Select Committee on Environment, Food and Rural Affairs Minutes of Evidence




  A1  King et al (2007) refer to the need for an intervention that would "reduce the prevalence of disease in both cattle and wildlife". Detailed analysis of the prevalence of M. bovis infection among badgers culled in the RBCT demonstrated that prevalence was increased following both proactive and reactive culls, where prevalence is the proportion affected (Bourne et al, 2007; Woodroffe et al, 2006b). We note that Sir David King's oral evidence to the Environment, Food and Rural Affairs Committee suggests that his team were envisaging a policy entailing reductions in badger density (ie the numbers of badgers per unit area) similar to those achieved in the RBCT (Q407, Environment, Food and Rural Affairs Committee, 2007). In this context, and in the absence of any data on the level to which population densities would need to be suppressed in order to reduce disease transmission among badgers in TB-affected areas of Britain, we do not consider badger culling able to fulfil King et al (2007) criteria for appropriate intervention.

  A2  Proactive culling, as conducted in the RBCT, substantially reduced the density of badgers (Woodroffe et al, 2007), and thus it probably reduced the density of M. bovis infected badgers despite increased prevalence. However localised reactive culling produced smaller reductions in overall badger density (Woodroffe et al, 2007) yet was still associated with elevated prevalence (Bourne et al, 2007); hence the density of infected badgers may not have been reduced and could have been increased.


  A3  We believe that clarity is critical to this scientific debate, and therefore note that the statement that "removal of badgers is the best option... to reduce the reservoir of infection in wildlife" is imprecise and potentially misleading. "Reduc[ing] the reservoir of infection in wildlife" could refer to reducing wildlife density, to reducing the prevalence of infection within the wildlife population, or to both. If this wording is intended to refer to a reduction in the prevalence of infection in wildlife, then it is erroneous: as described in paragraphs ISG25 and A1 above, there is strong evidence showing that RBCT culling increased, rather than reduced, the prevalence of infection in badgers (Bourne et al, 2007; Woodroffe et al, 2006b). If the wording is intended to refer to a reduction in the density of badgers, it is important to recognise the very strong evidence that such a reduction could decrease or increase TB risks to cattle (or both, Donnelly et al, 2006; Donnelly et al, 2003), depending on the form of culling. There is consistent evidence that if badger culling were to be conducted in a patchy, inefficient, or uncoordinated manner, then subsequent risks of transmission to cattle would probably be no lower—and could possibly be higher—than they were before culling even though badger densities were being suppressed (Bourne et al, 2007).


  A4  We are unsure of the evidence used by King et al (2007) to select 100 km2 as the minimal area for badger removal. While this was the scale of RBCT areas, it was clear from our analyses that, when surrounding areas were also considered, the benefits within the 100 km2 culled area were largely offset by the detrimental effects on surrounding land (Donnelly et al, 2007). Two extrapolations that we considered could be supported by the RBCT data indicated that the minimal area required to obtain a statistically significant overall benefit was either 265 km2 (Paragraph 5.41 and Figure 5.4A in Bourne et al, 2007) or 455 km2 (Paragraph 5.42 and Figure 5.4B in Bourne et al, 2007), depending on the assumptions used. Any such extrapolation requires untestable assumptions (by the very definition of extrapolation), yet it is clear from the RBCT data that 100 km2 is too small an area to be confident of an overall beneficial effect on cattle TB incidence.


  A5  As noted in paragraphs ISG13-ISG15 above, we strongly disagree with King et al(2007) statement that the evidence for a detrimental effect is limited to the area between 0.5 and 1 km outside the removal area.

  A6  Further, on the basis of RBCT and other data we consider it highly unlikely that the "soft boundaries" mentioned would either "reduc[e] the migration of badgers into the removal area" as stated, or markedly reduce detrimental effects on neighbouring land. These arguments are outlined in paragraphs ISG19 and A34-A35 of this document.


