An expert panel of nine responded to questions about aspects of TB control in cattle from delegates packed into the lecture hall at last November’s BCVA congress.
There was not total agreement over the interpretation of past studies and the make-up of the Eradication Group had not been announced. As usual, the BCVA had an advanced finger on the pulse of disease control.
The information and opinion advanced, within the eight technical papers, was detailed and carefully thought out. Having listened to the presentations, read the papers and followed the comments of the panel, there is much to consider.
The real questions, however, appear to relate to where we are and how do we move on? The levels of anguish realised by farmers who have herds failing the 60-day test repeatedly are real and the farmer with failures following years of being clear does demand some sort of explanation, or at least hope for a healthy future. Could the combined expertise of the panel offer confident take-home messages?
Professor Graham Medley and Dr Orin Courtenay introduced the work that has been carried out at the University of Warwick with cattle transmission, modelling and M. bovis in the soil.
Infection persists
Some of the figures noted were that half of the 48,000 cattle assessed for the data were purchased, that 697 herd tests were carried out with 156,000 animal tests yielding 723 reactors; 56% of the tests were carried out on 4% of the cattle and 84% were not tested during their lifetime.
Even when skin test negative cattle were introduced the infection persists on farm for five years. Cattle that had tested negative from a previously positive herd are likely to introduce TB.
One of the conclusions stated was that “individual animal tests are insufficiently sensitive to be certain that a non reactor is uninfected unless the history of the herds on which it has resided and their localities are considered”.
With regard to sensitivity (the proportion of true infections disclosed) and specificity (the proportion of true negatives not falsely disclosed), the diagnostic test is a compromise.
“There is more disquiet and discontent about a farmer who has a herd breakdown when there is no true infection, than about a farmer who has a clear test when there is, in reality, infection in the herd,” they said.
“We don’t believe that bTB can be controlled in cattle unless test sensitivity is increased. A more aggressive test and culling programme will have a positive impact on bTB in cattle.”
Their final conclusion: “We don’t believe that the impact of such a programme can be predicted with current data.”
Three sources of M. bovis have been considered by the researchers: cattle, badgers and the environment. It is indicated that even when badgers and cattle are removed, the organism remains in the soil but it may be in a dormant state and difficult to culture.
Once outside the mammalian host M. bovis is known to enter a state of non-replicating persistence. The organism is able to survive for many months and probably years.
Advances in the use of PCR to detect mycobacterial DNA in environmental samples have enabled bTB endemic farms to be sampled. The full results are in the BCVA proceedings but the positive M. bovis findings, from sett and latrine samples, are not necessarily matched by positive tests from captured badgers.
The removal of non excreting badgers would not be expected to reduce the environmental levels of M. bovis. Badgers are intermittent excretors and only yield positive samples in dung 25% of the time.
In discussion, the point was made that ingestion of the organism from pasture is not likely to cause bTB in cattle. Cattle “snorting” at latrines are more likely to produce an aerosol that could be inhaled and cause infection. The development of a sett test is expected to be of value to indicate where the application of a badger bait vaccine would be effective.
Clear messages
There was no question of misunderstanding the messages from the next speaker. In technical speak, Dr John Gallagher clearly stated that: “Assumptions which have been made in the mathematical modelling of the disease are challenged regarding the origins of infection of outbreaks, the universal infectiousness of reactors, the significance of multiple reactors and the differing nature of disease in cattle and badgers.
“It is suggested that these inputs into the ISG model have resulted in it being flawed. Also, the advice to further increase cattle controls is considered pointless. No progress in control of the epidemic will be achieved without a selective cull of infected badger communities.”
In non technical speak his message would be that if the current programme is continued the bTB problem will be greater year on year.
An increase in cattle test failures after being at pasture has been recorded. Frequent testing results in the removal of cattle reactors in the early stages of infection, before they present a risk to others. Disease in the badger can progress to emaciation and death.
