The prevalence of gross pathological damage in the livers of dairy cattle at processing plants in autumn in the North Island of New Zealand and an assessment of the gross liver pathology score as a method for estimating the prevalence of facial eczema

ABSTRACT

Aim

To measure the prevalence of gross pathological damage in the livers of dairy cows at slaughter in the North Island of New Zealand in 2018 and 2019 and to determine, using Bayesian latent class analysis, the specificity and sensitivity of gross liver pathology score (GLS) as a method for detecting moderate to severe facial eczema (FE) at processing plants.

Method

Meat inspectors at four processing plants located in the Northland, Waikato, Bay of Plenty and Taranaki regions of New Zealand graded all dairy cattle livers on particular days in April and May of 2018 and 2019 using a gross liver pathology score (GLS) from 0–5 (0 = no damage, 1= fatty liver, 2 = cholangitis and early cirrhosis, 3 = cirrhosis, 4 = extensive cirrhosis, 5 = regeneration). Forty livers from each grade were selected for histopathology. Sections from the apical margin of the right and the left lobes were scored using a histology biliary score (HBS; scored from 1–16) based on the classical histological lesions of FE focussing on changes that differentiate FE from fasciolosis. Mean HBS was collapsed into scores 2–6 (no and mild damage) and score 7–12 (moderate and severe) and GLS was collapsed into grades 0–2 (no or mild damage) and grades 3–5 (moderate, severe and chronic damage). A Bayesian latent class model was developed to estimate sensitivity and specificity of HBS and GLS. The diagnostic target was moderate or severe, gross and/or histological changes in the liver consistent with FE.

Results

A total of 2,899 dairy cow livers were graded at the four plants over the two study periods. There were 700/2,899 (24.1%) livers with at least some form of gross pathology damage (GLS≥1) and 130/2,899 (4.5%) livers with moderate, severe or chronic gross pathology damage (GLS≥3). The estimates (posterior median) for the sensitivity and specificity of GLS to liver damage were 0.844 (95% credible interval (CrI) = 0.757–0.905) and 0.932 (95% CrI = 0.866–0.973) respectively. Estimates for HBS were 0.834 (95% CrI = 0.765–0.892) and 0.778 (95% CrI = 0.707–0.854), respectively.

Conclusion and clinical relevance

GLS is a useful means of estimating liver damage consistent with FE at slaughter. Increased use of liver scoring at slaughter could be useful for monitoring the likely impacts of FE at the country and regional levels.

KEYWORDS:

• Facial eczema
• liver damage
• gross pathology
• histology
• processing plants

Acknowledgements

We would like to acknowledge Ministry of Primary Industries through Sustainable Farming Fund, New Zealand dairy farmers through DairyNZ Inc (Hamilton, NZ), Veterinary Enterprises Group Ltd, Fonterra, AgriTrade Ltd, CRV Ambreed, Dairy Cattle Veterinarians Society of the NZVA and Massey University for funding this study. We also express our sincere gratitude to the editor who provided constructive opinions on the choice of the uniform prior. Thank you to Gribbles Pathology and AsureQuality for their contribution to the collection of data for this study.

Notes

1 M. Lepherd, Gribbles Veterinary, Christchurch, NZ.

2 Supplemental data for this article can be accessed at https://doi.org/10.1080/00480169.2020.1826365.