BADGERS AND BOVINE TUBERCULOSIS
Content Updated:
19th June 2011
UPDATE (January 2013): This article was written back in June
2006 and much has happened to the debate around bovine tuberculosis and
badgers since then. This page will be updated in due course, but until
it is not currently at the top of my revision list. In the meantime, the
reader is directed to the
DEFRA's Bovine TB website. Badger biologist
Martin Hancox presented the anti-cull debate on his Great Badger and
Bovine TB Debate website and -- although this no longer appears to be
online -- the crux of his argument is available as a PDF (click
here to open it in a new window), while the National Farmer's Union also has a
channel discussing bovine TB that's worth a read. Please be aware that
there are very few impartial website on this topic and, as always, I
would advise the reader looks at as much of the evidence as possible
before making up their own mind. N.B. Many of the links
to other sites of interest at the bottom of this article were broken when
DEFRA had a tidy-up of their website; these have now been fixed.
 The first record of tuberculosis in Meles meles came from Switzerland
during the mid-1950s. In 1971, a dead badger recovered from the Cotswold
Hills in Gloucestershire (UK) was found to be infected with
Mycobacterium bovis, the bacteria known to cause tuberculosis in cattle
(that is to say, bovine tuberculosis, or bTb for short). This specimen
represents the first case of a badger infected with bTb from Britain.
In humans, Tb is typically caused by M. tuberculosis. However,
M.
bovis is a close relative of M. tuberculosis and can be equally
problematic for humans. During the 1930s, the incidence of M. bovis in
children (and cats) was high; the bacterium was contracted through the
consumption of unpasteurised milk. The implementation of milk
sterilization and pasteurising lead to a dramatic decline in human bTb
cases, although people in prolonged contact with cattle or their meat
(i.e. slaughter houses) have been known to contract M. bovis infections
– according to the Institute of Animal Health in Berkshire (UK), around
2000 people currently die of bTb each year, globally. Today, Tb as a
complex is the world’s biggest bacterial killer, estimated to kill three
million people annually.
At the bottom of this summary, I have provided several links to the
Department for Environment, Food and Rural Affairs (DEFRA, which
superseded the Ministry for Agriculture, Fisheries and Food, or MAFF, in
2001) reports, which will give you a much more comprehensive view of the
problem than I intend to provide here. Anyhow, for the uninitiated, here
are the basics.
Just how big of a problem is bTb in Britain?
Following a dramatic
reduction of infections post-1930s, incidences appear to be on the
increase. According to DEFRA’s figures, bTb is now “endemic in some
parts of Great Britain and increasing at a rate of 18% a year”, with
about 20,000 infected cows dispatched in the UK per annum (roughly 0.2%
of the
UK’s cattle population). This is unfortunate not only for farmers
(for whom bTb can represent the end of a livelihood) but also for one of
Britain’s best-loved mammals because, as DEFRA go on to state: “The main
wildlife reservoir for the disease in Britain is in badgers.” Indeed,
most authorities now consider that there are two main bTb reservoirs in
the UK: one in the badger population and the other in the cattle
population.
How is bTb spread?
Unfortunately, despite many years of fairly
intensive research into bTb, there are still many areas of its
epidemiology that we can only speculate upon; perhaps the most
significant of these is its transmission. We know that M. bovis
primarily infects the lungs’ and kidneys’ of badgers, suggesting that Tb
can be spread on the badgers’ breath and in their excreta. Indeed, a
summary of bTb cases published in Research in Veterinary Science during
2000 reported that of the 146 tuberculous (i.e. infected with bTb)
badgers studied, 51% had lesions in the lungs, 25% in the kidneys, 40%
on the lymph nodes and 14% in bite abscesses (implying territorial
disputes may spread infection). Lesions to the brain, spleen and gut
were infrequent (0.7%, 0.7% and 2%, respectively).
 At the badger-to-badger level, M. bovis is probably transmitted as an
aerosol (i.e. badgers breathe it in). However, the situation is less
clear at the badger-to-cattle level. Currently, the primary route of
transmission is considered to be through scent marks, especially urine.
