Bovine TB Past and present – is there hope for the future?
Roger Blowey FRCVS
To understand a disease, it’s often of value need to look at its history and how others have
resolved the same problem.
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1920’s – TB a problem in cattle and man; pasteurisation of milk
1940’s – no action; bigger issues were at stake nationally!
1950’s – voluntary TB accreditation.
1960’s – compulsory eradication
1970’s – down to 400 reactors per year, but then plateaued.
1971 – TB found in badgers in Glos
1973 – badger protection to stop badger baiting
1975-80 – Thornbury ; badger removal and TB disappeared for 10 years
1981 – protection of badger setts
1980 – Zuckerman report.
– Para 56 : “ … we believe it would be unnecessary and indeed a
waste of resources to seek further confirmation ….”
– 1997 – Krebs - total ban on badger culling outside of RBCT trial areas
• 1995 to 2012 – increasingly stringent testing + movement controls on cattle, but
TB increasing by around 18% per year
There must be a reason why TB fell so dramatically from the 1950’s onwards, and then
increased from the mid to late 1990’s, as shown on the attached graph, taken from the
DEFRA website. What changed in the 1990’s to produce this – progressively ineffective
measures aimed at removing TB infected badger setts.
The next chart shows badger control measures over the same period. They became
progressively weaker from the 1980’s until all culling was banned in 1998 at the start of the
Krebs trial.
There have now been six culling trials, and all have shown a reduction in bovine bTB, even
the most recent Krebs trial, which was least effective and achieved by far the poorest level of
badger culling
Area (sq km)
Thornbury
Gloucestershire (1970’s)
104
%
elimination
of badgers
by culling
100%
Reduction in TB herd outbreaks
Steeple Leaze, Dorset
12
100%
Clear for 7 years
Hartland Point
Devon
64
>80%
80-90% reduction – 10y
East Offaly, Ireland
738
>80%
88% reduction – 7y
Four Counties, Ireland
100 x 4
>80%
60-80% reduction
RBCTs (Krebs), UK (1998 –
2004)
100 x 10
30 – 70%
19 -23% reduction inside
22- 29% increase outside
Clear for 10 years
Note the very low success rate of culling in the Krebs trial, and consequently its relative
ineffectiveness in bTB control compared to Thornbury etc.
It is sometimes stated that poor efficacy of TB testing by vets and subsequent cattle to cattle
transmission is the major cause of the recent dramatic rise in TB. If this was the case, it does
not explain how the testing was so effective in the 1960’s, when we had the same test, the
same risk of transmission, and in my case, the same person doing the TB testing! I can think
of no other disease, and no other country, where attempting to control of the reservoir has not
been an integral part of the overall programme. If the test is that ineffective, ie if the upsurge
in TB is due to poor testing, it does not explain how
a) the skin test successfully eliminated bTB from Northumberland and parts of Cumbria
when it was taken there post FMD restocking and
b) all other countries worldwide have eliminated bTB using the same or similar test,
provided the wildlife reservoirs were dealt with.
There are now far fewer cattle than in the 1960’s, far fewer groups of cattle moving premises,
and far more stringent TB testing (premovement testing, culling of 2 x I/R etc), and yet TB
continues to rise.
Ideally we need to identify infected sets, cull all infected animals and destroy/collapse the set
to prevent re-colonisation. Personally I think culling of known infected badgers might be
more acceptable to the general public. If we fail to do this, bTB will continue to spread to
other badgers, to other forms of wildlife, and will eventually become a risk to all of us who
enjoy walking in the countryside. Action is needed for the benefit of our children and
grandchildren, and for the benefit of the badgers as much as anyone.
Maize silage and badger numbers
The increasing amounts of forage maize being grown since the 1980’s, an excellent feed
source for badgers, is considered to be one reason for the increase in badger numbers, and for
the spread of bovine TB in the badger population. Many farms now cut their maize early to
avoid further destruction of the crop by badgers.
In the 1940’s Neal in Somerset described one badger per sq km as ‘abundant’ and ‘excellent
badger territory’. In 1997 a survey in Westbury sub Mendip revealed 37 per sq km, and it is
estimated that since then there has been a further doubling of the national badger population,
ie we have moved from 1.0 per sq km to 74 per sq km.
The Perturbation Effect
As shown in the graph below there was undoubtedly an increase in bTB around the proactive
cull area in the Krebs trail BUT
1. This was only shown to occur in a narrow 2km band around the outside of the cull
area. It was not widespread
2. This rise in TB was seen primarily in the first 2 years. For the subsequent 3 years after
the cull had ended bTB was lower in the ‘perturbation zone’ than before the cull.
Hence overall there was beneficial effect in this 2 km zone, even though there was an
initial increase.
