The association of milk-solid production during the current lactation with liver damage due to presumptive ingestion of spores from Pithomyces chatarum by dairy cattle.

AIMS To determine the association between production of milk solids (MS) and liver damage from facial eczema (FE) in dairy cattle during autumn and to determine the most practical cut-off for serum gamma-glutamyl transferase (GGT) activity in predicting production loss. METHODS Farm history and Pithomyces chartarum spore counts identified herds likely to be affected by raised GGT activity in serum during autumn 2018 or 2019. In these herds, a pilot blood sample from 30 cattle was collected, followed by a full herd blood test within 2 weeks if in those 30 cattle one or more had GGT activities >300 IU/L. Individual MS production was measured within -5 - +12 days of a full herd blood test. Information about feeding Brassica spp. was collected from the farmer. Pooled sera from 10 randomly selected cattle from 10/11 farms with GGT >40 IU/L were tested for anti-Fasciola antibodies. STATISTICAL ANALYSIS The association of liver damage and production of MS was analysed using mixed linear regression. Potential risk factors included farm, cow age, MS at last herd test before the likely FE risk period, breed of cow and GGT activity. Subsequently, GGT activity thresholds, from 40-400 IU/L, were used to indicate varying severities of liver damage. For each threshold, a mixed linear model using herd test data produced estimated marginal mean differences in MS production for cows above or below threshold. The prevalence of animals above threshold was multiplied by the per cow loss to obtain the reduction in MS/day/100 cows for each cut-off. RESULTS The prevalence of animals with GGT activities > 40 IU/L ranged between farms from 11% (45/488) to 96% (139/145), and GGT activities for individual cows ranged from 3 - 6001 IU/L. From the model, an increase of 100 IU/L in GGT activity was associated with a decrease of 0.011 (95% CI = 0.010-0.012) kg MS/cow/day. A GGT activity threshold of 40 IU/L identified the largest association with MS production of 6.14 kg MS/day/100 cows. No evidence of significant liver fluke or brassica toxicosis was found. CONCLUSION AND CLINICAL RELEVANCE Liver damage was most likely caused by sporidesmin toxicity and was associated with substantial linear reduction in MS., When assessing the impact liver damage has on herd milk production, threshold and prevalence of animals exceeding threshold should be considered by the practitioner in assessing economically significant facial eczema.