Indeed, studies have found that while badger faeces (left) can contain up to
75,000 tuberculosis bacilli per cubic gram and badger pus up to 200,000
per millilitre (mL), urine may contain up to 300,000 per mL. Considering the
ranging behaviour of badgers, this implies that cows feeding on grass
along the periphery of fields (where badgers are more prone to scent)
are at a higher risk of picking up the disease than those grazing more
centrally. Moreover, badgers take very precise routes, frequently
re-marking the same areas and it is not unwarranted to think that a
build up of the bacteria (which can survive on the ground or in faeces
for days or months, depending on the conditions, while spores may
survive for decades) could occur in these areas and may persist even
after badgers have been removed. Consequently, several studies have
tried to assess how different parts of a field system might present
different levels of infection potential. Overall, it was found that
urination was more prolific at “crossing points” (i.e. the points where
badgers cross a linear feature like a fence or hedge) than other parts
of a pasture. Pastures with lots of linear features were, therefore,
found to have increased contamination with badger urine. It is perhaps
mildly reassuring, therefore, that field observations suggest direct
contact between badgers and cattle in the wild is rare and studies
looking at where cattle choose to feed have demonstrated that cows
generally avoid areas of grass contaminated with badger excreta.
However, this situation might be different when feeding from a trough
and it is not difficult to see how, in typically oligotrophic (low
nutrient) pastures, the added fertilization provided by nutrient-rich
faeces and urine may cause grass to grow lusher than in other areas of
the field.
One study recently published in the Journal of Dairy Science reports
that the type of grazing plan a farmer implements may affect how likely
his/her cattle are to come into contact with contaminated pasture. The
biologists write:
“If investigation is a major route of tuberculosis transmission, the
risk to cattle is greatest in extensive rotation-grazing [access to
whole field for up to 7 days at a time] systems. However if ingestion of
fresh urine is the primary method of transmission, strip-grazing
management [field subdivided for grazing for periods of less than 24
hours] may pose a greater threat.”
One might be forgiven for thinking that, in this case, a simple
double fence to keep the cattle away from this grass is the obvious
solution. However, this link has yet to be firmly established, let alone
ratified as the principle route of infection and M. bovis transmission
may occur by three main routes: urination or other scent marking
(including latrines); aerial; or through direct contact with either the
badgers themselves or ground they use (e.g. digging for worms).
Unfortunately, each of these contact routes would involve a different
control methodology. It is also important to remember that the process
of transmission is not a unilateral one; it is highly probable that
cattle can pass the disease to badgers just as easily as vice versa.
Are badgers the only transmitter of bTb?
 No. To further complicate
the issue, although badgers seem to have a considerable tolerance to the
infection, they are not the only species capable of transmitting the
bacterium. However, while badgers may not be the sole route of
wild-domestic transmission, they are one of the few species that can act
as effective vectors, because the disease is rarely fatal to them.
Despite the observation that, even in the most susceptible species, bTb
infection can persist for many months (even years) before it becomes
fatal, not all species that can become infected with the bacterium are
effective carriers (most die fairly soon after contracting the bacilli).
Badgers seem to have a remarkable tolerance to the bacterium and
generally between 50% and 80% of tuberculous individuals have no
observable lesions. Moreover, a paper published in The Veterinary Record
during 1998 reports that infected badgers generally have few sites of
infection and small numbers of the bacilli in their tissues. In the UK,
badgers are the only known maintenance host for M. bovis, although there
are some “spill-over hosts” (i.e. populations in which infection will
persist where a maintenance host is present in the ecosystem), including
ferrets and Red foxes.
A review of M. bovis infection in wild mammals from the UK, headed by
Central Science Laboratory (CSL) biologist Richard Delahay, lists ten
species (excluding the badger) from which this bacterium has been
isolated. Included in this list are Fallow deer (Dama dama -
right), Red fox
(Vulpes vulpes), Mink (Mustela vision), Mole (Talpa europaea), Brown
rat
(Rattus norvegicus), Ferret (Mustela furo) and Domestic cats (Felis
domesticus). A recent study of 1,307 Bank voles (Myodes
glareolus) found that only one yielded positive results for M. bovis,
leading the authors to suggest that this species is relatively
unimportant as a reservoir for bTb. Similar studies have shown that
rabbits and mice can be experimentally infected with bTb, although no
records are known from the wild.
