There are multiple websites, blogs and pages on the internet with information about bovine TB. Given the highly polarised and political arguments surrounding the subject I would recommend that anyone approaches information they read online with a certain amount of caution, particularly if it is unclear who the author is, or where they are getting their information. Personally I would recommend www.Tbhub.co.uk which is supported by AHDB, BCVA, DEFRA, Landex and the NFU, as a key website with reliable information on bovine TB. Throughout my work I will aim to get the content I produce (articles, fact sheets etc) uploaded to the TBhub where possible. This page acts as a ‘Blog’ for me to talk about some of this work as it progresses and also highlight other information on the subject of bovine TB.
TB in other livestock and domestic animals
March 1st 2019
Mycobacterium bovis (M. bovis) causes bovine TB in cattle and it can also infect badgers and a range of other wildlife (addressed in another fact sheet). But what about other livestock like sheep or pigs? Or domestic animals like dogs and cats? Can they also be infected? And if they can, is there a risk to cattle or wildlife?
To address these questions I have produced a fact sheet with input from veterinary experts from APHA. This can be downloaded by clicking the image below, or by going to the TB fact sheet page of this website.
In addition to this factsheet there are a number of other great information sources if you are interested in TB in other livestock or domestic animals.
Firstly the non-bovines section of the TBhub has really useful information and links to government advice.
The role of non-bovines is also discussed in length in the recent Godfray report (2018). See chapter 7 on page 80. Within this section there are also figures quoted of the numbers of other livestock found infected with M. bovis each year. As you can see below, the numbers are generally very low. To download spreadsheets containing these data go to the gov website here.
For a more in depth review of the subject there is a two part review by (Broughan et al. 2013). Part one reviews evidence/epidemiology of TB in other species. Part two then covers the different tests for TB available for non-bovines.
Broughan, J.M., Downs, S.H., Crawshaw, T.R., Upton, P.A., Brewer, J. & Clifton-Hadley, R.S. (2013) Mycobacterium bovis infections in domesticated non-bovine mammalian species. Part 1: Review of epidemiology and laboratory submissions in Great Britain 2004–2010. The Veterinary Journal, 198, 339-345.
Broughan, J.M., Crawshaw, T.R., Downs, S.H., Brewer, J. & Clifton-Hadley, R.S. (2013) Mycobacterium bovis infections in domesticated non-bovine mammalian species. Part 2: A review of diagnostic methods. The Veterinary Journal, 198, 346-351.
If you are interested, but cannot access these papers then feel free to contact me and I will be happy to send them to you.
Can M. bovis survive in silage?
February 11th 2019
TB transmission via contaminated feed
Bovine TB is caused by Mycobacterium bovis (M. bovis), but it is unclear how the bacteria spreads between hosts, whether that’s between cattle, or between cattle and wildlife. One potential route is by ‘indirect’ transmission, where an infected animal contaminates grass, stored feed, water or other parts of the farm environment, and then another animal interacts with this contamined material and becomes infected.
Studying these indirect transmission routes is very difficult, but research has shown that M. bovis can survive in different environments such as water, soil, and faeces and a range of stored feed types (Fine et al. 2011). Research from the United states has also shown that M. bovis can spread from infected deer to cattle via contaminated troughs and housing (Palmer, Waters & Whipple 2004). This is why current guidance in the UK is to keep badgers out of stored feed and housing to reduce opportunities for disease spread.
But what about silage?
Badgers may be attracted to open silage clamps, particularly those containing whole crop wheat or maize silage (click here for a list fo feed attractive to badgers), so it is advisable to reduce badger access if possible. BUT – silage could also potentially become contaminated in the field, either from infected badgers, or if contaminated slurry is spread on the field (click here for a factsheet about TB in slurry and faeces). Silage goes through an ensiling, or fermentation process, where oxygen levels are low and the pH becomes acidic. Previous reviews of the subject have highlighted that M. bovis could potentially survive in these conditions, but these reviews have also highlighted the lack of science in this area.
