Echinococcosis, or Hydatid disease, is an infection caused by tapeworms of the genus Echinococcus, a tiny tapeworm just a few millimeters long. Five species of Echinococcus have been identiﬁ ed which infect a wide range of domestic and wild animals. Echinococcosis is a zoonosis, a disease of animals that affects humans.
Hydatidosis or cystic echinococcosis (CE) is a zoonotic disease caused by the parasite. Of the ﬁve species Echinococcus granulosus (EG) which, from a taxonomic perspective, is currently considered a multispecies complex referred to as E. granulosus sensu lato (s.l.). the species identified in this complex are: E. granulosus sensu stricto (s.s.) (genotypes G1/G2/G3), E. equinus (genotype G4), E. ortleppi (genotype G5), E. canadensis (genotypes G6/G7/G8/ G9/G10) and E. felidis (“lion strain). E. granulosus s.s. (particularly genotype G1) is the most widely distributed species at global level and accounts for approximately 80% of human cases of hydatidosis.
E. multilocularis is widespread in the northern hemisphere, while two species, E. oligarthus and E. vogeli are found only in Central and South America. The fifh E. shiquicus, was discovered in 2006 in the People’s Republic of China. E. granulosus and E. multilocularis are most important for zoonotic risk. In contrast the Latin American species rarely infect humans, and the zoonotic status of E. shiquicus is unknown.
Like all tapeworms the life cycle involves two animals. A carnivore is the deﬁnitive host – where the adult worms live in the intestines – and almost any mammal, including humans, can be the intermediate host – where the worms form cysts in various organs.
Adult worms live in the small intestine of the deﬁnitive host. They reproduce releasing eggs into the environment in the faeces of the host animal. The eggs are well adapted to survive in the environment for as long as a year in cool moist conditions, but are susceptible to desiccation. Fresh eggs are sticky and may adhere to the fur of deﬁnitive hosts facilitating their spread.
The intermediate host ingests the eggs incidentally while grazing, foraging or drinking. The eggs hatch in the small intestine, become larvae which penetrate the gut wall, and are carried in the circulatory system to various organs. There the cysts, called hydatid cysts or metacestodes, are formed. The cysts, which contain larvae, either comprise ﬂuid ﬁlled bladders, which contain larval pre-tapeworms (protoscoleces), and cause the disease cystic echinococcos due to E. granulosus or alternatively, for E. multilocularis a multivesiculated lesion or mass containing protoscoleces that grows rapidly by exogenous budding and causes alveolar echinococcosis in rodents and other small mammals. Though slow growing in humans and long-lived animals (e.g. camels or horses) cysts of E. granulosus can reach a size of 10-20 centimetres, but in sheep are usually 2-6 cms.
The life cycle is completed when the cysts are ingested by a carnivore deﬁ nitive host (e.g. dog, fox, or wolf), the larvae (protoscoleces) are released from the cyst into the small intestine, and develop into adult tapeworms that produce eggs which are released into the environment in the faeces of the host animal within 25-80 days depending on the species and strain of Echinococcus.
How is the disease transmitted and spread?
The most widespread cycle exists for E. granulosus between dogs and sheep. When dogs are fed fresh offal or scavenge infected sheep carcasses containing cysts, they become infected, contaminate the pasture with their faeces, and sheep are re-infected as they graze. There is a similar cycle between dogs and horses, dogs and camels etc, and in wildlife, for example between wolves and moose/deer, and dingoes and kangaroos/wallabies. E. multilocularis is mainly transmitted within the predator-prey relationship between foxes and small mammals especially voles; cattle, sheep and pigs although sometimes exposed to infection only develop small non-viable lesions of E. multilocularis and are therefore not involved in transmission.
What are the clinical signs of the disease?
Tapeworms in the small intestine of the deﬁnitive host cause few ill effects. In the intermediate host, the cysts gradually displace or induce ﬁbrosis in normal tissue, and result in disease manifestation. The symptoms in humans depend on where in the body the cyst develops, and the size and numbers of cysts or metacestode mass.
In infected livestock with E. granulosus there can be reduced growth, decreased production of milk, meat and wool, reduced birth rate and losses due to condemnation of organs at postmortem examination. However, the cysts grow slowly so that many infected animals are slaughtered before the cysts ever cause disease problems.
There may however be multiple cysts of E. granulosus which can also occur in the brain, kidneys, bones, or testes causing more severe illness. Without control measures, infection rates can be very high in livestock and dogs, with associated signiﬁcant incidence in humans.
E.multilocularis (and the other species) rarely infect cattle, sheep and pigs and when exposure occurs the cysts may not be viable.
How is the disease diagnosed?
In view of the life cycle, the best means to diagnose echinococcosis in deﬁnitive hosts is the demonstration of the adult worm in the intestine at postmortem or in the mucus after a diagnostic test (arecoline purgation), or ﬁnding the proglottids (tapeworm segments) in faeces. In wild carnivores necropsy is commonly undertaken, for example foxes in E. multilocularis endemic areas. There are also tests for speciﬁc antigens in faeces (coproantigens) which are highly genus speciﬁc for Echinococcus and exhibit greater sensitivity than arecoline purgation.
In intermediate hosts, diagnosis depends on the post mortem detection of the cysts particularly in the liver and lungs for E. granulosus.
They are often detected during meat inspection, and can also be found with ultrasound examination but speciﬁcity of imaging is poor. Serological tests for cystic echinococcosis in cattle, sheep and pigs are not used routinely because of variable sensitivity and speciﬁcity. Post mortem examination of small mammals can be used to detect E. multilocularis cysts but prevalence are usually low, and fox prevalence data is therefore more useful.
Eggs can be sampled from soil (or faeces) for speciﬁc ampliﬁcation of DNA to detect the presence of Echinococcus. The diagnosis is performed according to the guidelines found in the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.
What is being done to prevent or control the disease?
The best control measure is to interrupt the life cycle of the parasyte. For E. granulosus this can be done by;
- preventing access of dogs to livestock carcasses or slaughter wastes from farms, households, abattoirs or butchers,
- treating dogs with an anthelmintic (praziquantel) to kill the adult tapeworm,
- detecting cysts at meat inspection, thus targeting infected farms or communities,
- vaccinating sheep (or other livestock) to protect against the development of the larval stage of E. granulosus.
Praziquantel is the drug of choice for systematic dog deworming, used at 5 mg/kg in a single dose. Given its bitter taste, masking the tablets within a piece of meat or pate is recommended. in some countries, the drug is delivered to the community by program personnel or in Healthcare facilities, where control programs are in place.
VACCINATION OF SHEEP
Vaccination of potential intermediate hosts with the EG95 recombinant vaccine developed by the Melbourne University may be a useful strategy to reduce the level of transmission as well as the incidence of infection in humans
DEVELOPMENT OF HEALTH INFRASTRUCTURE
Every establishment in which animals are slaughtered for family consumption, especially large livestock farms, should have a slaughter room available which prevents dogs’ access of and a pit with a lid or other system for the destruction or sterilization of viscera. Kennels may be built to keep the dogs while not working
MANAGEMENT OF CANINE POPULATIONS IN ENDEMIC AREA
In rural and some peri-urban areas as well as in small towns, dogs with no documented owner, feral dogs and stray dogs may be a source of Echinococcus spp. Un-owned dogs are those having no or unknown owner, while feral dogs are those living in rural areas as part of the fauna.
Management of these canine populations may include several strategies based on operational, financial, social and cultural assessments, and those considered most cost-effective by the countries to meet the required control objective.