Herds never TB restricted in South West England

In Devon, Cornwall and Gloucestershire less than 5% of dairy herds with 50 or more cattle have remained TB free between 1990 and 2012.

The following table shows such herds.

County	Herds	Never Restricted	Been Restricted	Restricted (%)	Never Restricted (%)
Cornwall	535	19	516	96.4	3.6
Devon	1000	46	954	95.4	4.6
Gloucestershire	152	3	149	98.0	2.0
Hereford & Worcester	137	7	130	94.9	5.1

Data in the above table were extracted from Reference 1.

The following table shows dairy and beef herds which have existed from 1990². At least 3 conditions have changed and these are as follows. Herd type now includes beef, the time period no longer includes 2012, and smaller cattle herds are included. The data may relate to all herds with 10 or more cattle^3,7 but this is an assumption and may not apply.

County	Herds	Never Restricted	Been Restricted	Restricted (%)	Never Restricted (%)
Cornwall	1532	415	1117	72.9	27.1
Devon	2427	738	1689	69.6	30.4
Gloucestershire	487	63	424	87.1	12.9
Hereford & Worcester	1064	242	822	77.3	22.7

Data in the above table were extracted from columns "1990 Herds Never Restricted" and "1990 Herds" in Reference 4.

Evidently the proportion of TB free herds have increased fivefold. Unfortunately more than one condition changed so it is not possible to isolate the impact of any one change. Regarding the impact of herd type, however, a study carried out on New Zealand herds showed a significant, threefold increase in bovine TB risk in dairy cattle relative to beef, conditional on the size of the local possum habitat.⁵

Judging by the data presented in the above table, about 25% of herds in the highest risk counties of South West England have not experienced a TB breakdown in 22 years. In Reference 6, Lord Krebs when asking a question in the House of Lords in June 2013 stated that 40% of farms in the highest-risk areas of the country do not get TB in their cattle.

Why are these proportions so different?

The following table shows dairy and beef herds which are cited in correspondence dated September 2011 to be clear of TB². The data may also relate to all herds with 10 or more cattle^3,7 for which the caveat stated above also applies.

County	Years in existence	Years clear	Years restricted	Years restricted (%)	Years clear (%)
Cornwall	43213	20403	22810	52.8	47.2
Devon	75530	37383	38147	50.5	49.5
Gloucestershire	15547	5655	9892	63.6	36.4
Hereford & Worcester	31986	13333	18653	58.3	41.7

Data in the above table were extracted from columns "Years Clear" and "Years In Existence" in Reference 4.

The above proportions for "years clear" represents the proportion of years over which a herd existed for which the herd was clear of TB. This exceeds the proportion of herds which remained free over their entire existence because the numerator also includes years when a previously restricted herd has been clear. For the purpose of calculating the proportion of TB-free herds these years should not be included because they are associated with herds which have not remained TB free. It can be seen that the proportion of "years clear" approximately equals 40%. Lord Krebs in Reference 6 described 40% as the proportion of farms which do not get TB in their cattle.

What quantity was Lord Krebs referring to when he made this statement in the House of Lords?

The proportion of herds which have remained free in Devon, Cornwall, Gloucestershire and Hereford & Worcester are shown above in the second table to be about 25% which is 15% less. In order to check understanding the requester in Reference 2 wrote again to DEFRA asking if his understanding of "59% of herds in Wales have never been restricted" is correct. This proportion is achieved when data is used in columns "1990 Herds Never Restricted" and "1990 Herds" taken from Reference 4. When data in columns "Years Clear" and "Years In Existence" are used as used in the table immediately above, this proportion comes to 69%. This confirms that data in the above table should not be used to calculate the proportion of herds which have remained clear over their entire existence.

Although Lord Krebs referred to the highest-risk areas in the country, perhaps Lord Krebs included other counties which have been less badly restricted. Let us include the counties of Wiltshire and Avon. When this is done, the total number of herds never restricted divided by the number of herds is as follows.

County	Herds	Never Restricted	Been Restricted	Restricted (%)	Never Restricted (%)
Cornwall	1532	415	1117	72.9	27.1
Devon	2427	738	1689	69.6	30.4
Gloucestershire	487	63	424	87.1	12.9
Hereford & Worcester	1064	242	822	77.3	22.7
Wiltshire	558	190	368	65.9	34.1
Avon	394	130	264	67.0	33
TOTALS	7560	2269	5291	70.0	30.0

Evidently, the overall proportion of herds after these less badly restricted counties are included comes to 30% which is still 10% less than the proportion quoted by Lord Krebs in Reference 6.

References

1. Bovine TB: Tb-free commercial dairy herds. ATIC0102. AHVLA, DEFRA. 24 June 2013.
2. Officially TB free herds - the facts. bovinetb.co.uk. 24 Dec 2011.
3. Dairy herd numbers in England. statistics@DEFRA.GSI.GOV.UK. May 2009.
4. Herds never TB restricted in the counties of England, Wales and Scotland. DEFRA. Dec 2011.
5. Risk factors for bovine tuberculosis in New Zealand cattle farms and their relationship with possum control strategies. www.sciencedirect.com. Preventive Veterinary Medicine Volume 86, Issues 1-2 , 15 August 2008, Pages 93-106.
6. Badgers. http://www.publications.parliament.uk. House of Lords. 18 June 2013.
7. Farm Data - Distribution of Cows by Herd Size. England. 2009 to 2011. Data sourced from Cattle Tracing System. Figures supplied by DEFRA. Table supplied by DairyCo.