Modelling the transmission and control of Johne's disease in Irish dairy herds with spring bunch calving

Context :

Paratuberculosis is a chronic bacterial infection of the intestine in cattle caused by Mycobacterium avium subspecies paratuberculosis (Map). In Ireland, the herd population dynamics is seasonal as lactating cows are mostly fed on pasture. In addition, calves can graze on pasture adjacent to cows or be completely separated from cows during the pasture period, which also influences their exposure to Map. Such a seasonality could impact Map spread and control at herd scale. To address this issue, a mechanistic modelling approach is suitable, enabling to easily compare scenarios. Therefore, to better understand Map spread in Irish dairy herds, a stochastic individual-based model was used which accounts for seasonal population dynamics. This model was used to investigate the probability of Map persistence over time, the within-herd prevalence, the relative importance of transmission pathways, and to assess test-and-cull control strategies.

Results :

We simulated a typical Irish dairy herd with on average 82 lactating cows, 112 animals in total. Each scenario was iterated 1,000 times to adjust variations caused by stochasticity. Map was introduced in the fully naive herd through the purchase of a moderately infectious primiparous cow. Infection was considered to persist when at least one infected animal remained in the herd or when Map was present in the environment. The probability of Map persistence 15 years after introduction ranged between 32.2-42.7% when calves and cows had contact on pasture, and between 18.9-29.4% when calves and cows were separated on pasture. Test-and-cull scenarios consisted of highly test-positive cows culled within 13 or 4 weeks after detection, and calf born to highly test-positive cows kept vs removed. The most effective strategy was to cull highly test-positive cows within four weeks of detection. Removing the offspring of highly test-positive dams did not affect Map persistence and the within-herd prevalence. Mean prevalence 15 years after Map introduction was highest (63.5%) when calves and cows had contact on pasture. Mean prevalence was 15% lower (absolute decrease) when cows were culled within 13 weeks, and 28% lower when culled within 4 weeks.

Perspectives :

Around calving, the infection rate was high, with calves being infected in utero or via the general indoor environment, the two main transmission routes. For the remainder of the year, the incidence rate was relatively low with most calves being infected on pasture when in contact with cows. Testing and culling was an effective control strategy when it was used prior to the calving period to minimize the number of highly infectious cows present when calves were born. The assessment of such control strategies should now be extend to the between-herd scale, in order to account for the diversity in herd management (size, renewal, etc.) and for the multiple introductions of Map into herds due to animal trade.

Valorisation :

This work is a collaboration between INRAE, Oniris Nantes and University College Dublin (UCD). It is part of the NexusMap project and funded by UCD.

Bibliographic references :

Biemans, F., Ben Romdhane, R., Gontier, P., Fourichon, C., Ramsbottom, G., More, S.J., Ezanno, P., 2021. Modelling transmission and control of Mycobacterium avium subspecies paratuberculosis within Irish dairy herds with compact spring calving. Preventive Veterinary Medicine, 186, 105228 doi.org/10.1016/j.prevetmed.2020.105228

Schematic overview of herd management in a typical Irish spring calving dairy herd.