ISSN 2398-2977      

Fasciola hepatica

pequis

Introduction

Classification

Taxonomy

  • Class: Trematoda.
  • Family: Fasciolidae. 
  • Genus: Fasciola
  • Species: hepatica.

Etymology

  • Hepatica: of the liver.

Distribution

  • Prevalence of F. hepatica is determined by temperature and environmental moisture, and the presence of suitable intermediate snail hosts.
  • F. hepatica occurs in Europe, North and South America, Caribbean, Asia, Australia and New Zealand, North Africa, highland regions of sub Saharan Africa.

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Clinical Effects

Epidemiology

Habitat

  • The intermediate host (lymnaeid snail), requires environmental moisture (damp soil at neutral or slightly acid pH) and moderate temperatures. They can also be found on the edges of streams, ponds, drainage ditches, and in permanently or temporarily wet areas of pasture.

Lifecycle

1. Egg Fasciola hepatica: egg 01.
2. Miracidium Fasciola hepatica: miracidium.
3. Larval stages within intermediate host (sporocyst, redia, cercaria) Fasciola hepatica: cercaria.
4. Metacercaria (encysted on herbage) Fasciola hepatica: metacercariae.
5. Juvenile flukes in liver parenchyma Migrating liver fluke.
6. Adult flukes in bile ducts Fasciola hepatica: adults.

Transmission

  • Infection is via ingestion of metacercariae encysted on herbage. 
  • Many mammalian species, both farmed and wild, are final hosts of Fasciola hepatica, and can contaminate pastures.
  • Any pasture with damp or wet areas, in endemic areas, is therefore a potential source of infection. 
  • Water sources may also occasionally be contaminated by "floating metacercariae".

Pathological effects

  • The immune response of horses infected with liver flukes has not been intensively studied.
  • In ruminants, infection results in a typical Th2 immune response, with eosinophilia and mastocytosis.
  • This response does not seem to be capable of protecting against further infection.
  • Many infected horses will show little or no pathology.
  • Enlargement and hyperplasia of bile ducts also occurs.

Other Host Effects

  • Low numbers of liver flukes infecting horses are unlikely to be pathogenic.

Control

Control via animal

  • Flukicide administration to livestock which may contaminate horse pastures. As most infected horses do not shed eggs, controlling infection in co-grazing ruminants is likely to be more effective at breaking the lifecycle than treating horses alone. 
  • Although clinical signs are most usually seen in autumn and winter (Europe), strategic dosing in the spring and summer can greatly reduce eggs on pasture, and therefore infection pressures.

Control via chemotherapies

  • The following drugs have been used specifically to treat liver fluke in the horse:
    • Triclabendazole Triclabendazole at 12 mg/kg PO treats flukes from 2 weeks of age. Resistance to triclabendazole is widespread in many countries
    • Closantel at 10 mg/kg PO covers fluke of 6-8 weeks and older.
    • Oxyclozanide at 10 mg/kg PO covers adults fluke only.
    • Nitroxynil at 7 mg/kg SC covers fluke of 6-8 weeks and older.

NO drugs are licensed for use in the horse.

Control via environment

  • Drainage of land decreases habitats available for the intermediate host, however this is often unacceptable for economic and environmental reasons.
  • Fencing off of local snail habitats (ponds, etc) can be a useful control method, although identifying snail habitats requires some experience, and temporary habitats can extend during wet years.
  • In many endemic areas "fluke forecasts" are provided which allow extra care to be taken in years when climatic conditions are particularly suitable for snail replication.
  • The most sophisticated forecasts now use computer modelling and satellite technology to provide detailed and accurate forecasts for individual areas.
  • In tropical areas F. hepatica infective stages are maximal towards the end of the rainy season. Infection is acquired as waters recede or from permanent waters. In cooler climates peak infection occurs in summer/autumn.
  • Application of molluscicides to pastures is not generally considered environmentally acceptable.
  • Elimination of this parasite is not feasible because of the large range of definitive host species, both farmed and wild.

Vaccination

  • Currently no vaccine is available for liver flukes.

Diagnosis

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Howell A K, Malalana F, Beesley N J et al (2019) Fasciola hepatica in UK horses. Equine Vet J 52 (2), 194-199 PubMed.
  • Raftery A G, Berman K G & Sutton D G M (2017) Severe eosinophilic cholangiohepatitis due to fluke infestation in a pony in scotland. Equine Vet Ed 29 (4), 196–201 VetMedResource.
  • Quigley A, Sekiya M, Egan S, Wolfe A, Negredo C & G Mulcahy (2016) Prevalence of liver fluke infection in Irish horses and assessment of a serological test for diagnosis of equine fasciolosis. Equine Vet J 49 (2), 183–88 PubMed.
  • Campe J, Ph Vyt & Ducheyne K (2011) Leverbot op een belgische stoeterij. Vlaams Diergeneeskundig Tijdschrift 80 (6), 403–6 ResearchGate.
  • Nelis H, Geurden T & Deprez P (2010) Fasciola hepatica bij het paard. Vlaams Diergeneeskundig Tijdschrift 79 (6), 436–44 VetMedResource.
  • Alves R M, Van Rensburg L J & Van Wik J A (1988) Fasciola in horses in the republic of South-Africa - a single natural case of fasciola-hepatica and the failure to infest 10 horses either with F hepatica or Fasciola-giganticaOnderstepoort J Vet Res 55 (3), 157–63 PubMed.
  • Owen J M (1977) Liver fluke infection in horses and ponies. Equine Vet J 9 (1), 29-31 PubMed.

Other sources of information

  • Dalton J P (1999) Fasciolosis. CABI Publishing, UK (a most comprehensive and up-to-the minute reference on all aspects of liver fluke and liver fluke disease).

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