Lifecycle Patterns

In endemic regions where a number of species of livestock are infected with cystic echinococcosis, it is important to determine which species are responsible for maintaining the life-cycle. This provides the basis not only for implementing targeted control efforts but, more fundamentally, for a reliable surveillance system, which is essential for successful control interventions (Gemmell and Roberts, 1995). This applies to parts of Europe, central Asia, China, Australia, Africa and the Middle East. For example, in Australia, where sheep, cattle, pigs and goats are susceptible to infection with the strain/species of Echinococcus present (Figure 22.4), husbandry factors suggest that sheep are the most important domestic intermediate host (Thompson, 1992). Cysts in sheep are usually fertile and dogs are likely to have access to sheep cysts as a result of direct feeding or scavenging. By contrast, sheep, cattle, buffalo and goats (Figure 22.5) all play a role in maintaining the life cycle of Echinococcus in many Middle Eastern countries, and where comparative studies have been undertaken, such

Fig. 22.4 Life-cycle and transmission patterns of Echinococcus granulosus in Australia
Fig. 22.5 Life-cycle of Echinococcus granulosus in the Middle East

as in Iran, data suggest that it is the same strain that infects these hosts (Hosseini, 1995; Fasihi Harandi et al., submitted for publication). In the Xinjiang Uygur autonomous region of China, approximately 50-80% of sheep, cattle, horses and camels are infected with hydatid disease, and dogs may become infected by ingesting cysts from any one of these animals, or they may have mixed infections of adult worms at any given time (Rausch, 1993). In Spain and Jordan, a range of domestic intermediate hosts are also commonly infected, but it has been shown that three distinct strains (probably different species) of E. granulosus are perpetuated in different life-cycle patterns (Figure 22.6), of which the sheep strain appears to be the most important to public health (Kamhawi and Hijjawi, 1992; Siles Lucas et al, 1994).

The problem of elucidating transmission patterns and putting strategies in place to interrupt the cycles is compounded in regions where wildlife are infected with Echinococcus and where there is the possibility of interaction with domestic hosts. Under such circumstances it is essential to determine whether species of wildlife act as reservoirs of the strain/species of Echinococcus to which livestock and humans are susceptible. In Australia, for example, wildlife are susceptible to the form of Echinococcus affecting sheep and other livestock. Interaction

Fig. 22.6 Life-cycles and transmission patterns of Echinococcus granulosus in Spain and Jordan
Fig. 22.7 Life-cycle of Echinococcus multilocularis in France and China

between wild and domestic cycles has been shown to occur on the Australian mainland (see below), and wild and feral animals appear to be important as sources of infection for cattle (Thompson, 1992; Lymbery et al., 1995; Schantz et al., 1995; Figure 22.4).

E. multilocularis also has a wide range of potential intermediate hosts (Rausch, 1995) and there is increasing concern over the interaction between the wild cycles of transmission and domestic definitive hosts. This is the case in France and China, where at least six genera of rodent intermediate hosts are involved in the transmission of E. multilocularis (Schantz et al., 1995; Figure 22.7) and where it is important to determine which are the most important for transmission in both wild and domestic environments.

Determination of the role of different hosts in the transmission of Echinococcus requires accurate characterisation of the species and/or strain infecting a particular host. Although morphological techniques have been, and will continue to be, of value in this respect, the application of DNA techniques, particularly those utilising PCR procedures, are proving to be very useful (Thompson et al., 2001). The adult stages recovered from definitive hosts may often be in too poor condition for morphological examination, and reliance on hook morphology is problematic for a number of reasons. There may not be sufficient discriminatory characters for differentiation based on hook morphology, and host-induced morphological variation may make specific identification difficult. It is for these reasons that molecular epidemiological techniques are providing valuable data for characterising the aetiological agents of echinococcosis and elucidating transmission patterns.

Molecular Epidemiology

In areas where there are several intermediate host species, it is important to know whether each harbours a different strain and whether there is the possibility of interaction between cycles. For example, in Britain, extensive studies have shown that E. granulosus is perpetuated in two distinct cycles of transmission, sheep-dog and horse-dog, and interaction is unlikely, since each cycle is associated with the perpetuation of a distinct strain/species exhibiting different intermediate host specificity characteristics (Thompson and Smyth, 1975; Thompson, 1991). Molecular characterisation of isolates of the parasite from horses and sheep has shown them to be genetically distinct, thus supporting the epidemiological observations (reviewed in Thompson et al., 1995).

In contrast, on the mainland of Australia (Figure 22.4), although E. granulosus is maintained in contrasting cycles of transmission involving either domestic or wild host assemblages, there is no evidence of genetic distinctness between the parasites maintained in domestic or wild host populations (Lymbery et al., 1990; Thompson and Lymbery, 1990, 1991; Hope et al., 1991). However, interaction between wild and domestic cycles of transmission has been demonstrated, in areas where they overlap, by the use of a novel 'transmission typing' procedure, which takes advantage of host-induced morphological variation. Host-induced morphological variation may be a complicating factor in identifying strains, but in this case alterations to hook shape have proved to be very useful in epi-demiological studies (Thompson and Lymbery, 1991), since they are induced during development in a particular species of intermediate host and are recognisable in the definitive host. These differences have been shown to be of great practical value in determining predator-prey relationships in areas of Australia where sylvatic and domestic cycles overlap (Constantine et al., 1993).

Molecular genetic techniques provide tools for characterising species and strains of Echinococcus in different endemic areas and, in addition, can be used to obtain information about population structure. Estimates of gene flow between populations of Echinococcus in different hosts or geographic areas can have valuable epidemio-logical applications. For example, it has been shown that gene flow is restricted between populations of E. granulosus on the mainland of Australia and in the island state of Tasmania, and these populations are now recognised as different strains (Lymbery and Thompson, 1988; Thompson and Lymbery, 1988). Despite the genetic differences between mainland and Tasmanian populations, however, migration between the populations was calculated to be of sufficient magnitude to be responsible for occasional breakdowns in the largely successful Tasmanian hydatid control campaign (Lymbery, 1995; Lymbery et al., 1997). In addition, Constantine et al. (1991) argued that the genetic distinctness of a population of E. granulosus on King Island, located between mainland Australia and Tasmania, made it unlikely to have originated from a recent introduction from either area.

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