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Carryover of bovine leukemia virus antibodies in samples from shared milk meters
O A Nekouei1, J Sanchez1, G P Keefe1
1Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, Prince Edward Island, Canada C1A 4P3.
Insights
Antibody carryover in milk samples from shared meters can lead to inaccurate bovine leukemia virus (BLV) test results. Adjusting diagnostic test cut-off values and creating a suspicious category can improve accuracy for meter-collected samples.
Area of Science:
- Veterinary immunology and diagnostics
- Dairy cattle infectious disease screening
- Bovine leukemia virus (BLV) research
Background:
- Dairy Herd Improvement (DHI) sampling offers convenient infectious disease screening in cattle.
- Shared milk meters in DHI sampling can cause antibody carryover, complicating diagnostic test interpretation for diseases like bovine leukosis.
- Accurate BLV screening is crucial for herd health and management.
Purpose of the Study:
- To assess the potential for antibody carryover against bovine leukemia virus (BLV) in milk samples collected via shared meters.
- To determine if adjusting diagnostic test cut-off values can enhance test characteristics for meter-collected milk ELISA results.
- To evaluate strategies for mitigating false-positive results due to carryover.
Main Methods:
- Eight dairy farms with varying BLV prevalence were selected.
- Simultaneous hand-collected and meter-collected milk samples were taken from 236 cows across 26 meters.
- Samples were tested for BLV antibodies using indirect ELISA; carryover was assessed based on preceding cow titers and meter sample results.
Main Results:
- At the standard cut-off, meter-collected samples showed higher positive rates (57.6%) than hand-collected samples (46.6%).
- Antibody carryover increased false-positive likelihood in meter samples, particularly for low-titer cows preceded by high-titer cows.
- Defining a 'suspicious' category and recommending retests reduced false-positive results.
Conclusions:
- Antibody carryover in DHI milk meter samples can significantly impact BLV diagnostic test accuracy.
- Adjusting assay cut-off values and implementing a tiered testing strategy (including a suspicious category) can improve the reliability of meter-collected samples.
- Careful interpretation and potential adjustments are necessary when using DHI meter samples for infectious disease screening.
Abstract:
Screening for infectious diseases of cattle using milk from the dairy herd improvement (DHI) sampling process is very convenient. However, when samples from shared milk meters are used, carryover of antibodies or other diagnostic targets can complicate the interpretation of the diagnostic test results for diseases, including bovine leukosis. The objectives of this study were (1) to assess the potential for carryover of antibodies against bovine leukemia virus (BLV) in milk samples obtained from shared meters, and (2) to determine if adjustment of the diagnostic test cut-off value would improve the test characteristics for meter-collected milk ELISA results. Eight dairy farms were randomly selected from herds with a wide range of BLV prevalence levels in Prince Edward Island, Canada. Within each chosen farm, 2 to 4milk meters were randomly selected. During the routine procedures of DHI sampling, 2 simultaneous milk samples, 1 hand-collected at the beginning of milking (after udder preparation) and the other from the corresponding milk meter, were taken from all lactating cows (n=236) that were milked at the selected meters (n=26). The sequence of cows using each meter was recorded. All samples were tested for BLV antibodies using a commercial indirect ELISA. Antibody carryover potential was assessed in meter-collected samples which were preceded by other cows using the same meters. Applying the hand-collected sample results as our reference standard, a new cut-off was defined for meter-collected samples to optimize the test characteristics. At the standard cut-off value of the diagnostic test, 110 (46.6%) of the hand-collected and 136 (57.6%) of the meter-collected samples were positive. For low-titer cows (e.g., true negatives), the likelihood of antibody carryover significantly increased as the titer of preceding cows increased, whereas this change was not substantial for high-titer cows. The odds of obtaining false diagnoses in meter-positive samples became larger with increase in the titer of preceding cows. A suspicious category for meter ELISA results was defined, and a retest was recommended for the cows falling into this category. This strategy effectively assisted in reducing the number of consequent false-positive results. When DHI-collected samples are used, carryover can affect the interpretation of dichotomous test results and may require adjustment of assay cut-off values.
