TB in Sheep
heep are spillover* hosts of infection in wildlife and cattle. Although traditionally considered as dead-end hosts, with the current level of knowledge, Defra state that it is not possible to rule out that some severely infected sheep might act as incidental vectors of infection for other sheep and other animals.
They also state that they continue to regard TB in sheep as an incidental finding in endemic bTB areas of little significance in the persistence of TB in cattle.
Recorded incidents of bTB in sheep in GB (annual)
|
2002* |
2003 |
2004 |
2005 |
2006 |
2007 |
2008 |
2009 |
|
|
No. of individual sheep sent to VLA for TB analysis |
2 |
0 |
11 |
4 |
0 |
0 |
6 |
9 |
|
No. of infected sheep diagnosed with M.bovis |
1 |
0 |
3 |
2 |
0 |
0 |
1 |
5 |
There are no recorded cases prior to 2002, even though records have been kept since 1997.
(Source: Defra http://www.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/tb/documents/tb-otherspecies.pdf)
Bovine TB is a notifiable disease in all farmed and pet mammals under the TB (England) Order 2007. There is a legal obligation for suspicions of the disease in any carcase to be reported to the local Animal Health office. Under the Order it is also compulsory to notify the VLA if M.bovis is identified by laboratory examination of samples taken from a mammal (other than humans).
The Government has legal powers to impose movement restrictions on TB affected sheep and to test any animals (not just cattle) for TB. However, there are no legal provisions to enforce the slaughter of reactors or contacts, nor pay compensation. No compensation therefore is paid for any sheep removed as reactors, contacts or clinically suspect animals. AH can conduct skin testing and post mortem reactors with voluntary agreement from farmers.
Defra have said that they are not planning an active ante-mortem TB surveillance programme in sheep flocks and that AH will continue to respond to incidental findings of Tb in sheep at post-mortem examination and meat inspection on a case by case basis.
TB testing in sheep is carried out using a comparative skin test, with the reagents injected into the inner thigh. This requires the sheep to be turned so can cause delays in the completion of the skin testing regime at high risk periods of the life cycle, i.e. when ewes are heavily pregnant.
New Zealand, like the Uk, has a major TB problem and, like us, they also have a lot of sheep. In 1987 they calculated a national prevalence rate of 0.05% in cattle and 0.6% in farmed deer. Given that there are over 67 million sheep in NZ, and crudely using an equivalent prevalence level to that of cattle for the national sheep flock, you would expect to see around 30,000 bTB infected sheep a year. This has not happened. In a slaughterhouse survey in the 1986/87 slaughter season, a total of 9.9 million lambs and 1.9 million adult sheep were examined for bTB. Of the 35 animals showing lesions suggestive of bTB, only 2 demonstrated cellular possibilities of bTB but neither gave a positive culture confirmation. This represented a sample prevalence for bTB of 0.00002%, or a factor of 2,500 less than expected.
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* Spillover Hosts – where TB occurs within the species only as long as there is input from an external source as opposed to Maintenance Hosts (Badgers, bovines, deer) where infection persists by vertical Pseudo-vertical or horizontal transmission within the species, without the need for input from other species.