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Are badger infections following, not leading, bovine TB infections in cattle?

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In an article by Damian Carrington (Ref: 1) in The Guardian, 27 May 2013, eminent scientist Sir John Bourne is quoted as saying that "Badger infections are following, not leading, TB infections in cattle."

Emerging evidence appears to support Sir John Bourne’s assertion. In Northern Ireland the Veterinary Epidemiology Unit, Department of Agriculture and Rural Development and the Veterinary Sciences Division, Agrifood and Biosciences Institute have been carrying out long term M. bovis surveillance in badgers killed by vehicles (RTA badger casualties) in Northern Ireland. A published paper (Ref: 2) of this research provides a useful time series dataset of TB prevalence in badgers in the province.

Reports of badgers killed by vehicles and collection of carcases are managed by a Wildlife Officer within the Veterinary Epidemiology Unit of the Department of Agriculture and Rural Development (DARD).

Carcases are submitted to one of two laboratories of the AgriFood and Biosciences Institute (AFBI), a non-departmental government body, who undertake all laboratory procedures. Following a standardised post mortem examination, a range of tissue samples are routinely collected. Bacteriological culture is undertaken on all samples using standard methods and genotyping performed where positive growth is observed, to confirm M. bovis to strain level.

As the study acknowledges, this data may be subject to bias due to sampling methodology. There is also a wide variation in annual sample sizes. Notwithstanding these caveats, this dataset provides a plausible indicator of the annual trend in TB prevalence in Northern Ireland badgers over a 13 year period.

Time series data from this study, along with published herd incidence data, allow comparison between relative trends of M. bovis infection in both badgers and cattle between 1999 and 2010. (2011 data has been released post publication)

The data appear to demonstrate a strong correlation in trends of infection levels in both cattle and badgers. This is not surprising as other research has shown that:
 Both cattle and badgers are common hosts for M. bovis
 Higher levels of M. Bovis infection exist in both cattle and badgers in ‘TB hotspot’ areas
 Spatial associations exist between similar strains of M. bovis found in local populations of both cattle and badgers. Ref: 3.

This study acknowledges, like all previous work in this area, that the data fails to provide any conclusive evidence of direction of transmission between species.

It is worth noting that, to date, not one herd breakdown has been proven to have been caused by badgers (Ref: 4). The direction and means of transmission of infection between cattle and badgers or whether statistical correlations in infection levels are due to a common risk factor remains a matter for conflicting hypotheses.

It is important to consider the significance of events that occurred during the time period in question, although this is often overlooked by many papers on the subject.

In 2001 a foot and mouth disease epidemic occurred in Great Britain and Northern Ireland. During this time the M. bovis testing regimen was suspended with the result that, after culling, depopulated herds were restocked with untested cattle.

It’s widely agreed that these events led to a tangible increase in levels of M. bovis in the Northern Ireland herd post foot and mouth. This is reflected in a clear rise in herd M. bovis incidence from 6.84% in 2000 to 9.92% in 2002.

By contrast, the introduction of a post foot and mouth M. Bovis eradication programme is credited with the significant fall in herd M. Bovis incidence in the province from the 9.92% post foot and mouth peak to 5.35% within only three years.

Significantly, no badger intervention programme has taken place in Northern Ireland and the 2008 Northern Ireland Badger Survey (Ref: 5) concluded that ‘neither the density of badger social groups nor the total abundance of badgers had changed significantly between 1990/93 and 2007/08’. Therefore the province’s badger population may be considered stable.

This understanding of the dynamics driving the herd incidence allows us to compare cattle and badger M. bovis trends in this context. The graph in Figure 1 at www.badgersni.org.uk compares trends in badger M. Bovis prevalence and herd M. Bovis incidence.

Prevalence and incidence are not the same but only a prevalence dataset for badgers is available.

Prevalence is the proportion of the total number of cases to the total population and is more a measure of the burden of the disease on the population.

Incidence is a measure of new cases arising in a population over a given period.

