A growing degree-day model for determination of Fasciola hepatica infection risk in New Zealand with future predictions using climate change models.

Infections of ruminants with Fasciola hepatica are considered to be of regional importance within New Zealand but there is very little recent information on its prevalence or severity other than anecdotal reports. Generally they are considered to be of secondary importance compared to gastrointestinal nematode infections. Utilizing data from Virtual Climate Stations (n=11491) distributed on a 5km grid around New Zealand a growing degree-day model was used to describe the risk of infection with liver fluke from 1972 to 2012 and then to apply the predictions to estimate the risk of fluke infections within New Zealand for the years 2040 and 2090. The growing degree-day model was validated against the most recent survey of infection within New Zealand in 1984. A strong positive linear relationship for 1984 between F. hepatica prevalence in lambs and infection risk (p<0.001; R(2)=0.71) was found indicating the model was effective for New Zealand. A linear regression for risk values from 14 regions in New Zealand for 1972-2012 did not show any discernible change in risk of infection over this time period (p>0.05). Post-hoc comparisons indicate the risk in Westland was found to be substantially higher (p<0.05) than all other regions with Northland ranked second highest. Notable predicted changes in F. hepatica infection risk in 2040 and 2090 were detected although they did vary between different climate change scenarios. The highest average percentage changes in infection risk were found in regions with low initial risk values such as Canterbury and Otago; in these regions 2090 infection risk is expected to rise by an average of 186% and 184%, respectively. Despite the already high levels of infection risk in Westland, values are expected to rise by a further 76% by 2090. The model does show some areas with little change with Taranaki predicted to experience only very minor increases in infection risk with average 2040 and 2090 predicted changes of 0% and 29%, respectively. Overall, these results suggest the significance of F. hepatica in New Zealand farming systems is probably underestimated and that this risk will generally increase with global warming following climate change.