1. Neither EU nor national law specify particular requirements for when a pig herd is Officially Tuberculosis Free (OTF) or when this status has been withdrawn or suspended as it does for bovines but Defra/APHA apply similar principles in the way control measures are applied to pigs. The main purpose of this is to ensure that occasional spill-over of infection into pigs primarily from badgers and cattle does not represent a risk of spreading disease and to protect the export market for British pigs.
2. Surveillance for TB in pigs in GB relies primarily on abattoir post-mortem examination (PME) with the Food Standards Agency (FSA) reporting suspect TB lesions in pigs under the terms of a Service Level Agreement (SLA). Additionally, suspect lesions may be detected at routine post mortem surveillance usually after suspecting clinical signs or during investigations for other problems. There is currently no programme of statutory TB testing for pig herds in GB.
1. In August 2015, Defra published its Veterinary Risk Assessment (VRA) for TB in Pigs within the Bovine TB in Non-bovine Farmed Animals - Call for Views (TR497). The executive summary includes the following:
1. In England, when TB suspect lesions are found in any species (including pigs), APHA must:
2. In Scotland and Wales, where M. bovis infection has been confirmed by PCR (or culture) of the bacterium in a pig herd, restrictions (TN02/TN02(Welsh)) will be served on the whole premises (unless served earlier on the basis of suspicion of disease and a VRA) and will remain in place until the Veterinary Lead Scotland (VLS)/Veterinary Lead Wales (VLW) is satisfied that the herd is free from infection or has demonstrated that the risk of infection is at an acceptable level within the herd, i.e. negligible or very low.
1. An epidemiological investigation visit should be carried out by APHA whenever a new incident of M. bovis infection is bacteriologically confirmed in a pig unit. The herd owner should encourage their private veterinary surgeon (PVS) to work with APHA Veterinarians to try and identify the source and the extent of spread within the herd. It may be possible to narrow down the groups of animals likely to be at risk of TB infection and reduce the number of animals to be tested using various surveillance methods for restrictions to be lifted.
2. Consider the following questions as part of the investigation:
1. All movements off TB-restricted pig premises must be direct to slaughter or to a biosecure finisher unit in the TB High Risk Area (HRA) of England. A General Licence to Slaughter (TN24c) may be issued but for a limited period before review e.g. three months. Unless there are unavoidable reasons (such as potential risks to animal welfare) for doing otherwise, applications for movements off to other premises in the Edge or Low Risk Areas (LRA) of England, to Wales and to Scotland should be rejected.
2. All movements must only be licensed after a VRA of the receiving premises has been completed by an APHA Veterinarian and the risk of spreading infection to TB-susceptible livestock and wildlife in the rearing area is deemed to be negligible or very low.
3. Completing VRAs for receiving premises can be time consuming and costly and it is recommended that where a pig keeper/company requests assessments of several finishing units, a limit of two premises can be stipulated for a period of six months, and no further premises will be reviewed within this period. The company must have carried out a preliminary assessment to ensure that the destination premises are biosecure and poses a negligible or very low risk to adjacent livestock. Once agreed by the company that a herd is suitable for receiving pigs from an infected premises, details of the premises will be passed to the relevant APHA office with a request to carry out a VRA.
4. Each premises will be visited by an APHA Veterinarian and a report with conclusions and recommendations made to the VHoFD (or their delegated officer).
5. Premises that are approved will be issued with a General Movement Licence from the breeder herd to the finisher (TN15(E)) and from the finisher to slaughter (TN24c).
1. The risk of disease entering the pig population is greatest for pigs raised in outdoor systems in the TB endemic areas, where there is greater opportunity for contact with M. bovis wildlife vectors unless biosecurity against badger contact is implemented, or those kept on farms where they are co-located with infected cattle or have contiguous contact with other livestock. For that reason, in cases where M. bovis infection in pigs is confirmed, an assessment of biosecurity on the premises is essential.
2. Consider the following biosecurity questions in all restricted pig premises, but especially in confirmed TB incident cases:
3. Regardless of the results of a TB testing programme aimed at lifting restrictions, the herd owner must be advised to remedy any biosecurity deficiencies identified at an early stage of the investigation before restrictions are lifted.The importance of biosecurity measures or the prevention of the introduction of further infection into the herd cannot be overstated. As the estimated probability of further infection being introduced into the herd increases so the confidence that the herd is free from infection decreases. For example, the probability that less than one animal is infected is greater than 90% in month nine after controls are started where there is no introduction of infection into the herd but less than 40% in month 20 where the probability of introduction is 0.25 (25%). (Source: Jessica Parry, Department of Epidemiological Sciences, APHA Weybridge).
1. If infection is entrenched and at high prevalence in a herd (many epidemiological groups and ages affected), the herd owner may decide to depopulate the herd, usually by culling out the breeders to slaughter and then having the rearing pigs finished, often on other fattening units in the same pyramid.
