Influence of vaccination against paratuberculosis on the diagnosis of caprine tuberculosis during official eradication programmes in Castilla y León (Spain)

The information generated from the official eradication programmes of caprine tuberculosis (TB) in Castilla y León, Spain, during 2018, has been used to assess the effect of vaccination against paratuberculosis (PTB) and the presence of this infection, on the single intradermal tuberculin (SIT) test results. Data from 121,665 goats belonging to 1936 different herds were analysed using generalized linear models. An epidemiological survey was conducted to know the herd immunization status against PTB and the date of last vaccination. All SIT test-positive animals were further investigated in order to confirm the diagnosis of TB, through bacterial culture, and PTB, by histopathological and qPCR analyses. SIT positivity was found in 39 (2.01%) herds and 507 (0.41%) goats. TB was confirmed by M. caprae or M. bovis isolation in 10 (0.51%) herds and 46 (0.038%) goats. PTB was diagnosed in 13 (33.33%) and 55 (10.84%) of the SIT test-positive herds and goats, respectively. Vaccination against PTB showed a significant influence on the results of the SIT test at herd level, with higher positivity detected among those herds vaccinated. However, this effect was not observed when the total number of animals was considered, where the highest positivity was found in unvaccinated goats. The time elapsed between vaccination and SIT test performance also influenced the results. The strongest effect was found when less than eight months elapsed between performing both activities, and to a lesser extent between 8 and 12 months. Conversely, no positive herds or animals were found when the time elapsed was higher than one year. No significant effect of the presence of PTB was observed. These findings demonstrate that the use of PTB vaccine does not result in false positives to a SIT test at individual level, provided that the time elapsed between the performance of both practices is higher than 12 months.