Recently a study was published in the United states by (Grooms et al. 2019), which directy addresses the question of M. bovis survival in silage:
Grooms et al. (2019) Survival of Mycobacterium bovis during forage ensiling. American Journal of Veterinary Research, 80, 87-94.
Given the interest in this subject, and the fact that this paper is a difficult to access journal, I have produced a summary of the paper below.
What did the study invovle?
The study involved experimentally adding M. bovis to samples of cut forage which was then ensiled in the lab.
Three different feed types were tested:
- Alfalfa – dry matter content at ensiling 36%
- ‘Mixed mostly grass’ – mainly orchard grass (not specified, but I believe this is cocks foot), with some alfalfa and clover. – 65% dry matter content at ensiling (this was higher than intended, the target was 40%)
- Maize (‘chopped corn’) – 32% dry matter content at ensiling Samples were spiked with M. bovis prior to ensiling, with samples arranged in replicate groups of six; four samples containing M. bovis and two with no M. bovis ( to act as controls).After each period of time the samples were then tested for M. bovis using two methods
Each group of six samples were then ensiled for one of ten different periods spanning 0 – 112 days (meaning 60 samples of each forage type). Samples (250g of each) were placed in thick vaccuum packed polythene bags, wrapped in black liners and and stored at room temperature (18-22°C) to simulate the ensiling process.
After each period of time the samples were then tested for M. bovis using two methods:
- Culture – M. bovis is grown in the lab
- PCR – tests for the presence of M. bovis DNA (click herefor a factsheet on PCR testing)
What did the study find?
This study had three key results:
- The study found that M. bovis could be cultured from Maize and Alfalfa silage up to two days into the ensiling process, and up to 28 days in grass silage. All samples tested after this period tested negative. The authors suggest that the longer survival in grass could be due to the higher (less acidic) pH recorded in these samples (pH was around 5 while maize and alfalfa was closer to 4). This could also be due to the higher dry matter content in the grass samples in the study.
- M. bovis DNA could be found in some samples of all three silage types up to the end of the study (day 112).
- None of the control samples tested positive on either culture or PCR
What do these results mean?
To quote the authors “these results suggest that properly ensiled forages would be an unlikely source for M bovis transmission to cattle”, as M. bovis could not be cultured after 28 days in any of the samples. M. bovis DNA was detected beyond this date, right up to the end of the study. However, it is not clear whether the bacteria are viable (ie if they could grow or infect an animal). One possibility is that the bacteria are ‘dormant’ and that changes in conditions in the future could make them infectious, but it is unclear if this is possible (and what condictions could causes this).
Although this study has some very interesting results, it is important to acknowledge that this is an experimental study, and no experiment is perfect. M. bovis was added at very high concentrations to the forage samples (tens of millions of backeria in each 250g sample), which is likely much higher than contamination would occur naturally. The primary method of testing for M. bovis (culture) is also not 100% perfect, so could potentially fail to detect the bacteria, even though it was present. It is therefore possible that M. bovis survival in natural conditions is longer or shorter than stated here. Nevertheless, this study provides the best information to date on the likely survival of M. bovis in silage.
Overall conclusion : The study suggests that properly ensiled forage is unlikely to be a source of infection in cattle, although the detection of M. bovis DNA up to the end of the experiment means that the risk cannot be totally ruled out.
The factsheet on TB survival in feed, water and soil has now been updated to reflect these new results.
What impact do badgers have on other wildlife?
January 22nd 2019
The badger population in England and Wales has increased in recent decades (Judge et al. 2014, Judge et al. 2017), although in cull areas in England there are likely to be declines. Badgers are our ‘largest predators’ and although their diet is mainly invertebrates (worms, snails etc) and plant matter, badgers will eat other mammals, amphibians and birds. Changes in badger numbers therefore have the potential to impact other species and this is an issue which receives a lot of attention in the ongoing debate around badgers and bovine TB. Much of the discussion is focused on hedgehogs and ground nesting birds, partly because while badger number have increased in recent decades, these species have undergone significant declines, and many people suggest these changes are connected.
