TB: Calm before the storm
DEFRA’s consultation on badger culling as a means to reduce the incidence of bovine tuberculosis in infected areas closes today (10 March).
Extensive advertising campaigns by the Royal Society for the Prevention of Cruelty to Animals and the Badger Trust in national and regional newspapers sought to garner a strong response from the public against a cull.
Meanwhile, attempts by the NFU and the National Beef Association sought to ensure an equally committed response from farmers and land owners in favour of a cull. In total DEFRA reckons to have received 10,000 responses.
For more than 20 years bovine TB has been spreading throughout Great Britain.
It is now endemic in the south west and parts of central England and south-west Wales as well as occurring sporadically elsewhere.
During this time the debate has raged over how the disease should be tackled and which species is the prime culprit for its spread. Consequently, views on both sides have become increasingly polarised.
And the problem is growing.
Last year exactly 30,172 cattle were slaughtered due to TB and 93m spent on testing and compensation.
Based on the long-term trend of an 18% increase in confirmed new herd incidences and a 20% annual increase in the number of cattle slaughtered, DEFRA forecasts suggest that under the current policy, by 2010 the number of cattle slaughtered each year will have risen to 66,000.
Furthermore, the cost to the Treasury of the current policy for the 10 years up to 2013 will reach 2bn.
There is also a human cost to this crisis.
An Exeter University study published in 2005 investigated the impact TB has on farmers in the south west.
It concluded that: “One in five calls to the Rural Stress Information Network arises from the direct and indirect effects of bovine TB”.
It added: “An outbreak of bovine TB can have a serious effect on the farm business concerned and movement restrictions, sometimes for extended periods, can make its impact much worse than that of an outbreak of foot-and-mouth.
It is the longer lasting effects which are the source of most of the damage to the farming industry by bovine TB.”
Methods of transmission
The exact extent of the badger’s involvement in spreading TB to cattle is not fully understood.
The Randomised Badger Culling Trial (RBCT) – also referred to as the Krebs trials – were supposed to quantify this, but when it became apparent that land owner compliance within the culling areas would be less than 100% it had to be abandoned.
However, a crude interpretation of the results from the RBCT and the Four Area trial in the Republic of Ireland allows us to estimate the badger’s involvement.
In the RBCT the culling of badgers reduced the incidence of TB by 20% after the initial cull and 25% after the first follow-up cull.
A more rigorous analysis of the Four Area trial data by the Independent Scientific Group (ISG) on bovine TB estimates the reduction in TB in cattle in the trial areas to be 58% when compared to the survey-only areas.
The best scientific estimate, therefore, is that badgers are responsible for 20-58% of TB spread in infected areas.
Cattle-to-cattle transmission is blamed for the remainder.
Cattle “nosing”, involving the ingestion of nasal discharges from contact animals, and inhalation of airborne particles represent the most common methods of transmission between cattle.
As TB is primarily a disease of the respiratory tract it is not surprising that 90% of tuberculous lesions in cattle are found in the thoracic cavity or the head.
For infection to be established in the respiratory tract an infective dose can be as low as 1-6 bacilli.
“You have to appreciate that Mycobacterium bovis is a disease of cattle and a very good one at that,” says Graham Medley, professor of infectious disease epidemiology at Warwick University.
“Studies have revealed that an infected animal can exhale large quantities of infected bacilli with each breath.”
While he admits that inhaling the breath of an infected animal does not guarantee infection it is easy to understand why, with nasal shedding occurring as soon as 21 days after infection, housed cattle, in sheds with poor ventilation and reduced levels of sunlight, are considered most at risk.
Initial studies by Prof Medley and his colleagues at Warwick also suggest those farms that keep manure in confined spaces near to buildings are more likely to suffer a breakdown.
Prof Medley is critical of the current preoccupation with badgers in spreading the disease.
“We’ve let TB get away from us in the UK and that needs to be challenged.
We’re learning all the time about the way the disease spreads.
