Exposure to liver fluke occurs frequently among horses in Britain, researchers report, and it may be an under‐recognised cause of liver disease.
Alison Howell and her colleagues, in a study reported in the Equine Veterinary Journal, set out to learn more about liver fluke infection among Britain’s horses.
Liver fluke, or Fasciola hepatica, is a common and widespread pathogen, well‐known for its effects on the health and productivity of ruminants.
Horses frequently graze the same pastures as sheep and cattle but are thought to be relatively resistant to liver fluke infection.
It is thought that fluke infections in horses often do not reach maturity and, even in those that do, eggs may not be excreted in their faeces.
This leads to difficulty in diagnosis using faecal egg detection methods.
However, there is evidence from case reports and experimental infections that horses can be adversely affected by liver flukes, with clinical signs including poor performance, fatigue, diarrhoea, poor appetite and jaundice.
At the same time, liver disease is common in horses and the cause can often be difficult to find.
These factors together led to a concern that liver fluke in horses may be responsible for some cases of undiagnosed equine liver disease.
The University of Liverpool study – the first to look at liver flukes in the UK‐wide horse population – had several phases, with the researchers using a cattle test for antibodies to liver flukes modified for use in horses.
Nine visits were made to an abattoir in England. On each visit, between 20 and 69 horses were slaughtered.
The liver of each animal was externally inspected for liver flukes. Only external inspection was possible because the meat inspection protocol in horses does not include incising the liver.
A total of 342 horses presenting to the abattoir were examined for fluke, of which 224 came from the UK and 104 from Ireland.
In total, testing showed that 9.8% (22) of the UK horses returned a positive antibody test for liver flukes, and 1.8% (4 horses) had flukes which could be seen in the liver by the scientists visiting the abattoir.
Eighteen percent (41) of the horses were New Forest ponies being culled for population control, and of these, 15% (6) tested positive for flukes.
Excluding the New Forest cull ponies from the overall UK abattoir population gave a fluke‐visual prevalence of 2.2% (4) and a sero‐prevalence of 8.7% (16).
The four livers in which flukes were seen were dissected along the bile ducts to extract all of the flukes. They had 3, 7, 31 and 42 flukes, respectively. The two horses with the heaviest burdens had visible thickening of the bile ducts, but no other pathological changes in their livers.
Of the Irish horses, 3.9% (4) tested positive based on the antibody test. No Irish horses had flukes found in the liver at post mortem examination.
The study team also sought out horses considered by their treating vet to have clinical signs and/or blood test results consistent with liver disease. These were recruited by advertising through social media and the veterinary and equine press.
A total of 109 cases, submitted from 49 vet practices, were considered suitable to use in the study.
Seventeen horses tested positive for liver fluke. This represented 11% of the horses with liver disease. A total of 180 control samples were tested, of which 2.5% returned positive results to the antibody test.
This indicated that horses with liver disease had significantly higher odds of showing seropositivity to liver flukes.
“We have demonstrated that exposure to liver fluke occurs frequently in horses and that it may be an under‐recognised cause of liver disease,” the study team concluded.
They also showed that the flukes isolated from horses are from the same population as those found in ruminants. “Horses, sheep and cattle often share the same grazing, which enables high levels of gene flow between flukes infecting the different species.
“This raises the question of whether the recommendation to co‐graze horses with ruminants, as a strategy to reduce nematode egg burden on pasture, is still appropriate.
“We believe that in most situations, the benefits of co‐grazing for nematode control outweigh the potential risk from liver fluke.
“The decision of whether to co‐graze should be made after considering the local environment and liver fluke infection history.”
They noted that many horses grazed with infected cattle and sheep without showing signs of infection.
“Whilst liver fluke infection in the horse can cause serious and chronic disease, if it is diagnosed early it can be easily and effectively treated.”
In the fluke-positive horses, clinical signs and serum biochemistry results were vague and variable, whilst several horses had no detectable abnormalities.
This, they said, was consistent with other studies.
“Although some abnormalities would be expected in the serum biochemistry of a liver-fluke-infected horse, the exact abnormalities would depend on the stage and intensity of infection and thus are likely to be more useful as a prognostic rather than as a diagnostic aid.”
Parasite diagnosis and control programmes should include consideration of fluke infection in certain circumstances, they said.
“A pragmatic approach to co‐grazing on fluke‐risk pasture is to remain aware of the potential for liver fluke infection, and to test and/or treat promptly if signs consistent with liver fluke are seen.”
Fasciola hepatica in UK horses
A.K. Howell F. Malalana N.J. Beesley J.E. Hodgkinson H. Rhodes M. Sekiya D. Archer H.E. Clough P. Gilmore D.J.L. Williams