So are badgers to blame for falling hedgehogs and ground nesting birds? And will culling badgers lead to changes in other species?
To address these questions I have produced a two-page fact sheet which can now be found on the factsheets page of this website, the TBhub and can also be downloaded by clicking the image below. This factsheet summarises the published literature on relationships between badgers with hedgehogs, ground nesting birds and also foxes.
Summarising these topics into two pages is always a challenge and it is not possibly to include every study or elaborate on every point. In addition to the contents of the fact sheet, below is some more details on hedgehogs and ground nesting birds.
Additional points on the effect of badger culling on hedgehogs.
Several studies have shown a negative association between badgers and hedgehog numbers (ie hedgehog numbers are lower in areas with higher badger activity/density), but the best evidence for the effects of badger culling on hedgehogs comes from this paper which I co-authored from Trewby et al. (2014)
Trewby, I.D., Young, R., McDonald, R.A., Wilson, G.J., Davison, J., Walker, N., Robertson, A., Doncaster, C.P. & Delahay, R.J. (2014) Impacts of Removing Badgers on Localised Counts of Hedgehogs. PLoS ONE, 9.
This paper involved conducting nocturnal spotlight surveys for hedgehog in RBCT cull areas. Hedgehog numbers were counted in pasture fields and amenity grassland (playing fields, village greens, parks etc), which is a key habitat for hedgehogs. Analysis of the data found that in cull areas there was a significant increase in hedgehog numbers in amenity grassland (hedgehog numbers roughly doubled), while in cull areas there was no such increase. This paper demonstrates that badger culling is likely to benefit hedgehog numbers. However, there are a couple of important points to consider:
- This increase was in amenity grassland only (essentially non-farmland). In pasture fields there were too few sightings of hedgehogs to conduct meaningful analyses, even after culling.
- The effect was significant (hedgehogs did increase), but the effect was quite variable (look at the figure).
So while culling does seem to benefit hedgehogs, it does not guarantee a doubling of hedgehog numbers across the whole cull area. Instead there is evidence for a variable increase in a specific habitat types.
Additional points on the causes of hedgehog declines
A recent study was published in nature scientific reports partly addresses this question.
Williams, B.M., Baker, P.J., Thomas, E., Wilson, G., Judge, J. & Yarnell, R.W. (2018) Reduced occupancy of hedgehogs (Erinaceus europaeus) in rural England and Wales: The influence of habitat and an asymmetric intra-guild predator. Scientific Reports, 8.
The study involved using tracking tunnels to survey for hedgehogs in areas where badger activity was also quantified in the recent national survey (Judge et al. 2014). The results found that hedgehog activity declined with increasing badger activity, consistent with badgers have a negative impact on hedgehogs. But this is not the whole story…
This study only found hedgehog activity in 22% of areas surveyed, suggesting hedgehogs were absent from a large part of the English countryside. The analyses also found that even in areas with no badgers present, hedgehogs were only present in 31% of areas surveyed, meaning around 70% of areas had no hedgehogs (even if there were no badgers recorded). This suggests that is a combination of factors affecting hedgehogs (ie badgers are important, but they are not the only factor) and the authors suggest that wider habitat changes are responsible. To quote the authors…..
“In summary, much of the blame for the perceived hedgehog decline in the UK has focussed upon the impacts of badgers as both a competitor but especially as a predator (e.g.34). Although our findings support the negative relationship between the two species, this relationship is likely to be complex, involving elements of predation, competition and avoidance; in the context of the latter, areas associated with human habitation appear to mitigate against some of the negative effects of badgers. At the same time, however, rates of hedgehog occupancy were low even in the absence of badgers, and badger setts were not recorded in 47.9% of sites surveyed. Collectively, this suggests that intensive management of rural areas is negatively impacting both these generalist terrestrial insectivores”
To further quote the authors….” the combined effects of increasing badger abundance and intensive agriculture may have provided a perfect storm for hedgehogs in rural Britain, leading to worryingly low levels of occupancy over large spatial scales.”
