Bovine TB in foxes
In June of this year a paper was published on TB in French foxes by Michelet et al. in the journal ‘Emerging and Infectious Diseases’.
As with all things TB related this resulted in a several articles in the press (most of them with over the top headlines) and the usual debate on social media. I thought it might be useful to produce a summary of what we know about TB in foxes. Is there a risk to cattle and what does this study actually mean?
We have known for some time that M. bovis (the bacterium which causes bovine TB) can infect foxes in the UK. The best data we have on this comes from Delahay et al. (2007) which involved analysing 756 fox carcasses collected across south-west England. Post mortem analysis and bacterial culture found that 24 foxes (3.2%) were infected with M. bovis.
Results from this study also found evidence of M. bovis infection in a wide range of other species including; stoats; polecats; shrews; mice; voles; squirrels and several deer species (badgers of course can also be infected, but this study was focused on ‘other’ wildlife). To people not familiar with this research these results might seem surprising or even alarming. BUT before we panic about the role of foxes or shrews, its important to consider what risk is actually posed by these species. In the paper by Delahay et al. (2007) the authors quantified the risk (relative to the risk from badgers) in relation to four parameters:
- How common is infection in the species? – based on the prevalence or % which are infected
- Is excretion and spread of M. bovis likely? – based on culture and the presence/distribution of lesions
- How common is the species? – are they widely distributed and do they occur at high densities?
- Is the species likely to come into contact with cattle? – or with environments used by cattle (pasture fields and farm yards)
The results from this paper suggest that deer present a potential TB risk to cattle, but the risk from other species are estimated as being very low. The risk from foxes was estimated as being less than 10% (relative to the risk from badgers), because foxes have a lower prevalence of disease, occur at lower densities, are less likely to contact cattle, and crucially, because foxes have few lesions (only one fox had visible lesions in this study) so are unlikely to excrete M. bovis.
So what does the new French study say?
The latest study from France involved analysing six fox carcasses from a TB endemic area in France using post-mortem analysis and bacterial culture. Interestingly the study also involved analysis of urine, faeces and oropharyngeal swabs (taken from the mouth/throat of the foxes) for M. bovis DNA, to look for evidence of excretion.
The study found no ‘TB-like visible lesions’ in foxes, but 4/6 foxes were culture positive (M. bovis was cultured from their tissues) indicating they were infected. Several foxes also had positive results for their swabs/faeces/urine (see below), either for M. bovis DNA or ‘MTBC’ (Mycobacterium tuberculosis complex – so possibly M. bovis but this was not clear).
What does this mean?
These results suggest that infected foxes may be able to excrete M. bovis, even if they have no visible lesions. This could suggest that the earlier studies (ie Delehay et al. 2007) may have underestimated the risk from foxes (partly because they assumed no visible lesions meant a low chance of excretion). BUT there a number of important things to consider:
- The sample size for this study is very small and it comes from another country where the TB situation is very different to the UK
- The molecular approaches used means that it is unclear whether the bacteria would be viable (ie if they could grow or infect other animals).
- Although there was ‘evidence of excretion’ we have no indication of the quantity of M. bovis. How would this compare to shedding from a deer or badger with multiple visible lesions and advanced infection?
- This still does not change the fact that foxes have much lower disease prevalence than badgers, as well as a lower density / lower rate of contact with cattle.
This study therefore raises some interesting questions and may (and I stress ‘may’ because of 1-3 above) suggest that foxes have a greater capacity to spread TB than previously thought. However, it seems likely that any risk posed by foxes is still significantly lower than that for badgers, which are the principal wildlife host in the UK.