A longitudinal study on the impact of Johne's disease status on milk production in individual cows.

Discipline: disease; Keywords: pathogen, faecal culture, blood serum tests, economic losses, culling.

How does infection with Johne's disease affect milk production in dairy herds? Smith and co-workers studied the status of the disease in three herds in the US and tried to predict the impact on milk production when the condition is latent or when the cows are actually shedding cells. The study was published in the Journal of Dairy Science, Volume 92 of 2009, pages 2653 to 2661, with the title: A longitudinal study on the impact of Johne's disease status on milk production in individual cows.

Johne's disease in ruminants is caused by intestinal infection by the pathogen Mycobacterium avium spp. paratuberculosis (MAP). It is estimated that approximately 68% of US herds is affected by the pathogen which costs the dairy industry between $200 and $250 million per year. In South Africa the high incidence in sheep flocks is of concern; however, the prevalence in dairy herds is largely unknown but certainly present. Infection with MAP usually occurs in calves where the pathogen enters a latent, non-shedding stage of varying length. The latent stage is then followed by a period of low and intermittent shedding of MAP with no obvious clinical symptoms. If allowed to progress, clinically apparent Johne’s disease may develop, with a high level of MAP shedding. Detection of MAP in the live animal is by faecal culture and blood serum tests. Post-mortem it is by culture of tissues, including intestinal epithelium and lymph nodes. If the prevalence in the herd is high, high-shedding animals are culled immediately to limit environmental contamination and the transmission of the pathogen to herd-mates, particularly calves. The problem though is that low-shedding animals are usually retained because of the low sensitivity of the tests and therefore the condition usually remains in the herd.

The question addressed in the present study was to estimate the loss in milk production over time of individual cows as affected by their Johne’s disease status, whether shedding MAP or latent. Three commercial herds known to be infected were monitored over three years through quarterly serum testing, biannual faecal culture and culture of tissues at slaughter. Milk production data were collected from the Dairy Herd Improvement Association in the US. Interestingly, infected cows produced more milk than uninfected cows before they began shedding MAP cells, which further indicate to the unlikelihood that latent cows will be culled. Cows infected with MAP had monthly decreases of 0.05 to 1kg in daily milk production relative to uninfected cows (about 225 l per 300-day lactation), with greater decreases in progressive disease categories. Cows with faecal culture results of more than 30 cfu's (cells) per gram produced about 4 kg less milk per day (1200 l per 300-day lactation) compared with uninfected cows. The results clearly show that Johne’s disease status affects milk production in all infected cows, with increasing losses as the disease progresses. The indicator losses above should be useful to calculate economic losses in the herd and as basis to make decisions on culling.