  A7  While King et al (2007) recommend monitoring of cattle TB incidence in the proposed culling areas, we are concerned that no detailed consideration has been given to the precision of the analysis of such monitoring data. Without knowing the spatial extent over which any such culling would occur, it is impossible to provide statistical guidance. More importantly, it would be extremely difficult to obtain reliable estimates of the effects of culling on TB incidence (particularly any detrimental effects in neighbouring areas) if no unculled comparison areas were available. Indeed, this is one of the reasons why Krebs et al (1997) recommended that a randomised controlled experiment be conducted, on an appropriate temporal and geographical scale, to evaluate the impacts of badger culling on cattle TB.


  A8  As noted in paragraphs ISG20-ISG22 above, the results of Cox et al (2005) have been misinterpreted.


  A9  King et al (2007) comment that "...cattle movements alone cannot explain the persistence of the geographically compartmentalised areas of high incidence of cattle TB, nor their gradual expansion over the last decade..." but provide no evidence in support of this statement. In considering the potential contribution of cattle movements to the geographical spread of TB, it is worth noting that, in western England and Wales, 43% of cattle movements occur over a distance of less than 20 km (Mitchell et al, 2005). Hence, cattle movement is likely to contribute to local as well as long-distance spread of infection.


  A10  This paragraph notes that "most of the areas had only four removal operations". This is incorrect. The number of proactive culls conducted in each triplet were: A five culls, B seven culls, C six culls, D four culls, E six culls, F five culls, G five culls, H five culls, I four culls, J four culls, giving a mean of 5.1 and a mode of five culls per triplet.

  A11  Consideration is given in this paragraph to the non-significant trend for the benefits of culling within trial areas to increase on later culls. While mathematical modelling could be used to extrapolate the likely benefits of conducting further annual culling operations, this would again require untestable or uncertain assumptions (including, but not limited to, how badger recolonisation and birth rates might change with repeated culling, what proportion of breakdowns were due to badger-to-cattle transmission, and how badger ranging behaviour might change with badger density). While detailed mathematical models of the cattle-badger disease system have been constructed (eg Smith et al, 2001), considerable detailed sensitivity analyses of parameter values and model structure would be required before their predictions of future benefits could be considered reliable.


  A12  The RBCT did demonstrate that repeated culling carried out by skilled members of the Defra Wildlife Unit delivered significantly decreased TB incidence in cattle within the 100km2 RBCT trial areas (Donnelly et al, 2006). However, these beneficial effects were accompanied by detrimental effects in neighbouring areas (Donnelly et al, 2006).

  A13  With regard to extrapolation of effects of culling over larger areas, paragraphs 5.41-5.42 and Figure 5.4 of Bourne et al (2007) provide what we consider to be reliable extrapolations from our results.


  A14  We are unsure why the increased incidence of cattle TB observed on land neighbouring RBCT trial areas was judged "hard to interpret". We have published a body of research on this issue and the evidence is considered "substantial" and "strong" by scientific authorities (eg Royal Society, 2006; Shepherd, 2005; Shepherd et al, 2005). We have published a number of peer-reviewed papers showing that culling leads to disrupted territorial organisation and expanded ranging by badgers (Pope et al, 2007b; Woodroffe et al, 2006a), as well as immigration of badgers into culled areas (Woodroffe et al, 2007). These ecological changes are associated with reduced clustering of infection in both badgers and cattle (Jenkins et al, 2007), and with increased prevalence of infection in badgers (Bourne et al, 2007; Woodroffe et al, 2006b). The causality of the relationship between culling-induced changes in badger ecology and increased TB incidence in cattle is suggested not only by these broad patterns, but also by details including modification of the effects by factors such as geographic barriers to badger movement, and the practice of non-simultaneous culling (Woodroffe et al, 2006b). Indeed, it was the observation of expanded ranging by badgers living immediately outside proactive culling areas (Woodroffe et al, 2006a) that prompted us to investigate cattle TB incidence on these lands (Donnelly et al, 2006). It is unfortunate that none of this ecological work is cited by King et al (2007).