Additional points on badgers and ground nesting birds
The decline of farmland birds in the UK and the rest of Europe has been extensively studied for decades and there are a number of great reviews of the subject (see Donald, Green & Heath 2001; Newton 2004). Predation can be a factor for some species, but changes in the wider countryside are widely believed to be the primary factors driving declines in birds, as well as wildflowers, insects and other animals. For a summary of the factors affecting farmland birds see this page from the RSPB. For a summary of the factors driving changes in UK wildlife see the State of nature report, compiled by a large number of conservation and research organisations.
The British Trust for Ornithology (BTO) also has records of population trends, along with summaries of likely causes of population changes on the website ‘bird trends’, for example see this page on skylarks.
I stress that by mentioning the above I do not aim to ‘blame farmers’ for these changes, as changes in farming practices and the wider countryside are driven by many economic, social and political factors. There are also many good examples (contained in the report above) of how certain farming practices can have huge benefits for wildlife.
The above reports, papers and sources relate to large landscape changes in bird populations. As stated in the summary sheet there will be examples of localised areas (such as nature reserves or breeding colonies) where badgers, foxes and other predators can have impacts on breeding birds such as terns. This in part is because many bird species (particularly ground nesting species) are confined to small protected areas or fragments of suitable habitat, making them vulnerable to predators.
Bovine TB biosecurity videos
January 11th 2019
In 2011 FERA (now APHA) created a series of videos on bovine TB and farm biosecurity.
- Introduction –badger behaviour and TB infection in badgers.
- Identifying badger activity – badger setts, runs and latrines.
- Biosecurity at pasture – measures to reduce interactions between badgers and cattle at pasture.
- Biosecurity in farm buildings (part 1) – measures to reduce badger access to farm buildings and yards.
- Biosecurity in farm buildings (part 2) – measures to reduce badger access to farm buildings and yards.
The videos contain a wealth of useful information, especially the biosecurity at pasture and in farm building videos (videos 3-5), which feature several practical examples and interviews with farmers who have used measures on their farms. The effectiveness of measures at reducing badger activity in yards and buildings is largely based on a paper by Judge et al. (2011) and many of the measures are also outlined in a series of factsheets on the TB hub.
As these videos are now a few years old, there are a couple of areas which need updating based on research published in recent years (ie after the videos were created).
Video 1 – Introduction
Video 1 – corrections /updates
At 3:15 the video states…….
“Local badger population densities vary considerably throughout the country, from 20 adults per km2 in the Southwest to less than 1 adult per km2 in less favourable areas. The last national survey of the badger population in 1997 estimated that it was around 325,000. There is the perception that in some areas the badger population has increased dramatically since then, but there is no evidence currently available to support this.”
A national badger survey carried out from 2011-2013 (Judge et al. 2014; Judge et al. 2017) estimated that average badger densities in the south west were around 6 badgers per km. In particularly high density areas badger densities can be 20 per km2, as quoted in the video, but this is more the upper value, rather than the average across the whole south west region.
This more recent study estimates that the population is 424,000 for England and 61,000 for Wales (Judge et al. 2017). It is important to remember that this was conducted in 2011-2013, which was prior to the roll out of culling across England. These population estimates use modern genetic based methods (which makes it difficult to compare the numbers to previous studies), but as the authors state “our results are consistent with a marked increase in the badger population of England and Wales since the 1980s”. Comparisons of main sett numbers (comparing 2011-13 to the 1985-88 surveys) also suggest a large increase in the badger population, with a 103% increase in England, but no change in Wales (Judge et al. 2014).
Video 2 – Identifying badger activity
Video 2 – corrections / updates
At 1:10 the video states…
“The relationship between badgers, cattle and bovine TB is extremely complex. Badgers undoubtedly become infected with bovine TB and can pass it on, but what proportion of cattle herd breakdowns are caused by badgers is unknown.”
