Carryover of bovine leukemia virus antibodies in samples from shared milk meters

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.

DNA carryover in milk samples from routine milk recording used for PCR-based diagnosis of bovine Staphylococcus aureus mastitis

“…aureus would be expected to be low at higher Ct-values and minor carryover may easily affect or modulate the Ct-value for cows milked subsequently using the same milk meter. Nekouei et al (2015) have reported a similar finding for the diagnosis of bovine leukemia virus based on the ELISA. The authors concluded that carryover of bovine leukemia virus antibodies at shared milk meters was significant.…”

“…It is very difficult to avoid carryover during test sampling at routine milk recordings for many reasons, including high costs or interference with test results. These issues have been thoroughly described by Nekouei et al (2015) and Walsh et al (2013) for the detection of leucosis.…”

“…aureus IMI at routine sample collection. Nekouei et al (2015) reported that carryover of antibodies or other diagnostic targets (e.g., DNA) could complicate interpretation of the diagnostic test results for diseases when samples from shared milk meters are used. Carryover could bias the diagnosis of Staph.…”

Dairy Cows Experimentally Infected With Bovine Leukemia Virus Showed an Increased Milk Production in Lactation Numbers 3–4: A 4-Year Longitudinal Study

“…Evermann et al found that the experimental calves seroconverted to BLV within 8 and 14 weeks, respectively, depending on the equivalent of inoculation (Evermann et al, 1986 ). Moreover, it is convenient to screen for BLV infection using milk from the DHI sampling process (Nekouei et al, 2015b ). In this study, the anti-gp51 antibody was available to be detected since 45 DPI, which emphasized the early diagnostic importance of real-time qPCR.…”

Diagnostic Measures of Disease Progression in Cattle Following Natural Infection with Bovine Leukemia Virus

“…The ELISA assay was categorized as negative, suspect, or positive based on a numerical optical density (OD) reading. The general purpose of the suspect category was to express caution in the test result because residual milk carryover (milk from the previous cow tested) may cause false positives [ 40 ]. However, in this study, 85% (131/154) of ELISA-suspect observations were PVL positive.…”