  A15  King et al (2007) go on to recommend that "measures... should be put in place to minimise that increase [in cattle TB]". This appears to ignore the detailed consideration in paragraphs 10.8-10.48 of Bourne et al (2007) which concludes that no practical measures are likely to be able to achieve this. Further details are given in paragraphs A33-A35 below.


  A16  As mentioned in paragraph ISG16 above, we consider it inappropriate to exclude data from the period between the first and second culls. The reason for this is detailed in the peer-reviewed Supplementary Information of Donnelly et al (2006), which we reproduce here.


  The main text presents analyses from two time periods, one dating from the completion of the initial cull (which shows statistically significant effects both inside and outside trial areas), and one dating from completion of the first follow-up cull (which shows a significant effect inside, but a non-significant trend outside). The reasons for considering these two time periods, and their implications for the interpretation of our findings, merit further comment.

  We used incidence from the date of the initial cull as our primary analysis, mainly because this measure is the most relevant to policy: the effects detected reflect what one could expect to achieve from a proactive culling policy implemented on the timescale measured. As described briefly in the main text, there were two reasons for performing secondary analyses which excluded data from before the first follow-up cull. First, this excluded breakdowns that might have originated prior to the onset of culling, even though (given annual testing) they were not detected until after culling had begun. Such breakdowns would lead to under-estimation of culling-induced effects on incidence in the first year—essentially this would bias estimates of increases or reductions toward no effect. Our findings of statistically significant effects dating from completion of the initial cull, both inside and outside trial areas, therefore indicate the strength of both positive and negative effects of culling.

  An additional reason for performing the secondary analyses was that a more complete badger removal would have been achieved from the date of the first follow-up cull. This more complete cull would be expected to generate a greater reduction in cattle TB inside trial areas, and the results of the secondary analysis are indeed consistent with this prediction, albeit with a wider confidence interval due to the smaller dataset.

  In contrast with the situation inside trial areas, however, the circumstances of incomplete badger removal that would have occurred between the initial cull and the first follow-up could be expected to increase any detrimental effects of culling, if such effects were caused by disruption of badger territorial organisation at artificially reduced population densities. The frequency of potentially infectious contacts between cattle and badgers will be related to both the density of badgers, and the ranging behaviour of those badgers. We have hypothesised that, where densities are substantially reduced, contact rates will be reduced despite expanded ranging behaviour, but that smaller reductions in density will generate increased contact rates if (as observed) they are also accompanied by expanded ranging. In this scenario, we would expect detrimental effects to be particularly marked following the initial cull since densities were probably reduced to a lesser extent during this period. Our secondary analysis excluded this potentially important time period and this, along with the reduced sample size, helps to explain why the culling effect in `neighbouring areas' was found to be weaker.


  A17  As noted by King et al (2007), "the detrimental effect was not spread over all of the area [up to] 2km outside the removal area" although we wish to clarify that the analyses to which this comment refers considered distance from the boundary of the trial area, rather than the treatment area (Donnelly et al, 2007). Trial area boundaries were delineated mainly along property boundaries, so that herds could in principle be classified unambiguously as located inside or outside the trial area. Treatment areas, within which culling was conducted, were slightly larger than trial areas, and delineated according to the estimated boundaries of social group territories so that all badgers using farms inside the trial areas could be targeted (Bourne et al, 2007). As a consequence, some culling was conducted on land immediately adjoining trial areas, but outside their boundaries, and this almost certainly explains the (non-significant) beneficial effect of proactive culling among herds on land 0-0.5 km outside RBCT trial areas.

  A18  Our decision to analyse data from all land up to 2 km outside trial areas (except when it was within 2 km of more than one trial area), rather than treatment areas, was a deliberate one, taken to be conservative and to avoid any accusations that data were selected or excluded in order to obtain a particular result. However, the occurrence of apparently beneficial effects within 500m of the trial area boundary would be expected to offset, to some extent, the detrimental effects observed at greater distances and could therefore lead to under-estimation of detrimental effects on unculled land.

