High Mycobacterium bovis Exposure but Low IGRA Positivity in UK Farm Workers

Background

Between 1999 and 2021, 505 culture-confirmed cases of M. bovis disease in humans (zoonotic tuberculosis, TB) were diagnosed in England. We aimed to estimate the prevalence of M. bovis infection in persons exposed to TB-infected cattle in England and identify any risk factors associated with latent TB infection (LTBI) in this population.

Methods

We co-developed a retrospective cohort study in Southwest England, a bovine TB high risk area, with members of the UK farming community. A questionnaire captured participant characteristics, behaviours and farming practices. Linkage with historical herd testing data was used to categorise participants as low, medium or high risk for TB exposure. Interferon gamma release assay (IGRA) positivity with Quantiferon was used to determine LTBI status and linked to questionnaire data.

Results

We recruited 90 participants at agricultural shows and a standalone event. Participants were farmers/farm workers (76/90) and veterinary professionals (10/90). Median age was 45.5 years (IQR: 19–77); 63% were male; 67% reported BCG vaccination. M. bovis exposure was via direct contact with infected cattle and consumption of raw milk. One participant in the high-risk group was IGRA positive, all other participants were IGRA negative. Estimated IGRA positivity rate was 1.1% (95% CI 0.058%–7.0%) in all participants and 4.0% (95% CI 0.21%–22%) in participants with high exposure levels.

Conclusions

We found limited LTBI in individuals in contact with TB-infected cattle in England, despite high and prolonged exposure. We identified a high-risk group of farmers who should be prioritised for future engagement.