Where better to start than to dive in at the deep end and
take a closer look at what must have been (and still is) the biggest wildlife
storey in the news over recent years, the case of badgers and bovine
tuberculosis. With recent news that the government is about to give the green
light to a full scale badger cull in England,
which could see in excess of 100,000 animals culled, I thought I would give my view
on it.
What is Bovine TB?
Bovine Tuberculosis is bacterial disease caused by the
bacillus Mycobacterium bovis and is
similar in many ways to Mycobacterium
tuberculosis the human form of the bacterium. In-fact it is likely that M.bovis evolved from M.tuberculosis and was originally spread
from humans to cattle following their domestication. Cattle then passed the now
mutated disease back to humans and on to other mammals including badgers. The
disease basically consists of the formation of tiny tubercles on internal
organs, most often found in the lungs, within these the bacteria multiply. When
one bursts it spills out bacteria spreading the disease around the hosts body. TB often causes
the animal to become weak, lethargic and eventually die. What makes it complex,
certainly in the case of badgers is some animals appear not to show signs and
suffer, where the disease lays dormant acting as a reservoir for the disease. Badgers
seem to have a remarkable tolerance to the bacterium and generally between 50%
and 80% of tuberculous individuals have no observable lesions. In some studies
badgers with TB and ones without it co-exist in the same sett over years
without it spreading between them. Some animals do succumb to it and often
leave the sett in a very poor condition. Approximately 35,000 cattle are
slaughtered each year as a result of Bovine TB, making it a real problem for
British Farmers.
Why Badgers?
People often argue that mammals other than badgers carry TB
and could contribute to the spread of Bovine TB and act as reservoirs. This is
true, however the significance of this is unlikely to be high in most species
with the exception of perhaps deer. A host species need to be able to carry the
infection and interact with cattle regularly. Deer populations are on the increase
and could certainly become a more significant reservoir for the disease in the
future, especially given that they are capable of ranging further more quickly
in comparison to badgers, which are relatively sedentary especially in areas
with high population densities. However deer have not always been as numerous
in comparison to how prevalent bovine TB has historically been, especially the
large species like red deer and fallow deer. Badgers are undoubtedly a key
reservoir for the disease as they are relatively vulnerable to infection and
their biology means they often come into closer contact to cattle, in
comparison to many other wild mammals. Badgers regularly feed in grazed pasture
where they are directly close to cattle, they also urinate on the pasture
causing an indirect risk, they regularly feed on stored cattle food, drink from
water troughs and some radio-tracking studies have even noted them as
physically interacting with cattle. In Britain, 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 Red foxes. Despite this the exact
transmission from badger to cow and back from cow to badger is still poorly
understood and may involve a number of transmission route ways. It is widely
accepted that cow to cow routes are the most significant way in which cattle spread
the disease, although cattle are less likely to act as long term reservoirs due
to health checks.
Badger Culling - What we know
With the evidence suggesting that Bovine TB was linked to
badgers, the idea of culling as a control measure was investigated. This has
occurred almost continually in one way or another since the 1970's, usually in
the form of trials. Thankfully unselective and inhumane methods such as the use
of hydrogen cyanide to gas setts was soon outlawed. This was replaced by live
trapping and shooting, which still operates to an effect today. In the 90's
Professor John Krebs began a Randomised Badger Culling Trial, this trial had a
key aim to determine the extent to which the removal of badgers reduced TB in
cattle. The trial used two culling regimes, Proactive - where badgers were
eliminated as far as possible from the area and Reactive - where culling occurred
in a small area as a direct result of a TB outbreak. In addition no cull
control areas were also studied, something which previous trials had not
investigated in sufficient detail. Culling trials in Ireland did however show
that by culling 100% of badgers in an area you can reduce TB, this is unethical
and not a practical solution to the problem.
The results of this trial offered conclusive and interesting
evidence (which appears is being ignored once again). The Reactive culling
resulted in an increase in the rate of TB in cattle by on average 27 per cent.
The proactive cull resulted in a lower rate of TB in cattle in culled areas,
than in control areas, showing the removal of badgers did reduce TB. However
only by a rather low 20%, not a lot given the time and money spent culling. What
makes matters even more interesting is the rate of bovine TB actually increased
by 25% ( on average) in a 2km band around the culling area. The increase in TB
surrounding the cull area effectively cancels out the results of culling inside
the cull area. Certainly good enough data to abort a cull and look at other
ways forward, and look at a genuine was to control the disease in cattle, in my
opinion.
The reason for this is known as the perturbation effect.
Culling alters the spatial social organisation of badgers, which results in
abnormal behaviour, not only in culled areas but in areas surrounding the cull
site. Badgers are territorial animals which more of less constantly remain within
the territory, or in close proximity to its boundary. Culling causes badgers to
increase their territories size, range further and disperse more frequently.
Larger territories often have more poorly defined boundaries increasing the (indirect
and direct) interaction of animals from neighbouring setts. All of this moving
around also causes TB to move around and spread at an increased rate.
Ways forward
I personally think the most realistic way of reducing the
disease in cattle is to deal directly with the cattle rather than badgers.
Cattle are easier to work with, easier to conduct research on and easier to
access. Research into vaccinating cattle needs to be continued and looked at as
a realistic way forward. However it is not as straight forwards as it would
seem in that so-far no 100 effective vaccine has been found. Vaccinating cattle
would also mean that you would no longer be able to conduct TB tests on
vaccinated herds as they would be diagnosed as TB-positive. So any development
of a vaccine would also require the development of a new TB test. Recent
promising advances have been made using Bacille Calmette-Guerin (BCG), the
vaccine used on humans. To make things worse EU Legislation forbids vaccination
of cattle as it may increase the spread of the disease, as vaccinated cattle
cannot be reliably tested. Trials are ongoing in Gloucestershire to test the
efficiency of vaccinating badgers. In my opinion administering drugs to a wild
animal like a badger would be very difficult, it would involve applying an oral
vaccine to bait, which would be impractical as it would be impossible to ensure
the whole sett had fed on the relative quantity of vaccine. This would then
have to be regularly carried out to vaccinate young animals weaned after the
initial bait was set up and animals new to the sett/area, which have dispersed
from other areas. In addition to cattle vaccination, increased bio-security of
stock sheds and food stores would help reduce the risks of infection i.e.
increase ventilation and have lower numbers together, as would better fencing
where possible (a method much cheaper than a national cull believe it or not). Certainly stricter farm bio-security, continued research into a more
efficient vaccination for cattle and tighter measures of cattle movement would
be positive steps forward.
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