Bovine TB infection status in cattle inGreat Britain in 2018 · • Scotland remained Officially TB...

28 March 2020 | VET RECORDPB VET RECORD | 28 March 2020 373

Surveillance

KEY POINTS

• In Great Britain (GB), the herd incidence of bovine TB decreased in 2018 to its lowest level since 2014. Approximately eight new incidents of TB were detected for every 100 herds that underwent a full year of surveillance in 2018, compared to 9.4 in 2017 and 7.7 in 2014.

• There was substantial variation across GB at country, surveillance risk area and county level. Differences in surveillance and control policies make it inappropriate to directly compare TB in cattle between the countries of GB and therefore these are best considered separately.

• Scotland remained Officially TB Free, with 36 incidents (0.7 TB incidents per 100 herd-years at risk [HYR]) detected. This was a slight decrease on 2017 (40 incidents, 0.9 incidents per 100 HYR).

• There has been a steady decline in TB incidence in Wales since 2012, and, overall, in 2018 seven incidents of TB were detected for every 100 HYR.

• By TB area in Wales, TB incidence: ◊ Fell in the High TB East area from 12.4 incidents per 100 HYR in 2017 to 10.8 in 2018. ◊ Rose in the High TB West area from 12.5 incidents per 100 HYR in 2017 to 12.7 in 2018. ◊ Rose in the Intermediate TB North area from 7.9 incidents per 100 HYR in 2017 to 8.5 in 2018. ◊ Remained stable in the Intermediate TB Mid and Low TB areas.

• TB incidence in the Low risk area of England in 2018 remained very low at 0.8 incidents per 100 HYR.• The TB epidemic in the High risk area (HRA) of England has plateaued since 2011; in 2018, the

overall number of TB incidents in the HRA decreased to 18.5 per 100 HYR, compared to 19.8 in 2017. This reduction was observed in almost all counties of the HRA.

• TB incidence in the Edge area of England has been escalating for the past few years, although there was only a slight increase in incidence between 2017 and 2018, from 9.1 to 9.2 per 100 HYR. The recent rising trend has been attributed to more intensive surveillance since 2013, the purchase of (undisclosed) infected cattle, and the development of areas of endemic badger and cattle infection.

Bovine TB infection status in cattle in Great Britain in 2018

ABOUT THIS REPORTThis report is produced for Vet Record by the APHA and presents the key descriptive epidemiological parameters for bovine TB in cattle in Great Britain (GB) from 1 January to 31 December 2018. It provides summary information on the epidemic, including the main statistics and context for their interpretation, as presented in the epidemiological surveillance reports for England and Wales.1,2 Descriptive statistics including herd incidence, herd prevalence and incident duration were measured using TB surveillance data routinely collected by the APHA. This report also updates the previous annual summaries for 2012 to 2017 published in Vet Record.3–8 The aim is to provide a concise description of the epidemiology of TB in cattle in GB on an annual basis and to put the surveillance data into context.

New incidents in 2018Bovine TB was detected in 4378 of the 74,457 live herds in Great Britain (GB) in 2018, down from 4643 in 2017. A slight downward trend in the number of new TB incidents in GB has been seen since 2011, partly due to a reduction in the number of active cattle herds.

In GB in 2018, England had the highest proportion of new incidents, with 7.3 per cent of English herds affected. In Wales, 6.2 per cent of herds experienced a new TB incident. In Scotland, only 0.3 per cent of herds experienced a new incident in 2018 (36 herds of a total of 13,267) (Tables 1, 2).

Herd incidence Within GB there are substantial differences in the behaviour of the epidemic at country, risk area (see Fig 1 for the location of risk areas in GB) and county level (Figs 2,3). The incidence rate of TB in cattle herds represents the occurrence of new infection events detected in the cattle population over a defined time period, and is expressed as the number of newly detected TB incidents per 100 herd-years at risk (HYR). The methodology for this calculation is described in Perrin et al.8

Between 2012 and 2018, TB incidence mostly plateaued in GB (on average 8.5 TB incidents per

100 HYR annually). There was a peak in TB incidence in 2017 of approximately 9.4 TB incidents per 100 HYR, falling to 8.2 TB incidents per 100 HYR in 2018.

Risk areaA notable decrease in the TB incidence rate was observed in the High risk area (HRA) of England and the High TB East (HTBE) area of Wales in 2018 compared to 2017 (19.8 to 18.5 per 100 HYR and 12.4 to 10.8 per 100 HYR, respectively), with the incidence rate in 2018 similar to that reported in these areas in 2016 (18.7 and 10.8 per 100 HYR, respectively).

