Relationship between presence of cows with milk positive for Mycobacterium avium subsp. paratuberculosis-specific antibody by enzyme-linked immunosorbent assay and viable M. avium subsp. paratuberculosis in dust in cattle barns

Abstract

Paratuberculosis, or Johne's disease, in cattle is caused by Mycobacterium avium subsp. paratuberculosis, which has recently been suspected to be transmitted through dust. This longitudinal study on eight commercial M. avium subsp. paratuberculosis-positive dairy farms studied the relationship between the number of cows with M. avium subsp. paratuberculosis antibody-positive milk and the presence of viable M. avium subsp. paratuberculosis in settled-dust samples, including their temporal relationship. Milk and dust samples were collected in parallel monthly for 2 years. M. avium subsp. paratuberculosis antibodies in milk were measured by enzyme-linked immunosorbent assay (ELISA) and used as a proxy for M. avium subsp. paratuberculosis shedding. Settled-dust samples were collected by using electrostatic dust collectors (EDCs) at six locations in housing for dairy cattle and young stock. The presence of viable M. avium subsp. paratuberculosis was identified by liquid culture and PCR. The results showed a positive relationship (odds ratio [OR], 1.2) between the number of cows with ELISA-positive milk and the odds of having positive EDCs in the same airspace as the adult dairy cattle. Moreover, the total number of lactating cows also showed an OR slightly above 1. This relationship remained the same for settled-dust samples collected up to 2 months before or after the time of milk sampling. The results suggest that removal of adult cows with milk positive for M. avium subsp. paratuberculosis-specific antibody by ELISA might result in a decrease in the presence of viable M. avium subsp. paratuberculosis in dust and therefore in the environment. However, this decrease is likely delayed by several weeks at least. In addition, the data support the notion that M. avium subsp. paratuberculosis exposure of young stock is reduced by separate housing.

Fig 1

Different relationships tested between the ELISA results and the number of positive EDCs to investigate what number of M. avium subsp. paratuberculosis milk ELISA-positive cows per TD sampling was most related to EDCs positive for viable M. avium subsp. paratuberculosis. The EDC results were modeled in relation to milk samples from the same TD (1) and at −4 weeks (2), −8 weeks (3), +4 weeks (4), and +8 weeks (5). The numbers at the top of the scales represent periods of dust sampling, while the numbers on the lower side of the scales represent milk sampling. The ovals represent the shifting of the number of the ELISA-positive cows forwards and backwards in time relative to the EDC observation. The model numbers are listed below the scale in parentheses (next to the relevant oval).

Fig 2

(a to h) Number of cows with milk positive for M. avium subsp. paratuberculosis-specific antibody (MAP) by ELISA (⧫) and the number of EDCs positive for viable M. avium subsp. paratuberculosis detected per farm per TD. Farm 6 moved to a different location during the study (July 2010) (asterisk). Farms 3 (c) and 7 (g) had all EDCs under the same roof. Summer periods at each farm are underlined on the x axis.