  A19  To provide further clarity on this point, we have repeated our analyses of the incidence of cattle TB on land up to 2km outside trial area boundaries, excluding herds occupying land outside the trial area but inside the treatment area. These analyses, which are presented in Table 1 below, reveal estimated detrimental effects slightly stronger than those reported in our published papers. These results confirm that our inclusion, in primary analyses, of herds outside the trial areas but inside the treatment areas was conservative and probably led to slightly under-estimation of detrimental effects on unculled land.

  A20  As discussed in paragraphs ISG13-ISG15 above, it is inappropriate to judge the overall detrimental effect of culling by considering confidence intervals for individual subgroups of data. Also, as noted in paragraphs ISG16 and A16 above, and in paragraphs A40-A44 below, we consider it appropriate to include all data, from the completion of the initial proactive cull, in primary analyses of the effects of culling.

Table 1

  Comparison of estimated detrimental effects of proactive badger culling on the incidence of cattle TB on farms up to 2km outside RBCT areas, when herds falling outside the trial areas but inside the treatment areas are included (as in our published analyses, Bourne et al, 2007; Donnelly et al, 2007) or excluded. Confidence intervals and p values are adjusted for overdispersion as described in our published work.

Including herds inside treatment area
Excluding herds inside treatment area
Time period
(95% CI)
(95% CI)

Using VetNet location data
First cull to one year after last cull
Second cull to one year after last cull
First cull to second cull
Using RBCT location data
First cull to one year after last cull
Second cull to one year after last cull
First cull to second cull


  A21  King et al (2007) found it "hard to compare data" from our report as "figures are presented as percentages rather than as absolute numbers of herd breakdowns". We are surprised at this concern. It is standard to report percentage differences in risks, and then for these to be translated into absolute numbers for specific considerations, such as cost benefit analyses. Indeed, we give specific calculations for 100 km2 areas in Bourne et al (2007) paragraph 5.39. These indicate that, over a five-year period of culling, 116 fewer confirmed breakdowns would have occurred within 10 circular 100 km2 areas, and 102 additional breakdowns would have occurred in the 10 associated 83.5 km2 neighbouring areas. This gives a net overall benefit of 14 fewer confirmed breakdowns (Donnelly et al, 2007). Detailed consideration of the costs and benefits of this scale of operation was presented in Chapter 9 of Bourne et al (2007).

  A22  King et al (2007) suggest that the potential role of cattle herd densities was not considered in our analyses. In fact, the inclusion of the (ln transformed) number of herds as an independent variable in our log-linear regression analyses allowed us to characterise the relationship between herd density and the incidence of cattle TB, since all trial areas were of similar size.

  A23  The full raw data on the numbers of herds, breakdowns and historic breakdowns were provided in supplementary data files published with both papers on the effects of proactive culling (Donnelly et al, 2007; Donnelly et al, 2006). While we did not specifically report herd densities (per km2, for example) in Bourne et al (2007), we did provide the number of cattle herds at baseline in Tables 5.1 and 5.7, and the sizes of trial areas are given in the peer-reviewed Supplementary Information of Donnelly et al (2006). Hence, relevant data have been publicly available since December 2005 (when Donnelly et al (2006) was published online), allowing interested parties to examine herd density effects in further detail had they so wished.


  A24  As noted in paragraphs A33-A35 below, King et al (2007) comment that "the overall beneficial effect on incidence of cattle TB will be maximised if steps are taken to minimise that detrimental effect" fails to take account of the absence of practical measures likely to minimise detrimental effects.

  A25  As noted in paragraph A7 above, we are concerned that it would be extremely difficult to obtain reliable estimates of the effects of badger culling on the incidence of cattle TB by "monitoring of these effects up to 2 km outside the removal area" if no comparable unculled comparison areas were available. Indeed, this is one of the reasons why Krebs et al (1997) recommended that a randomised controlled experiment be conducted to measure the impact of badger culling on cattle TB.


