Frequently Asked Questions

A2. No. The current skin test for bovine Tb is an effective test.  There is a 1 in 1,000 chance that a non-infected animal will be wrongly classed as a reactor.  It is the accepted standard laid down in both national and international legislation for determining the existence of infection in the cattle herd.  Studies have shown that the test is on average 80% sensitive at standard interpretation rising to 93.5% sensitive at severe interpretation.

A3. Very definitely not.  Such animals are infected and can infect others.  A significant proportion of cattle infected with M. bovis enter a state comparable to the latency defined for the majority of people infected with M. tuberculosis.  A large proportion of the skin (and gamma interferon) test reactors from herds with culture-confirmed bovine TB, are culture-negative and have no gross visible lesions.  Many of these animals are likely to be carrying infection that is not detectable by the culture methods employed and that has the potential to re-emerge at a later date, especially when animals are stressed e.g. by movement to a new herd, high production demands etc.  Such cattle, even if not infectious at the time of slaughter, might be so at a later stage if left in the herd.  They should therefore be considered as potential disease transmitters that pose a threat to the disease security of the herd.

A4.  The skin test is best used as a herd test but has value in controlling the spread of disease when used as an individual animal test.  Any diagnostic test that is applied to individual animals for disease screening will have a better chance of detecting infected groups of animals (herds) than individual infected animals, because one only needs to find a single infected animal to declare a whole herd as infected.  Therefore, the sensitivity of a test at the herd level will always be at least as high as its sensitivity at the individual animal level. 

A5.  A positive gamma interferon result indicates the presence of replicating M. bovis organisms.  There is evidence that they are more likely to be in the early stages of infection.  Therefore, failure to find post-mortem evidence of disease does not mean that the animal in question was free of the infection.

A6.  No.  Detection of M. bovis by culture is affected by many factors including the sampling process, with visibly lesioned animals giving a greater chance of detecting infection.  Animals at early stages of disease and latently infected animals do not present with visible lesions at post-mortem and will result in some animals escaping detection.

A7.    No.  The risk of the gamma interferon test identifying a false positive animal is 3 in 1,000, this risk is further reduced when the test is applied in a herd known to be Tb infected.  It is a common misconception that, as 82% of gamma interferon test positive animals do not show post-mortem evidence of Tb in the slaughter house or laboratory, they were “false positives”.  A failure to find post-mortem evidence of disease does not mean that the animal in question was free of infection.

A8.  There is no conclusive evidence to support this.  On the other hand fluke, through compromising immunity might make animals more susceptible to infection and/or might make infected animals less likely to react to the skin test (infected animals may therefore be missed).

A9.   Yes.  Exposure to Johne’s disease can cause cross reactivity when using the skin and gamma interferon tests for bovine TB.  

A10.  It is likely that any infective agent that suppresses an animal’s immune response mechanism such as occurs in cattle when infected with BVD virus, will increase the likelihood of establishment and progression of any additional disease such as TB.  For instance, concurrent TB infection is frequently seen in people infected with human HIV (AIDS) infection, but there has been limited work to demonstrate a similar risk for cattle with BVD.

A11.  There is undoubtedly some undetected infection – no test is 100% accurate and not all animals are tested.  Despite this, test and slaughter regimes based on the skin test have been successfully used in other countries to control bovine Tb where there is NO wildlife reservoir.

A12.  The evidence is that animals may become infectious – can pass on infection- very soon after they have themselves been infected (perhaps in days).  This may be followed by periods when animals are less infectious with intermittent excretion of tubercle bacilli.  These animals can eventually progress to clinical cases.  Infected animals should be regarded as a risk to others.

A13.  No. The extent of cattle to cattle transmission varies depending on the area and level of infection. There is no evidence to support this theory.  In low bovine TB incidence areas, there is evidence that cattle to cattle transmission could be responsible for around 80% or more of cases.  However, the situation is quite different in the high incidence areas of the country where 85% - 90% of all confirmed breakdowns occur.  Some herds in these areas are also infected by purchased cattle (several studies have shown around 7-16% Green et al.  2008 and ISG), but wildlife is a major source of new herd infection and in many counties wildlife may be a more important source than cattle.  It is impossible to put precise figures on these possible sources.

A14.  No.  There is an increasing body of evidence to suggest badger visits to farmyards and buildings may impose a comparable disease transmission risk to that posed by contamination of grazing land.

A15.  With the routine testing of cattle and reactor removal the transmission of TB from cattle to badgers is a low risk, as cattle are unlikely to be shedding large amounts of TB organisms into the environment.  This is only likely in uncontrolled cattle TB situations e.g. during FMD and pre 1930’s when a dedicated testing and slaughter regime was not being carried out.  The ISG reported an increase in prevalence in both cattle and badgers following the 2001 FMD epidemic.

A16.  Generally accepted principles of disease transmission indicate that it is possible that infected, infectious cattle that stray into the herd can infect others almost immediately.  Infection in these circumstances is a chance process and while transmission on the first day is possible, it is more likely the longer an infected animal is in contact with other cattle and if this contact is close or in confined spaces (as TB is primarily a respiratory disease).  However, it is very difficult to ascribe date or source of infection in a long latent period disease such as bovine TB.

A17.  Reactor cattle are infected with M. bovis and thus infectious to other cattle.  Development of bovine Tb disease may take many months or years but transmission of infection may be immediate.  Therefore the strict and immediate isolation of reactors is extremely important.

A18.  No.  Currently there is no validated test and even if one were available detection of M. bovis directly from badger excretions is difficult, largely because of the low levels and intermittent nature of excretion of M. bovis by infected animals.

