Bovine TB in Ireland and how it compares to that in the UK

The Irish Republic is the closest equivalent to the UK in that the Irish Republic has badgers (albeit at a lower density) and a bovine TB problem. Estimated badger populations in Great Britain, the Irish Republic and Northern Ireland are shown in the table below.

Country	Badger population	Year when estimated	Reference
Great Britain	300,000-350,000	2003	3
Irish Republic	72,000-95,000	2009	4
Northern Ireland	33,500	2007/08	5

This page illustrates the extent of bovine TB in the Irish Rebublic, illustrates how the situation changed between 1996 and 2006, compares this with the change in Great Britain and Northern Ireland, and gives a measure of what the Irish Republic have been doing to address the problem.

Cattle-related actions are similar in both the Irish Republic and Northern Ireland, focusing on surveillance (the detection of new cases, through field and factory surveillance) and control (the resolution of existing cases, through herd restriction, reactor removal, ongoing testing, etc.). However badgers are routinely culled in the Irish Republic but not in Northern Ireland.

The prevalence of bovine TB in cattle

Distribution of bovine TB reactors in the Irish Republic and Northern Ireland in 1996 and 2006

The above images were taken from Reference 1, where maps are presented every 2 years, which can either show a reduction or increase every 2 years. These images suggest that the problem in Northern Ireland between 1996 and 2006 became more established whereas the problem in the Irish Republic reduced slightly.

Distribution of bovine TB reactors in Great Britain in 1996 and 2006

Distribution of bovine TB reactors in Great Britain in 1996 and 2006

The above maps have been taken from Reference 2. The problem has clearly deteriorated in South West England, Wales and the West Midlands, is becoming more established in Central and Northern England, and is spreading into new areas of Eastern England. New incidences in Eastern England which are remote from established areas are more likely to be due to cattle-to-cattle transmission than badger-to-cattle transmission¹⁸.

Number of cattle reactors due to bovine TB in the Irish Republic.

Number of cattle reactors due to bovine TB in the Irish Republic.

Percentage of cattle each year which were TB reactors in the UK and the Irish Republic from 1956.

In the above graph the proportion of cattle culled was obtained by dividing the number of cattle culled by the total number of cattle which existed in each year as supplied in Reference 20 except for data labelled "Northern Ireland 2" where incidence numbers are used. Such incidences were calculated by dividing the number of reactors disclosed by the number of animals tested each year. Data was sourced as shown in the table below.

Northern Ireland	References 20, 25 and 29
Northern Ireland 2	Request, reply and received data
Irish Republic	Reference 32
Great Britain	Request	Reply	(See reply)
	References 20, 22 and 24

It should be noted that there are likely to be anomolies between what data presented in these graphs represent. For instance some data include cases discovered in slaughterhouse inspections and other data do not. In Great Britain, slaughterhouse cases reported to Animal Health in 2009 and 2010 accounted for about 3% of the total number of animals reported to be infected by TB in each year.

Data for the year 2001 is not shown in the graphs for Great Britain and Northern Ireland because sufficient testing was suspended during this year due to Foot and Mouth as to cause significant under reporting of cases.

Northern Ireland operates an annual testing policy throughout the country so in theory the number of animals tested should equal the number of existing animals in the country. The differences between the two sets of numbers for Northern Ireland are more clearly shown in the graph below which shows data from the same sources as in the graph above but from 1998 only. Such data was re-downloaded from the above sites in Mar 2013 when data for 2012 were included for all countries. Regarding the above graph, cattle slaughtered pre 1990 in Northern Ireland do not include abattoir cases.

In the Irish Republic, approximate values for the number of TB reactors between 1961 and 1964 were read off a graph shown in Fig 1 of Reference 19. Unlike all other years for which the number of existing cattle was taken from reported numbers for each year, the number of existing cattle between 1961 and 1964 was assumed to be constant and equal to 5,000,000. This number lies roughly half way between the reported numbers for 1960 and 1965.

Proportion of cattle slaughtered each year due to TB from 1998

Proportion of cattle slaughtered each year due to TB from 1998

The influence of Foot and Mouth

Foot and Mouth occurred in 2001 and, as can be seen in the above graph, in the following 3 years TB infection levels increased substantially in Northern Ireland whereas they noticeably reduced in the Irish Republic. The extent to which TB testing was interrupted due to Foot and Mouth and the percentage of cattle which were culled in each country due to Foot and Mouth are shown in the tables below.