The very high levels of organisms excreted by the badger in urine indicate high pasture contamination. “Improving diagnostics to kill even larger numbers of cattle which are essentially acting as sentinels of disease in the local badgers will do nothing to reduce the numbers of new outbreaks, nor will it resolve the deteriorating problem in badgers,” he said.
Professor Simon More (University College Dublin) said the steady year on year number of cattle reactors in Ireland indicated a level of control, but that eradication would not be possible in the presence of wildlife reservoirs.
Following two clear herd tests, farmers in Ireland are free to trade but about 10% of these herds fail at the next test. The badger is considered to be an “upstream driver” of infection with badger to cattle transmission being relatively independent of efforts to minimise cattle to cattle transmission.
The eradication programme in Australia took 27 years but proved effective and included a major change in the movements of cattle from infected herds and the way cattle were traded. A herd was not considered free of disease until there were two years of clear tests. The emphasis was on the herd not the individual animal.
Pre-movement testing of individuals was not applied. In Ireland the movement of cattle is considerable but only 6-7% of current herd restrictions are attributed to the introduction of an infected animal. Residual infection in cattle appears to be particularly important in Ireland.
Contact between infected badgers and cattle needs to be reduced. Detailed information about transfer of infection between badgers and cattle is lacking. Removal of badgers or vaccination of badgers is considered the way forward. Focused removal of badgers in Ireland has not shown adverse infection prevalence in either badgers or cattle.
Work towards an effective vaccine and delivery method for badgers is ongoing. When a vaccine becomes available, current control methodologies, particularly with respect to cattle to cattle transmission, will need to be critically reviewed.
Biosecurity scoring
The application of a biosecurity scoring tool by veterinary surgeons in Wales was outlined by Dr Stephen van Winden (RVC). Factors listed for scoring included the introduction of cattle, mixing of herds, manure management, occasional contact, local herds and land use, contact with other animals, shared or borrowed equipment, visitors and use of protective clothing.
Following initial visits, farm management changes were advised and a reduction in the introduction of animals and fewer contacts with other animals were noted. Involvement of the local veterinary practice with a client to consider on-farm improvements to reduce bTB were considered a worthwhile contribution to disease control.
Peter Jinman recalled the origins and development of the TB campaigns. Both he and the audience found it sobering that in 1908 there was a call for the Government to control TB. Progress was made with disease reduction but now there is a year on year increase with the slaughter levels rising. In England the mass slaughter of badgers is currently not being considered as part of disease control.
Dr Michael Welsh (Agri-Food and Biosciences Institute) presented the findings from an interferon-gamma testing programme in Northern Ireland. A skin test negative animal that was IFNg positive is more likely to subsequently have confirmed disease within 12 months than a skin test negative and IFNg negative animal.
Herd owners identified as having a problem bTB history in their cattle participated voluntarily in the programme and 60% of the IFNg positive animals were slaughtered. Cattle detected earlier are less likely to show visible lesions so the herd owner needs to be aware that lower levels of disease confirmation after slaughter can be anticipated.
Discussion
Carl Padgett chaired the discussion and Andy Biggs joined the panel of speakers. Both are now part of the TB Eradication Group so it was highly relevant that the first question asked was, “What does the panel believe should be the priorities for the group?”
Mention was made of a 20-year programme, targeting persistently infected areas, improvements in test sensitivity and greater control over the whole programme. Observations were made on the relevance of zoning and risk-based trading as needing to be “something that the industry wants to do”.
A figure was also given that 70% of cattle test negative when they are positive and that cattle genetics indicate that some strains of M. bovis may not show using the current skin test antigens.
It was also indicated that it is very difficult to clean up contaminated pasture. By culture, the indications are that the organism dies out on pasture within 24 hours but molecular tests indicate months or years. Abattoir testing as a means of bTB surveillance was said to not be sufficiently sensitive.
The discussion was pointed and intense and there was considerable frustration at the current levels of disease.
Clearly, although bTB may have been with the industry for generations, there is much that is not understood and greater efforts are anticipated.