We know cattle movements are responsible for a large proportion of the disease spread and our work suggests that farmers who buy steers are at greater risk,” he adds.
He speculates that this is because cattle destined for sale are less likely to be tested.
“We will watch with interest the impact of pre-movement testing,” he adds.
However, it is the nature of cattle-to-cattle contact that defines the level of risk.
Direct contact represents the biggest threat, but airborne transmission is also a route to infection.
For airborne transmission to occur however, each particle needs to be 1-5 micron in size.
This can only occur in a limited number of ways, not all of which are fully understood, but spreading infected slurry using an irrigator gun may be a potential source of atomised M. bovis.
Studies in Ireland recovered the organism from a distance of up to 800m away from the gun.
The bacteria can also survive for up to 315 days in slurry, meaning slurry stored on infected farms will, almost certainly, always be infected with the bacteria.
Another study carried out in Ireland by Liz Wellington, professor of environmental microbiology at Warwick University, found M. bovis in the soil of a test farm two years after infected badgers and cattle had been removed.
DEFRA is funding a trial to investigate further these findings.
Solutions
Biosecurity
There are a number of measures suggested to reduce their risk of infection from wildlife.
For example, farmers are advised to refrain from feeding cattle at pasture with troughs.
Troughs offer a valuable source of food for badgers and they have been observed urinating in and around troughs to mark territory.
Other suggestions like raising water troughs have been shown to have little effect.
Improved farm biosecurity through pre-movement testing of bought-in stock followed by a period of quarantine is suggested as a first step towards ensuring that TB is not unwittingly introduced to the herd.
Other ideas include erecting electric fences close to ground level to ensure badgers remain excluded from cattle yards and feed stores.
Measures such as quarantining, however, present benefits beyond TB control.
With the cattle industry scaling up its efforts to tackle Johne’s disease and Bovine Viral Diarrhoea (BVD) the benefit of such an investment could be reaped elsewhere.
“Just compare the biosecurity levels in the pig and poultry sector to those in the cattle industry,” says John Bourne, chairman of the Independent Scientific Group on bovine TB.
“It’s living on a piece of string and it’s quite astonishing how long the cattle industry has got away with the level of disease it has.”
TB is a serious disease and deserves to be given the respect other zoonoses command, he says.
Vaccine
Some observers believe a vaccine will provide the panacea, but such beliefs may turn out to be folly.
Even the Badger Trust, which campaigns tirelessly against culling, accepts that a vaccine is not a realistic solution now nor in the future.
“We cannot see it being developed for badgers because of the difficulties in inoculating badgers below ground before they get infected,” said the Badger Trust’s Trevor Lawson when giving evidence to the House of Commons’ Environment, Food and Rural Affairs Select Committee in February.
A vaccine for cattle would also create problems because of the danger it might mask disease.
Vaccinating cattle for TB would jeopardise the UK’s ability to trade cattle as it would almost certainly lose its TB-free status.
All realistic experts estimate a suitable vaccine – for either species – is at least 15 years away.
Lessons from other countries
For the past nine years Australia has boasted TB-free status, but achieving this took 27 years of resolute determination on the part of Australian cattle farmers and state and federal governments.
It also demonstrates the importance of a co-ordinated approach that incorporated a wildlife cull, strict movement controls and regular testing.
Although there were a large number of feral water buffalo carrying the disease they covered a small area of open land across the northern areas of the Northern Territory and Western Australia.
For this reason, and the fact that they were large targets to shoot, they were easily eliminated.
This, combined with a rigorous programme of tuberculin skin and gamma-interferon tests followed by the slaughter of infected herds, achieved a significant reduction in the disease.
The testing procedure was rigidly policed and enforced with strict movement controls that permitted only clean herds to trade freely while infected herds could only trade direct to slaughter.
The policy also enjoyed full farmer support.
Those who broke ranks or refused to lend support to the wildlife cull were portrayed as pariahs by their peers.
Similarly, tough restrictions are in place in New Zealand.
Although the possum is recognised as an important reservoir for the disease, culling is again combined with a test and slaughter policy for cattle.
