BCG vaccination of cattle reduces transmission of bovine tuberculosis

Bovine tuberculosis (bTB) is an economically important disease of livestock with zoonotic transmission. The development and deployment of vaccines such as Bacillus Calmette-Guérin (BCG) contributes to accelerating control and the ultimate elimination of bTB across the globe. The BCG vaccine is routinely used to protect children against Mycobacterium tuberculosis. The vaccine comprises of attenuated Mycobacterium bovis.

Effectiveness against transmission of BtB

To assess the effectiveness of the vaccine, a natural transmission experiment was conducted by an international research group under the name ETHICOBOTS (Ethiopia Control of Bovine Tuberculosis) consortium. The experimental design for this transmission experiment was developed and published earlier in cooperation between modellers from Cambridge and professor Mart de Jong from QVE (Conlan et al., 2018). The purpose of this experiment is to estimate the transmission rate parameters to and from vaccinated and unvaccinated calves over a one-year exposure period. The results of the experiment were that the BCG vaccine also reduces transmission from vaccinated animals that subsequently become infected (74%; 95% CrI: 48-98%). This indirect efficacy of the vaccine was higher than the direct protection against infection (58%; 95% CrI: 34-73%). The estimated total efficacy amounted to 89% (95% CrI: 74-96%).

Model

Within the research project, a mechanistic transmission model of bTB spread within the Ethiopian dairy sector was developed to show the effect of the estimated reduction in transmission on bovine tuberculosis prevalence at population level. “This model shows that there are prospects of elimination of the disease when cattle in Ethiopia would be routinely vaccinated”, says professor De Jong. Due to a combination of direct and indirect protection resulting from vaccinating all newborn calves, the effective reproduction ratio can be brought below 1 in Ethiopian dairy. However, according to the model the elimination resulting from that will take a long time. “The path of elimination will be slow and the pathogen will remain within herds for a very long time. According to our simulation up to 50 years.”

Potential

Based on the research findings and the model the international research group that was involved in the project is hopeful the BCG vaccine can play an important role in protecting livestock. “The evidence for indirect effects of BCG in cattle and the implications of its use in accelerating bTB control programs, are hopeful”, conclude the researchers. As circumstances in animal husbandry change rapidly over time, it remains to be seen what the actual effect will be once the suggested strategy will be applied. “Our scenarios should be considered as a projection to illustrate the potential of vaccination, not a forecast.”