TB transmission from badgers to cattle – new study published
Badgers are a wildlife host for M. bovis and they are implicated in the transmission of the bacteria to cattle, but proving that badgers can actually infect cattle is actually quite difficult.
Evidence of transmission between species include the following studies:
- Experimental captive trials – a study by Little et al. (1982) demonstrated that cattle housed with infected badgers became infected with M. bovis (in an artificial setting).
- Spoligotyping data – Genetic strains (or spoligotypes) of M. bovis often have localised distributions, suggesting a local source of infection. Strains of M. bovis in cattle also typically match local strains in the badger population, suggesting some degree of transmission between species (Goodchild et al. 2012).
- Outcomes of culling trials – Changes (both positive and negative) in TB incidence in cattle following the RBCT suggest that badgers can transmit M. bovis to cattle (summarised here).
During the RBCT there was also an increase in TB in badgers linked to the pause in TB testing during the foot and mouth outbreak (Woodroffe et al. 2006), suggesting cattle can transmit infection to badgers. The occurrence of disease outbreaks in badgers in the low risk area well away from the main epidemic (such as in Cumbria) also suggests that the movement of infected cattle can spread the disease to wildlife.
Despite the above evidence, there is still much uncertainty over the rates of disease spread from badgers to cattle, and from cattle to badgers. Can badgers really transmit M. bovis to cattle? Can cattle infect badgers? This fuels some of the debate over the role of wildlife in bovine TB.
A new study by published in elife sheds light on rates of transmission from badgers to cattle and cattle to badgers:
Crispell J, Benton CH, Balaz D, De Maio N, Ahkmetova A, Allen A, et al. Combining genomics and epidemiology to analyse bi-directional transmission of Mycobacterium bovis in a multi-host system. eLife.
What did the study involve?
The study involved the detailed genetic analysis of M. bovis isolates from badgers in Woodchester Park in Gloucestershire, the badgers in this study area are regularly trapped and sampled to test for M. bovis infection (for a description of the woodchester study see this review I co-authored). Isolates of M. bovis were also analysed from slaughtered cattle testing positive for bTB within 10 km of the woodchester study area.
Using WGS (whole genome sequencing – where the DNA of the M. bovis is analysed in detail) the authors constructed a phylogenetic tree to measure how closely the different M. bovis isolates were related. A phylogenetic tree is similar to a family tree, it shows how different isolates of M. bovis group together (into ‘clades’) and can be used to infer the direction of transmission from samples higher up the family tree to those on the lower branches. For example if M. bovis from a badger sample is located above cattle samples in the phylogenetic tree (i.e. the badger sample is the ancestor of the cattle samples) this indicates transmission from badgers to cattle
The authors also used a range of statistical modelling tools to estimate the rates of transmission both within species (cattle to cattle, badger to badger) and between species (cattle to badger, badger to cattle) that best explains the genetic patterns observed in their data.
What were the results?
The phylogenetic tree constructed from the genetic sampling provides good evidence of disease transmission between the species. If there was no transmission from badgers to cattle (or visa versa) there would be distinct genotypes of M. bovis in badgers and cattle, with the different species clustering together in separate groups, on different branches of the phylogenetic (family) tree. This study found that this was not the case, the genotypes of M. bovis were very closely related to the M. bovis in badgers, and several groups on the phylogenetic tree contained a mix of badgers and cattle. This provides good evidence of transmission between the species.
The figure above shows the phylogenetic tree from the paper, this is like a family tree but it is so wide it is curled into a circular shape so it can fit in one image. The different colours are clades, or groups with distinct genotypes.
The main group within the phylogenetic tree containing the majority of the woodchester badger isolates (purple on the figure below) also contained 16 cattle isolates, which had genetic signatures consistent with badger to cattle transmission events. In several other cases M. bovis isolates from cattle were above those from badgers in tree, this is consistent with there also being cattle to badger transmission of M. bovis.
The results from the analyses reporting rates of transmission within and between species were also very interesting. The best fitting model (statistically this was the best one at explaining the data) had the following results:
It is important to emphasise that these are estimates from the best fitting model, several of the other models had slightly different results, although these models did not fit the data quite so well. For example, in the second best fitting model transmission rates between badgers and cattle were estimated at being close to equal.
What can we conclude from this study?
- This study provides some of the best evidence to date that badgers can act as a source of M. bovis infection in cattle, but also that cattle can act as a source of infection for badgers.
- Although badgers are more likely to infect cattle this does not mean that badgers are the main source of infection for cattle, or more important than other sources. The majority of disease transmission in this study was within species, both cattle to cattle and badger to badger. This confirms that transmission within the cattle population is also important for spreading bTB.
- Given the high levels of transmission within the badger population and the genetic patterns observed, this study also provides evidence that M. bovis can persist within badger populations for long periods of time without the need for reinfection from cattle. This suggests that without some form of management infected badgers could act as a source of infection for long periods of time.
BUT as with all studies there are caveats and limitations. One obvious limitation of this study is that the data comes from a single study site in Gloucestershire. This is an area with a relatively high density of badgers, and a long history of TB infection in both badgers and cattle. In other areas with different badger populations and cattle populations, the rates of transmission could be different to those estimated here. For more information on the implications of this study read expert opinion on the study at the science media centre.
What does this tell us about the current strategy for controlling TB in the UK? The authors state the following in the final section of the paper….. “In line with a recent evidence review (Godfray et al., 2018), our research also suggests that coordinated bTB control in both cattle and badgers may be necessary to control infection in cattle.”
But this study does not address the type of control strategy used, to quote one of the authors Prof Richard Delahay…..“What the study shows is that in this area both species are involved and we’ve got transmission going from badgers to cattle and cattle to badgers and circulating within both of these populations. Under that scenario, you would imagine you would have to control the disease in both species but how you do that is a completely different question.”