Some evidence suggests that sex and age may affect susceptibility to
M. bovis, while the stage of disease progression seems to influence
mortality. In a paper to the Journal of Zoology in 2000, a group of
scientists fronted by David Wilkinson (CSL) report that, while
ELISA-Positive badgers (i.e. incubating bTb, but not excreting the
bacilli) at Woodchester Park did not have a substantially higher
death-rate than uninfected individuals, infected badgers that were
excreting M. bovis did. The study also found that the progression of the
disease is considerably more rapid in males than it was in females;
females ultimately seemed to cope with the disease better than the
males. Similarly, the susceptibility of badgers to M. bovis infection
may be related to age, although studies to date have yielded mixed
results. One 1991 paper found that infection rate was slightly higher in
cubs than in adults (46% and 39%, respectively), while another paper
from 1998 found the opposite (13% of cubs and 20% of adults).
There are also data from Ireland to suggest a connection between
irregular bacula development (potentially affecting reproduction?) and incidences of bTb infection.
How does bTb manifest itself?
Regardless of how or from which species
the infection arrives, upon entering cattle the bacterium targets the
lungs, manifesting as a severe pulmonary infection. Overall, the
symptoms of Tb typically include loss of appetite, weakness, weight
loss, fever and caseous (looking cheese-like) lesions in the lungs, on
the bronchomediastinal lymph nodes (little ‘filters’ in the neck near
the thymus) and other organs. Swelling of the lymph nodes can lead to
lameness (especially if it leads to skeletal and synovial lesions).
How do you test for bTb?
Testing for bTb post mortem (which is the
only way bTb can be confirmed in badgers) can be done
histopathologically (looking at tissue sections under the microscope,
culturing of bacteria etc.). Testing live animals can be achieved
through a variety of methods. In the UK, the most common test is the
Tuberculin -- or Delayed Hypersensitivity -- Skin Test, which is
typically used in conjunction with the Gamma Interferon Blood Test (for
details on these tests, see the links below). It should be noted that
the Gamma Interferon test is only viable for bovine livestock and that
research is continually underway looking for more accurate (one problem
with using a vaccine is that vaccinated cattle will show positive for
bTb infection) and more rapid testing procedures. One such blood test is
the Enzyme-Linked Immunosorbent Assay (ELISA), which shows promise not
only in its wide species application, but also in its ability to test
badger faeces for M. bovis.
Is the UK alone in its battle against bTb?
No, bovine TB is not just
a UK problem; it is also prevalent in North America, Africa, New Zealand
and Australia. In the USA, deer and elk are considered to be the primary
reservoirs, while buffalo are the main reservoir in Southern Africa,
Brushtail possums and ferrets in New Zealand and feral pigs and feral
Water Buffalo in Australia. Additionally, sporadic reports of M. bovis
isolates from free-ranging wildlife are known from other countries (e.g.
Red foxes in Spain).
 What can be done to reduce/eradicate bTb?
Until quite recently,
culling was considered the main answer to the bTb epidemic. Badgers were
culled using cyanide gas between 1975 and 1981 -- when gassing was
outlawed -- although research indicated that not only was gassing
inhumane, but it was also ineffective at combating TB transmission. It
also cost more to gas the badgers than the government saved with any
resulting decline in bTb. Since 1975, some 30,000 badgers have been
killed in a bid to eradicate and research bovine TB, even though only
20% of them had any sign of the disease.
It has been widely implied that culling badgers is still the most
obvious and practical way to control bTb infections in cattle. Although
it may seem inherently logical that reducing the wild reservoir would
lead to a reduction in infection rate, the reality is considerably more
complex. Recently, the National Farmers Union (NFU) has thrown its
support behind DEFRA’s culling trial, in which they plan to instigate a
substantial cull of badgers in southwest England so as to assess its
impact on reducing cattle infection. This action was deemed worthwhile
largely on the results of a study from Ireland, frequently referred to
as the “Four Counties Trial” (FCT). The FCT, found that culling badgers
did indeed lead to an decrease in the incidence of bTb in the Irish
herd. However, one of the main findings of this -- highly controversial
-- study was that bTb infections can only reduced significantly if you
cull 100% (or very close to) of badgers. In other words, you can only
solve the problem this way if you kill all the badgers! As such, the
authors of the FCT paper state that culling is not the way forward in
combating bTb!
