On farm strategies to reduce the impact of Johne’s disease in British dairy herds

Johne’s disease (JD) is a chronic disease of cattle caused by the bacteria Mycobacterium avium subsp. paratubercullosis (MAP). A study conducted by RVC as part of the AHDB Dairy Research Partnership  estimates that more than 60% of the dairy herds in GB are infected. Young calves become infected via contaminated colostrum, milk and other feedstuffs. This is followed by a long incubation period. Infected cows shed MAP in milk and in their faeces contaminating the environment and spreading the infection. At the time when this project was started, different sets of recommendations had been issued. However, there was considerable uncertainty on the actual impact and cost-effectiveness of control measures under British field conditions.

Key Findings:

1: Review of scientific evidence and expert opnion to assess preventable economic losses in GB dairy herds

• There is a lack of field studies regarding the impact of specific management practices on within-herd MAP prevalence
  • Due to: the chronic nature of the infection, which requires lengthy follow-up of individual animals to assess the relationship between exposure at early stages of life and infection
• Experts considered, based on their experience, that management practices addressing the faecal-oral (others) MAP-transmission route have a relatively high impact.
• Reducing time spent with dam was viewed to have relatively high impact of addressing faecal-oral transmission whilst cleanliness of dams' udders and legs was viewed upon as relatively ineffective.
• Management practices to reduce colostrum and milk transmission were not seen as particularly effective.
• Estimates of the preventable economic losses (at 2013 prices):  
  • Outdoor calving provided the greatest savings annually (median: £4,184) followed by limiting faecal exposure at the calving house and reducing the time the calf spend with the dam.
  • Addressing faecal-oral (others), tended to be economically attractive, whilst addressing faceal-oral (own dam) gave mixed results depending on the management practice.
  • The median economic benefit of using colostrum replacement only and milk replacer only was similar: £1,692 and £1,700 respectively, these tended to be less effective in terms of preventing economic losses.

2. Statistical analysis of JD milk ELISA test results

• The average age when infected cows test positive for the first time, was 4.0 years.
• 90% of the infected cows had seroconverted by the end of their 6th year.
• 87.5% of the cows diagnosed as high-risk during the study period were over 3 years old when diagnosed as such.
• High-risk cows were kept in the herd during an average of 153 days following their identification as high-risk, and 31 % of them were kept in the herd over a year after classified as high-risk.

3. Cohort study of heifers born in JD infected herds

By following up a large cohort of cows from their birth until the end of their second lactation the main findings which should be taken into consideration by farmers and industry when develoing their JD control programs are:

• Whether the dam is infected or not is a major factor influencing JD risk and this is not limited to cows that have already tested positive.
  • The offspring of cows that are infected but have not yet tested positive is also at high risk of infection.
  • Current JD management advice focuses on the offspring of cows that have been confirmed as positive. The results obtained in this study strongly suggest that JD infected cows can infect their offspring before they are found as positive by current diagnostic tests.
  • This has major implications for JD control, which should consider previous heifers born to a positive dam.
• For calves that are born to non-infected cows, the time spent in the calving yard and the cleanliness of the yard and of the cows appears to be the main contributor to infection.
  • A long time in the yard is particularly risky if the yard is not clean
  • A dirty yard is particularly risky if calves spend a long time
  • Farmers should consider that these two factors when trying to prevent infection in calves born to seronegative dams
• JD negatively affects milk production as early as in the 2nd lactation, i.e. before the cows show other clinical signs.
  • JD positive cows gave 10% less milk than JD negative herdmates during their 2^nd lactation. Up to second lactation JD does not appear to affect other production parameters such as age at first calving or number of days open.

Aims and Objectives:

To identify best practice measures for the control of JD and determine the extent to which implementation of these measures reduces the prevalence of JD and is cost beneficial for British dairy herds.

To achieve this overall aim 3 interrelated research studies were:

1. A study of the scientific evidence available at the start of the project which was combined with the opinions of expert to assess preventable economic losses in GB dairy herds.

2. A detailed statistical analysis of a large dataset including more than 1.6 million JD milk ELISA test results from GB dairy herds in the last 5 years.

3. An extended field study of more than 500 dairy heifers born on 6 GB dairy herds from birth until their 2nd lactation.