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Permin and J. This parasite is found worldwide and causes serious public health problems in certain parts of the world Schantz, In addition there are economic losses from the condemnation of affected organs.
Echinococcosis is a cyclozoonosis that requires two vertebrate hosts to uphold the life cycle Figure 1. Humans can accidentally become intermediate hosts by ingesting the eggs of the tapeworm. While most cysts develop in the liver and lungs, other organs arid tissue Figure 2 may become affected Soulsby, The wide variety of animal species that can act as intermediate hosts and the domestication and spread of some of these animals from Europe to other parts of the world has given Echinococcus granulosus a worldwide distribution.
It has been extensively studied in a number of different geographical areas and is now present in Asia, Africa, South and Central America and the Mediterranean region McManus and Smyth, Some countries, such as Iceland and Cyprus, have already eradicated or are close to eradicating the disease. Control measures in New Zealand and Australia Tasmania have significantly reduced the prevalence of E.
Although control programmes resulting in a marked decrease in the incidence of the disease have been carried out in some countries, little effect has been achieved worldwide. There is some evidence that the disease is spreading because of a lack of meat control, dog management and appropriate legislation Gemmell ; Schwabe, However, collaboration between veterinarians and public health workers is essential for the successful control of hydatidosis Thompson and Allsopp, Aetiological agents and taxonomy At present, four species of the genus Echinococcus are recognized and regarded as taxonomically valid : E.
These four species are morphologically distinct in both the adult and the larval stages. In addition, several different strains of E. The development of strains may be a result of the fact that tapeworms are hermaphrodites, which reproduce themselves through cross- or self-fertilization.
A single mutant can therefore produce large genetically identical populations that differ from the original genus. These populations are referred to as strains McManus and Smyth, Only E.
Life cycle and host-parasite relationships The adult tapeworm is found in different parts of the small intestine of the definitive host Figure 1 , from where segments containing eggs are passed with the faeces Figure 3. When ingested by the intermediate host, the eggs are immediately infective, releasing larvae that penetrate into the lymphatic or vascular system, reaching the liver or lungs and possibly other organs Figure 4. The hydatid cyst Figure 5 develops slowly over several months, forming an outer laminated membrane and an inner membrane called the germinal layer.
From the germinal membrane brood capsules develop, each containing one or several invaginated heads protoscolices that can develop into the adult tapeworm upon ingestion by the definitive host Soulsby, Host range It is likely that E. It has now adapted: into a "domestic" cycle, however, commonly involving dogs and sheep FAO, ; Soulsby, Epidemiology Echinococcus granulosus is primarily maintained through domestic Figure 6 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, ; Soulsby, Segments detached with the faeces may perform rhythmic contractions and relaxations that assist egg-expulsion FAO, 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 epidemiological implications since a single dog can thus infect many sheep over a wide area Gemmell and Lawson, It has also been shown that flies and possibly other insects may mechanically transport eggs over considerable distances, having been contaminated during feeding or egg-laying activities in or on the dung Lawson and Gemmell, 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, Clinical effects 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 cyst is slow in growing and animals are often slaughtered before it manages to create sufficient pressure on the tissue or organs Thompson and Allsopp, 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. New segments develop from the neck region of the parasite after the gravid segments are detached - E. Note the brood capsules on the germinal layer of the cyst - Kyste d' E.
Remarquer les capsules d'incubation sur la membrane germinale du kyste - Quiste de E. 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 tapeworms of the Taenia family FAO, Serological screening has recently proved to be a powerful tool in detecting infected dogs Gasser et al.
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 A number of anthelmintic drugs have proved to be effective against adult stages of E. The best drug currently available is praziquantel Schantz, ; WHO , 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. Control Echinococcosis can be controlled through preventive measures that break the cycle between the definitive and the intermediate host.
These measures include dosing dogs, inspecting meat and educating the public on the risk to humans and on avoiding feeding offal to dogs, as well as introducing legislation. None of these measures will work in isolation, however. The disease can be controlled successfully through health education and appropriate legislation only when people understand the life cycle of the parasite.