  A26  We consider the evidence for the reduction in badger density achieved through proactive culling to be considerably stronger than "an informed guess". Multiple indices of badger abundance gave similar estimates of the impact of proactive culling (Woodroffe et al, 2007). Several of these indices were based on densities of badger field signs; these were shown to correlate with the numbers of badgers captured per unit area on initial culls, indicating that they are likely to be reasonable measures of badger abundance (Woodroffe et al, 2007). Another index used, the density of road-killed badgers retrieved, is a more direct measure of badger abundance and gave similar results (Woodroffe et al, 2007). Improved methods for measuring badger density were developed while the RBCT was in progress (Frantz et al, 2004; Hounsome et al, 2005; Wilson et al, 2003) but were not available at the start of the study and so could not be used to monitor culling effects throughout the course of the study. Nevertheless we consider the close agreement between estimates based on different measures to indicate that the effects of culling on badger density were estimated reasonably reliably (Woodroffe et al, 2007).


  A27  King et al (2007) rightly note that clustering of M. bovis infection in badgers "was disrupted over the course of the trial and... the prevalence of infection in badgers... increased". Their report fails to note, however, that similar changes in the clustering of infection were also observed in cattle: in reactively culled areas, and on land neighbouring proactively culled areas, cattle infections became less clustered as successive badger culls were conducted (Jenkins et al, 2007). This change in the geographical distribution of cattle infections reflects expanded ranging behaviour by badgers in the same areas (Woodroffe et al, 2006a), contributing to the large body of evidence linking detrimental effects of culling to disruption of badger social organisation.


  A28  The conclusions reached in this paragraph are difficult to interpret since it is not clear whether they refer to badger populations inside the culling areas, to those immediately outside, or to both. In either case, the statement that "The ISG considered that the disruption of badgers and the increased ranging behaviour was a permanent effect... However, there is a reasonable possibility that the disruption is transient" is misleading in that it implies that the ISG gave no consideration to temporal trends. In fact, Woodroffe et al (2006b), Woodroffe et al (2007) and Pope et al (2007a) sought evidence for temporal trends in the effects on badgers, Donnelly et al (2007) and Bourne et al (2007) sought evidence for temporal trends in the beneficial and detrimental effects on cattle, and Jenkins et al (2007) evaluated temporal trends in the relationship between infections in the two host species. In particular, the Discussion section of Donnelly et al (2007) includes several paragraphs proposing an ecological mechanism that might explain the temporal trends observed in cattle.

  A29  The first investigation of any transience of the effects on cattle was not a "simple regression" as stated, but rather a detailed consideration of the stratified data presented in Figure 5.2A of Bourne et al (2007). Any modelling beyond the linear regression suggested by the pattern observed over the first four culls would require untestable assumptions and would thus be unlikely to give consistent results over the possible range of parameter values and model structures consistent with the known data. The evidence of the RBCT is clear over the first four annual culls.

  A30  While King et al (2007) are correct that "the data do not discount this theory" [that disruption of badger spatial organisation is transient] outside culling areas where data are limited, data from inside culling areas suggest that disruption was sustained. As successive proactive culls were conducted, an increasing proportion of badgers were captured close to culling area boundaries, indicating sustained badger immigration into culled areas with no evidence of this effect levelling off on later culls (Woodroffe et al, 2007). Genetic studies likewise show an increasing proportion of the badger population engaging in long-distance movements following successive culls, confirming sustained disruption of badger populations (Pope et al, 2007a). Finally, the prevalence of M. bovis infection among badgers rose, and the spatial distribution of infection became less clustered, on successive culls, once again with no evidence of lesser effects on later culls (Jenkins et al, 2007; Woodroffe et al, 2006b) despite declining badger density over the same time period (Woodroffe et al, 2007). All of this evidence indicates that "disruption of badgers" was sustained throughout the course of the RBCT, contrary to King et al's suggestion. In contrast, evidence cannot discount the hypothesis that detrimental effects for cattle may have declined on later proactive culls (Donnelly et al, 2007) as the suppression of badger densities was sustained. The reduction in spatial clustering of cattle infections likewise appears to have been greatest between the first and second culls (Jenkins et al, 2007).