There was a slight increase in incidence rate in the Edge area of England (9.1 to 9.2 per 100 HYR),

and in the Intermediate TB North (ITBN) area in Wales (7.8 to 8.5 per 100 HYR) in 2018 compared

D. Duncan, A. Brouwer, K. A. Harris, J. R. Lawes, R. Avigad, Department of Epidemiological Sciences,

J. Dale, Bacteriology,

P. A. Upton, Data Systems Workgroup,

APHA, Woodham Lane, New Haw, Addlestone, Surrey KT15 3NB

email: [email protected]

373-380 TB surveillance 2018.indd 373 25/03/2020 12:18

on May 27, 2020 by guest. P

rotected by copyright.http://veterinaryrecord.bm

j.com/

Veterinary R

ecord: first published as 10.1136/vr.m1242 on 27 M

arch 2020. Dow

nloaded from

http://veterinaryrecord.bmj.com/

28 March 2020 | VET RECORD374 VET RECORD | 28 March 2020 375

SURVEILLANCE

to 2017. While these were only modest increases, they contribute to a rising trend that has been seen since 2013 (over five consecutive years) in the Edge area of England,

and since 2016 in the ITBN area of Wales. In the ITBN area of Wales, the incidence rate in 2018 was nearly as high as that reported in 2012 (8.8 per 100 HYR).

TABLE 1: Key bovine TB occurrence and other epidemiological parameters, by country, in 2018

England Scotland WalesGreat Britain

total

Overview

Total new herd incidents (2017 values in brackets) 3607 (3816) 36 (40) 744 (787) 4387 (4643)

Number of open cases (ongoing TB incidents) at the end of 2018 (2017 values in brackets)

2874 (3035) 12 (17) 682 (661) 3568 (3713)

Herd incidence per 100 herd-years at risk (2017 values in brackets) 9.4 (11.0) 0.7 (0.9) 7.5 (7.8) 8.2 (9.4)

Average monthly prevalence (%) (2017 values in brackets) 5.9 (5.8) 0.1 (0.1) 5.4 (5.1) 4.8 (4.7)

Duration

Median duration of all TB incidents* restrictions (days) (25-75th percentile)

199 (165–307)

140 (81–174)

216 (168–326)

201 (165–311)

% persistently infected herds*† 7.3 0 6.2 7.1

% persistently infected herds (OTF-W only)*† 10.2 0 7.0 9.5

% persistently infected cases open at the end of 2018† 14.1 16.7 16.0 13.7

Number of herds 49,235 13,267 11,955 74,457

Recurrence

% incidents involving previously infected herds, within past 36 months 51.8 8.3 37.0 48.9

*That closed in 2018†Persistently infected cases are incidents with a duration >550 daysOTF-W Official TB Free herd status withdrawn

The drivers of infection in the Edge area of England are discussed in more detail in the APHA’s Edge area county reports.9

Incidence in the High TB West (HTBW) area of Wales has been falling in recent years, from 17.9 incidents per 100 HYR in 2012 to 12.7 per 100 HYR in 2018, although there has been a slight increase in the past two years compared to the lowest level of 12.3 per 100 HYR in 2016.

In the Intermediate TB Mid (ITBM) area of Wales there was a small drop in incidence from 3.8 incidents per 100 HYR in 2017 to 3.4 per 100 HYR in 2018.

Comparatively, incidence in the Low risk area (LRA) of England and in Scotland remained low and consistent with previous years (around 1 and 0.2 incidents per 100 HYR, respectively) (Fig 2).

CountyBoth herd size and local herd incidence are closely associated with the risk for a particular herd to become infected with TB, and these

TABLE 2: Key bovine TB occurrence and other epidemiological parameters, by country risk areas, in 2018

England Wales

High risk area

Edge area*

Low risk area

High TB East area

High TB West area

Intermediate TB North area

Intermediate TB Mid area

Low TB area

Overview

Total new herd incidents (2017 values in brackets) 2761 (3033)

717 (606)

129 (123)

251 (299)

332 (330)

67 (63)

60 (68)

34 (28)

Number of open cases (ongoing TB incidents) at the end of 2018 (2017 values in brackets)

2258 (2511)

553 (468)

64 (56)

209 (238)

354 (325)

54 (42)

45 (40)

20 (16)

Herd incidence per 100 herd- years at risk (2017 values in brackets)

