Number of cattle tested and slaughtered for bovine TB

The following graphs were generated from data in the following table. In each graph, data is smoothed using a Hanning window over 4 points.

Great Britain	-	-	See Reference 25
Northern Ireland	Request (Sent 14Feb11)	Reply (Received 28Feb11)	Received data
Irish Republic	Request (Sent 12Feb11)	Reply (Received 29Mar11)	Compiled data
New Zealand	Request (Sent 14Feb11)	Reply (Received 23Feb11)	Received data

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NUMBER OF CATTLE TESTED

Defra, the Welsh Government and the Scottish Government carry out with Animal Health a national review of TB testing intervals on an annual basis.


The above graph for Great Britain shows that for each year, except for 2001 when Foot and Mouth occurred, the number of cattle tested is a minimum in Summer and a maximum in Winter/early Spring. This may be influenced by the fact that it is easier to test in the Winter months when stock is housed than in the Summer months when stock has to be brought in from the fields.


In Northern Ireland Foot and Mouth only appears to have significantly reduced the number of cattle tested in one month whereas in Great Britain testing in 9 months was reduced.


In the above grapth for the Irish Republic, data is presented only for years for which data is available for all 12 months in the year.

Unlike data for Great Britain and Northern Ireland the above data shows no increase in testing during the Winter months. In fact the opposite is true. The following is an extract from an email received from DAFF which explains why.

The absence of a "Winter" surge in testing in Ireland is probably due to the pro-active management of testing by the Department. Under our annual testing programme, scheduling of testing is divided into 5 phases spread through the calendar year so as to avoid overloading of workload of vet practices at any time during the year. We tend to finish our annual testing in November and historically have recommenced the round early in Spring so there is least amount of testing in Winter.


The above graph for New Zealand shows that the number of cattle tested each year reaches a maximum in April. This is in the middle of New Zealand's Autumn.

New Zealand kindly provided the following two reasons for the very strong seasonal cycle shown in the above graph.

Firstly the vast majority of our 14,000 odd dairy herds are seasonal milkers. Therefore the herds are "dried-off" in the Autumn and are then "dry" (ie not milking until they calve some 6 - 8 weeks later). Thus there is an increase in the number of dairy herds wanting to be tested during the drying off period. Nevertheless, the majority of dairy herds would be tested from about late October through to March.

As an aside, testing of dairy herds is achieved by undertaking the TB test at milking time so that there is minor inconvenience to the farmer and cows. Given the caudal fold site is used, its easy to test on a rotary platform as the rear end of the cow come past and is presented to the tester. As discussed with you before, this enables one tester to test a large herd of say 1,000 cows between 4 am and 8 am in the morning during milking. With herring-bone sheds, the depth of the pit means its difficult to reach the base of the tail, thus testers walk on stilts to raise them up to the correct height to competently undertake the caudal fold test.

Secondly, our beef herds are weaning in the Autumn and because the cows and calves are being yarded for weaning, farmers also want their TB tests to be conducted while the cows are in the yards.

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NUMBER OF CATTLE SLAUGHTERED


The above graph shows for Great Britain the impact of Foot and Mouth which occurred in 2001. The large number of cattle slaughtered between Mar 2008 and Mar 2009 was probably influenced by a return to use of the more potent Weybridge tuberculin which started to be phased out at the end of 2005. Stocks of the Weybridge tuberculin ran out in September 2009. Great Britain is currently (17Dec2011) using the Lelystad tuberculin exclusively. The influence of the Weybridge and Lelystad tuberculins is discussed in more detail here.

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RISK OF AN ANIMAL TESTING POSITIVE


The above graph for Great Britain shows the probability of an animal testing positive. This probability was calculated by dividing the number of acattle slaughtered by the number of cattle tested each month. Evidently the probability was about one in a hundred during Foot and Mouth which 2 years later reduced to about one in every hundred and fifty. It is clear that the probability of testing positive each year is greatest in mid Summer and least in mid Winter. In Reference 1 it states that the immune response can be detected within 3 weeks of infection but the incubation period of the disease can be from a few weeks to a lifetime. In Reference 2 it states that TB infection is usually slow to develop and it can take months or even years before it is detected by the tuberculin skin test in a high proportion of infected cattle. Experimental challenge studies show that skin tests can become positive within a few weeks of infection with a high dose of bacteria, but natural transmission studies demonstrate that the period between infection and skin test conversion can in fact often extend over a period of many months.


A greater proportion of cattle test positive in Summer than in Winter. Possible reasons for this behaviour include the following.

• The large volume of testing puts a considerable burden on veterinaries. In order to complete workloads, testing procedure in the busy Winter months may not be followed as closely as in Summer months. For example, non-compliance with the hard-to-follow, European Union stipulation of cutting hair and measuring skin thickness is likely to be widespread.¹⁰ More seriously, a court case has been brought against a veterinary who falsely alleged that tests had been carried out on cattle which had not been tested¹² and against a veterinary for using untrained staff in order to get the work done.¹³ Reference 10, which reviews TB test procedures, states that there is a lack of supervision and monitoring of Local Veterinary Inspector performance and that there is an urgent need to carry out a thorough review of the present 'Manual of Procedures'. In addition to this, Reference 14 gives the impression that local veterinaries are failing to classify herds as infected by stating that, in Northern Ireland, in-house staff who were found to be 1.5 and 1.8 times more likely than Private Veterinary Practitioners to classify a herd as a breakdown herd.
• March is an active time for badgers and interaction between cattle and badgers may be large at this time of year. Although the delay between becoming infected and reacting to the skin test varies considerably, if infection rate is particularly high in March, this may result in reaction rate being high in July/August. However if cattle infection is primarily due to cattle-to-cattle transmission whilst cattle are housed during winter months, if the average delay is 7 months, this would lead to the risk of a positive herd test being a maximum in summer and a minimum in winter as is observed. It would appear from this that if the average delay is closer to 4 months, then the route may primarily be badger-to-cattle. If the spread of the delay centres roughly on 7 months, cattle-to-cattle transmission may play a stronger part.
• A larger proportion of tests carried out on cattle in Summer months are not routine. These non-routine tests may be brought on by cattle showing symptoms of being ill. Such tests result in a higher proportion of cattle tested being higher risk in the Summer months when total test numbers are low.