2. After any essential biosecurity improvements, the herd may be repopulated with new pigs.
3. There are issues to be considered:
1. The single intradermal comparative cervical test (SICCT) is used for International Trade and has been used in pig herds affected by TB breakdowns. It is not validated under UK conditions and may be difficult to perform in large numbers of pigs. Skin testing of pigs is performed on the base of the ear. It is slow, laborious, requires good restraint such as snares and restraint pens. It is potentially quite dangerous and impractical if to be used on large numbers of pigs.
2. There would be little point using skin testing on fattening pigs which are destined for imminent slaughter (unless a small targeted epidemiological group with confirmed cases) but on small numbers of breeding pigs, especially if rare or of high value, skin testing may be considered as part of a surveillance programme to lift restrictions if combined with post mortem examination of culled/slaughtered pigs.
3. This option should be made available to all herd owners, provided the testing is not judged to pose unacceptable safety risks.
1. The APHA Department of Epidemiological Sciences have investigated the effectiveness of abattoir surveillance to demonstrate freedom from M.bovis infection in pig herds. The main conclusions were that it is unlikely that a reasonable probability of freedom from TB infection could be demonstrated by abattoir surveillance alone due to the estimated low sensitivity of post-mortem meat inspection and the fact that only a small proportion of the herd is inspected at any one time.
2. The sensitivity of traditional post mortem meat inspection, based on visual inspection, palpation and incision techniques, has been estimated as 25% in fattening pigs and 50% in adults. A higher sensitivity can be expected in adults due to longer lifespan and opportunity for exposure to TB and for the development of detectable lesions. The sensitivity of visual-only post mortem meat inspection is likely to be much lower than using traditional methods and would not provide any basis for assessing herd freedom from M. bovis infection.
3. When replacement pigs are being constantly moved into a herd coupled with poor biosecurity (and thus potential exposure to an ongoing or persisting source of infection), the greater the probability of reintroducing infection and the less confidence that the herd is free of infection over time. Replacing pigs sent to slaughter will increase the time it takes to establish freedom from infection.For example, evidence for freedom from infection accumulates faster where an infected pig herd is isolated and there is no replacement of pigs removed for slaughter. In rearing/fattening pigs after 25 weeks (assuming that no infected pigs are detected and there is no further introduction of infection) the probability that there is less than one infected pig in the herd is 0.73 in the pig herd that does not receive any new pigs compared to 0.57 in the pig herd where slaughtered pigs are replaced. For breeding pigs, after 20 months (assuming than no infected pigs are detected and there is no further introduction of infection) the probability that there is less than one infected pig is 89% in the herd that does not receive any new pigs compared to 62% in the herd where slaughtered pigs are replaced. However the evidence accumulates slowly. At six months, the probability of freedom, where there is no incoming infection, is 0.54 in the herd managed with and without replacement. Furthermore, to achieve a probability of freedom of at least 0.70, over 75% of the adult pig herd that is slaughtered without replacement, will have been removed and it would still take 20 months to achieve a 0.96 probability of freedom (Source: Jessica Parry, APHA Weybridge).
4. In cases where abattoir surveillance is the only option that the herd owner is willing to accept, they should be advised that it is unlikely that restrictions will be lifted from the herd for at least 24 months (or until all contemporaries of the index case have been slaughtered and undergone post-mortem inspection), during which all adult culls will need to be subject to full incision and palpation meat inspection. If the turnover of adult culls is low, longer than 24 months may be necessary. Post Mortem Inspection (PMI) of adult fallen stock should be included.
1. This option is one which may be combined with one or more of Options 1 to 3 above.
2. Although it is not permissible in TB restricted cattle herds, it may be possible to apply restrictions to just one part of a pig herd (not co-located with cattle) in which infection has been confirmed and groups of animals can be considered epidemiologically separated from each other.
3. Where the whole herd has been restricted it may be possible for restrictions to be lifted on parts of the herd at different times (i.e. partial de-restriction), following a favourable, thorough, and documented VRA (with risk being assessed as negligible to very low). All this is without prejudice of the implications that the TB movement restrictions will continue to have on the whole holding (CPH), e.g. for the purposes of export health certification of live pigs and pig products. The VRA should be documented on the standard template.
4. Consideration should be given by the keeper to separate the restricted herd into smaller groups of pigs on which surveillance measures could be conducted separately and where biosecurity could be controlled. Unweaned piglets are extremely unlikely to maintain and spread infection prior to being weaned from the adults for finishing but testing of these smaller groups may still be warranted.
1. Polymerase Chain Reaction (PCR) Tests - faecal or sputum samples could be collected from pigs (oral fluid sampling using ropes from groups of pigs reduces handling problems). This test is promising but the sensitivity is thought to be low and TB infected pigs do not shed bacilli consistently in body excretions and fluids. Whilst this technique and test is not validated at present, it may have a role to play alongside more conventional methods of surveillance such as abattoir sampling.