18.5 (19.8)

9.2 (9.1)

0.8 (1.0)

10.8 (12.4)

12.7 (12.5)

8.5 (7.8)

3.4 (3.8)

1.4 (1.1)

Average monthly prevalence (%) (2017 values in brackets) 11.5 (11.8) 5.6 (4.6) 0.3 (0.3) 7.3 (7.3) 10.1 (9.5) 5.0 (4.4) 2.0 (2.0) 0.7 (0.4)

Duration

Median duration of all TB incidents† restrictions (days) (25-75th percentile)

202 (166–320)

200 (167–284)

146 (77–174)

207 (166–315)

245 (178–362)

208 (158–285)

181 (147–298)

161 (113-216)

% persistently infected herds †‡ 8.1 4.7 1.7 5.4 8.6 1.8 5.7 0

% persistently infected herds †‡ (OTF-W only) 10.9 6.8 5.0 55.8 9.22 2.22 7.11 0

% persistently infected cases open at the end of 2018‡ 15.7 8.3 4.7 12.0 21.2 14.8 2.2 0

Number of herds 20,265 8832 20,138 2862 3251 953 2048 2841

Recurrence

% incidents involving previously infected herds, within past 36 months

57.7 38.1 6.3 40.3 42.3 23.0 25.9 8.8

* Edge area values for both 2017 and 2018 are provided for herds within the 2018 Edge area boundaries† That closed in 2018‡ TB incidents that had lasted >550 daysOTF-W Official TB Free herd status withdrawn




j.com/

Veterinary R


arch 2020. Dow

nloaded from



SURVEILLANCE

Counties with the highest number of TB incidents were Devon, Cornwall and Staffordshire, which also had high herd numbers. There are areas, however, in north and east England, Scotland and Wales that have higher herd densities but low incidence, such as North Yorkshire, Ayrshire and Anglesey.10

Herd size and typeThe higher likelihood of detecting infection in particular locations and in larger herds plays a role in explaining why TB is found proportionately more frequently in dairy herds than beef herds. Consistent with previous years, the majority of herds in GB in 2018 were classified as beef herds (80.3 per cent), and these tend to

factors make a strong contribution to the spatial pattern of the TB epidemic (Fig 3). Herd-level incidence was highest in west and south-west England, and Pembrokeshire in Wales.

In 2018 the highest incidence rate was detected in Wiltshire, with 22.1 TB incidents per 100 HYR (down from 25.0 in 2017).10 Other counties with high incidence rates included Devon (20.6 incidents per 100 HYR), Hereford and Worcester (20.3 incidents per 100 HYR), Shropshire (19.9 incidents per 100 HYR), Staffordshire (19.4 incidents per 100 HYR) and Gloucestershire (19.4 incidents per 100 HYR) in England and Pembrokeshire in Wales (16.4 incidents per 100 HYR).

be smaller than dairy herds. Beef herds predominate in all herd size categories in Scotland; however, in England and Wales, 50 per cent or more (71 per cent and 87 per cent, respectively, in the HRA of England and HTBW incidence area of Wales) of all herds with greater than 300 cattle are dairy herds.10 An increase in herd size together with a reduction in the number of herds has occurred across GB in recent years, which affects both the epidemiology of TB and between-year differences.

In GB the probability of TB infection increased with herd size, with lower incidence rates observed in herds with fewer than 300 animals compared to herds with more than 300 (Fig 4). This effect remained after adjusting

RISK AREAS AND TESTING FREQUENCY

England is divided into three surveillance risk areas for disease control and eradication purposes. The High risk area (HRA) is largely in south-west England and TB is considered to be endemic in much of this region (Fig 1). Herds in the HRA have been tested annually since 2013.

The Low risk area (LRA) is located in the north and east of England where TB is rare. The default TB testing interval for most herds in the LRA is four yearly, supplemented with postmovement testing for cattle travelling from higher risk areas.

Between these two regions, a group of 10 contiguous counties plus East Sussex make up the Edge area, which acts as a buffer for disease spread from the HRA, with annual or six-monthly routine TB testing of herds (Fig 1).

