27 July 2011

4 minute read

Europe

The liver fluke, Fasciola hepatica, is a highly pathogenic flatworm parasite responsible for severe production-limiting disease in both sheep and cattle. Acute fluke can cause the sudden death of previously healthy animals, especially sheep, whereas chronic fluke can significantly reduce lambing and calving percentages and liveweight gain in sheep and beef cattle.

The incidence of fluke has increased dramatically in Scotland over the past 10-15 years and the disease has spread from west to east and into previously fluke-free areas.

One of the main problems with current fluke control regimes is the lack of sensitive and convenient diagnostic tests for use in the field and for monitoring the effectiveness of flukicide treatment.

A new diagnostic test for detection of fluke, the so-called coproantigen ELISA, has recently been produced in Belgium. This test can detect tiny quantities of fluke secretions in the faeces of the host animal. Because these secretions are only produced by live fluke, this test should be a very accurate and an immediate indicator of fluke infection and, possibly, treatment success.

Dr Philip Skuce, Senior Research Scientist at Moredun Research Institute, said examination of suspect cases post mortem or carcasses at slaughter provides evidence of fluke or fluke damage. This can provide useful feedback to the farmer but does not help make a diagnosis in the live animal.

"Blood-based tests, such as detection of liver enzymes or antibodies to liver fluke, have several disadvantages, most importantly the fact that a blood sample is needed. In addition, blood-based assays cannot be used to evaluate treatment success because they can stay positive long after the fluke are killed," said Dr Skuce.

"Faecal egg counting, a routine diagnostic method for roundworms, does not work well in fluke. This is primarily because the test only identifies egg laying adult parasites, not the juvenile fluke that cause most of the damage, especially in sheep.

"Moreover, fluke do not release eggs consistently. Once released, fluke eggs can be stored in the gall bladder and so do not appear consistently in the animal’s faeces. As a result, an animal with an active fluke infection may have a negative egg count or, conversely, an animal may have a positive egg count after successful flukicide treatment.

"Triclablendazole (TCBZ) is the drug of choice for acute fluke because of its activity against the very damaging young fluke (~ 2 days old), but there are several anecdotal reports of TCBZ treatment failure. Therefore, a test that indicates treatment success reliably is urgently needed."

Two farms with a history of fluke infection and reported lack of TCBZ efficacy were recruited for this study. On each farm, sheep were randomly divided over two treatment groups. One group was treated with TCBZ and the other group was treated with an alternative flukicide, Closantel.

All faecal samples were tested by the traditional faecal egg count (FEC) method and by the new coproantigen ELISA.

Both tests gave essentially the same overall result: Closantel was effective in both flocks, but TCBZ was not. Animals from the TCBZ treated groups were subsequently retreated with Closantel and again, both tests showed that Closantel had worked in both flocks.

Dr Skuce said: "Although both tests gave essentially the same overall result, there were several differences between the tests. For example, the coproantigen ELISA was easier and faster to conduct in the laboratory. Furthermore, the coproantigen ELISA clearly showed treatment outcome as early as seven days post treatment, whilst fluke eggs were still detected in several ewes at that time.

"So, the coproantigen ELISA allows for rapid follow-up after treatment. This is important because it allows animals that do not respond to treatment to be identified so that they can be retreated with a different and more effective product before disease occurs. This study also reinforces the need to monitor the efficacy of any flukicide treatment to promote best practice for parasite control.

"Given the success of the QMS-funded evaluation of the coproantigen ELISA on two farms, we would like to use this test to diagnose fluke infection and determine flukicide efficacy on a larger number of sheep farms across Scotland."

To make this possible the next step will be to evaluate whether this test can be used on pooled or composite faecal samples.

"We want to determine the feasibility of using farmer-collected composite samples to estimate fluke prevalence and flukicide efficacy using the coproantigen ELISA. This would require sampling of faeces from a group of 20 sheep, followed by treatment with flukicide, and a post-treatment sampling of the same 20 sheep," added Dr Skuce.

Sampling kits would be shipped by mail in the same manner as for roundworm detection. Veterinarians or farmers interested in participating in this study should contact [email protected] or [email protected].

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