Johne's disease is a chronic illness in ruminants, endemic in many countries, and is responsible for significant economic losses in the dairy sector, according to DairyGlobal, citing a recent study on the subject.

The disease is caused by Mycobacterium avium subspecies paratuberculosis (MAP), which typically infects calves that remain latently infected for a long period, making early detection of the infection particularly challenging.

Transmission from cow to calf can occur in utero through milk/colostrum or fecal-oral routes, so understanding the different transmission pathways to calves is important for informing control recommendations.

The aim of a longitudinal study was to measure the association between dam-related transmission routes and the environment on a calf subsequently serologically tested positive for MAP.

The study, conducted by researchers from the Royal Veterinary College, included 439 dairy calves from the United Kingdom from 6 herds enrolled between 2012 and 2013. These calves were tracked from birth until 2023.

Individual calf data were recorded at birth. During the follow-up, individuals entering the milking herd were tested quarterly for the presence of MAP antibodies using milk ELISA.

Cox regression models were used to measure the association between exposure from the dam (in utero and/or colostrum) or from the environment (long time in a dirty calving area) and the time until the first detection of MAP infection.

An association between calves born from positive females and the likelihood of having a MAP-positive test remained after excluding the potential transmission of MAP through colostrum (Risk Ratio: 2.24; 95% CI: 1.14-4.41).

Calves less likely to be infected with MAP through in utero or colostrum had a 3.68 times higher risk (95% CI: 1.45-9.33) of a positive test result when spending more time in a dirty calving area.

The effect of the dam's infection status on transmission to calves precedes maternal seroconversion and persists after excluding the potential role of colostrum transmission.

Therefore, the association between time spent in a dirty calving area and the likelihood of a positive MAP test result highlights the role of environmental contamination as a source of infection in addition to dam-related transmission.

In conclusion, researchers found that:

• The risk of infection is partially determined by the dam's infection status.
• Calves born from infected females have a higher risk of MAP infection, regardless of the dam's calving test status.
• Prolonged time in a dirty calving area increases the risk of MAP infection.
• The dam's impact on MAP risk extends beyond colostrum transmission.
• The persistence of MAP in commercial dairy herds results from a combination of dam-related and environmental factors.

(Source: This research was published in Preventive Veterinary Medicine.)