Associations between Mycobacterium avium subsp. paratuberculosis antibodies in bulk tank milk, season of sampling and protocols for managing infected cows.

Casey L Cazer,Kellie M Cicconi-Hogan,Rebecca M Mitchell,Michael Gamroth,Pamela L Ruegg,Roxann M Richert,Ynte H Schukken

doi:10.1186/1746-6148-9-234

Abstract

BackgroundThe objective of this study was to identify associations between the concentration of Mycobacterium avium subsp. paratuberculosis (MAP) antibodies in bulk milk and potential risk factors in herd management and herd characteristics, explaining high MAP antibody titers in milk. An extensive questionnaire was administered to 292 organic and conventional dairy farms from New York, Wisconsin and Oregon. Bulk milk samples were taken from each farm for MAP enzyme-linked immunosorbent assay (ELISA). A general linear model was constructed with MAP ELISA value as the outcome variable and the management factors and herd characteristics as independent variables, while at the same time controlling for the study design variables of state, herd size, and production system (organic or conventional). High bulk tank MAP ELISA value may be due to either a high prevalence of MAP in a herd with many cows contributing to the antibody titer or due to a few infected cows that produce large quantities of antibodies.ResultsResults of the regression models indicated that bulk milk ELISA value was associated with season of sampling and the presence or absence of protocols for managing MAP-positive cows. The concentration of MAP antibodies in bulk milk varied seasonally with a peak in the summer and low concentrations in the winter months. When compared to farms that had never observed clinical Johne’s disease, keeping MAP-positive cows or only culling them after a period of delay was associated with an increase in optical density.ConclusionsThe seasonal variation in MAP antibody titers, with a peak in the summer, may be due to a seasonal increase in MAP-bacterial load. Additionally, seasonal calving practices may contribute to seasonal fluctuations in MAP antibody titers in bulk tank milk. Keeping MAP-positive cows increases the antibody titer in bulk milk, likely due to direct antibody production in the infected cow and indirect triggering of antibody production in herdmates.

Highlights

The objective of this study was to identify associations between the concentration of Mycobacterium avium subsp. paratuberculosis (MAP) antibodies in bulk milk and potential risk factors in herd management and herd characteristics, explaining high MAP antibody titers in milk
Johne’s disease, a chronic disease caused by infection with Mycobacterium avium subsp. paratuberculosis (MAP), costs the US dairy industry $200 to $250 million annually due to increased cow replacement costs and reduction in milk production [1] and decreased fertility in highshedding animals [2]
Van Weering et al [10] demonstrated that certified MAP-negative herds had a low sample/positive (S/P) ratio on bulk milk Pourquier enzyme-linked immunosorbent assay (ELISA) tests and they showed that the likelihood of a herd having a MAP-infected animal increased with increasing bulk milk ELISA S/P ratio

Summary

Introduction

The objective of this study was to identify associations between the concentration of Mycobacterium avium subsp. paratuberculosis (MAP) antibodies in bulk milk and potential risk factors in herd management and herd characteristics, explaining high MAP antibody titers in milk. The magnitude of an ELISA test result for MAP antibodies in the milk of individual cows has been reported to be related to the likelihood of an animal testing positive on a fecal culture for MAP [7]. Van Weering et al [10] demonstrated that certified MAP-negative herds had a low sample/positive (S/P) ratio on bulk milk Pourquier ELISA tests and they showed that the likelihood of a herd having a MAP-infected animal increased with increasing bulk milk ELISA S/P ratio. The sensitivity can be improved by using modified protocols [8,12] Together, these published results on bulk milk and individual cow milk provide a logical validation for the use of bulk milk ELISA corrected optical density (OD) as a continuous outcome value to scale the risk of MAP infection in the lactating herd

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