It is true that there is a lot of uncertainty over the proportion of breakdowns which are caused by badgers (as opposed to by infected cattle). A study published after this video has estimated the contribution of badgers to TB breakdowns using data from the RBCT (Donnelly & Nouvellet 2013). This study found the following:
- Badgers to cattle transmission is estimated to cause 6% (1-25%) of cattle breakdowns
- Further cattle to cattle transmission means that TB from badgers is then transmitted further, so that the overall contribution to cattle breakdowns is 52% (9-100%).
There is a lot of uncertainty around these numbers (hence the wide ranges in the brackets), for more details about this study see the fact sheet and post on this website.
At 4:02 the video states……
“In high density areas there are usually 3 to 6 setts per territory, but in lower density areas there can be up to 40.”
I think this statement must have been a slight mistake, of course there will not be almost 10 times the number of setts in low density areas. Although the density of badger groups (main setts) varies among land types in the UK (higher in the SW and lower in the N/E), numbers of setts per main sett (ie setts per group) recorded in the national sett survey are actually similar in high density and low density areas (Judge et al. 2014).
Video 3 – Biosecurity at pasture
Video 3 – corrections / updates
At 2: 30 the video states….
“While contact with faeces, urine and other excretions from infectious badgers are a real risk of disease transmission the M. bovis bacteria may only remain infectious on pasture for a few days to a few weeks depending on the weather. Therefore, it is highly unlikely that any faeces taken up during the silaging process will still be infectious by the time it is fed to cattle.”
Several different experimental studies have investigated the survival times of M. bovis in different environments, which are summarised in several fact sheets on this website.
Although it is true that risk from silage are probably lower than other sources, longer term survival of M. bovis in silage cannot be ruled out (at least based on the limited available science on the subject). See the survival of M bovis in feed water and soil fact sheet.
Video 4 & 5 Biosecurity in farm buildings (no updates required)
If you are interested in free biosecurity advice contact the TB advisory service at http://www.tbas.org.uk/
TB trends in the badger cull areas
October 31st 2018
Recently a report was published by APHA which contains summary TB statistics for the first 10 licensed ‘badger control’ or cull areas. Click here to download this report.
The report contains measures of cattle TB incidence (the rate that new breakdowns occur) and TB prevalence (the % of herds under restrictions). These statistics are provided for the three years prior to each cull starting (this gives an idea of any trends or patterns prior to culling) and for each year during the culls.
The report covers up to the end of 2017, so provides data for the first 10 badger cull areas which started between 2013 and 2016. Cull areas licensed in 2017 and 2018 have not been underway for long enough for the data to be available, but they will be included in subsequent reports. The map to the right and table below show the details of the current badger cull areas active in England (in 2018).
|County||Year started||size (km2)|
The statistics in this report are potentially quite interesting and give an idea of the TB trends in these areas, indicating whether TB is increasing, decreasing or stable. However, it is very important to note that on their own these data cannot tell us why these changes are occurring. This is clearly outlined by the authors of the report who state that “these data alone cannot demonstrate whether the badger control policy is effective in reducing bovine TB in cattle”. Unfortunately several organisations and individuals have ignored this and used these latest figures as evidence the cull are “working”. They may very well be working, but these latest statistics can not prove this for a number of important reasons.
If you look at any of the TB statistics (either in various reports, using the TB dashboard, or using ibTB) you can see that the numbers vary up and down over time and from area to area. It is therefore possible that TB could increase or decrease due to other factors which are unrelated to culling. This is clearly demonstrated by the fact that TB trends are often not stable in cull areas in the years prior to culling.
To demonstrate that culling is causing changes in TB in cattle we need to compare the rates or numbers of breakdowns in cull areas to areas without culling (see example below). There may be a benefit from culling (ie fewer TB breakdowns than would have occurred without culling) even if the raw trends in the data (ie the green arrow below) are decreasing, stable or even increasing (as the increase may be much less than in areas without culling, as in the bottom left graph – suggesting a benefit). Likewise, even if TB is decreasing this does not mean that culling is the cause, as it may have declined anyway due to other factors (as in the top middle and top right graphs).