2. Blood tests - serological tests are theoretically available but none have been optimised or validated for use in pigs under UK conditions, and performance data is lacking. These are quicker to perform than skin testing but in pigs are still laborious to carry out and extremely time consuming. Furthermore, the pig industry currently has higher priorities than attempting to validate such tests. The interferon-gamma test used in bovines (BovigamTM) does not work in pig blood. Pig specific reagents would be necessary in order to develop an interferon-gamma release assay specific for Suidae.
3. It is important to note that negative surveillance results from these tests, do not overturn positive post mortem (PM) results or standard skin test findings, nor will they speed up the lifting of restrictions. They are ancillary tests which are useful and promising and provide useful background information to assist in confirming the results of other tests.
1. Pigs are spillover hosts of M. bovis and the risk of onward spread from infected pigs is considered low, but not negligible.
2. Abattoir surveillance alone is insufficient to release an infected herd from movement restrictions - a minimum of 24 months surveillance is likely to be necessary, depending on turnover of the adult herd.
3. Skin testing of the whole herd would provide evidence of freedom (two herd tests with negative results).
4. Blood testing (and PCR) as a back-up to other surveillance methods could be investigated but are back-ups only.
5. A combination of skin testing the adult herd (or possibly any exposed epidemiological groups), post-mortem meat inspection of cull sows/boars and/or post mortem examination of fallen stock may be acceptable.
6. Ensure any replacements new stock for TB infected pig herds are tested on entry, sourced from low prevalence areas of GB or use AI.
7. Implementation of identified and necessary biosecurity measures to prevent recurrence of infection is essential for the consideration of lifting of restrictions.
8. Restrictions with licenced movements.
9. No surveillance programme will give 100% certainty of freedom, however, the aim is to provide confidence, by using any of the above-mentioned surveillance methods or a combination of thereof, that the risk is negligible to very low to make informed decisions about lifting restrictions.
1. Can all or some of the adult stock be skin tested (preferably twice)?
2. Can all adult culls be post-mortemed? (needs to be done over 24 months with no lesions detected - otherwise reset clock).
3. Is depopulation of the breeding herd a possible option?
4. Continue to monitor finishing pigs in the abattoir (all slaughter pigs in contact with infected pig(s) will need a traditional post mortem inspection (visual, palpation and incision) - if segregated from adults and new rearers will take six months, if not then monitor over 12 months).
5. Only licence pigs to slaughter directly or to finishing units in the HRA of England and the receiving unit to have a VRA (limit to two farms in any six month period).
6. Require the keeper to make premises biosecure from wildlife and agreed other conditions - if unable to make premises biosecure, restrictions to remain in place for a minimum of 24 months to allow all breeding stock to have been culled and no lesions found at slaughter, otherwise clock resets.
7. An Action Plan will be agreed with individual keeper and presented to the VHoFD for discussion/amendment with Case Vet and keeper. The relevant Veterinary Advisor (VA) in Defra, Scottish Government (SG) or Welsh Government (WG) may be consulted.
8. A VRA must also be completed by the Case Vet and submitted with the action plan to the VHoFD.
9. Where it is not possible to agree an action plan with the keeper, the herd will remain under permanent movement restrictions.
1. When skin testing is part of an Action Plan, two consecutive tests with negative results obtained at minimum intervals of 60 days are required following identification of a positive M. bovis animal. The testing programme may be limited to a specified group if the VRA allows this.
2. The first test in a breakdown will be interpreted using the severe test interpretation (TB64(ES)/(TB64(W))) as appropriate to increase sensitivity.
3. An exception is where infection with M. bovis has not been confirmed, e.g. a Check Test. In these cases standard test interpretation (TB64(ES)/TB64(W)) should be used as appropriate.
4. The second test in a breakdown is considered on a case-by-case basis, based on the results of the first test and the results of any PMEs on reactors from that test. This should be a local decision and it can be made in consultation with the Veterinary Advisor (VA) in APHA, Scottish Government (SG) or Welsh Government (WG) if necessary, and considering the following:
5. Tests must be reinterpreted at severe if reactors are found to be VL at PME.
6. Any Inconclusive Reactors (IRs), or animals with reactions that are in the same epidemiological group, may be considered for reclassification as Direct Contacts (DCs).
7. Any IRs remaining on the premises should be:
8. If the second qualifying (restriction-lifting) herd test identifies only NVL reactors, only one further clear herd test at standard interpretation would be required.
9. Any check test carried out less than 60 days after the death of a tuberculous pig will not count towards the two negative herd tests normally required after tissue proves positive for M. bovis by PCR test or culture.
10. If only NVL and PCR (or culture) negative reactors are identified at a privately paid test, or a check test of pigs which are co-located with infected cattle (as recommended by a VRA), or animals traced from an infected herd, etc. (i.e. any situation where live TB testing takes place for whatever reason in the absence of evidence of confirmed M. bovis infection in the herd), then only one more skin test will be necessary to lift the restrictions.
11. If infected animals are identified, on a premises where cattle are located the APHA will TB test any cattle present on the breakdown premises. Cattle on neighbouring premises (contiguous) and any traced animals will also be considered for testing.