Animal or herd incidence is considered a better indicator of disease trend in the national herd. Herd incidence is used in figure 1 as the dynamic of M. Bovis infection in cattle in this context is more closely associated with herds than with individual animals.

The other significant difference is that, when detected on routine testing, reactor cattle are removed and slaughtered. However, because of widely acknowledged inadequacies in the test used, some 20% of infected animals may be undetected and remain on farm with the potential to contribute to disease persistence in the herd.

We have a graph (at www.badgersni.org.uk) that shows a strong correlation in M. bovis infection in both species - showing relative trends of bovine TB infection in cattle and badgers (Note that these are relative trends with separate incidence/prevalence ranges and axes.). If we accept that this correlation is not merely a coincidence, it may be considered reasonable to infer that a common factor (or factors) is driving levels of M. bovis in both species.

The respective levels rise and fall at very similar rates. Badger TB levels rise in line with the post FMD rise in cattle. Similarly, badger TB levels fall in line with the ensuing fall in cattle TB.

This suggests that the M. bovis prevalence in badgers is driven by the same factor, or factors, implicated in influencing levels of M. bovis in cattle.

The only known variable is in the cattle population due to industry dynamics. We are not aware of any tangible or anecdotal evidence of any probable variable in the badger population.

Therefore the apparent close correlation of TB infection trends in both species, in this context, and the absence of any known factor(s) influencing levels of badger infection, suggest that levels of TB in badgers are driven by TB in cattle.

This supports the view that badger infections are following, not leading, TB infections in cattle.

Furthermore, in an early analysis of data from the RTA survey, no association was detected between TB in the badgers and that of the nearest two herds. However, the likelihood of a herd having experienced a breakdown TB in the previous four years in herds within three kilometres of a positive carcase was significantly higher than those around a TB-negative carcase.

This may suggest that the presence of reactors in a herd is an indicator of risk of TB being detected in the local badger population within the following four years.

The rate of decline in M. bovis infection in both species also merits closer analysis. Whilst the reduction in herd incidence is acknowledged to be a direct result of a robust M. bovis reduction programme in cattle, infection in the badger population has fallen commensurately with level of infection in cattle.

M. bovis infection in badgers appears to have decayed significantly over a five year period in the absence of high levels of M. bovis in cattle.

As no badger intervention took place, the level of infection in badgers appears to decay naturally in line with the reduction of cattle TB levels. This suggests that M. bovis is not self sustaining in the badger population and that badgers are not a persistent reservoir for bovine TB as the disease prevalence in badgers would appear to decrease naturally with time as the chance of re-infection from cattle decreases.

These observations may have important implications for any M. bovis eradication programme in cattle especially the relevance, efficacy and cost of any badger intervention.

The apparent poor ability of M. bovis to sustain itself naturally in the badger population may improve the effectiveness of any badger vaccination strategy. Research indicates that culling, as currently practised, is likely to have the opposite effect, increasing the proportion of badgers infected (Ref: 6).

Mike Rendle
Northern Ireland Badger Group

1 Counting the cost: fears badger cull could worsen bovine TB crisis Damian Carrington, The Guardian, 27 May 2013 http://www.guardian.co.uk/environment/2013/may/27/badger-cull-bovine-tuberculosis
2 Mycobacterium bovis surveillance in European badgers (Meles meles) killed by vehicles in Northern Ireland: an epidemiological evaluation, Abernethy et al. http://www.dardni.gov.uk/m-bovis-surveillance-european-badgers-rta-epi-eval.pdf
3 Whole Genome Sequencing Reveals Local Transmission Patterns of Mycobacterium bovis in Sympatric Cattle and Badger Populations, report by Biek et al.
4 The author cannot find any evidence of any herd breakdown having been conclusively proven to have been caused by badgers. No authoritative veterinary, government or industry source has offered such evidence and this statement remains unchallenged to-date.
5 Badger Survey of Northern Ireland 2007/08, Reid et al. http://www.dardni.gov.uk/badger-survey-of-ni-2007-08.pdf
6 Culling and cattle controls influence tuberculosis risk for badgers, Woodroffe et al. http://www.pnas.org/content/103/40/14713.full

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