It is of the utmost importance that the government be involved, through the Ministries of Health, Agriculture and the Interior, for example. So far, the only successful control programmes have been those where the Ministry of Agriculture has been the responsible control authority Gemmell, Recently, a recombinant vaccine has been developed.
When fully tested, it will become a valuable tool in the vaccination of sheep M. Lightowlers, personal communication , breaking the life cycle of echinococci. With an efficient vaccine the attack phase, as described later, can be shortened considerably.
The data should include the size of the rural dog population, incidence and age prevalence of hydatidosis in humans, reinfection rate of rural dogs and number of veterinarians and technicians needed to treat and test every 10 rural dogs. Attack The attack phase is labour-intensive and therefore very costly. The duration of this phase will depend on the strategy utilized, however, and according to the experience gained, it will take at least ten years and often more.
Public education programmes, including instruction in the importance of not feeding uncooked offal to dogs, and strengthened general meat control must be implemented simultaneously. In order to ensure success, it is paramount that funding is secured for the entire length of the programme before it is started.
Consolidation The consolidation phase transfers activities from indiscriminate dog dosing to the quarantining of infected farms or farms deemed to be at risk. This transfer should be accompanied by the introduction of penalties for having infected dogs, if appropriate.
Maintaining eradication During this phase, all special activities will cease and the normal resources of the meat inspection services of the Ministry of Agriculture will be used to prevent reintroduction. Maintenance of eradication is secured through the continuous surveillance monitoring of human, canine and livestock populations. Discussion and conclusion Control programmes have been carried out successfully in several countries, including Australia Tasmania , New Zealand, Cyprus and the Falkland Islands, where E.
Other countries are planning to institute similar control programmes. It is very important to support and implement new control programmes so as to prevent further spread of the disease. Research in possible vaccines is essential in order to supplement the existing methods of breaking the Echinococcus life cycle.
Both WHO and FAO are interested in collaborating with national governments in developing national or regional control programmes. Bibliography Cook, B. The epidemiology of Echinococcus granulosus in Great Britain. Gasser, R. Serological screening of farm dogs for Echinococcus granulosus infection in an endemic region. Gemmell, M. Hydatidosis control: a global view. The epidemiology and control of hydatid disease. Thompson, ed.
The biology of Echinococcus and hydatid disease , p. Lawson, J. The potential role of blowflies in the transmission of taeniid tapeworm eggs.
Parasitology , McManus, D. Hydatidosis: changing concepts in epidemiology and speciation. Today , 2 6 : Morris, D. Comparison of albendazole and praziquantel therapy of Echinococcus granulosus in naturally infected sheep.
Human echinococcosis hydatidosis, or hydatid disease is caused by the larval stages of cestodes tapeworms of the genus Echinococcus. Another species, E. Two exclusively New World species, E. Many genotypes of E. The known zoonotic genotypes within the E. Research on the epidemiology and diversity of these genotypes is ongoing, and no consensus has been reached on appropriate nomenclature thus far.
File:Ciclo biológico Echinococcus granulosus.jpg
Human echinococcosis hydatidosis, or hydatid disease is caused by the larval stages of cestodes tapeworms of the genus Echinococcus. Echinococcus granulosus sensu lato causes cystic echinococcosis and is the form most frequently encountered. Another species, E. Two exclusively New World species, E. Many genotypes of E.
Correspondence to :. In Chile is a zoonosis of great importance. Hence, it was discovered that in endemic areas of hydatidosis there could be several strains and genotypes of EG. In addition, there is evidence that some strains and genotypes are more infectious for human beings than others. This interesting phenomenon of the biology of EG has been studied using molecular biology techniques based on polymerase chain reaction PCR and DNA sequence analysis, which has made it possible to characterize the cestode species complex called EG sensu lato s l as being comprised of EG sensu stricto s. Genotypes G1-G3 , E. The aim of this manuscript is to describe the different genotypes of EG described in humans and different livestock host reported in the literature.