  A31  As in paragraph 37 of King et al (2007), it is not clear whether the conclusions reached in this paragraph refer to badger populations inside, or immediately outside, the culling areas. If they refer to badgers inside the culling area then, as detailed above, the statement that "the likelihood of uninfected badgers being exposed to infectious badgers will... be reduced, ideally to a level at which TB cannot sustain itself within the badger population" is at odds with the available evidence, which has been published in peer-reviewed scientific journals. Woodroffe et al (2006b) showed that the prevalence of infection among badgers increased as successive proactive culls depressed badger density (Woodroffe et al, 2007). Moreover, the finding that these infections were also less spatially clustered on later culls (Jenkins et al, 2007) is consistent with the hypothesis that disruption of badger spatial organisation led to increased badger-to-badger transmission (Woodroffe et al, 2006a). While in principle it is to be expected that extremely low population densities would limit contact between badgers and, hence, badger-to-badger transmission of infection, there are no data to indicate the level of population reduction needed to achieve this in TB-affected areas of Britain. Moreover, there is evidence for widespread cattle-to-badger transmission in Britain (Jenkins et al, 2007; Woodroffe et al, 2006b), and this could well contribute to continued infection in very low-density badger populations despite low badger-to-badger transmission rates.

  A32  As noted in paragraphs A2 and A3 above, substantially lowering badger density by sustained, simultaneous, coordinated culling across very large areas could reduce the incidence of cattle TB inside culled areas, even though the remaining badgers might experience increased prevalence of infection. However, since the relationship between badger density and TB risk to cattle is strongly non-linear, culling in smaller areas, for shorter time periods, or in an uncoordinated manner will all seriously undermine any beneficial effects of culling and have the potential to generate detrimental effects.


  A33  All of the measures proposed in this paragraph were considered systematically in Chapter 10 of Bourne et al (2007), and found to be unworkable. For example, we noted that too few existing barriers to badger movement occur in southern and western Britain for these to be used to delineate culling zones over a meaningful proportion of TB-affected areas, and that the mitigating effect of such barriers on the incidence of cattle TB (as opposed to infection prevalence in badgers) was unproven. We also drew attention to the cost of constructing badger-proof electric fences (Poole et al, 2002), and the impracticality of fencing any but the smallest culling areas given the number of roads traversing the British countryside.

  A34  We wish to comment in particular on the suggestion of using "...soft boundaries such as arable land with no cattle..." to minimise detrimental effects on cattle TB. This sort of approach—which we considered in paragraph 10.22 of Bourne et al (2007)—would not prevent disruption of badger populations immediately outside culling areas (since badgers regularly occupy arable land). However, this measure could reduce the impact of such disruption on cattle by ensuring that few or no cattle inhabited the areas where disease transmission from badgers was most likely.

  A35  In our view, any such "soft boundaries" would have to be substantially wider than the 1 km proposed. We observed changes in badger ranging behaviour, and detrimental effects on cattle TB, up to 2 km outside RBCT trial area boundaries (Donnelly et al, 2006; Woodroffe et al, 2006a). Bait-marking studies can be used to derive conservative estimates of badger home range sizes inside proactively culled areas: the mean value was 0.77 km2, equivalent to a circle with a diameter of approximately 1km. Adding one standard deviation to this mean home range size gives an area of 1.34 km2, equivalent to a diameter of about 1.3 km. A 1 km-wide band would therefore be fairly small in relation to the scale of badger ranging, and would be regularly traversed by local badgers. We therefore considered somewhat wider bands in Bourne et al (2007). To illustrate the scale of effects, if culling were to be conducted within circular 100 km2 areas, each surrounded by a cattle-free buffer 2 km wide, each buffer would cover 83.5 km2. Buffers would be proportionally smaller (though absolutely larger) for larger culling areas, and would also be larger where culling areas were not circular. We concluded that the costs (in the broadest sense) of excluding cattle from such large areas of the British countryside would be likely to out-weigh the benefits (Bourne et al, 2007).