A19.  No. It is quite impossible, as with cattle, to identify infected badgers on the basis of appearance and behaviour. Only in the very late stages of disease do animals show clinical signs and these are non-specific and may reflect diseases other than TB.

A20.  No.  It is impossible to identify infected setts without the capture of animals from that sett and detailed diagnostic tests.  There is currently no validated test for use in the field.

A21.  There is anecdotal evidence pointing to a genetic variation for resistance of cattle to infection of M. bovis.  However this has not been properly quantified in the cattle population in the UK and it remains a possibility that such genetic variation exists.

A22.  There is no evidence to either support or dismiss this theory.

A23.  The Randomised Badger Culling Trial (RBCT) has shown that culling badgers leads to a decrease of about 23% in cattle herd breakdowns in the culled areas, with a trend to a stronger effect (about 40%) in the central areas.  The question of how much bovine TB is caused by badgers has not been answered accurately through the RBCT as culling could not be conducted with 100% efficacy.  However, indications given by the Independent Scientific Group (ISG) were that the final results from the RBCT showed at least 40% was due to badgers. 

A24.  Infected baggers are able to reproduce and raise young successfully and live for several years.  However, based on knowledge of the pathology and extrapolation from the disease in other species, there is evidence that indicates that the disease will have a progressively increasing negative effect on the physical well-being of the badger.

A25.  Modelling suggests that if disease in cattle is reduced then disease in badgers will also be reduced.  On the other hand, there is evidence that TB is a self sustaining infection within the badger population and once introduced, the infection persists withthat species without the need for input from other infected species such as cattle.

A26.  No, there is a good case for starting a vaccination programme even though a proportion of animals are infected.  The key objective is to reduce transmission risks – between badgers and from badgers to cattle.  Although desirable, there is no need to vaccinate all badgers or stop them becoming infected to have an impact on transmission.  The BCG vaccine will reduce the risk of uninfected badgers becoming infected but would not offer protection to already infected badgers, nor will it harm them.  Even if infected badgers were present in the population at the time of vaccination, one would still expect the disease pressure on cattle to reduce over time as infected badgers die off naturally.  The typical life span of a badger is between 3-5 years.

A27.  The Government is investigating the scope and potential for making changes to EU trade regulations which would allow vaccination of cattle against bovine TB.  Indications are that it is unlikely, and certainly not in the near future.

A28.  The greatest TB risk to cattle in wild mammals is from badgers which are the main wildlife host

A29.  Wild deer in GB are generally considered a sentinel or ‘spillover’ host of infection in cattle rather than the source of disease in cattle.  Overall TB prevalence in wild deer is low and the ecology and behaviour of wild deer makes it unlikely that they would have any close direct contact with cattle.

A.30. Currently pigs are considered spillover hosts, they become infected through direct or indirect contact with infected badgers, deer or cattle, their carcases and excreta.  Evidence indicates that pigs become infected only when the prevalence of infection in the natural hosts is relatively high and pig populations cannot sustain the infection in the absence of infected cattle or a wildlife maintenance host.

A31.  For the majority of the population, the risk of people contracting TB from cattle in GB is considered very low.  At present, less than 1% of all confirmed cases of TB in humans are due to infection with M. bovis.  The majority of these cases are considered to be due to reactivation of latent disease contracted before widespread milk pasteurisation or from infection contracted abroad.  Somewhat greater risk is involved in some occupations where there is direct exposure to infected animals.

A32.  No.  Unless milk is pasteurised it is possible that it could be a source of infection.

A33.  There is a low risk to the public in general but many owners of these animals are not aware of the zoonotic risks associated.  One of the potential mechanisms of transmission between camelids and man could be through aerosols generated if an infectious camelid “spits” while being handled by a person.  Camelids are not regularly tested for TB compared to cattle and are spillover hosts to M. bovis, with the prevalence of infection in these species low compared to cattle and badgers.

A34.  No.  Whether or not there is a possible relationship between trace element supplementation and decreased susceptibility to infectious diseases such as bovine TB has yet to be proven.  Deficiencies of trace elements should be corrected as a matter of good husbandry practice.

A35.  TB is mainly a respiratory disease, caught by breathing in the bacteria and direct transmission can occur through, for example, nose to nose contact.  However, there is also evidence that indirect transmission is possible, for example through contact with infected saliva, urine, droppings, pus from TB abscesses etc.  It is difficult to identify the relative importance of each route of transmission of the disease and for this reason emphasis should be put on efforts to reduce the risk of cattle and badgers coming into direct and indirect contact.

A36.  There is anecdotal evidence that badgers are attracted to maize and maize silage and in areas where maize is grown it often forms a major part of their diet, but there is no evidence to suggest that reducing the amount of maize / changing from maize to grass silage can reduce bovine Tb to an extent that would justify what would be significant changes to farm management practices.

A37.  Research from the Veterinary Laboratories Agency (VLA) has shown that the ensiling process does, with time (6-12 weeks), kill the M. bovis bacterium.  As with the effect of all such processes on bacterial survival, the longer that the organism is exposed to the hostile acidic conditions in silage, the higher the proportion that will be killed or rendered non-viable.

A38. Yes – Slurry has the potential to spread bovine TB but this is highly unlikely under the conditions existing in the UK as a result of the current cattle controls.  The risk is mitigated by the dilution effect of slurry, the PH and the storage process, plus the spreading on land and exposure of organisms to the environment.

A39. TB infected cattle can shed M. bovis bacteria in faeces, urine and in coughed-up sputum.  C & D is a key part of TB risk reduction and in the control of other infectious diseases.