Six-year longitudinal study of Fasciola hepatica bulk milk antibody ELISA in the dairy dense region of the Republic Ireland.

Completion of the F. hepatica lifecycle is dependent on suitable climatic conditions for development of immature stages of the parasite, and its snail intermediate host. Few investigations have been conducted regarding temporal variations in F. hepatica status in Irish dairy herds. The current study aimed to conduct a longitudinal study examining annual and seasonal trends in bulk milk seropositivity over six years, while also investigating associations with soil temperature, rainfall and flukicide treatment. Monthly bulk milk samples (BTM) were submitted by 28 herds between March 2009 and December 2014. In all, 1337 samples were analysed using a Cathepsin L1 ELISA. Soil temperature, rainfall and management data were obtained for general estimating equation and regression analyses. A general decrease in milk seropositivity was observed over the six year study period and was associated with an increased likelihood of treating for liver fluke (OR range=2.73-6.96). Annual and seasonal analyses of rainfall and F. hepatica BTM status yielded conflicting results. Higher annual rainfall (>1150mm) yielded a lower likelihood of being BTM positive than annual rainfall of <1000mm (OR=0.47; P=0.036). This was most likely due to farmers being more proactive in treating for F. hepatica in wetter years, although a 'wash effect' by high rainfall of the free living stages and snails cannot be ruled out. Higher seasonal rainfall (>120mm), however, was associated with increased ELISA S/P% values (Coefficient=9.63S/P%; P=0.001). Soil temperature was not found to influence F. hepatica to the same extent as rainfall and may reflect the lack of severe temperature fluctuations in Ireland. Flukicides active against both immature and mature F. hepatica were approximately half as likely to record a positive F. hepatica herd BTM status than a flukicide active against only the mature stage of the parasite (OR≅0.45; P<0.01). This study highlights the importance of examining both annual and seasonal F. hepatica data, which can vary significantly. Additionally, it highlights the progress that can be achieved in fluke control by application of a continuous BTM monitoring program.