  A36  This paragraph contains a statement fundamental to King et al's (2007) conclusions, namely that our own "...view that this benefit [of proactive culling] was largely offset by the increase in incidence outside the removal area is unsound". We consider this statement to be inconsistent with the data available. As discussed elsewhere in this document, King et al's downplaying of the detrimental effects of culling appears to be based upon a number of misunderstandings including inappropriate interpretation of confidence limits (paragraphs ISG13-ISG15), exclusion of data from the initial time period which cannot be justified on the basis of any statistical bias toward overestimation (paragraphs ISG16 and A16) or time delay between performing badger culling and detecting its effects (paragraphs A40-A44), and failure to take full account of ecological data which offer a plausible and consistent explanation for both the detrimental and beneficial effects observed (paragraph ISG18). In particular, the increased incidence of cattle TB observed in herds up to 2 km outside proactively culled trial areas was a consistently observed phenomenon and, given its magnitude, largely offset the benefits of reduced TB incidence among cattle within proactively culled trial areas (Donnelly et al, 2007). This offsetting is clearly demonstrated for culling areas up to 300 km2 in Figure 5.4 of Bourne et al (2007).


  A37  As mentioned in paragraphs ISG13-ISG15 above, counting how many confidence intervals of stratified analyses include zero is statistically inappropriate.

  A38  The occurrence of some badger removal immediately outside RBCT trial areas, and the way in which data from these areas were included in analyses, should not have been "unclear" given the detailed descriptions of methodology provided in Bourne et al (2007). The "trial" and "treatment" areas are defined in paragraphs 2.11 and 2.12, and illustrated in Figure 2.1. Paragraphs 5.5 and 5.25 indicate that primary analyses concerned herds inside or outside trial (rather than treatment) areas and paragraphs 5.15 and 5.33 make explicit reference to distances from the "trial area boundary". Moreover, paragraph 5.33 states that "...herds... less than 0.5 km outside the trial area boundary appeared to experience a benefit... this... was unsurprising, because badger culling extended just beyond the boundaries of the trial areas to target social groups judged... to occupy home ranges falling partially inside the trial areas...".

  A39  To provide further clarity on this point, we have repeated our analyses of the incidence of cattle TB on land up to 2km outside trial area boundaries, excluding herds occupying land outside the trial area but inside the treatment area. These results are presented in paragraph A19 above.


  A40  King et al (2007) note that "it would be reasonable to expect... a time lag between removal of badgers and detection of changes in infection in cattle" but that "this time lag does not seem to have been taken into account when the ISG collected data on cattle TB incidence immediately after the first proactive removal".

  A41  Our reasons for including data between the first and second proactive culls in our primary analyses are detailed in paragraphs ISG16 and A16 above. Since we considered it likely that the effects of culling might change over time, our publications also presented estimates of effects from the date of completion of the second cull (Bourne et al, 2007; Donnelly et al, 2007; Donnelly et al, 2006) and explicitly investigated changes across different time periods (Bourne et al, 2007; Donnelly et al, 2007). The suggestion that a possible time lag "does not seem to have been taken into account" is therefore incorrect.

  A42  We discussed this possible time lag in detail in the peer-reviewed Supplementary Information of Donnelly et al (2006). Available data indicate that such time lags could be short. This is because (i) behavioural data show that local reductions in badger density affect ranging behaviour within a few days or weeks (Cheeseman et al, 1993; Roper & Lu­ps, 1993; Woodroffe, Macdonald & da Silva, 1995), allowing contact with additional cattle herds; and (ii) once infected, cattle become responsive to the tuberculin test after approximately three weeks (Thom et al, 2006). Hence, if badgers can infect susceptible cattle rapidly on contact, increased cattle incidence would be detectable two to three months after badger culling.