Several studies from the UK confirm the idea that culling is not a
plausible alternative to other strategies, including vaccination and
better biosecurity. Two papers to the journal Nature (one in 2003 and
the other in 2005) have demonstrated that far from helping the
situation, anything less than a wide-scale, blanket (i.e. as close to
total eradication as possible) cull will only make the situation worse. The data, collected and analysed by a team of 14 scientists from across
the country, show that not only did culling have a negligible impact on
bTb incidence within the study areas (roughly 19% decline), it actually
lead to a 25% increase in cases of bTb in peripheral areas (Note: it has
been argued that this increase actually happened before the cull and was
simply part of the epidemic growth). The biologists suggest that culling
badgers causes a breakdown in the clan’s social cohesion; badgers leave
the clan and move into neighbouring areas, taking any infection with
them. Indeed, a paper to Biological Conservation by Linda Sadlier and
Ian Montgomery of The Queen's University of Belfast, found a significant
negative relationship between the severity of disturbance and sett size
in Northern Ireland's badger clans. In other words, as the disturbance
got worse, the number of adult badgers in the clan declined. The authors
consider that this migration from the main sett was a result of a
disturbance-induced disruption in territorial behaviour. That
said, there have been examples where targeted culling of badgers by MAFF
has apparently succeeded in eradicating bovine TB on a local scale and
highly targeted culling may have a role to play in the reduction of
infected animals within a population.
Another proposal has been to impose restrictions on cattle movements.
In fact, a recent paper to the journal Nature reported that, although
the current distribution of bTb infection in cattle most closely mirrors
the areas of highest badger density, the current spread of the disease
most closely mirrors the patterns of cattle movement across the UK.
Indeed, this paper's models suggest that the disease has spread more
rapidly than the badger population itself. Consequently, this means
tighter restrictions on nationwide cattle movements as well as more
rigorous bio-security measures, regulating movement between farms and
how cattle are handled at market, will be required.
It is worth pointing out that there is evidence that bTb infection
can induce behavioural changes in badgers. Studies looking at the
ranging behaviour of infected badgers have found that these individuals
range further than their uninfected conspecifics. In their 1981 paper to
the Journal of Zoology, CSL biologists Chris Cheeseman and Peter
Mallinson report that badgers infected with M. bovis became solitary,
often taking up residence in outlier setts and ranging more widely than
was usual (similar results have been found in primates infected with
this pathogen). More recent work has found that infected badgers can
have home ranges as much as 50% larger than uninfected individuals; they
also range over a greater proportion of the territory than healthy clan
members, foraging, on average, 65% further away from the main sett.
Consequently, infected badgers may be more likely to stray onto
neighbouring farmland and potentially into barns and farmyards, which
would put them in direct contact with cattle.
The discovery that badgers infected with bTb move around more than
uninfected individuals coupled with the potential for disease
transmission through direct personal contact has lead to the suggestion
that bTb could be significantly reduced (if not actually eliminated) by
excluding badgers from cowsheds (with better fencing) and feeding
troughs (with better designs). Indeed, visits to farms by badgers can be
common, especially in periods of low rainfall, when the soil is dry and
digging for earthworms is all but impossible. For example, in a 2002
paper to the Proceedings of the Royal Society of London, Ben Garnett and
Richard Delahay of the CSL and Tim Roper Sussex University in Brighton
report on the use of cattle farm resources by badgers in
Gloucestershire. The biologists found that during 449 hours of
observation (59 half-nights and 17 full nights), at least 26
individually identifiable badgers from two clans made 139 separate
visits to farm buildings, using cowsheds, feedsheds, barns, haystacks,
slurry pits, cattle troughs and farmyards. In some instances the badgers
were seen to approach to within 2m (6ft) of penned cattle as well as
consume cattle feed, silage and defecate in cattle troughs. Moreover,
tests on the clan found three of the visiting badgers to be tuberculous
and excreting M. bovis.
 Based on Garnett
et al. results, it may intuitively seem like the best
solution would be to take precautions to exclude badgers from farms.
