocularis, E. oligarthrus andE. vogeli. The taxonomic status of intraspecific variants is uncertain. Some authorities consider that the majority of described subspecies are invalid taxonomically. However, concern is expressed that previous taxonomic considerations ofEchinococcus have not taken into account the nature of its reproductive mechanism which, like that of most cestodes, defies the application of traditional taxonomic concepts and definitions derived from dioecious cross-fertilising organisms. At the present time intraspecific variants ofEchinococcus are referred to as strains. The existence of such strains, particularly ofE. granulosus, in different parts of the world is firmly established. Their importance in the epidemiology of hydatidosis has high-lighted the needed to establish criteria for their differentiation and characterisation. The limitations of morphology as a sole differential criterion are emphasised. Attention is given to recent studies on strain variation which have demonstrated the value of applying several different criteria in which the developmental, physiological and biochemical characteristics of different strains are compared in conjunction with morphological investigations.

A review of the taxonomy and speciation of the genus Echinococcus Rudolphi 1801.

The present material of buffalo–dog origin could not be transmitted to the laboratory animals, such as white rats, white mice, rabbits, golden hamsters, guinea-pigs and chicks. Secondary development took place only in mice. The strobilar phase was infective to a cat but did not show sexual differentiation even at 30 days, and worms were further observed to be at a degenerative stage. The buffalo was highly suited to the perpetuation of the parasite, forming larger cysts and massive production of scoleces; nearly 90% of the cysts were fertile. The strobila had mostly two segments and not more than three. The number and distribution of testes were more posterior to the genital pore and were also observed posterior to the vitellaria, occasionally. The position of the genital pore in the gravid as well as mature segments was both anterior and posterior but with an average of 52.8%. Uterine sacculations were observed to range from 28 to 54. Echinococcus material of buffalo–dog origin does not appear to fall into a definite ‘subspecies’ pattern. It is suggested that it be merely recognized as a further ‘mutant’, the existence of which can be accounted for on theoretical grounds. The authors are thankful to the Director, Indian Veterinary Research Institute, and to Dr H. D. Srivastava, Head of Division of Parasitology, I.V.R.I., for providing the necessary laboratory facilities.

B V Rao

Papers

On the biology and morphology of Echinococcus granulosus (Batsch, 1786) of buffalo-dog origin