Epidemiology of Hydatidosis
Echinococcus granulosus is primarily maintained through domestic and sylvatic life cycles. The life cycle (Figure 1) is complex, involving two hosts and a free-living egg stage. The dynamics of the transmission of the parasite are determined by the interaction of factors associated with these two hosts and with the external environment.
In the definitive host the parasitic burden is determined by the number of protoscolices ingested. Natural resistance varies in dogs and there is evidence that acquired immunity develops slowly. This immunity is likely to affect the number of established worms (FAO, 1982; Soulsby, 1982).
Segments detached with the faces may perform rhythmic contractions and relaxations that assist egg-expulsion (FAO, 1982) and eggs may be dispersed over considerable distances away from the faeces. Since sheep generally avoid grazing near areas contaminated with Dog faeces, this dispersal mechanism enhances the chances of eggs being ingested by the grazing animals. This has important epidemiology implications since a single dog can thus infect many sheep over a wide area (Gemmell and Lawson, 1986).
It has also been shown that flies and possibly other insects may mechanically transport eggs over considerable distances, having been contaminated during feedings or egg-laying activities in or on the dung (Lawson and Gemmell, 1985).
The survival of the infective egg is influenced by environmental factors, such as humidity and temperature. While eggs may survive for several months under moist conditions and moderate temperatures, desiccation is detrimental and they will only survive a short time when exposed to direct sunlight and dry conditions.
The number of infective eggs ingested by the intermediate host is therefore determined by the level of contamination and the infectivity of the eggs. Furthermore, the number of eggs that develop into hydatid cysts is controlled by the immune system of the host (Thompson and Allsopp, 1988).
Clinical Effects of Hydatidosis
The adult echinococcus is considered to be rather harmless to the definitive host, except when it occurs in large numbers, which may cause severe enteritis.
The effect of the hydatid cyst on the intermediate host depends on the size and location of the cyst. There are few available data on the clinical effects of the cystic hydatid disease in animals since the cysts is slow in growing and animals are often slaughtered before it manages to create sufficient pressure on the tissue or organs (Thompson and Allsopp, 1988).
The hydatid cyst is normally well tolerated in humans until its development results in pressure on adjacent tissue or organs. The cyst may also burst into the peritoneal or thoracic cavity, which can cause anaphylactic shock or give rise to many new cysts.
Parasitological and Immunological diagnosis in animals and humans
In the definitive host, a post-mortem examination is the most reliable method is diagnosis. Examination of the faeces after using arecoline as a purgative is less reliable, although proglottides in the faeces is conclusive. Egg counts are not specific because of the similarity of eggs from other tapewarms of the Taenia family (FAO, 1982).
Serological screening has recently proved to be powerful tool in detecting infected dogs (Gasser et al. 1990) and is superior to the arecoline testing.
In the intermediate host, diagnosing hydatidosis is possible through scanning, radiology, serology and postmortem examination. The post-mortem examination of sheep is usually an important component in monitoring the efficiency of control programmes.
Treatment of Hydatidosis
A number of anthelmintic drugs have proved to be effective against adult stages of E. granulosus in the final host. The best drug currently available is praziquantel (Schantz, 1982; WHO 1992), which exterminates all juvenile and adult echinococci from dogs.
Unfortunately, surgery is the treatment of choice at present, but several of the benzimidazole compounds have been shown to have efficacy against the hydatid cyst in the intermediate host. Long-term treatment with albendazole has a particularly marked effect on the cysts (Morris et al., 1990; McManus and Smyth, 1986), while long-term treatment with praziquantel only has a limited effect with few changes in the germinal layer of the cyst.