Research and analysis

Cumbria (low risk area) year end report 2022

Updated 20 November 2023

Introduction

The Low Risk Area (LRA) of England was established in 2013. In 2014 this area was incorporated into the UK government’s strategy to achieve Officially Tuberculosis-Free (OTF) status for England by 2038. A key action was to recognise the different levels of bovine tuberculosis (TB) in different parts of the country and to vary the approach to control accordingly. Overall, the LRA has a very low and stable incidence of infected herds. The current strategy seeks to rapidly control infection when it arises through:

• high sensitivity testing of affected herds
• temporarily enhanced local surveillance (radial and hotspot testing)
• mandatory pre- and post-movement testing of cattle entering the LRA from higher risk areas of the UK

The aim is to preserve the favourable disease status of this area so that its counties can be declared OTF as soon as possible.

This report describes the frequency and geographical distribution of TB in 2022 in cattle herds in Cumbria, which is part of the LRA. In 2022, 15% of all new TB incidents in the LRA were detected in Cumbria.

TB in cattle and other mammals is primarily caused by the bacterium Mycobacterium bovis (M. bovis) and the disease is subsequently referred to as TB in this report. Although other sources may refer to TB ‘breakdowns’, this report will use the term ‘incidents’ throughout.

This report is intended for individuals involved in the control of TB, both locally and nationally. This includes, but is not limited to farmers, veterinarians, policy makers and the scientific community.

Details of the data handling methodology used in this report, a glossary of terms, and the TB control measures adopted in the LRA, can be found in the explanatory supplement for the annual reports 2022.

Types of TB incident

Unless otherwise specified, this report includes all new TB incidents detected during the reporting period, 1 January to 31 December 2022. This includes both ‘Officially Tuberculosis-Free Status Withdrawn’ (OTF-W) and ‘Officially Tuberculosis-Free Status Suspended’ (OTF-S) incidents.

OTF-W incidents are those involving at least one skin test reactor (an animal positive to the Single Intradermal Comparative Cervical Tuberculin, or SICCT test) with either typical lesions of TB identified at post-mortem (PM) meat inspection, or at least one animal with M. bovis-positive culture results from tissue samples collected from carcases during the PM inspection, or both.

OTF-S incidents are triggered by reactors to the skin test, but without subsequent detection of TB lesions or positive culture results in any of those animals.

Cattle industry

Cumbria has over 436,000 cattle, equally distributed between beef and dairy farms. Cattle herd sizes vary across the region, as shown in Appendix 1.

There are 10 livestock markets in Cumbria, 7 of which are approved by APHA to hold slaughter gatherings of cattle from OTF herds that are eligible for, but have not been subjected to, pre-movement TB testing (known as ‘TB pre-movement testing-exempt’ markets). TB dedicated sales or TB slaughter gatherings in which cattle from TB-restricted premises may be sold are not permitted in the LRA. The county has one Licensed Finishing Unit (LFU).

There is frequent movement of cattle from Cumbria into Scotland, as well as a significant number of cattle entering the county from Northern Ireland and the Republic of Ireland. These are generally destined for beef finishing or fattening units. Pedigree breeding bulls from Northern Ireland also move onto Cumbrian farms.

New TB incidents

In 2022, the total number of new TB incidents (22) increased by 4 compared to 2021 (18). The number of total incidents in this county has been on a decreasing trajectory since 2017, when there were 33 incidents. This is shown in Figure 1. In 2022, the number of OTF-S incidents increased from 11 to 15, whilst OTF-W incidents were unchanged compared to 2021 (7).

At the time of writing this report the county has 3 active TB hotspot areas, Confirmed Hotspot 21 (HS21) in the central eastern area, Potential Hotspot Area 26 (PHA26) in the southern area and Potential Hotspot Area 29 (PHA29) in the eastern area. The latter was formally established early in 2023, whereas HS21 and PHA26 had been in place since 2016 and 2019 respectively. The areas covered by all 3 hotspots are shown in Figure 3 and described in more detail later in this report.

