Comparison of Diagnostic Testing Strategies for Failure of Passive Transfer (FPT) in Scottish Dairy Calves

Denholm, K. , Haggerty, A., Ellis, K. and Mason, C. (2022) Comparison of Diagnostic Testing Strategies for Failure of Passive Transfer (FPT) in Scottish Dairy Calves. 31st World Buiatrics Congress 2022, Madrid, Spain, 4-8 September 2022. pp. 184-185.

Abstract

In dairy calves, failure of passive transfer (FPT) results in an increased risk of calfhood morbidity and mortality (Tyler et al. 1999; Pardon et al. 2015). Furthermore, calves with FPT have reduced liveweight gains following weaning (Furman-Fratczak et al. 2011) and reduced first milking lactation yields (Denise et al. 1989). Background: Serum IgG levels of ≥10g/L are indicative of adequate passive transfer (Tyler et al. 1999; Virtala et al. 1999). The reference test for measuring IgG in serum is radial immunodiffusion (RID) which directly measures the IgG in a sample (Cuttance et al. 2017). In Scotland, this test is prohibitively expensive so indirect measures such as total protein (TP) tests are often substituted. Zinc sulphate turbidity (ZST) testing is another indirect measurement of IgG in serum (Hogan et al. 2015) and is commonly used in Scotland by clinicians and disease investigation centres to diagnose FPT in calves; despite a growing body of evidence to suggest its inferiority to other tests (Hogan et al. 2015), in particular its poor specificity. Brix refractometry has been used as an inexpensive and convenient calf side measure of FPT, which can also be used to measure colostrum quality (Quigley et al. 2013). Objectives: To explore the performance of various diagnostic testing strategies for FPT in Scottish dairy calves. To redefine optimal cutpoints for indirect testing strategies, including Brix refractometry, total protein (TP) and zinc sulphate turbidity (ZST). To challenge the status quo of indirect measures of FPT offered in commercial laboratories and disease investigation centres in Scotland. Materials and methods: Serum samples were collected from 370 calves between 24 hours and 7 days old from 38 dairy farms in Lanarkshire, Stirlingshire and Dumfries and Galloway. Calves were excluded if they exhibited signs of ill health; in particular, dehydration. Samples were tested using ZST, Brix refractometry, TP and RID. Receiver operating characteristic (ROC) curve analysis was used to determine the optimal cutpoints for correctly predicting FPT using concentrations of TP, percentage Brix and ZST. The optimal cut-point for each test was calculated using the Youden’s index, where the sum of sensitivity plus specificity is maximised and equal weight is given to false positive and false negative results. Results: False positive rates were high using indirect testing, particularly when ZST was used. FPT prevalence was 29.5%, 30.8% and 46.4% for TP, Brix and ZST respectively, compared with 14.1% for direct measurement of IgG concentration using RID. The results from this work confirm the poor test specificity of the ZST test and recommend a lowered cutpoint to improve test performance, in line with other published, international work. Existing cutpoints (from international literature) for indirect FPT tests were redefined using ROC (receiver operating curve) analysis and the accuracy of each test improved. Conclusions: Despite over-estimation of the prevalence of FPT in this population of calves, it is still useful to monitor FPT using indirect tests due to their convenience and cost effectiveness; however, it is recommended that test cutpoints are lowered for all indirect measures, particularly ZST to improve performance. It is recommended that clinicians take at minimum of 12 serum samples from a cohort of 1-7 day old calves at regular intervals to monitor the effects of any interventions to reduce the prevalence of FPT on a farm. Over-interpretation of indirect testing at one time point is a risk, especially if low numbers of calves are sampled.

• Item Type: Conference or Workshop Item
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Haggerty, Alexandra and Denholm, Mrs Katie and Ellis, Dr Kathryn
• Authors: Denholm, K., Haggerty, A., Ellis, K., and Mason, C.
• College/School: College of Medical Veterinary and Life Sciences > School of Biodiversity, One Health & Veterinary Medicine