On 1 January 2018, two changes were made in the Edge area. First, the area was expanded westwards to fully incorporate five counties that were previously split between the HRA and Edge area: Cheshire, Derbyshire, East Sussex, Oxfordshire and Warwickshire. This brought herds in the previously HRA portions of these counties under Edge area surveillance testing and case management policies (including compulsory interferon- gamma testing in all herds with Officially TB Free status Withdrawn

incidents), and made these areas eligible for support from the Badger Edge Vaccination Scheme.13

Second, six-monthly routine surveillance testing, which had been in force in central and north Cheshire since 2015, was expanded to the whole of Cheshire, Berkshire west, Derbyshire west, Hampshire north west, Oxfordshire and Warwickshire.

In the remainder of the Edge area, routine surveillance testing is performed annually (Fig 1).

All herds in Wales have been tested annually since 2010 (following the 2008 TB Health Check Wales). Additionally, approximately 300 herds have been tested six-monthly in the Intensive Action Area in north-east Pembrokeshire since May 2010.

In 2017, Wales was divided into five surveillance TB areas, representing regions of high, intermediate and low TB for disease control and eradication purposes (Fig 1).

Scotland adopted risk-based testing in 2012, meaning herds that meet certain risk-mitigating criteria – eg, a large proportion of stock slaughtered, rarely purchasing from a higher risk area, and herd sizes of fewer than 50 animals – are exempt from four-yearly routine testing and are monitored entirely through slaughterhouse surveillance (herd sizes of fewer than

Fig 1: TB risk and incidence areas and six-monthly testing areas of Great Britain in 2018

50 animals have been exempt since January 2017; before this date, the exemption cut off was 20 animals).12




j.com/

Veterinary R


arch 2020. Dow

nloaded from



SURVEILLANCE

for the effects of herd type and location. Conversely, for herd type, the incidence rate is very similar between beef and dairy herds after adjusting for herd size and location.

Herd prevalenceHerd prevalence (the proportion of herds under restriction at a point in time due to a TB incident) based on live herd numbers was similar in England (5.9 per cent) and Wales

(5.4 per cent), equating to 2906 and 649 herds under restriction, respectively (average across the year).10 In both countries this was similar to recent years.

Scotland had very few herds under restriction on average in 2018 (0.1 per cent or 19 herds).

Similarly to incidence, substantial differences in TB prevalence were observed between counties within each country.

The geographical distribution of herd-level prevalence (derived as a proportion from the status of the closest 100 herds) is shown in Fig 5, and in England and Wales prevalence followed a similar geographical pattern to herd incidence (Fig 3). The highest herd-level prevalence was observed in the HRA of England and the HTBW area of Wales.

Other descriptors of disease impactIncident durationIncident duration is the number of days that a herd is under movement restrictions due to a TB incident, and can be influenced by a number of factors. TB incidents identified as having a longer duration can be a result of: • changes in the incident management policy (such as the application of interferon-gamma [IFN-g] testing within a herd or the

CLASSIFYING TB INCIDENTS

A new TB incident (or ‘herd breakdown’) refers to a case of bovine TB (defined by at least one skin test reactor and/or a culture-confirmed slaughterhouse case) being disclosed in an unrestricted cattle herd within the reporting period, in this case 2018. Herds are designated free of TB by their Official Tuberculosis Free (OTF) status, which is awarded if TB has not been found. By contrast, if surveillance has revealed either strongly suspected or confirmed Mycobacterium bovis infection, the herd’s status becomes OTF suspended (OTF-S) or OTF withdrawn (OTF-W), respectively, and movement restrictions are applied.

OTF-W statusA new OTF-W incident refers to the detection of TB in an OTF herd where at least one animal slaughtered for disease control purposes has postmortem evidence of M bovis infection (judged by the presence of visible lesions typical of TB and/or identification of M bovis in culture), or where at least one suspect slaughterhouse case had positive results from laboratory tissue culture for M bovis.

Additionally, in Wales, not every new TB incident necessarily requires postmortem confirmation to be classified as OTF-W. When an incident is considered to have a very high probability of infection according to defined epidemiological criteria, it is referred to as ‘OTF-W-2’. From 2011 onwards, when this change in policy occurred, all OTF-W and OTF-W-2 incidents in Wales have been considered as part of the same cohort, are managed in the same way and contribute to OTF-W statistics in analysis, though the different definitions should be considered when comparing statistics across countries.