In HS21 there were 5 new TB incidents in 2022, 4 of which were OTF-S incidents and one OTF-W incident. In 2022, there were 7 new TB incidents, 4 OTF-S incidents and 3 OTF-W incidents in the area that became PHA29 in January 2023. There were no new TB incidents in PHA26 in 2022.

Figure 1: Annual number of new TB incidents (OTF-W and OTF-S) in Cumbria, from 2013 to 2022

Year	OTF-W incidents	OTF-S incidents	Total
2013	5	9	14
2014	5	15	20
2015	11	20	31
2016	16	16	32
2017	12	21	33
2018	9	18	27
2019	5	24	29
2020	4	19	23
2021	7	11	18
2022	7	15	22

There were no reports of TB in other farmed or captive non-bovine species in Cumbria during 2022.

The headline cattle TB statistics for Cumbria is summarised in Appendix 2.

Disclosing test type

Most new TB incidents were identified through risk-based enhanced surveillance testing, including 7 by radial testing (RAD) and 5 by specific hotspot testing (HS). These 2 test types were also responsible for disclosing the greatest proportion of incidents in 2021 (8 by radial testing and 4 by hotspot testing).

One incident was disclosed by a tracing test due to its links to another OTF-W incident, as displayed in Figure 2. Four incidents were detected using routine herd testing (RHT), which was an increase from one incident in 2021. Slaughterhouse surveillance identified 2 new TB incidents in 2022, increasing from one in 2021.

Of the incidents identified by radial testing, all but one was located in PHA29. The other incident was found around an OTF-W incident to the east of HS21.

Figure 2: Number of new TB incidents (OTF-W and OTF-S) in Cumbria, in 2022 according to the surveillance method that detected them

Disclosing test type	Number of new TB incidents	Total
6 month test	1	1
Hotspot test	5	5
Pre-movement test	1	1
Radial test	7	7
Routine herd test	4	4
Slaughterhouse	2	2
Tracing test	1	1
Whole herd test	1	1

Duration of TB incidents

A total of 21 TB incidents were resolved during 2022. Of these, 11 were new TB incidents that started in 2022, 7 started in 2021 and 3 started in 2020.

Of the 21 TB incidents that closed in the 2022, 4 were OTF-W incidents. Three of these were resolved within 240 days, and the other one resolved within 550 days. The median duration was 189.5 days (interquartile range 168 to 230.5).

Most OTF-S incidents (11 out of 17) were resolved within 150 days and the median duration was 137 days (interquartile range 102 to 182). There were 2 resolved OTF-S incidents that were under restrictions for increased periods of time (544 and 558 days).

One of these herds required 6 rounds of short interval skin testing to regain OTF status, and due to the high number of animals and multiple days required to complete each skin herd test, this resulted in an extension of the period under TB movement restrictions. The other case was a herd with chronic infection in which the lifting of restrictions was delayed due to the need to wait for some positive animals to a non-official TB antibody test to be privately culled by the farmer.

The median duration for all incidents that closed in 2022 was 150 days (interquartile range 106 to 193). This is shorter than the duration of incidents that closed in 2021; 173.5 days (interquartile range 98 to 185). For the whole LRA, the median duration of TB incidents that closed in 2022 was also 150 days (IQR 98 to 194).

Geographical distribution of TB incidents

All but 2 of the 22 new TB incidents detected in Cumbria during 2022 were in the eastern half of the county. Most of these occurred along the Eden Valley to the northeast of Penrith, but some were to the north of the county and some to the south of the Lake District National Park, where the land is more suitable for cattle farming. This is shown in Figure 3.

