Variation of the milk antibody response to paratuberculosis in naturally infected dairy cows

A longitudinal study was performed to determine the course of the milk antibody response in cows presumably infected with Mycobacterium avium subsp. paratuberculosis. Milk samples were collected repeatedly (1 to 10 times) from all lactating cows in seven Danish dairy herds. A total of 4,289 observations from 812 cows was analyzed after exclusion of samples collected after 280 days in milk (DIM). The level of antibodies in the milk samples was assessed using an indirect ELISA. A piece-wise linear random coefficient regression model was specified. The model controlled for the effect of herd, breed, laboratory effects, and age at first calving to estimate parity-specific antibody responses in relation to DIM. Separate antibody profiles were estimated for fecal culture-positive and fecal culture-negative cows. The resulting population average models showed higher antibody levels for fecal culture-positive cows and higher antibody levels with increasing parity. On average, the antibody response was high at the beginning and end of lactation. However, evaluating the cows individually indicated that most cows actually had quite stable ELISA levels throughout lactation, with some cows having higher levels than others. Thus, two criteria seem applicable to assess whether a cow is infected: stability and ELISA level. The random coefficients for each cow were highly significant. Thus, the study suggests that all cows can be classified into one of the four categories by combining the cow-level ELISA characteristics "stability" and "level" as an aid in the diagnosis ofparatuberculosis and thereby substantially increasing the sensitivity of the ELISA.