Section 16 TB and Deer in Ireland
- Colm Brady
- Veterinary Inspector, ERRAD Division, DAFM, Backweston Campus, Celbridge, Co. Kildare, Ireland
Tuberculosis, a chronic progressive disease which results in considerable production losses in cattle as well as in other species, is caused by the bacterium Mycobacterium bovis. This agent may be transmitted in a number of ways by infectious animals.
Wildlife reservoirs have been implicated as a source of infection for grazing cattle. Infected badgers are considered maintenance hosts and are directly implicated in the transmission of Mycobacterium bovis to cattle in Ireland (Griffin et al. 2005). With respect to deer, their role in acting as a maintenance host for Mycobacterium bovis is considerably less clear, in most areas of Ireland, there is no evidence in support of deer acting as maintenance host for TB (More 2019).
In certain areas of County Wicklow, a higher prevalence of TB in deer has been found. Recently, DAFM, in cooperation with the NPWS and local stakeholders groups, has been carrying out research into the prevalence of TB in deer populations. The findings of these ongoing studies will be reported in due course.
Between 2018 and 2019 Tralee RVO led an investigation into TB in deer within its management region. To this end, 184 separate deer carcass specimens were sent from the south west of Ireland for post mortem examination to Cork RVL. Most deer specimens were comprised of head and pluck (composed heart and lungs), liver and kidneys; the remainder consisted of heads only.
Post mortem examination involved dissection and inspection of lymph nodes in the head (submandibular, parotid, and retropharyngeal), bronchial and mediastinal lymph nodes, lungs, liver, hepatic lymph node and kidneys when available.
Where suspect gross lesions suggestive of TB were found, these were sampled and submitted to the TB Section at the CVRL Backweston for histopathology and/or bacteriological culture testing.
Figure 16.1: Chronic granulonatous inflammation. Langhan’s multinucleated giant cell (red arrow) and dystrophic calcification (green arrows) due to necrosis in a TB granuloma. Photo: Cosme S'anchez-Miguel.
Of all of the cervine samples submitted, one sample yielded Mycobacterium bovis on culture, a second sample yielded an atypical non-tuberculous Mycobacterium spp.; the remaining samples had negative cultures. In addition, histopathological examination of these samples did not reveal any lesion suggestive of TB, corroborating bacteriological findings.
In conclusion, of the 184 carcass specimens submitted to the DAFM laboratory service from Tralee RVO, Mycobacterium bovis infection was confirmed in one submission, a prevalence of 0.54 per cent.
During the period 2014–2016, Drumshanbo RVO led an epidemiological study of TB in wildlife, as part of its enhanced response to an outbreak of TB in cattle herds in north Co. Sligo. As part of the investigation, 145 badgers were culled in the vicinity of TB breakdown herds and were subjected to both gross post mortem examination and bacterial culture of lymph node tissue. Tuberculosis was confirmed in 26.2 per cent of these badgers. In addition to the badger work, 17 deer from the same study area shot under licence from the NPWS were subjected to similar testing, none were found to have TB (Doyle et al. 2018)
This low TB prevalence was reflected in the findings of the largest post mortem survey (and therefore statistically most robust) of TB in deer carried out in Ireland. A 1997 publication by Quigley et al. (1997) describes this study where 14,842 farmed deer were slaughtered and examined post mortem in Irish abattoirs between January 1993 and September 1996. Lymph nodes in the head and pluck were visually examined and gross lesions were removed, submitted to the laboratory for histopathology and cultured. TB caused by Mycobacterium bovis was confirmed in 41 of the slaughtered deer, a 0.28 per cent prevalence. The same study looked at 340 wild deer culled from Glenveagh National Park Co. Donegal and found a prevalence of Mycobacterium bovis infection of 2.8 per cent.
Several DAFM coordinated studies have been carried out to estimate the prevalence of TB in deer in Ireland. The largest of these studies found a TB prevalence of 0.28 per cent in farmed deer. Two recent epidemiological studies, led by two separate Regional Veterinary Offices, found TB incidences of a similarly low order, 0.54 per cent in the south west and 0 per cent in North Sligo. Data from Co. Donegal outlined above suggest a somewhat higher, albeit still low, TB prevalence in wild deer. Research from Wicklow is ongoing and will be reported when completed.
Over many years, research into TB in deer has focussed on disease prevalence, perhaps future research, using newer research tools, should focus on dynamics, ecological drivers and the spread of infection, both within deer populations and between deer and other mammalian species.
References
Griffin, J.M., D.H. Williams, G.E. Kelly, T.A. Clegg, I. O’Boyle, J.D. Collins, and S.J. More. 2005. “The Impact of Badger Removal on the Control of Tuberculosis in Cattle Herds in Ireland.” Preventive Veterinary Medicine 67 (4): 237–66. doi:https://doi.org/10.1016/j.prevetmed.2004.10.009.
More, Simon J. 2019. “Can Bovine Tb Be Eradicated from the Republic of Ireland?: Could This Be Achieved by 2030?” Irish Veterinary Journal 72 (1): 3. doi:10.1186/s13620-019-0140-x.
Doyle, Rob, Tracy A. Clegg, Guy McGrath, Jamie Tratalos, Damien Barrett, Ada Lee, and Simon J. More. 2018. “The Bovine Tuberculosis Cluster in North County Sligo During 2014-16.” Irish Veterinary Journal 71 (1): 24. doi:10.1186/s13620-018-0135-z.
Quigley, F. C., E. Costello, 0. Flynn, A. Gogarty, J. McGuirk, A. Murphy, and J. Egan. 1997. “Isolation of Mycobacteria from Lymph Node Lesions in Deer.” Veterinary Record 1 (141). British Medical Journal Publishing Group: 516–18. https://veterinaryrecord.bmj.com/content/vetrec/141/20/516.full.pdf.
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