OTF-S statusAn OTF-S incident occurs where, despite infection being suspected through the detection of skin test reactors, the above criteria for OTF-W status are not met. However, it should be borne in mind that the probability of true infection in test-positive animals is very high in areas where incidence is high, regardless of whether disease could be ‘confirmed’ postmortem. 14

Fig 2: Herd incidence of bovine TB – the number of new TB incidents per 100 herd-years at risk between 2009 and 2018 by (a) country; (b) risk area of England; and (c) incidence area of Wales

(a) (b)

(c)




j.com/

Veterinary R


arch 2020. Dow

nloaded from



SURVEILLANCE

interpretation of short-interval tests required to regain Official Tuberculosis Free [OTF] herd status); • a high within-herd prevalence of infection at the outset of a new incident;• continued cattle-to-cattle transmission despite removal of identified infected animals; and • continued reinfection (eg, from badgers or contiguous/neighbouring herds).

Additionally, the logistics of staff availability and testing protocols may also have an impact.

The median time taken to resolve an incident varied between risk areas, but increased in all regions in 2018 compared to 2017. The longest median duration was in the Welsh HTBW area, at approximately eight months, followed by the HTBE and ITBN areas in Wales and the English

HRA and Edge areas, all with a median duration over six-and-a-half months. The lowest median duration was reported in the LRA of England and Scotland where incidents took just over four-and-a-half months to resolve, followed by the Low TB (LTB) area of Wales with five months.

TB incidents that last more than 550 days (around 18 months) are referred to as ‘persistent’ and may be subject to additional control measures. Across GB, 7.1 per cent of incidents that closed in 2018 were considered persistent. However, there are substantial geographical differences across GB; the proportion

of persistent TB incidents was highest in the HTBW area of Wales (8.6 per cent) and HRA in England (8.1 per cent) (Table 1).

Number of reactors: skin test- and IFN-g test-positive animalsThe proportion of TB incidents with two or more animals reacting positively to a tuberculin skin test or IFN-g test was 66 per cent for GB, with higher proportions observed in the HTBW area of Wales (74 per cent) and the Edge area of England (71 per cent) and lower in the LTB area of Wales (37 per cent). Only one or two reactors are identified in most incidents, although some incidents have much larger numbers, which affects the mean.

The median number of reactors identified per incident that closed in 2018 was three in GB, and highest in the HTBW area of Wales (four). The median number of reactors was three for the ITBN area in Wales and the HRA and Edge area of England, two for the Welsh HTBE area and one for the LRA of England, ITBM and LTB areas of Wales and for Scotland.

RecurrenceRecurrence was defined as a herd found to be infected with TB in 2018 which had a history of one or more TB incidents within the previous 36 months. Recurrent TB incidents can occur due to either enduring risk factors or new risk pathways of infection. Recurrent incidents can also be due to residual cattle

Fig 3: Herd incidence of TB in Great Britain in 2018, represented as a spatial kernel applied to the herd incidence of the 100 closest herds to each herd location, which smooths the effect of political boundaries. Symbology is represented by 12 quantiles which are evenly distributed throughout the classifiers

Fig 4: Herd probability of infection by herd size, type and location, as measured (ie, unadjusted), and adjusted to account for the other two factors; measured by the incidence rate ratio (IRR)

INTERPRETING LONG-TERM TRENDSBefore the foot-and-mouth disease (FMD) epidemic of 2001, between 1986 and 2000 the number of new TB incidents detected in Great Britain per quarter doubled approximately every five years. This was followed by a substantial increase in cases in the wake of the FMD epidemic, when on-farm TB surveillance was temporarily suspended and many farms across the country were restocked with cattle that had not recently been tested for TB.

Subsequently the epidemic slowed, with a doubling time of 11 years from 2003 to 2010.7 Multiple measures have been used to monitor the epidemic since 2010, including the number of new incidents, incidence rate, prevalence and incident duration. These are affected by changes in the cattle population and strategies for tackling TB, and caution should be exercised when examining trends over time.

To demonstrate that there is a true change in the frequency of TB there would need to be a marked and sustained trend in the incidence and prevalence of infected herds for at least five consecutive years (to avoid historical cyclical trends associated with testing frequency), as well as changes in their geographical distribution.




j.com/

Veterinary R


arch 2020. Dow

nloaded from



SURVEILLANCE

infection in the herd from a previous TB incident.

In 2018, most recurrent TB incidents had experienced either one or two TB incidents within the previous 36 months. Recurrent incidents are generally more concentrated in areas where TB is endemic, for example in the HRA of England and HTBW area of Wales (Fig 6). Incidents with just one additional case of TB in the past 36 months are not shown in Fig 6 due to the high frequency of such incidents. There were eight recurrent incidents in the LRA of England, and three each in the LTB area of Wales, and Scotland.