The changes in TB incidence in the RBCT (randomised badger culling trial) were estimated by comparing areas with culling to matched controls without culling, producing the following results below (Taken from Godfray et al. (2013) ).
Again I stress that the above figures are changes in TB incidence (rate of new breakdowns) relative to the control areas without culling.
To date the only study that has compared TB incidence rates in cull areas to the comparison areas without culling is the study by Brunton et al. (2017) which uses only the first two years of data and is also summarised here. There are many limitations and caveats to this work, but to date this is the only evidence that that badger culling is causing changes in TB in cattle. An updated report/paper using four years of data is currently being prepared by APHA. Until then the data in this recent report may be interesting, but unfortunately they cannot answer the question “are the culls working?”.
Bovine TB in foxhounds
September 18th 2018
Back in 2017 there were several articles in the press on the subject of TB in foxhounds, specifically at the Kimblewick hunt. This resulted in an investigation by APHA, the results of which were published in a recent paper in the journal ‘Transboundary and Emerging diseases’’.
The publication of this paper resulted in a second wave of articles in the press and (rather predictably) a second wave of discussion on social media, much of it questioning whether fox hounds may play a significant role in the transmission of TB to cattle. The paper is open access, so available for anyone to read (click on the link above), but I still thought it might be useful to produce a quick summary of what the paper says and what the results mean for the spread of TB in the UK.
What did the study involve?
The investigation was triggered because a hound was put down (it was suffering from anorexia, excessive thirst and urination) and post mortem suggested that this animal was infected with Mycobacteria. APHA then became involved and confirmed that the animal was infected with Mycobacterium bovis (which causes TB in cattle), specifically genotype ‘10:a’.
Following this a “test and cull” policy was applied at the kennels (test the hounds and remove those with a positive reaction) with the aim of containing the spread and removing any other potentially infected animals. Hounds were tested using two tests:
- IGRA (interferon gamma release assay), essentially a modified version of the interferon gamma or blood test used on cattle (click here for factsheet on the gamma test).
- DPP (dual path platform) Vet TB test an antibody (serological) blood test developed for deer but regularly used by APHA to test badgers.
All dogs testing positive for either of these tests were euthanased and then subjected to gross post-mortem examination (PME – carcass inspected for lesions). Samples from some of these euthanased animals (but not all) were also taken for mycobacterial culture (where M. bovis is grown from samples in the lab).
Both the IGRA and DPP test for an immune response which indicates that the animal has been exposed to M. bovis. Although, it should be noted that neither of these tests have been specifically validated for use on dogs.
What where the results?
The results for this study are quite complicated as different numbers of hounds tested positive for various combinations of IGRA, DPP and culture. Overall the results suggest that large numbers of hounds at the kennels were infected with M. bovis and several were in advanced stages of the disease, with the bacteria cultured from several organs and also from urine. Several people in contact with the hounds were also tested, and one was diagnosed with TB (it is not possible to know if this was from the hounds or other sources). The main results are sumarised in the infographic below:
The testing at the kennels ran from the end of 2016 to July 2017. Following the results above it was deemed that the remaining 57 hounds were unlikely to be infected/infectious, so voluntary restrictions at the kennels were lifted.
How did the hounds become infected?
The hunt operates across several counties in the edge and low risk area in England, where TB incidence in cattle is relatively low (compared to the high risk area – for a map of the risk areas click here). It is difficult to identify an exact source of infection, but various options were considered and ranked as part of an epidemiological investigation conducted by the authors of the study.
The potential sources of infection were:
- Movement of infected hounds from other kennels within the high risk area. Hounds at these kennels may have become infected from eating fallen stock (raw meat from culled cattle) and then carried the infection with them. This was viewed as the most likely route.
- Feeding fallen stock infected with M. bovis. Rather than the infection moving in from other areas it is possible that the hounds at this kennel were directly fed meet from cattle infected with M. bovis. This was viewed as possible, but relatively unlikely, based on the TB history and status of where cattle fed to the hounds were sourced.