Whole genome sequencing (WGS) replaced genotyping at APHA in 2021. There were 5 new OTF-W incidents associated with clade B3-11 of M. bovis, which has a homerange across much of the North Midlands, extending up into Greater Manchester. Three of the B3-11 incidents were in the Eden Valley area where PHA29 has now been established (Figure 3). Four OTF-S incidents were also detected in this area. Another of the OTF-W incidents with clade B3-11 was detected just outside PHA29 to the north-west, and the fifth one was in the south of Cumbria, where the cattle density is high (Figure 3).

One OTF-W incident was caused by infection with clade B6-23 of M. bovis, to the east of HS21. This is the clade associated with HS21. The radial zone triggered by this incident identified one OTF-S incident.

It was not possible to grow M. bovis in bacteriological culture from the VL reactor cattle removed from the only new OTF-W detected in HS21 and so the WGS clade for this case could not be determined.

Figure 3: Location of cattle holdings in Cumbria with new TB incidents (OTF-W and OTF-S) in 2022, and cattle holdings with pre-2022 OTF-W incidents still ongoing at the beginning of 2022, overlaid on a cattle density map. Note that ‘OTF-W Introduced 2022’ refers to OTF-W incidents in which cattle movements were the most likely source of infection.

As displayed in Figure 4, OTF-W incidents in Cumbria in 2022 were attributed to different sources of infection. One incident was likely associated to the introduction of undetected infected cattle, albeit cattle were sourced from the LRA of England and from Scotland. The remaining 6 OTF-W incidents were therefore thought to have resulted from locally acquired infection. However, it is sometimes challenging to decide which pathways mediating this are most likely involved, hence some TB incidents have a source described as ‘unknown’.

Of the 3 OTF-W incidents in PHA29, fomites (farm objects, vehicles or material contaminated with the bovine TB bacterium) were the likely source of infection for one incident, but exposure to infected wild deer and contiguous cattle contact were also considered possible. For the other 2, there was no clearly predominant risk pathway hence the ‘unknown’ source status. However, exposure to infected badgers, exposure to infected wild deer, and contact with contiguous cattle herd and cattle movements were all found to be possible. Exposure to infected badgers or other wildlife, and contiguous cattle contact could also not be ruled out for most OTF-S incidents in PHA29.

The only OTF-W incident detected in HS21 remained culture negative and so WGS analysis could not be used to aid investigations. Several risk pathways were found to be possible, but there was no clearly predominant one identified.

Outside HS21 and to the east, there was an OTF-W incident with clade B6-23 (previously termed genotype 17: z), which is the same clade of M. bovis most commonly found within HS21. The isolate was found to be very closely related to isolates from HS21, but although cattle are sometimes purchased, a specific risk pathway via cattle movements could not be identified. Nevertheless, cattle movements, exposure to infected badgers and wild deer were all considered possible, but a clearly predominant risk pathway could not be determined for this incident.

In 2022, an OTF-W incident (clade B3-11) identified through slaughterhouse surveillance, occurred in the south of the county approximately 2 miles to the east of the boundary of PHA26. This incident was about 4 miles away from a previous OTF-W incident of clade B3-11 in 2021, which was in turn 4 miles to the south-east of the boundary PHA26 and was attributed to cattle movements. The source of infection for the 2022 incident remains unknown. See section about PHA26.

Figure 4: Map of the source of infection pathway recorded with the highest level of certainty, for OTF-W incidents, and the location of OTF-S incidents in Cumbria which started in 2022. Local cattle refer to residual infection and contiguous cattle risk pathways. Purchased refers to cattle movement risk pathways. Wildlife refers to both badger and other wildlife risk pathways.

Figure 5 shows the hotspot areas and radial surveillance zones around OTF-W incidents that were active, completed or not instigated in Cumbria during 2022. OTF-W incidents in Cumbria have become more localised to the east of the county in recent years, mainly within PHA29 and HS21. There were no new OTF-W incidents since 2020 towards the west of the county. As such, radial zones in the west half of Cumbria are now completed, as can be observed in Figure 5.