In 2018, approximately half of new incidents in GB were recurrent (Table 1). Recurrence has been stable in England (overall) in recent years, fluctuating between 51 and 53 per cent since 2011 (52 per cent observed in 2018).

The proportion of recurrent TB incidents in Wales has been decreasing since 2015, from 50 per cent down to 37 per cent in 2018.

The smaller number of incidents in Scotland creates greater fluctuation in the percentage of TB incidents that are recurrent; however, there have been fewer than 10 such incidents in every year since 2007 and usually fewer than five.

Logistic regression was performed to determine if the odds of having a TB incident in the current year were increased where a herd had a history of incidents in the previous 36 months. Across all risk areas of GB herds with a history of TB were more likely to be found infected in 2018 than herds with no TB history (odds ratio [OR] >1). However, in the ITBN (OR 1.71, 95 per cent confidence interval [CI] 0.89–3.29) and LTB (OR 1.91, 95 per cent CI 0.57–6.44) areas of Wales, CIs spanning 1 suggest this could have occurred by chance.10

Surveillance efficacySurveillance for TB is classified into four ‘streams’ to help monitor the performance of different types of surveillance in identifying new incidents.11 ‘Routine’ and ‘Slaughterhouse’ surveillance look for TB in herds (or animals) that are not expected to be at increased likelihood of infection. ‘Area and herd risk’ (eg, short interval, tracing, inconclusive reactor and contiguous

Fig 5: Spatial representation of herd prevalence of bovine TB in Great Britain in 2018 derived from the proportion of herds under restriction on December 15, 2018. Symbology is represented by 12 quantiles which are evenly distributed throughout the classifiers

herd tests) and ‘Trade and other’ (eg, postimport, pre- and postmovement and private tests) testing targets cattle thought to be at higher risk of infection, and/or those where infection would have a greater impact if present.

In England, most TB incidents in the HRA (47 per cent) and LRA (40 per cent) were detected through ‘Area and herd risk’ tests, whereas in the Edge area the majority were detected through ‘Routine’ surveillance (57 per cent).

In Wales, over half of TB incidents in the ITBN (76 per cent), HTBW (62 per cent) and HTBE (59 per cent) areas were detected by ‘Area and herd risk’ tests, and such tests also detected most incidents in the ITBM area (35 per cent). In the LTB area of Wales and in Scotland, most TB incidents were detected through ‘Routine’ surveillance (65 and 64 per cent, respectively).

‘Trade and other’ testing (mainly statutory pre- and postmovement TB tests) generally detected between 3 and 13 per cent of all TB incidents. In all areas trade testing was an important preventive measure in reducing the spread of infection.

Slaughterhouse surveillanceRoutine postmortem meat inspection of cattle carcases looking for tuberculous lesions complements the surveillance testing programme of live cattle. A slaughterhouse case occurs where suspect lesions of TB are found in a carcase and Mycobacterium bovis is isolated from those lesions by laboratory culture. When suspect lesions are found, the OTF status of the herd of origin is automatically suspended (OTF-S) pending culture results. Slaughterhouse cases with positive culture results lead to the withdrawal of OTF herd status (OTF-W). If the culture results are negative, a TB herd incident is only generated if TB-positive cattle are found at any subsequent skin check test.

The proportion of all OTF-W incidents detected at routine slaughter varies by country and TB risk area. The HRA of England has been relatively stable over

TB CONTROL MEASURESDisease surveillance and control measures in cattle include routine and targeted surveillance of herds using the single intradermal comparative cervical tuberculin test (commonly known as the tuberculin skin test), slaughterhouse surveillance, pre- and postmovement testing, further testing of infected and/or at-risk herds, movement restrictions and the rapid isolation and slaughter of infected and/or suspect animals.

Good on-farm biosecurity and controlling incursion of TB from known wildlife reservoirs (mainly the badger) is also a key component of disease eradication in England and Wales. Tools to achieve this include bespoke farm-level interventions identified though site visits by Cymorth TB (Wales) and TB Advisory Services (England High risk area and Edge area), supporting badger vaccination in Wales and England, and industry-led licensed badger culling in parts of England.15




j.com/

Veterinary R


arch 2020. Dow

nloaded from



SURVEILLANCE

recent years, fluctuating between 15 and 21 per cent since 2012. In the Edge area, the percentage of slaughterhouse incidents has shown a steep decline from nearly 30 per cent in 2011 to 11 per cent in 2018, consistent with the increased intensity of live animal testing here since 2013.