If you have any thoughts on possible causes, or see any flaws in the explanations given above, please submit a comment here .


In the Irish Republic in 2010 it can be deduced from the above graph that the number of tests to every animal tested positive was twice the number in Great Britain.


The above graph for New Zealand shows that the risk of testing positive is greatest in New Zealand's mid Winter which in terms of month matches that in Great Britain but in terms of season is exactly opposite to Great Britain. In New Zealand the risk of an animal testing positive is at least 3 times greater in July than it is in February. July is New Zealand's mid Winter. As noted in Reference 9, New Zealand is different from the UK in that it is blessed with a climate that eliminates the need for housing cattle during Winter months. Unlike in the UK, the proportion of positive tests does not reach a peak exactly when the number of cattle tested reaches a minimum. Peak risk actually occurs about 6 weeks before the number of cattle tested reaches a minimum. This timing may be influenced by the delay between an animal testing positive and being slaughtered. Regarding data for Great Britain, DEFRA were unable to provide an estimate of this delay saying that supplying this estimate would require significant effort¹¹.

Also the number of tests to every animal tested positive is twenty times greater in New Zealand than in Great Britain. (New Zealand in March 2011³ announced that this number will be reducing) This may be influenced by New Zealand's test which is easier to apply⁴. New Zealand apply the skin test to the caudal fold area (which is just underneath the base of the tail) whereas Great Britain applies the skin test to the neck. Also New Zealand has more incentive to put more of their resources into disease testing and eradication because a greater proportion of New Zealand's Gross Domestic Product (GDP) is tied up in agriculture. See the table below.

Country	Contribution which agriculture makes to GDP (%)
UK	0.7 5
Irish Republic	1.0 6
New Zealand	5.6 7

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SOME NOTES ON THE TEST INTERVAL IN GREAT BRITAIN

In April 2011 it was reported^15,16,19,20 that 103 cattle were slaughtered on a farm in Cumbria when a test 18 months earlier was classified as clear. If the test was performed properly and the results diagnosed correctly, this would indicate the speed with which cattle can become infected, regardless of the source of infection, and raises the question of whether a shorter test interval is required. In parts of Great Britain where herd incidence is currently low, the test interval can be a great as 4 years. At the time TB was discovered on the farm in Cumbria where over 100 cattle were slaughtered, the test interval was 3 years.¹⁸

In the Intensive Action Area of Wales in May 2010 a number of measures were introduced to address bovine TB. This included reducing the test interval from 12 months to 6 months¹⁷.

Regarding the number of tests performed in Great Britain compared to the number in other countries, it can be deduced from the data plotted in the above graphs that in 2009 the number of tests to every animal tested positive in Great Britain was overall about one half of that in the Irish Republic and about one twentieth of that in New Zealand. This is an indication of how much more other countries are spending to address the problem. Even though Great Britain is testing less than other countries, the cost of testing in Great Britain in 2010 was still the biggest cost and as such was larger than the compensation paid to farmers for slaughtered animals.

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REFERENCES

1. Myth busting and Bovine TB by Richard Gard 2009
2. Bovine Tuberculosis in Cattle and Badgers. A Report by The Chief Scientific Adviser, Sir David King submitted to the Secretary of State, DEFRA on 30 July 2007
3. TB movement controls and testing reduced in New Zealand
4. Comparison of skin test applied in New Zealand with that applied in the European Union
5. UK's economic profile. Last available data in March 2011
6. Ireland's economic profile. Last available data in March 2011
7. New Zealand's economic profile. Last available data in March 2011
8. CVO statement on the reduction in the number of new TB incidents in Great Britain. A report issued by the Chief Veterinary Officer in August 2006.
9. Dairy Production in New Zealand. Published by University of Illinois in 1998.
10. Review of TB testing procedures: Report for Defra and the Welsh Assembly Government report no.: 22115204-02 Final Report rev 3, 12th June 2006
11. Average time between test and slaughter dates. A reply from DEFRA to an FOI request made on 14 Feb 2011.
12. Royal College Of Veterinary Surgeons versus John Richard Owen-Thomas for events in Sep 2009.
13. Royal College Of Veterinary Surgeons versus Iwan Parry for events in Jan/Feb 2007.
14. The Control of Bovine Tuberculosis in Northern Ireland
15. Cattle culled on Cumbrian farm
16. Tests to find source of bovine TB in Cumbrian herd
17. Progress and implications of the additional cattle control measures in the Intensive Action Area
18. Test interval for farm in Cumbria
19. Bovine TB testing zone extended around Cumbria farm
20. Testing for bovine TB extended in Eden
21. Cattle slaughtered in the IAA
22. DEFRA Stats - Wales 2009
23. DEFRA Stats - Wales 2010
24. IAA statistics and epidemiological information
25. Incidence of TB in cattle in Great Britain - GB dataset. Table 2 Animals.