Irish Republic	Request (Sent 30May11)	Reply (Received 30Jun11)
Northern Ireland	Request (Sent 07Apr11)	Reply (Received 22Apr11)

Country	Number of months over which testing was suspended or reduced	Number of cattle culled due to Foot and Mouth	Number of cattle in the country	Percentage of cattle culled due to Foot and Mouth
Irish Republic	1.5	1,181	6,408,000	0.02
Northern Ireland	1	50,565	1,679,100	3.01
Great Britain	9¹¹	581,802¹⁷	8,923,000⁶	6.52

In summary, TB testing was suspended for 4 weeks in Northern Ireland and 6 weeks in the Irish Republic. However Northern Ireland culled 3.01% of their cattle (50,565 cattle out of 1,679,100) whereas the Irish Republic only culled 0.02% of their cattle (1,181 cattle out of 6,408,000). It follows that there would have been a lot more cattle movements during restocking in Northern Ireland than in the Irish Republic. This probably caused significantly more cattle to cattle transmission of bovine TB in Northern Ireland and may explain why the prevalence of bovine TB increased much more in Northern Ireland than in the Irish Republic.

The influence of culling

The following graph shows the number of badgers (reported by DAFF) which have been culled in the Irish Republic each year between 1995 and 2010.

Irish Republic

Request (Sent 28Nov10)

Reply (Received 21Feb11)

Received data

Number of badgers culled each year in the Irish Republic

Between 1996 and 2006 about 4,000 bagers were culled each year. If the badger population was about 80,000 and the average life expectancy of a wild badger is assumed to be 4 years, then there was a 16/80 chance of any one badger being culled during its lifetime during this 10-year period, that is, 1 in 5 badgers were culled.

Consider the differences between the proportions of cattle culled in Northern Ireland and the Irish Republic in the 5 years between 2008 and 2012 as presented in the table below. If these differences are due to culling alone, culling badgers in the Irish Republic would have saved on average 9,137 cattle each year.

Year	Number of cattle in the Irish Republic	% of cattle culled in Northern Ireland	% of cattle culled in the Irish Republic	% difference	The number of cattle saved
2008	6150163	0.517	0.486	0.03	1906
2009	6025656	0.513	0.395	0.12	7078
2010	5751564	0.399	0.351	0.05	2724
2011	5819281	0.512	0.318	0.19	11246
2012	6145469	0.671	0.301	0.37	22731

Average number of cattle saved each year = (1906+7078+2724+11246+22731)/5 = 9137

In 2012, the proportion of cattle slaughtered due to TB in the Irish Republic became less than half the proportion of cattle slaughtered due to TB in Northern Ireland. Although this situation existed between 2002 and 2004, this difference has increased over the last 2 years. Also in 2002 to 2004, unlike now, incidence in Northern Ireland may have been subjected to temporary high incidence levels due to relocation of TB-infected cattle after Foot and Mouth. If this gap continues to grow, it will become increasing likely that this difference is due to the badger culling policy which the Irish Republic have been increasingly investing in since the mid nineties. This will be interesting because (as details show below) the Irish Republic are implementing a very localised badger culling policy which in England and Wales would be expected to introduce significant perturbation of infected badgers.¹⁰

The following gives some background as to why perturbation may be an issue. Cattle incidence in Great Britain between 1986 and 1998 significantly increased as is shown in the graph below taken from Reference 12. (Foot and Mouth occurred in 2001.)

Incidence of bovine tb in Great Britain between 1956 and 2006

Incidence of bovine tb in Great Britain between 1956 and 2006

In 1986 the culling strategy changed from that of 'clean ring' where badgers are first culled on land occupied by affected cattle herds, then on adjoining land, expanding outwards until no further infected animals were captured¹⁴, to one where badgers were only culled on the infected farm. (A history of English culling strategies is given on Page 5 of Reference 14 and also in Reference 10.) Records of the proportion of diseased badgers made during the RBCT¹⁰ showed that during the course of the trial, detected disease levels in culled badgers doubled¹⁵. The report in Reference 13 explains that in the Irish Republic, badger setts are first identified within 2 km of a tuberculosis-affected farm and then badgers are culled within a 500 metre radius of each sett. The aerial photographs shown on Page 13 of this report show that this distance is typically equivalent to about 3 or 4 medium size fields. If badgers are migrating out from these areas as a result of culling, such perturbation may be increasing disease levels in badgers due to their greater interaction. Perturbation however is only thought to exist in badger populations where badgers exist in social groups. In more favourable habitats where population density is high, badgers form into social groups to share duties such as defending precious food resources. This can be done more effectively in groups.³⁰ Badger population densities in Ireland may be considerably less than in Great Britain¹⁶. If social grouping is also less extensive then perturbation will be less significant. Unfortunately estimating a badger population is very difficult and estimates for Ireland have varied over a very wide range.³¹ If the culling is causing perturbation and increased disease levels in surviving badgers, this will be to a certain extent offsetting the benefits of reduced frequency with which cattle come into contact with less densely populated badgers.

References

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Last Modified 10 May 2013 12:30