Between 2011 and 2018, the proportion of OTF-W TB incidents detected at slaughterhouses in Welsh herds varied between 6 and 12 per cent, with the highest proportion in 2018 detected in the ITBM area (24 per cent) and the lowest proportion detected in the ITBN area (3 per cent).

In areas where longer routine testing intervals apply by default (LRA of England and Scotland), a larger percentage of TB incidents were disclosed at the slaughterhouse in 2018. In the LRA of England 30 per cent of OTF-W incidents were

disclosed at slaughterhouse, while in Scotland 40 per cent of incidents were detected in this way.

Slaughterhouse surveillance is particularly important in Scotland following its attainment of OTF status in 2009, as it has replaced live animal testing in herds where a sufficient proportion of cattle go for slaughter and the risk of introduction through cattle movements is low.12

M bovis was recovered from the majority of GB slaughterhouse cases (67 per cent), although there were regional variations in this percentage. In Scotland and the LRA of England fewer than 25 per cent of slaughterhouse cases yielded M bovis on culture (21 and 17 per cent, respectively); in contrast, M bovis was detected in 77 per cent of slaughterhouse cases in the HRA of England.10

M bovis genotypingGenotyping of M bovis isolates is used to differentiate the strains of the bacterium responsible for TB incidents in cattle and other species and is performed using two molecular typing techniques (spoligotyping and variable number tandem repeat genotyping).

In 2018, molecular typing was carried out on 2667 M bovis isolates from new cattle TB incidents across GB. The full genotype was obtained in 97 per cent of the isolates from England and 93 per cent of isolates from Wales. Maps displaying the distribution of the major genotypes are presented in the GB data report.10

Eighty-one per cent of genotyped isolates from cattle in England were from incidents located within their ‘home range’ (the geographical area in which a genotype is most frequently recovered). A total of 84 different genotypes were identified in England, with genotype 17:a the most common (19 per cent), followed by 25:a and 11:a (both 17 per cent).

In Wales, 80 per cent of breakdown isolates were within their home range, with 28 different genotypes identified. Most often observed was 9:b (38 per cent), followed by 17:a (29 per cent) and 9:c (12 per cent).

Genotypes 25:a and 9:d were the most frequently observed in Scotland (25 per cent each); however, as the source of infection in Scotland is often purchased animals, this distribution differs from year to year.

M bovis isolates (n=225) from 182 incidents involving non-bovine species were genotyped in 2018. Incidents affecting badgers (112) and deer (38) were the most frequent. There was a reduction in the number of isolates from alpacas (nine down from 13 in 2017) and pigs (eight down from 12).

Other species affected in incidents in 2018 included goats and cats (five incidents each) plus others experiencing just one TB incident.

In general, the genotypes of M bovis found in non-bovine species subject to scanning TB surveillance reflect the frequency and geographical distribution of those found in the local cattle population.

ConclusionThe burden of TB borne by cattle populations varies widely across GB.

Scotland is officially free of TB, and the incidence remains low.

Over the past decade TB has been plateauing in England and declining in Wales, where an overall downward trend has been intermittently punctuated by short-term increases. In both countries the total number of new incidents fell in 2018 compared to 2017. However, patterns vary by TB risk area and in the Edge area of England

OFFICIAL REPORTSBoth the England TB epidemiology report1 and Wales TB epidemiology report2 are published online, together with a further 17 reports focusing separately on the epidemiology of the Edge area and Low risk area counties.9 Together, this portfolio of reports describes in detail the burden of TB in GB cattle during 2018. Scotland has been officially TB free since 2009, so does not have a separate report. However, in addition to the detail provided in this article, data for England, Scotland and Wales are available in a further data report, published in a Microsoft Excel table format, ‘Bovine tuberculosis in Great Britain: Surveillance data for 2018 and historical trends’.10 This presents the data used to compile current and previous surveillance reports.

Fig 6: Geographical distribution of recurrent bovine TB incidents in 2018 for herds with two or more previous incidents in the three years before the 2018 incident




j.com/

Veterinary R


arch 2020. Dow

nloaded from


28 March 2020 | VET RECORD380 VET RECORD | 28 March 2020 PB

SURVEILLANCE

and more recently the HTBW and ITBN areas of Wales, TB incidence has risen.

The increase in new TB incidents reflects both introduction of disease through infected cattle purchases and the development of new areas of local spread (endemic) infection, and in some areas may reflect changes in surveillance intensity leading to the detection of more cases.