Figure 5: Hotspot areas and radial surveillance zones around OTF-W incidents that were active, completed or not instigated in Cumbria during 2022, by year of initiation.

Potential or confirmed TB hotspots

Confirmed Hotspot 21

HS21 was established in 2016 and subsequently confirmed in the spring of 2017, after the detection in the carcases of 3 badgers found dead of the same strain of M. bovis as that associated with the cattle incidents in the area (clade B6-23, previously termed genotype 17:z) (Figure 5).

There was only one new OTF-W incident in HS21 in 2022 (from which M. bovis could not be isolated despite the detection of visible lesions of TB at PM inspection) and 4 new OTF-S incidents. There were no B6-23 cattle isolates identified within the hotspot in 2022, or for the previous 3 years. For 2 of the 4 OTF-S incidents, cattle movements were the most likely risk pathway. For the other 2 OTF-S incidents and the one OTF-W incident in HS21, the risk pathway remains unknown, however exposure to wildlife, movements of cattle from LRA, and contiguous cattle contact could not be ruled out. Residual infection within the herd was also possible for one of the OTF-S incidents that had an OTF-W incident in 2020.

As described previously, one new OTF-W incident caused by WGS clade B6-23 was, however, detected to the east of HS21 outside the hotspot boundary. The isolate was found to be very closely related to B6-23 isolates from HS21, but although the affected farm purchased some cattle, a specific risk pathway via cattle movements could not be identified. Cattle movements, exposure to infected badgers and deer were all considered possible, but a clearly predominant risk pathway could not be determined. This incident triggered a radial zone in which one OTF-S incident occurred, again of unknown origin.

Badger disease control measures have been taking place in the area since 2018 and there have been no further M. bovis-culture positive badgers detected since 2019. After 4 consecutive annual seasons of intensive badger culling licensed by Natural England, vaccination became the sole TB control method for badgers in this hotspot in 2022. Read more about wildlife surveillance within hotspot areas. Surveillance of road killed badgers and wild deer is still being carried out in this hotspot.

In 2020, cattle herds in the outer section of the hotspot became eligible to move to annual (instead of 6-monthly) TB surveillance testing, subject to certain eligibility criteria. In 2021 and 2022, the area within HS21 where this applies was extended to include more farms. The epidemiological situation in the area continues to be closely monitored as it provides the evidence required to inform changes in cattle testing measures.

At the end of the year, there were 3 cattle TB incidents still open in this hotspot.

Potential Hotspot Area 26

Potential Hotspot Area (PHA26) was established in 2019, following a cluster of TB incidents in South Cumbria near Cartmel Fell, between Kendal and Windermere (Figure 5). The 2 OTF-W incidents triggering PHA26 were both WGS clade B3-11 (formerly genotype 25: a).

Wildlife surveillance was implemented in the area in August 2019. However, at the time of writing there had only been 7 submissions and M. bovis had not been identified in any of these. A summary of the wildlife carcases collected for TB testing from Potential Hotspot Areas up to 18 January 2023 is available on GOV.UK. No enhanced TB control measures have been applied to cattle herds in PHA26 to date, apart from the radial testing zones triggered around the OTF-W incidents from 2019. Nevertheless, the area continues to be closely monitored for any further developments.

There were no new OTF-W or OTF-S incidents in PHA26 in 2022.

Potential Hotspot Area 29

An increase in OTF-W incidents along the Eden Valley was detected in 2021. Additional OTF-W incidents were disclosed in 2022, leading to the establishment of Potential Hotspot Area 29 (PHA29) at the beginning of 2023. Clade B3-11 was detected in the 4 incidents from 2021, which after further WGS were identified as being closely related. The 3 new OTF-W incidents in 2022 also yielded clade B3-11 and were genetically related to the M. bovis isolates from 2021. Geographical and phylogenetic evidence combined provide strong evidence of local spread.

