Bovine tuberculosis transmission in the United States, inferred from population genetic structure

Background/Question/Methods

Despite control and eradication efforts, bovine tuberculosis continues to be identified at low levels among cattle in the United States. We evaluated possible external sources of infection by characterizing the population genetic structure of bovine tuberculosis from a national database of reported infections, comparing strains circulating among US cattle with those of imported cattle, and farmed and wild cervids.

Results/Conclusions

Farmed cervids maintain a Mycobacterium bovis strain genetically distinct from those in cattle, and cattle occasionally become infected with this strain. In contrast, wild cervids host many of the same strains found in cattle, and the data do not show a clear transmission direction. Cattle from Mexico hosted a higher overall richness of strains than US cattle, and a number of those strains are found in both US and Mexican cattle. However, these two populations appear to be well-mixed with respect to their bovine tuberculosis lineages, and higher resolution data is necessary to infer the direction of transmission. Patterns of both host and geographic distributions were highly variable among strains, suggesting that different sources or transmission mechanisms are contributing to maintaining different strains.