  A43  King et al (2007) state that naturally-acquired infections entail longer delays to skin test responsiveness than do experimental infections, but provide no data to support this assertion. We are not persuaded that time to responsiveness could be estimated for natural cases since infection dates would be unknown. Thom et al (2006) used infective doses of M. bovis that resulted in disease similar to that observed in naturally infected cattle, and we therefore consider their experimental findings the most reliable data currently available.

  A44  We also wish to note that this concern about time delays refers to our hypothesis about the mechanism whereby badger culling prompts detrimental effects in cattle, not to the existence of detrimental effects themselves. Since there is strong and highly consistent evidence that detrimental effects occur, and since these are costly for farmers, for the farming industry, and ultimately for the taxpayer, it is vital that they be taken into account in developing TB control policy.


  A45  As discussed in paragraphs ISG18-ISG19 and A14 above, we consider it unfortunate that King et al (2007) state that they were "not fully persuaded by" our explanation for detrimental effects of culling yet fail to cite any of our peer-reviewed papers which provide strong support for this hypothesis. As noted in paragraph A14, there is consensus within the scientific community that evidence in support of our hypothesis is "substantial" and "strong" (eg Royal Society, 2006; Shepherd, 2005).


  A46  As detailed in paragraphs A28-A30 above, the ISG gave explicit consideration to the possibility that the effects of culling might change over time.


  A47  King et al's (2007) statement that RBCT data should not be used " either support or rule out a reactive removal strategy..." contrasts with their earlier conclusion that "...the minimum overall area within which badger removal should take place is 100 km2" (the average area targeted by reactive culling was 8.8 km2).

  A48  King et al (2007) dismiss our findings regarding reactive culling, partly because this part of the RBCT was "stopped before robust results could be obtained". We note that the decision to halt reactive culling was taken by Defra ministers; the ISG had recommended that culling be continued while recognising that this might be difficult for Defra to justify (Bourne et al, 2005).

  A49  Despite this, the consistency of our original results (Donnelly et al, 2003) with the findings of subsequent analyses indicate that our conclusions concerning reactive culling are indeed "robust". Subsequent analyses provide (i) evidence that detrimental effects occur on land neighbouring proactive culling areas (Donnelly et al, 2006), making an overall detrimental effect predictable where culling areas are small and hence exceeded in extent by the areas of neighbouring land (Bourne et al, 2007); (ii) evidence that the detrimental effect of reactive culling disappeared following cessation of culling, indicating that the effect was not due to a systematic bias between trial areas unrelated to reactive culling (Bourne et al, 2007); (iii) evidence that herds located in close proximity to reactively culled land experienced elevated TB risk, even after controlling for the effect of contiguous breakdowns (Bourne et al, 2007); (iv) evidence that repeated reactive culling was associated with spatial spread of infection in cattle (Jenkins et al, 2007); and (v) evidence that repeated reactive culling, like proactive culling, was associated with elevated infection prevalence in badgers (Bourne et al, 2007). This information indicates that it is extremely unlikely that a future reactive culling strategy could contribute to the control of cattle TB, and would probably exacerbate disease spread. Given this evidence we consider it remarkable that King et al (2007) failed to "rule out a reactive removal strategy".


  A50  In a comment about the use and interpretation of confidence intervals, King et al (2007) comment that our inclusion of "decimal points... may give the impression of more certainty than is the case." While we agree that the individual values make sense only to fewer digits, our reason for giving more in this case had a scientific basis: it was to allow any reader wishing to make some additional calculations with the limits to do so without appreciable loss of information from rounding errors.


  We wish to thank former ISG research assistants Helen Jenkins and Tom Johnston for conducting the additional analyses presented in paragraph A19.


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