Abstract:
The influence of milk yield and milk composition
on the diagnosis of Mycobacterium avium ssp. paratuberculosis
(MAP) by milk ELISA in the context of
the total IgG secretion patterns in milk throughout
lactation and serum concentrations were investigated.
A 2-yr trial was performed in which 1,410 dairy cows
were sampled monthly and MAP milk ELISA status
and milk yield and composition were determined. Data
were analyzed by mixed model analysis. Milk yield
was found to significantly influence ELISA results
expressed as sample-to-positive (S/P) ratios. For each
5-kg increase in milk, the S/P ratio has to be multiplied
by 0.89; therefore, high milk yield can change the MAP
milk ELISA outcome of a cow in early infection from
positive to negative. Parity influenced ELISA outcome
significantly, indicating that cows with a parity >1 are
more likely to be identified by milk testing. Also, herd
was an important predictor, showing that herd prevalence
influences the milk ELISA strongly. Other factors
influencing the S/P ratios were protein concentration,
somatic cell count, and days in milk. The IgG concentration
and mass excreted per day were determined
longitudinally in a subset of 41 cows of which samples
and data of a complete lactation were available. Again,
the IgG concentration in milk was mainly influenced by milk yield. The total IgG mass secreted per day in milk
was found to be relatively constant, with a mean of
8.70 ± 5.38 g despite an increasing IgG concentration
in serum at the same time. The variation of IgG concentration
in milk can be mainly attributed to dilution
through changes in milk yield. This supports the assumption
that concentrations of MAP-specific antibodies
are influenced by changes in milk yield similarly. In
conclusion, we confirmed that antibody concentrations, and therefore MAP ELISA outcome, were influenced by
milk yield, herd, and parity. To enhance performance,
milk ELISA tests should either be performed in early or
late lactation, when milk yield is low. From a management
perspective, sampling should be done during early
lactation before cows are bred again. Based on the slow
progressive infection dynamics, only first-parity cows
should be preferentially tested at the end of their first
lactation to avoid false-negative results.