Estimated effects of proactive culling on the incidence of confirmed cattle TB
breakdowns inside trial areas and up to 2 km outside trial area boundaries (DEFRA)
Vaccination
It is generally accepted that vaccination will have no effect on badgers already infected with
TB, and around 30% of all badgers examined from Gloucestershire in the Krebs were
infected. We cannot be sure that we will vaccinate all badgers in a set (it is thought that a
minimum of 70% vaccinated will be needed), and it is improbable that the young uninfected
cubs, the ideal candidates for vaccination, will be caught in the traps and therefore
vaccinated.
The most commonly reported trials on vaccination are papers by Chambers et al. If you look
closely at the data it is clear that all vaccinated badgers eventually developed and excreted
bTB. Hence the vaccine cannot be anything more than a delaying mechanism. We know that
after 3 years of vaccination by Glos Wildlife Trust on their Cotswolds farm the cattle broke
down with TB
A report released by DEFRA (Ref 1) states that
"If only 50% of badgers can be trapped and injected with a vaccine which is only 50%
effective, and only 50% of farms are involved the disease control benefit becomes rapidly
diminished in any given year - 50% of 50% of 50% = 12.5% of the potential available
benefit. While there will be a benefit, as any level of vaccination will produce a benefit, it will
take substantially longer to appear in terms of reduced cattle breakdowns and vaccination
will have to continue for a much longer time in order to accrue the benefit."
Other Wildlife reservoirs
There is no doubt that bTB occurs in other species including deer, outdoor pigs, wild boar (in
Portugal) and of course cats and dogs.
Unfortunately for the badger it is the only species that does not ‘wall off and contain’ the
infection. For example TB occurs in wild boar hunted in Portugal, but the lesions ‘calcify’ so
excretion of infection is unlikely. In badgers the TB spreads all over the body, lesions are not
‘walled off’, and as a consequence infected badgers excrete huge amounts of infection in their
urine, faeces, saliva etc. , making them the most probable reservoir of infection.
Reduced TB in Wales
There was a 33% reduction in TB cattle reactors in 2013 vs 2102, but this effect was seen
over the whole of Wales, and not the 1% of the country currently in the pilot vaccination area,
so the reduction is likely to be due to other factors such as a concerted attempt to remove
reactors over the previous two years. There was an overall 8% decrease in TB reactors in
England from 2012 to 2013, so this may be natural variation that occurs with any disease.
Cattle Movements and cattle numbers
It is said by some that one factor increasing the spread of bTB since the 1970’s is the big
increase in the number of cattle movements. I do not agree with this because
1. I have yet to see any data to prove the alleged large increase in animal movements
since the 1970’s
2. Total UK cattle numbers peaked at 15.4m in the 1970’s, had fallen to below 10m by
2005, and have been falling at around 2% pa ever since (see table below taken from
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http://www.parliament.uk/briefing-papers/SN03339.pdf). As all farmers are aware,
when cattle numbers were at their peak, TB was at its lowest level.
If we expressed the number of TB reactors in the 1970’s (400 pa) as a proportion of
total UK cattle numbers (15m) and compared this with today’s figures (38,000
reactors from just 8m cattle) the current bTB explosion is even greater
There is often confusion between a) the numbers of animal movements and b) the
number of animals moved. In the 1970’s cattle were brought from large numbers of
small farms into a central market, mixed, sold, and moved onto a range of small units.
Some animals were bought by dealers and could pass through another 2 or even 4
markets over the course of the following week or two. In today’s era of larger herds, if
a dairy farmer wants more cows, he does not go to the market on a weekly basis he
buys one herd of say 100 cows. This represents just one animal movement (but 100
animals have moved). An epidemiological understanding of this in relation to the risk
of disease spread is well established and widely used in the pig industry.
If cattle movements were the major factor in terms of the spread of bTB we would see
bTB outbreaks occur randomly over the UK, and these could be mapped in terms of
animal movements. This is not the case. The spread of bTB has been a slow and
seemingly unstoppable outward movement from the minor 1970’s residual reservoirs
of Gloucestershire and Cornwall. This is typical of the progressive spread of disease
in the wildlife reservoir.
The explosion in bTB occurred especially from the late 1990’s. I am unaware of any
evidence to demonstrate that this was associated with an explosion in the number of
recorded cattle movements.
When bTB HAS been spread by cattle movements, eg Cumbria and Northumberland
post FMD in 2001, then the simple skin test has rapidly eradicated the TB from cattle
herds in all cases where no wildlife reservoir exists.
Closed herds, ie those that are entirely home bred or very rarely purchase cattle, have
a similar incidence of bTB breakdowns to herds that purchase cattle
Our Aim
Freedom from TB in cattle, badgers and all other livestock. Even if it has no befit to cattle,
the elimination of TB in badgers would be very much to the benefit of the badger.
Roger Blowey May 2014