Enhanced surveillance and case management policies in the Edge area of England seek to control the epidemic by preventing further spread of the disease. The impact of policies introduced in the Edge area in 2018 (including expanded six-monthly testing and wildlife control measures) will require further assessment in the coming years, when a greater body of evidence is available to thoroughly examine the disease trends.

AcknowledgementsThe authors would like to thank Emily Rimdap, Zoë Pembery and James Tiller, who assisted with the compilation of the annual surveillance reports; Monika Klita and Karen Gover for their technical expertise in genotyping the strains; Graham Smith; their policy colleagues; and their

veterinary advisers, Ricardo de la Rua-Domenech, James McCormack, Paul Schroeder, Richard Griffiths, Gosia Siwonia, Matthew Curds and Martyn Blissitt, for their advice and guidance in the production of the surveillance reports and for reviewing this summary.

The compilation of the annual TB surveillance reports is funded by Defra and the Scottish and Welsh Governments (project SB4500). TB genotyping is funded by Defra project SB4020.

References1 APHA. Bovine tuberculosis in England in

2018: Epidemiological analysis of the 2018 data and historical trends. 2019; https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/844813/england-tb-epi-report-2018a.pdf (accessed 21 November 2019)

2 APHA. Bovine TB: annual surveillance reports: Summaries of cattle data and the success of measures taken to tackle bovine TB. 2019; https://gov.wales/bovine-tb-annual-surveillance-reports (accessed 21 November 2019)

3 Harris KA, Upton PA, Goodchild AV, et al. Bovine TB infection status in cattle in Great Britain in 2012. Vet Rec 2014;174:600–4

4 Broughan JM, Harris K, Brouwer A, et al. Bovine TB infection status in cattle in Great Britain in 2013. Vet Rec 2015;176:326–30

5 Lawes JR, Harris KA, Brouwer A, et al. Bovine TB surveillance in Great Britain in 2014. Vet Rec 2016;178:310–5

6 Harris KA, Brunton L, Brouwer A, et al. Bovine TB infection status in cattle in Great Britain in 2015. Vet Rec 2017;180:170–5

7 Harris KA, Brouwer A, Upton P, et al. Bovine

TB infection status in cattle in Great Britain in 2016. Vet Rec 2018;182:538–44

8 Perrin LD, Harris KA, Reynolds M, et al. Bovine TB infection status in cattle in Great Britain in 2017. Vet Rec 2019: 184:371–8

9 APHA. Bovine TB epidemiology reports, 2018: Descriptive reports on the epidemiology of bovine tuberculosis in the low risk areas (LRA) and edge areas of England. 2019; www.gov.uk/government/publications/bovine-tb-epidemiology-reports-2018 (accessed 21 November 2019)

10 APHA. Bovine tuberculosis in Great Britain: surveillance data for 2018 and historical trends (Microsoft Excel). 2019; https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/844816/gb-data-report-2018.xlsx (accessed 21 November 2019)

11 APHA. Bovine tuberculosis in Great Britain in 2018: Explanatory supplement to the annual reports. 2019; https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/844814/epi-report-tb-supplementary-reporta.pdf (accessed 21 November, 2019)

12 Anon. Changes to bovine TB testing 04/10/2011; 2011; http://wayback.archive-it.org/3011/20130202194429/http://www.scotland.gov.uk/News/Releases/2011/10/04085344 # (accessed 27 October 2015)

13 DEFRA. Badger vaccination scheme relaunched in fight against bovine TB. 2017; www.gov.uk/government/news/badger-vaccination-scheme-relaunched-in-fight-against-bovine-tb (accessed 21 November 2019)

14 Goodchild AV, Downs SH, Upton P, et al. Specificity of the comparative skin test for bovine tuberculosis in Great Britain. Vet Rec 2015; doi:10.1136/vr.102961

15 DEFRA. Strategy for achieving bovine tuberculosis free status for England. 2014; www.gov.uk/government/uploads/system/uploads/attachment_data/file/300447/pb14088-bovine-tb-strategy-140328.pdf (accessed 21 November 2019)

Make sure you’re complying with waste regulationsDownload the BVA guide to handling veterinary waste from www.bva.co.uk/guides




j.com/

Veterinary R


arch 2020. Dow

nloaded from


Bovine TB infection status in cattle inGreat Britain in 2018 · • Scotland remained Officially TB...

Documents

Transcript of Bovine TB infection status in cattle inGreat Britain in 2018 · • Scotland remained Officially TB...