The seroprevalence and spatial distribution of liver fluke infection in a sample of West Coast and Canterbury dairy herds

The West Coast region of the South Island of New Zealand is recognised as having a high prevalence of liver fluke infection, however, few studies have tested this assumption. This study aimed to estimate the seroprevalence and spatial distribution of liver fluke infection in most West Coast and some Canterbury dairy herds using bulk milk ELISA. Herds were bulk milk sampled on three occasions, twice, a week apart in March 2017 (n = 430 and n = 99) and once in October 2017 (n = 412). The concentration of liver fluke antibodies was estimated using the IDEXX Fasciolosis Verification ELISA (IDEXX Europe BV, Hoofdorp, The Netherlands) on these bulk milk samples. Herds were categorised according to the test instructions, with "medium" (≥20 % cows infected) or "strong" (≥50 % cows infected) results designated as production limiting. All herds were also sent a short questionnaire about awareness of liver fluke and treatment in their herds. The agreement between the IDEXX results for 99 herds sampled twice in March 2017, and between the IDEXX results for March and October for all herds was assessed with a Kappa test. There was a substantial agreement between the two March tests, kappa = 0.7 (95 % CI 0.58-0.82) and moderate agreement between the March and October samples kappa = 0.41 (95 % CI 0.34-0.48). The results of the questionnaire were linked to the IDEXX results, and the distribution of herds categorised by their IDEXX result plotted. At the March and October samplings 0/35 (0 %) and 0/27 (0 %) Canterbury herds, 248/395 (63 %) and 193/385 (50 %) of West Coast herds had a production limiting seroprevalence of liver fluke infection. Mapping showed that West Coast herds with a production limiting infection were clustered in coastal areas, whereas those without, were clustered in inland valleys. The odds of a coastal West Coast dairy herd having a production limiting seroprevalence of liver fluke were 10.7 (95 % CI, 6.6-17.3) times that of an inland herd in March and 8.6 (95 % CI, 5.4-13.8) times in October. There was no effect on IDEXX ELISA SP% (p = 0.92) of herds which were treated for liver fluke at dry off soon after the March test compared to their SP% in the following October test, soon after calving. The results show that liver fluke infection is a potentially serious problem for West Coast dairy herds especially for those on coastal river plains prone to flooding.