However, subsequent work by the same scientists has found that this is
easier said than done. For example, in their 2003 paper to Applied
Animal Behaviour Science, Garnett and his colleagues report that the
maximum height to which badgers in their clan at Woodchester Park
(Gloucestershire) would climb to gain access to a trough was 115 cm
(almost 4 ft). Unfortunately, according to Garnett et al. research,
raising a trough to this height would place it out of the reach of
calves, young heifers and bullocks. If the data presented by Garnett et
al are representative -- and offhand, climbing height may vary in
relation to how hungry a badger is when it comes across the trough -- it
seems that if exclusion of badgers is going to be a workable alternative
to culling, new designs for feeding troughs and cattle sheds are going
to be necessary. Garnett et al. make a few suggestions as to how a
trough may be designed to deter badgers, including rolling bar to the
rim or placing a pressure plate on the floor; ultimately these require
experimentation.
The fact that bTb is a global problem might hold the key to its
resolution here in the UK. Currently, many authorities consider that the
only way to eliminate M. bovis from the British herd is to vaccinate the
reservoirs (i.e. the badger and cattle). Although vaccination of badgers
is fraught with complications, work in New Zealand has shown some
promise of an attenuated M. bovis bacillus Calmette-Guerin (BCG)
vaccination for cattle. Experiments by virologists at the Wallaceville
Animal Research Centre in New Zealand and the UK’s National Institute
for Medical Research and Veterinary Laboratory Agency have demonstrated
that a BCG booster vaccination can induce protection (a 70% reduction in
pathology) against bTb. However, even in humans (where it is the most
widely-used vaccine in the world), the results are highly variable and
given that in excess of 75% of the cattle reservoir would require
inoculation, it is clear that there is much work still to be done. Although a marketable vaccine may still be a way off, in June 2005 the
British Farming Minister announced to the House of Commons that a
three-year vaccine field trial is due to commence in an area of
southwest Britain later this year (2006).
So, what’s the upshot of all this?
The ideal state is one where bovine TB has been eradicated from Britain. The route to this state is far from clear, but will almost certainly
involve a multifaceted approach, including vaccination of cattle and
badgers, restrictions on cattle movement, and overall improved
biosecurity. Culling will likely play a roll in this strategy, but
how sizeable this roll is remains to be seen. One point is, however, clear: if culling is to be implemented, it must be
undertaken with great caution and regiment. Culling is a highly
contentious issue, curiously more so for badgers than many other
species, but as there are continued setbacks in the development and
implementation of vaccines for both badgers and cattle, lethal control
becomes increasingly appealing to those desperate for a solution to the
inexorable rise in cattle TB infection. Moreover, a vaccine only benefits healthy
individuals (cow or badger), which invariably means that infected
animals must either be culled out of the population (often a complex
and expensive operation) or uninfected animals must be protected from
infection until the infected animals naturally die off; the result is
that vaccination, certainly of badgers, would need to be carried out in
several waves annually for several years. The logistics and costs
associated with such a scheme are vast -- essentially, badgers must be
tested and either vaccinated or culled in accordance with whether they
are clear or infected -- and thus more wide-scale culling (aimed at
reducing badger numbers and thus the probability of an infected animal
being present in any given area) is presented as an aid to slow the
spread of the disease. It is important to remember, however, that while
some previous highly targeted culling has proven successful in
eliminating TB locally, there are plenty of data showing how
inappropriate (particularly uncoordinated) culling can do more harm than
good. There is no magic bullet for the problem of bovine TB and, I
suspect, vaccination and culling will both play a part in the final
solution.
External Links
DEFRA Animal Health
DEFRA Consultation on badger culling
DEFRA
Tuberculosis in cattle
Gamma Interferon Blood Test
Impact of localized
badger culling on tuberculosis incidence in British cattle (Nature
abstract)
International Study Group
Final Report on Bovine TB (PDF Document)
National Farmer’s Union Bovine Tuberculosis
Positive and negative effects of widespread badger culling on
tuberculosis in cattle (Nature abstract)
Tuberculin Skin Test
Tuberculosis Vaccine Trial (BBC News)
Update on Measures to Tackle Bovine TB
(DEFRA)
Return to TOP
|
