ISSN 2398-2977      

Liver: fasciola hepatica infestation

pequis

Synonym(s): Fascioliasis, Distomatosis, Distomiasis


Introduction

  • Cause: ingestion of plant material containing metacercariae Fasciola hepatica Fasciola hepatica.
  • Diagnosis: serum antibody ELISA is the most reliable method. Fecal egg detection requires use of a sedimentation technique, however, most infected horses do not shed liver fluke eggs.
  • Signs: horses are relatively resistant to infection with F. hepatica, and many infected horses are asymptomatic. Common signs include lethargy and weight loss, and less frequently, signs associated with liver insufficiency.
  • Treatment: most commonly, triclabendazole (12-15 mg/kg PO) treats fluke from 2 weeks of age, or closantel (10 mg/kg PO) covers fluke of 6 weeks and older. Less commonly used alternatives include oxyclozanide (10 mg/kg PO) and nitroxynil (7 mg/kg SC injection).
  • Prognosis: excellent if clinical cases treated promptly.

Pathogenesis

Etiology

  • Infection with metacercariae Fasciola hepatica: metacercariaeof F. hepatica or F. gigantica
  • Numbers and survival of intermediate hosts and metacercariae are highly temperature and moisture-dependent.

Predisposing factors

General

  • Grazing pastures also grazed by infected sheep or cattle, in areas where the climate favors intermediate host survival.

Pathophysiology

  • Horses grazing pastures contaminated with liver fluke metacercariae → ingest metacercariae encysted on herbage → juvenile flukes penetrate intestinal epithelium, traverse peritoneum and penetrate liver capsule → migrate in liver parenchyma for up to 12 weeks and longer → some may reach patency and grow to maturity in bile ducts → produce eggs which are excreted in the feces. 
  • There is infiltration of infected liver tissue with eosinophils and mast cells
  • Proteases secreted by migrating flukes, as well as simple mechanical damage, contribute to the pathological process. 
  • Host immune responses may also contribute to liver damage, eg by inducing oxidative stress
  • In the chronic stages of infection, mature flukes in the bile duct induce hyperplasia of bile duct epithelium. 
  • Aberrant flukes, which may infect tissues other than the liver, can also potentially be pathogenic.
  • Infection pressure is dependent on the suitability of the environment and microclimate for the snail intermediate host.
  • Many infections in horses go undiagnosed. 
  • Light infections are unlikely to cause significant disease.

Timecourse

  • The minimum pre-patent period (time between infection and appearance of eggs in feces) is 12 weeks.
  • This may be much longer in horses.

Epidemiology

  • Horses are rarely clinically affected, but infection may be relatively common. 
  • Epidemiology is highly dependent on the suitability of climate and habitat for the intermediate host, namely, snails of the genus Lymnea.
  • In most areas, the snails have a period of peak activity lasting a few months, coinciding with suitable temperature and moisture levels, which allows for one liver fluke life cycle per year.
  • The intermediate host stage of the parasite requires similar temperature and moisture.
  • Once the parasite egg reaches pasture, wet conditions are required to enable it to mature and hatch into a miracidium which then exits the fecal pat and makes its way towards an intermediate host. It penetrates the body of the snail host and undergoes several hundred-fold clonal replication. Cercariae then exit the snail and encyst on herbage as the infectious stage, metacercariae.
  • Metacercariae can survive up to several months although hot, dry or cold temperatures will kill a proportion. They do not survive for more than a few weeks in well-made hay.

Diagnosis

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Treatment

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Prevention

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Outcomes

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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 et al (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.
  • 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-gigantica. Onderstepoort 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. ISBN: 0851992609.

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