Zoning controls prevent the transfer of cattle from infected areas to clean zones.
Ireland employs similarly tough action along the lines of New Zealand.
“It simply astounds me that cattle movements in Britain are not restricted along the lines of New Zealand,” said Leigh Corner, a New Zealander working at University College Dublin.
“Why do you continue to allow the movement of cattle from infected areas to clean areas?” he asks.
Improved diagnostics
Although badgers unquestionably play a significant role in the spread of the disease and an extensive cull of badgers in infected areas will be necessary for the long-term reduction of TB, evidence is mounting that suggests improved detection with increased testing frequency coupled with tough cattle controls and faster removal of infected cattle is likely to result in a large reduction in the incidence of TB.
The tuberculin skin test was originally developed as a herd test (which it is very good at) but its suitability as an individual animal test is continuously questioned.
Its true accuracy is often debated, but it is considered to return false-positives in up to 34% of cases (if this were commonplace, however, slaughterhouse detection would reveal more falsely identified animals).
Although the needless loss of a healthy animal is frustrating for those affected, from a disease control point of view the number of false-negatives is of more concern.
Prof Bourne is confident that 11% of cattle in the south west that have passed a tuberculin test would be latent carriers of the disease.
Hence his regular assertion that “any farmer who buys cattle from the south west needs his head examined”.
Statistical modelling by the ISG suggests a 6% improvement to the accuracy of the skin test would be sufficient to reverse the number of new herd incidences.
A reduced testing interval would return a similar result, according to Christl Donnelly, a statistician at Imperial College London and a member of the ISG.
“An increase in testing from annual to every 10 months in the Western region would be sufficient to turn the current increasing trend in TB incidence in cattle into a decreasing trend, according to our modelling,” said Prof Donnelly.
The trouble is that the skin test is already as good as it can be.
But when used in conjunction with the gamma-interferon test sensitivity increases to 88% and specificity to 97% – this would vastly improve the detection of infected animals.
However, until the consultation was announced DEFRA had consistently refused to expand the role of the gamma-interferon test insisting that it had not yet been evaluated fully for use in the field.
Where to now? Until the publication of the RBCT results in December 2005 the government claimed there was insufficient evidence on which to base a sustainable policy capable of reducing the incidence of disease in both cattle and badgers. Farmer groups disagreed citing the Thornbury trials (1975-82), Steeple Leaze (1976), Hartland (1984) and Ireland’s East Offaly trial (1989-94) as justification for a cull. Conservation groups such as the Badger Trust and the RSPCA point to the scientific data on the role of cattle movements in spreading the disease and lessons from other countries such as Australia that focused on tough cattle controls as more suitable measures. The conclusions of the RBCT combined with evidence from the Four Area trial in Ireland, lessons from Australia and on-going work in New Zealand has generated a science base of sufficient quality to devise a policy relevant to the situation in Great Britain. Many believe the evidence suggests a policy should include:
Some will claim these measures are obvious and should have been implemented years ago. It’s even been suggested that the RBCT was just a government excuse for inaction. But the truth is that there was simply not the depth or quality of evidence available to support the tough measures required in the face of divided opinion. For example, previous beliefs that small-scale culling of badgers on a reactive basis would be sufficient have been vanquished. We now know that if culling badgers is to have a long-term impact on the incidence of TB it requires the virtual elimination of badgers from large tracts of land. The wholesale slaughter of badgers is neither considered sustainable – nor is it acceptable under the Bern Convention (an international agreement on the preservation of natural habitats and the mammals that inhabit these environments) – but nor is slaughtering tens of thousands of cattle each year. So until a solution acceptable to all parties is developed the debate will continue to rage and the government will continue to face tough political decisions that receive intense scrutiny. With the necessary scientific data published and the consultation closed the decisions that remain are entirely political. We can only hope that Britain’s farmers and their representatives have been successful in persuading the ministers at DEFRA to make the necessary investment in overseeing a cull and expanding the use of the gamma-interferon test. |