There are no enhanced TB cattle control measures in PHA29 yet, apart from the radial testing zones triggered around OTF-W incidents. Wildlife surveillance of badgers and wild deer started in early 2023 and several submissions have been received, however M. bovis has not been identified in local wildlife so far. It is important that as many carcases as possible are reported to inform epidemiological investigations. APHA is continuing to test ‘found-dead’ badgers and deer carcases reported through the Defra Rural Service Helpline (03000 200 301).

As described above, during 2022, there were a total of 7 new TB incidents in cattle in PHA29, 4 OTF-S and 3 OTF-W.

At the end of the year, there were 5 cattle TB incidents still open in PHA29.

Of the 3 OTF-W incidents in PHA29, fomites were the likely source of infection for one incident, but exposure to infected deer and contiguous cattle contact were also considered possible. For the other 2, there was no clearly predominant risk pathway hence ‘unknown’ status, however exposure to infected badgers, exposure to infected deer and contiguous cattle contact, and cattle movements were all found to be possible.

Exposure to infected badgers or other wildlife and contact with contiguous cattle herds could also not be ruled out for most OTF-S incidents in PHA29.

Skin test reactors and interferon gamma test positive animals removed

As presented in Figure 6, the number of reactors removed for TB control purposes in Cumbria decreased compared to 2021 (from 106 to 38). Of the 38 cattle that were removed for TB control purposes in 2022, 28 were skin test reactors, a decrease from 37 in 2021. A further 10 were positive on the interferon gamma (IFN-γ) test, a decrease from 69 in 2021. This is a large reduction compared to a peak in the number of reactors removed in 2018, when 225 IFN-γ and 57 skin test reactors were removed.

Figure 6: Number of cattle that tested positive to the skin test or interferon gamma test and were removed by APHA for TB control reasons in Cumbria, from 2017 to 2022.

Year	Skin test	Interferon gamma test	Total
2017	59	199	258
2018	57	225	282
2019	77	113	190
2020	37	2	39
2021	37	69	106
2022	28	10	38

Main risk pathways and key drivers for TB infection

Evidence collected during APHA veterinary investigations into the source of infection within herds was used to inform this understanding. In 2022, 18 out of 22 (82%) of new TB incidents in Cumbria received a preliminary or final APHA veterinary investigation to identify the source of infection.

It can be challenging to retrospectively establish the route of infection for a TB incident herd. Ideally this investigation includes a thorough on-farm investigation and scrutiny of routinely collected data, such as cattle movement records, and the results of WGS where available. Up to 3 hazards and risk pathways were selected for each incident investigated. Each of these potential sources were given a score that reflects the likelihood of that pathway being the true one, based on the available evidence.

Details of the protocol used for these investigations, and the subsequent methodology used to calculate the weighted contribution of the different suspected sources of M. bovis infection can be found in the explanatory supplement to the annual reports 2022.

The key drivers of the TB epidemic in Cumbria during 2022 were identified as follows:

• purchase of undetected infected cattle
• exposure to probable infected wildlife
• contact with contiguous infected cattle herds

The movement of undetected infected cattle continued to be the main pathway of infection for cattle herds in 2022, with a weighted contribution of 17%, see Appendix 3. All pathways and the weighted contributions are given in Table 5, Appendix 3. The attribution to cattle movements is largely due to the risk pathways for OTF-S TB incidents in 2022. Although these were distributed across the county, it is particularly pertinent to HS21, where wildlife intervention in previous years has significantly reduced the likelihood of exposure to infected wildlife. Moreover, it reiterates the importance of implementing risk-based trading practices and correct biosecurity measures in the county.

Exposure to infected wildlife was considered to be another important risk pathway. Potential exposure to infected badgers had a weighted contribution of 11%. This appears to be relatively less important in 2022 compared to previous years (20% in 2020 and 14% in 2021). Other infected wildlife, however, had a higher weighted contribution compared with previous years, increasing to 15% in 2022 compared to 9% in 2021. This can be explained by the creation of an additional PHA29, where the status of disease in the local wildlife population is being surveyed.

