Because the liver fluke spends much of its complicated life cycle outside on pasture – either as eggs, cysts or larval stages – inside its mud snail intermediate host, its prevalence, seasonality and geographic spread are largely dictated by the prevailing weather conditions, especially temperature and rainfall.
Dr Philip Skuce, principal scientist at the Moredun Research Institute, Edinburgh, says the wet summer of 2012 and the ‘horrendous’ fluke season which followed, are remembered all too well.
However, he adds, since then summers have been drier but there are still implications for fluke risk to consider this year.
Dr Skuce says: “The pioneering work of Dr C.B. Ollerenshaw and colleagues at the central veterinary laboratory in Weybridge, back in the 1950s, established most liver fluke infections in the UK at the time were as a result of summer infection of snails, with little or no infection surviving the harsh winters.
“However, that pattern of infection seems to have changed somewhat in recent years after a run of significantly warmer and wetter winters. This allows at least some of the snails to survive and carry infection into the spring.
“Perhaps more significantly, it also allows a larger proportion of the infective cysts to survive on pasture and, with many farmers taking advantage of grazing opportunities mild winters present, animals will still be at risk from picking up fluke infection over winter and into spring.”
Dr Skuce says this was certainly the case in 2012 to 2013 and there have been several anecdotal accounts of acute fluke disease in sheep in June this year – three to four months earlier than farmers and scientists would expect to see it.
He says: “This can only have come about as a result of sheep picking up over-wintered cysts or fresh cysts shed by over-wintering snails. This being said, the last two summers have been significantly hotter and drier than usual, so we would anticipate less infection of snails and fewer infectious cysts going onto pasture in autumn.”
Dr Skuce says this means the ‘typical’ fluke infection risk should be lower than of late, and certainly lower than 2012, but more of the infection will have survived winter and could cause problems in spring and summer.
“Fluke risk has, therefore, become a year-round issue and we know it changes from year-to-year and farm-to-farm.”
In terms of fluke control this year, Dr Skuce advises sheep farmers to determine the fluke risk on individual farms and work closely with
the farm vet or animal health adviser to include fluke control in annual animal health planning.
He says: “Practical farm management practices, such as improving drainage of low lying ground, rolling poached areas, removing reeds and rushes, fencing ‘fluky’ areas – i.e. anything to reduce potential snail habitats and interaction between snails and livestock – will help improve your pasture and reduce the overall fluke burden on your farm.”
Dr Skuce says it is also a good idea to monitor sheep over the course of the season to build up a picture of fluke on your individual farm.
“This year’s lambs make ideal ‘sentinels’ for doing this, as they will not have seen fluke before. Collecting fresh faecal samples off the ground and submitting them for fluke egg counting is an excellent starting point for doing this.
“If you decide to treat, make sure you use the right product at the right dose at the right time. Triclabendazole remains the drug of choice for treating sheep for fluke in the autumn, as it can kill fluke right down to about two days old, but check it is working effectively for you by conducting a post-drench check or, better still, a faecal egg count reduction test. Your vet will be able to advise on this.
“There are reports of triclabendazole failure in the field and resistance to this product has been confirmed on some farms, but equally, it still works well on other farms. We have heard numerous reports of farmers switching to less suitable products without testing the efficacy of either.
“If you use a product other than triclabendazole, for example closantel or nitroxynil, these can only kill fluke down to about six to seven weeks of age, so be aware you may have to follow up with further treatments if your sheep are still out grazing.”