- Exposure to infected wildlife or cattle (either directly or by M. bovis contaminated environment). According to the authors, most of the area operated by the hunt has low levels of TB in cattle and no evidence of TB in wildlife (although little data is available on this). This was viewed as an unlikley source of infection, partly because it is very rare to find dogs infected with M .bovis, even in areas with high levels of TB in badgers and cattle (which suggest that transmission risks via these routes are low).
It seems likely that the feeding of infected fallen stock to hounds was a likely source. This has resulted in the following changes outlined in the paper….
“As a proportionate risk mitigation strategy, DEFRA has introduced tighter restrictions on the collection and feeding of fallen stock to hounds in registered kennels. Since 10 October 2017 the feeding of offal from livestock species to dogs from recognized kennels or packs of hounds has been banned in England (Anonymous, 2017). Hunt kennel operators must also carry out additional examinations for lesions of TB in fallen stock originating from “high risk” premises”
What do the results of this study mean?
TB has been previously found in small numbers of dogs, but the general assumption has been that dogs are dead end hosts which are unlikely to spread infection further. This study challenges this previous assumption – it provides evidence of transmission between dogs and demonstrates excretion of M. bovis by infected animals. BUT it is important to stress that this study is highly unusual and is the first example of such a large outbreak of TB with evidence of spread through a dog population. It may be that this was the result of the specific conditions at these kennels.
This study therefore raises the possibility that dogs may be able to infect other species such as cattle or wildlife. It also highlights that current practices of feeding fallen stock to dogs has a potential risk of disease transmission. However, it would be wrong to conclude from this one study that large numbers of hounds are infected in the British countryside and are contributing significantly to the hundreds of TB breakdowns that occur every year. If other kennels are infected to the same extent as in this study (with multiple animals with clinical signs of TB) then this should be detected, as happened here. Even if there is a risk from infected hounds, it seems likely that this risk is localised and significantly lower than the risks from other infected cattle, or from infected wildlife (principally badgers), both of which are far more numerous, widespread, and are the main sources of TB in the UK.
Bovine TB in foxes
September 3rd 2018
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.
Skin testing and Gamma testing factsheets now online
June 22nd 2018
The primary test used for bovine TB in cattle in the UK is the tuberculin skin test or SICCT (single intradermal comparative cervical tuberculin test). In recent years there has also been an increase in the use of the interferon gamma or blood test. No test is perfect and it is important that people understand the pros/cons and limitations of the tests used to control TB in cattle. To try and explain the tests and dispel a few common myths i have created two new fact sheets in collaboration with the TB advisory service, APHA and with input from experts in the field of cattle testing.
How much do badgers contribute to bovine TB in cattle?
June 11th 2018
There is overwhelming evidence that badgers are involved in the transmission of bovine TB to cattle. What is much less clear is ‘how much’ of the problem is down to badgers, relative to other factors like the movement of infected cattle or the inaccuracy of the TB tests. One study which has attempted to quantify the contribution of badgers to bovine TB in cattle is this study by Donnelly and Nouvellet (2013).
It is a study which is often mentioned in discussions about bovine TB and badgers. As with other studies and statistics you often hear if from both ‘sides’ either as a justification that badgers are a significant part of the problem…. “badgers cause half of breakdowns”….or badgers are only a tiny part of the problem….. “only 6% of breakdowns are due to badgers”. To try and add some clarity I have produced a one page summary below. If you are interested I have added more detail on the methods below the summary, or alternatively click on the links to the original paper above.
A bit more detail on the study methodology
The paper uses a mathematical model based on an early paper by Donnelly and Hone (2010). The model involves a series of equations. Several parameters in these equations are already known or can be estimated from other data sources (on the left of the figure below). Other unknown parameters are estimated using the model itself, essentially by solving the equations to find the missing values. See a list of model parameters below.
The authors use the data produced by this model to then produce estimates for two key figures
1) The overall % contribution of badgers to cattle TB incidence
2) The % of transmission which is badgers to cattle.