Finally, contact with contiguous infected cattle herds was the third most important risk pathway, weighted contribution 12%. This emphasises the need for improved biosecurity practices.

There was a high degree of uncertainty around the source of new incidents in 2021. Other or unknown sources had a weighted contribution of 28%. This category is added to those incidents in which there was high uncertainty around the selected pathways as alluded to earlier in the report (see explanatory supplement for methodology).

Forward look

To further reduce the TB incidence and maintain LRA status, collectively, Cumbrian farmers need to remain vigilant with the cattle they bring on to their farms, herd biosecurity and adherence to TB surveillance measures. With regard to cattle movements, farmers need to be aware that unknowingly bringing on TB infected cattle can result in the infection being seeded into local wildlife populations, thus potentiating further local spread. Existing, as well as new clusters arising within the county, will be areas of particular focus, whilst aiming to resolve herd incidents as quickly as possible and with the least impact on farm businesses as possible.

Appendix 1: cattle industry demographics

Table 1: Number of cattle herds by size band in Cumbria as of 31 December 2022 (RADAR data)

Size of herds	Number of herds in Cumbria
Undetermined	23
1 to 50	1,015
51 to 100	515
101 to 200	542
201 to 350	379
351 to 500	169
Greater than 500	165
Total number of herds	2,808
Mean herd size	156
Median herd size	82

Table 2: Number (and percentage of total) of animals by breed purpose in Cumbria as of 31 December 2022

Breed purpose	Number (and percentage of total) cattle in Cumbria
Beef	217,705 (49%)
Dairy	201,247 (46%)
Dual purpose	17,850 (4%)
Unknown	10 (0.002%)
Total	436,812

Appendix 2: summary of headline cattle TB statistics

Table 3: Herd-level summary statistics for TB in cattle in Cumbria in 2022

Herd-level statistics	2020	2021	2022
(a) Total number of cattle herds live on Sam at the end of the reporting period	3,386	3,242	3,278
(b) Total number of cattle herds subject to annual TB testing (or more frequent) at the end of the reporting period (any reason)	473	390	475
(c) Total number of whole herd skin tests carried out at any time in the period	1,389	1,205	1,068
(d) Total number of OTF cattle herds having TB whole herd tests during the period for any reason	1,141	947	885
(e) Total number of OTF cattle herds at the end of the report period (herds not under any type of Notice Prohibiting the Movement of Bovine Animals (TB02) restrictions)	3,345	3,201	3,232
(f) Total number of cattle herds that were not under restrictions due to an ongoing TB incident at the end of the report period	3,375	3,231	3,266
(g.1) Total number of new OTF-S TB incidents detected in cattle herds during the report period	19	11	15
(g.2) Total number of new OTF-W TB incidents detected in cattle herds during the report period	4	7	7
(h.1) Of the new OTF-W herd incidents, how many occurred in a holding affected by another OTF-W incident in the previous 3 years?	1	1	0
(h.2) Of the new OTF-W herd incidents, how many could be considered secondary to a primary incident based on current evidence?	1	0	0
(h.3) Of the new OTF-W herd incidents, how many were triggered by skin test reactors or 2 time inconclusive reactors (2xIRs) at routine herd tests?	1	1	1
(h.4) Of the new OTF-W herd incidents, how many were triggered by skin test reactors or 2xIRs at other TB test types (such as forward and back-tracings, contiguous, check tests)?	3	5	4
(h.5) Of the new OTF-W herd incidents, how many were first detected through routine slaughterhouse TB surveillance?	0	1	2
(i.1) Number of new OTF-S incidents revealed by enhanced TB surveillance (radial testing) conducted around those OTF-W herds	6	4	4
(i.2) Number of new OTF-W incidents revealed by enhanced TB surveillance (radial testing) conducted around those OTF-W herds	0	4	3
(j) Number of OTF-W herds still open at the end of the period (including any ongoing OTF-W incidents that began in a previous reporting period)	3	3	4
(k) Number of OTF-W herds still open at the end of the period that are within a finishing unit	0	1	1
(l) New confirmed (positive M. bovis culture) incidents in non-bovine species detected during the report period (indicate host species involved)	1 (sheep)	0	0

Table 4: Animal-level summary statistics for TB in cattle in Cumbria during 2022

Note (c) Reactors detected during year per incidents disclosed during year, reactors may be from incidents disclosed in earlier years, as any found through testing during the report year count here.

