The pathogenesis and epidemiology of TB in wild deer
For the following reasons deer are unusual and possibly unique amongst European ungulates, including farmed livestock, in respect of the pattern of TB infection:
- Deer may be infected with both bovine and avian TB and both may cause severe disease in individual animals.
- Respiratory and alimentary routes and sites of infection appear to be equally important.
- Significant numbers, often the majority, of clinical TB lesions in deer fail to stain positive by normal ZN laboratory staining methods.
- In an infected population of deer, although a proportion will show gross lesions, many infected animals only show signs of subtly swollen lymph nodes and many are asymptomatic carriers.
- Pooled lymph node culture from asymptomatic deer in contact with individuals showing gross lesions often reveal very high infection rates in the whole group.
- Fulminating TB infection is often seen in young deer [<2yo], whilst older, carrier animals may have few or no obvious lesions.
- When clinical or post mortem signs are obvious, they may manifest in a in wide range of lesions, from classical mammalian caseous or calcified TB nodules to very large abscesses with liquid pus. Any internal abscess within a deer must be taken as suspicious of TB.
- Recent and ongoing evidence suggests that there may be a few "hotspot" areas in lowland England where significant levels of TB are found in wild deer. These areas are usually associated with high TB infection rates in cattle and high deer densities.
The environmental and ecological interaction of deer with farmed livestock
Of the six species of deer found wild in the UK, the three larger species, red, sika and fallow are much more socially organised than the three smaller species, roe, muntjac or Chinese Water Deer. The large deer tend to aggregate into herds; the smaller deer tend to be more solitary.
In contrast to the environmental and agricultural situation in North America and much of Europe, where the large deer occupy forest, woodland or wilderness areas, in the UK herds of red, fallow and sika frequently have a close association with farmed livestock, grazing the same pasture and subsisting on the same crop fodders. There is very close contact between wild deer and cattle in large areas of lowland Britain. This intimate co-existence of large wild ruminants with domestic livestock is very uncommon in other western countries with advanced livestock husbandry.
Although bovine TB has been reported in the smaller deer [roe, muntjac Chinese water deer], the most frequent reports and serious foci of infection have occurred in red, fallow and sika. Young adult deer, particular young stags and fallow bucks are the individuals most likely to travel considerable distances and also appear particularly vulnerable to TB infection.
The British Deer Society is particularly concerned by evidence of significant levels of TB infection in deer in the Hereford/Gloucester areas and in south east Exmoor. Both are areas where high cattle TB breakdown rates occur.
Deer to deer transmission of TB has been shown to occur by both the respiratory and alimentary routes, the latter occurring through both faecal contamination of pasture and by susceptible deer sharing feedstuffs with infected animals. Although deer to cattle or badger to deer infection has not been demonstrated, neither has it been investigated.
The current legislation in respect of TB in wild and park deer
The Tuberculosis [Deer] Order 1989 as amended and the Tuberculosis [Deer] Notice of Intended Slaughter and Compensation Order 1989 were framed to respond to the problem of TB in farmed deer. The mandatory controls requiring the marking and retention of records of all deer moved have been relaxed since inception.
The Orders provide for control of infection on premises and for elimination of infection in managed herds in which intradermal skin testing is possible. The Orders fail to address the possibility of an outbreak of infection in a wild population of deer.
Under the current Orders it is mandatory for any suspicion of TB in any deer [whether farmed, park, or wild] to be reported to Defra, but the powers of control arising from the confirmation of infection apply only to enclosed premises.
Section A restrictions may be placed upon premises on which TB in deer has been confirmed, and will only be lifted when the remaining deer are tested clear by intradermal testing. This clearly has no bearing upon wild deer and is also unsatisfactory in the case of enclosed park deer, which can not be handled.
Under current legislation a TB infected deer park may remain under Section A restrictions indefinitely, with Defra having no further powers to limit the spread of infection to wildlife. BDS has experience of a deer park remaining under Section A restrictions for twelve years, until the owners were persuaded to remove all the deer to allow the restrictions to be lifted [over 30% of the deer were clinically affected by TB at post mortem examination].
Under the current Orders Defra has no power to investigate the possibility of infection in wild deer, even if TB positive carcasses are reported and investigated. There are no enforceable means by which Defra can assess the level of TB infection in a given wild population, once the presence of TB has been confirmed from post mortem submissions. The British Deer Society has evidence that in south east Exmoor the level of TB infection in red deer may be as high as 50% in some limited localities. (Source: The British Deer Society)