You may be thinking…”aren’t they the same thing!!??” Not quite. We know that there is some degree of cattle to cattle transmission. This means that herds directly infected by badgers can potentially spread the disease further by the cattle to cattle route. Or to turn that round, if we can estimate the change in TB in cattle following badger culling (say 50%) then the % which is direct badger to cattle must be less than that (ie less than 50%) as whatever rate of TB comes from badgers will then be amplified by the spread through the cattle population. For example, if you imagine that badgers infect 5 herds directly, but cattle from these herds then transmit the infection to a further 10 herds (these are just rough numbers for an example), then badgers would contribute to TB in 15 herds in total (5+10), but only 5 directly. The model in the paper uses the data available to estimate the rate of cattle to cattle and badger to cattle transmission that are most likely, or will best fit the data.
As with any model there are limitations and assumptions and the values produced are estimates with a degree of uncertainty. This model uses data from the RBCT areas, so it is representative of those areas at that time. The contribution is likely to be different in parts of the country such as the low risk area and potentially different in other areas too, such as parts of the HRA where the cattle or badger population differs from the RBCT areas.
The model also assumes that the cattle to badger transmission is ‘negligible’, clearly some cattle to badger transmission does occur, but again, how much is unclear. As TB in badgers increased following foot and mouth (during which there was no TB testing) the authors of this study also ran the model excluding the initial proactive culls undertaken in 2002 after the 2001 epidemic (so only using 7/10 triplets). However, the exclusion of this data increased the uncertainty in the model.
The paper also reports the results from an earlier analysis by Jenkins et al. (2010) which looked at changes in TB incidence following the proactive culls in the RBCT. This can be viewed as another estimate of the overall contribution of badgers to cattle TB, as this is the level of reduction achieved if the badger-to-cattle route is reduced (by culling the badger population).
What do the results mean?
Firstly it is worth stressing that there is a large amount of uncertainty around the estimates produced, particularly the overall contribution of badgers as estimated by the model. The confidence intervals for this estimate are from 9% to 100%, with the 52% figure essentially in the middle of this. This is likely to reflect the uncertainty around the numbers/model itself, but also the fact that the data comes from 10 different RBCT areas, which themselves will vary. As stated above the data are specifically from the RBCT areas and different rates of transmission may occur in other areas. As such, the exact values quoted should be treated with a note of caution.
Both the analyses support the conclusion that badgers play a major role in maintaining M. bovis in cattle as they contribute to a significant % of breakdowns and culling (ie removing or reducing the badger source) can lead to significant declines within cull areas. The results of the model also support the conclusion that cattle to cattle transmission is important in disease transmission as it is responsible for roughly half of breakdowns directly (ie the other 50% not caused by badgers) AND because it amplifies the impact of transmission from badgers, further spreading TB through the cattle population. If accurate, these results suggest that reducing badger to cattle transmission at a proportion of farms could potentially lead to significant benefits in TB reduction. It also suggests that any cattle measures which minimise cattle to cattle spread could also have significant benefits.
Responses to the consultation on badger culling in the LRA
May 25th 2018
What was the consultation?
The consultation was aimed at getting views on 6 questions relating to the culling of badgers in the low risk area (LRA) in England. Broadly speaking this is in Northern and Eastern England where TB incidence is low (for a map of the different TB areas click here).
Who responded to the consultation?
Most of the 832 people who responded were from wildlife or animal welfare organisations, or were members of the public. Unfortunately very few farmers (3% or around 24) responded.
What did they say?
Most people were against proposals for culling in the LRA, which is not surprising given the makeup of the groups/ people responding.
What was the Governments response?
Taken from section 3 of the document….“The government’s view remains that enabling badger control in the LRA where disease in badgers is linked with infected herds is a rational extension of the TB strategy to eradicate bovine TB. The consultation responses have not provided new or compelling evidence to change that view.”Essentially meaning that the government still intends to cull in the LRA (where badgers are linked to infection).
Which areas might be culled?
This is not stated, but the document does mention that there have been 18 (or 21 – the numbers are not consistent) potential hotspot areas in the LRA from 2004 – 2017 and one of these has confirmed infection in the badger population.