Introduction

Classification

Taxonomy

• Class: Cestoda.
• Family: Taeniidae.
• Genus: Echinococcus.

Active Forms

Resting Forms

Clinical Effects

Epidemiology

Habitat

• Adult tapeworms in anterior small intestine of dog .
• Infection can also occur in foxes.
• Eggs in environment: feces, soil, water, etc.
• Metacestodes (hydatid cysts) in sheep liver and lungs, horse liver  and lungs. Cattle, goat, pig may also be infected.
• In human, hydatid cysts are found most commonly in the liver, but lungs and other organs, including brain and bone, can be also be infected .
• On very rare occasions an Echinococcus cyst has been identified in a dog.

Lifecycle

• 1. Adult.
• 2. Egg.
• 3. Hydatid cyst containing brood capsules and protoscoleces .

Transmission

To sheep

• Ingestion of eggs on pasture. Eggs in dogs' feces can be washed onto pasture or can be eaten by flies and deposited over the pasture.

To dog

• Sheep strain: ingestion of protoscoleces in fertile hydatid cyst in viscera when the dog is scavenging or is fed uncooked offal.
• Horse strain: infection seen in hound kennels in which horse carcasses are fed to the hounds.

Pathological effects

• Very little or no protective immunity develops within the dog population.
• No protective immunity develops in the sheep population, although artificial infection of sheep with more than 40,000 eggs will induce a protective immunity lasting for about 3-6 months.

In dog definitive host

• Adult worms, even in large numbers, in dogs cause little pathology and essentially no clinical signs are apparent.
• There is crypt hyperplasia, increased numbers of mast cells and goblet cells, and villous atrophy.

In sheep/horse intermediate host

• Hydatid cysts cause space occupying lesions in the lungs and less often in the liver of sheep  , but these usually cause no apparent loss in production. Cysts normally occur in the liver of horses  and cattle.
• Rarely cysts occur in other locations causing a variety of clinical signs.
• Occasionally a cyst is recorded clinically in the horse, ie behind the eye.

In humans

• Many infections are asymptomatic, but symptoms may result from pressure atrophy induced by large cysts may compress or invade the biliary system, veins, bronchi, etc.
• Patients present most commonly with abdominal or chest pain. Cysts in the brain or heart can be life-threatening .
• Rupture of a cyst causes a variety of symptoms including those of anaphylaxis.

Control

Control via animal

• Anthelmintic treatment of dog at a maximum interval of 6 weeks.
• Pick up dog feces (Echinococcus spp eggs are immediately infective).
• Prevent dogs from roaming.
• Uncooked offal should not be fed to dogs and dead sheep should be disposed of according to local regulations so they cannot be scavenged.

Control via chemotherapies

• Praziquantel Praziquantel.

Control via environment

• Egg resistant to many disinfectants.
• Desiccation or dry heat kills eggs.

• Sodium hypochlorite solution (3.75% or higher) for at least 2-3 hours can be used to clean concrete surfaces.

Vaccination

• No vaccine is available for either the dog or sheep.
• Recently, an antigen that has induced protection experimentally in sheep has been identified, cloned and expressed. It is likely to become available for use in control programs around the world in the next few years, although it yet requires field testing.

Other countermeasures

Control programs

• Control programs against Echinococcus granulosus (sheep strain) have been introduced in many countries. Measures can include:
  • 6-weekly praziquantel treatment of dogs.
  • Destruction of stray dogs.
  • Education of the population, eg not to feed sheep to dogs.
• This can break transmission from dogs to sheep within 1 year but must continue for 10-15 years until all infected sheep have been removed (at least 2 sheep generations).
• Echinococcus granulosus has been eliminated from Iceland (free status), Tasmania and New Zealand (provisionaly free status). Programs are underway in other countries such as Greek Cyprus and some South American countries.

Effects of control on other dog/sheep cestodes

• When attempting control of E. granulosus, consideration must be given also to Taenia ovis Taenia ovis and Taenia hydatigena Taenia hydatigena. The stability of these latter 2 parasites can be disturbed and result in increased intensity and prevalence of their cysts and economic loss to the sheep industry.

Endemic E. granulosus

• Where there is no control, E. granulosus may be in a stable endemic steady state.
• Sheep remain sucseptible to infection so sheep continually acquire infection with age (as in nature they are never challenged with enough eggs at any one time to stimulate immunity).
• Mathematically, this gives E. granulosus a reproduction ratio of about 1.3-1.6 and it is in endemic steady state.
• If a control campaign is instigated that reduces the reproduction ratio to < 1 the parasite population will gradually decline to extinction.
• Hence, praziquantel rapidly pushed E. granulosus to extinction.

Hyperendemic Taenia hydatigena and Taenia ovis

• Control measures aimed at E.granulosus may have an impact on the occurrence of T.hydatigena and T.ovis.
• Normally the prevalenc and intensity of these parasites is controlled by the development of immunity in sheep. However, if the prevalence is reduced by control measures, this may reduce the level of immunity present in the sheep population and thereby the parasites may attain a hyperendemic level with the reproduction ratio >>1.

Diagnosis

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Buishi I, Walters T, Guildea Z et al (2005) Reemergence of canine Echinococcus granulosus infection, Wales. Emerg Infect Dis 11 (4), 568-571 PubMed.
• Lloyd S, Walters T M, Craig P S (1998) Use of sentinel lambs to survey the effect of an education progamme on control of transmission of Echinococcus granulosus in South Powys, Wales. Bull World Health Organ 76 (5), 469-473 PubMed.
• Cabrera P A, Parietti S, Haran G et al (1996) Rates of reinfection with Echinococcus granulosus, Taenia hydatigena, Taenia ovis and other cestodes in a rural dog population in Uruguay. Intern J Parasitol 26 (1), 79-83 PubMed.
• Lloyd S, Martin S C, Walters T M et al (1991) Use of sentinel lambs for early monitoring of the South Powys Hydatidosis Control Scheme - prevalence of Echinococcus granulosus and some other helminths. Vet Rec 129 (4), 73-76 PubMed.

Other sources of information

• Eckert J et al (2002) WHO/OIE Manual on echinococcosis in humans and animals: a public heath problem of global concern. Published by WHO and OIE, Paris.
• www.esccap.org: website for European Scientific Counsel: Companion Animal Parasites, guidelines for worm control in cats and dogs.
• www.wales.nhs.uk: Hydatid disease (Echinococcosis) background inforomation on hydatid disease and overview of the current situation in Wales.
• Wales Assembly Government Website: an overview of the current hydatid control program in Wales, questionnaire for the public, downloadable leaflet and poster.
• Gemmell M A & Roberts M G (1998) Cystic echinococcosis (echinococcus granulosus. In: Zoonoses - biology, clinical practice and public health control. Oxford University Press. pp 665-688.
• Thompson R C A & Lymbery A J (eds) (1995) Echinococcus and Hydatid Disease. Wallingford, CAB International. pp 477 (Complete overview).