Echinococcus granulosus of camel origin: development in dogs and parasite morphology

Abstract

The developmental characteristics ofEchinococcus granulosus of camel origin were studied in four dogs artificially infected with protoscolices originating from hydatid cysts isolated from the lung of a camel (Camelus dromedarius). Two dogs each were necropsied 34/35 and 41 days post-infection (p.i.); one dog had a low worm burden and the others were heavily infected (27,625–41,150 worms). At day 35 p.i., 20% of the parasites had developed three segments and the uterus of the vast majority of the total population was full of developing eggs in the terminal segment. At day 41, up to 58% of the parasites contained mature eggs (embryonated eggs with fully developed, “thick-shelled” embryophores). Morphological studies revealed the following major characteristics for 35 day-old worms: the mean length of the terminal segment accounted for 54% of the total worm length; the position of the sexually mature segment was always terminal; the female reproductive system possessed an elongated ovary with compact lobules; the female ducts were also compact; the Mehlis' gland was covered by the vitelline gland and the testes were distributed throughout the segment, with 1 row posterior to the vitelline gland. The camel isolate can readily be distinguished from the horse and sheep strains, but it is similar to the cattle strain in some respects, particularly in its precocious development. However, the camel isolate differs from the cattle strain in the position of the sexually mature segment, arrangement of the testes and structure of the female reproductive system. As in the cattle strain, the metacestodes in the principal intermediate host are mostly localised in the lung and have a high fertility rate. Detailed morphological characteristics and biological features of the sheep, cattle, horse and camel isolates ofE. granulosus are presented. Increasing evidence is being accumulated that local populations ofEchinococcus granulosus may vary in morphological and biological features and that the occurrence of intraspecific variants or “strains” can play an important role in the epidemiology of echinococcosis (Rausch 1986; McManus and Smyth 1986; Thompson 1986; Thompson 1988; Thompson and Allsopp 1988; Eckert and Thompson 1988), because strains differ in their infectivity to final and intermediate hosts and their transmission patterns. Moreover, the pathogenic significance of strains to humans may also vary. The camel is an important host ofE. granulosus and is commonly infected throughout Africa and the Middle East (Schwabe 1986). The camel has attracted much interest as an intermediate host ofE. granulosus, particularly in its role as a reservoir of infection in humans. In addition, it has been proposed that the camel form ofE. granulosus may be a different subspecies or strain (Pandey et al. 1986). However, very little comparative work has been carried out to characterise isolates ofE. granulosus from camels (Thompson and Lymbery 1988). The aim of the present work was to study the morphological and biological characteristics of intestinal stages ofE. granulosus of camel origin obtained from artificially infected dogs. In addition, data were collected on the metacestodal stage.