The number of reported liver fluke cases soared in the last quarter of 2012, compared to the same period in 2011, according to Animal Health and Veterinary Laboratories Agency (AHVLA) figures.
Between October and December 2011, AHVLA said there was seven diagnosed cases of liver fluke, a figure that increased to 69 over the same three-month period last year. Experts say these high figures are down to mild, wet weather.
"The wet summer followed by the mild winter has provided perfect breeding conditions for fluke," said Neil Roberts, partner at a vet group in Yorkshire.
"One of the problems farmers have is there is a variety of fluke treatments and some will only kill the adult flukes. They also all have a 56-day meat withdrawal on them, so this is a problem for those fattening lambs."
Mr Roberts added that farmers should not rely on triclabendazole products all to time, to avoid developing resistance.
The Sustainable Control of Parasites in Sheep (SCOPS) group has urged farmers to continue to monitor their flocks for symptoms of fluke infection, which include an unexpected drop in fertility levels or growth rates, pale gums and bottlejaw.
The parasites fluke metacercarieae thrive in mild wet conditions, and are only killed when exposed to temperatures below -18°C for several days.