Note (g) Slaughterhouse (SLH) cases confirmed by culture of M. bovis, not all cases reported are submitted for culture analysis. All cases reported are from any period prior to or during restrictions.

Animal-level statistics (cattle)	2020	2021	2022
(a) Total number of cattle tested with tuberculin skin tests or additional IFN-γ blood tests in the period (animal tests)	224,679	204,784	185,351
(b.1) Reactors detected by tuberculin skin tests during the year	37	37	28
(b.2) Reactors detected by additional IFN-γ blood tests (skin-test negative or IR animals) during the year	2	69	10
(c) Reactors detected during year per incidents disclosed during year	1.7	5.89	1.73
(d) Reactors per 1,000 animal tests	0.17	0.52	0.21
(e.1) Additional animals slaughtered during the year for TB control reasons (dangerous contacts, including any first time IRs)	5	3	1
(e.2) Additional animals slaughtered during the year for TB control reasons (private slaughters)	5	2	2
(f) SLH cases (tuberculous carcases) reported by the Food Standards Agency (FSA) during routine meat inspection	6	6	7
(g) SLH cases confirmed by culture of M. bovis	0	1	2

Appendix 3: suspected sources of M. bovis infection for all of the new OTF-W and OTF-S incidents identified in the report period

In 2022, 18 out of 22 (82%) new TB incidents in Cumbria received a preliminary or final APHA veterinary investigation to identify the source of infection.

Each TB incident could have up to 3 potential risk pathways identified. Each risk pathway is given a score that reflects the likelihood of that pathway bringing TB into the herd. The score is recorded as either:

• definite (score 8)
• most likely (score 6)
• likely (score 4)
• possible (score 1)

The sources for each incident are weighted by the certainty ascribed. Any combination of definite, most likely, likely, or possible can contribute towards the overall picture for possible routes of introduction into a herd.

If the overall score for a herd is less than 6, then the score is made up to 6 using the ‘Other or unknown source’ option. Buffering up to 6 in this way helps to reflect the uncertainty in assessments where only ‘likely’ or ‘possible’ sources are identified.

Table 5 combines the data from multiple herds and provides the proportion of pathways in which each source was identified, weighted by the certainty that each source caused the introduction of TB.

The output does not show the proportion of herds where each pathway was identified (this is skewed by the certainty calculation).

WGS of M. bovis isolates can be a powerful tool in identifying a likely source of infection, however WGS clades have not been determined for OTF-S herds. As a result of varying levels of uncertainty, only broad generalisations should be made from these data. A more detailed description of this methodology is provided in the explanatory supplement for the annual reports 2022.

Table 5: Suspected sources of M. bovis infection for all new OTF-W and OTF-S incidents identified in Cumbria in 2022

Please note each TB incident could have up to 3 potential pathways so totals may not equate to the number of actual incidents that have occurred.

Source of infection	Possible (1)	Likely (4)	Most likely (6)	Definite (8)	Weighted contribution
Badgers	12	0	0	0	11%
Cattle movements	2	4	0	0	17%
Contiguous	9	1	0	0	12%
Residual cattle infection	2	1	0	0	6%
Domestic animals	0	0	0	0	0%
Non-specific reactor	3	1	0	0	6%
Fomites	2	1	0	0	6%
Other wildlife	12	1	0	0	15%
Other or unknown source	3	0	0	0	28%