Discussion

This study provides the first detailed insight into how cervical dislocation of laboratory rodents is currently performed across UK research institutions. We found marked inconsistencies in both technique and the use of tools to aid application, with mice predominantly killed via manual operation (i.e. without the aid of tool), while rats were more often killed using mechanical aids. A wide range of improvised tools (i.e. not designed for the purpose of killing) were reported, including pens, scissors, and cage scrapers. Across all laboratory rodents, there was little or no consensus regarding which physical actions are essential for a successful dislocation (i.e. the necessity of a stretch and/or a twist), or in institutional standard operating procedures (SOPs).

One of the most striking results from this study was the divergence in approach between mice and rats. Most participants reported relying on manual operation solely for mice (manual 69.5% vs mechanical aid 25.9%), whereas mechanical tools dominated the killing of rats (manual 15.0% vs mechanical aid 67.5%). This division is not reflected in current legislation, where rodents (i.e. both rats and mice) are grouped together under Schedule 1 of the Animals (Scientific Procedures) Act 1986) (Home Office 1986). The Act permits cervical dislocation up to 500 g for rodents, but imposes additional requirements for rodents over 150 g, such as the use of anaesthesia or sedation. Although mice are unaffected by these weight thresholds due to their smaller size (with typical adult bodyweights rarely exceeding 40 g [Home Office 2014]), typical laboratory rats quickly exceed 150 g (Home Office 2014), meaning the use of a sedative or anaesthetic are often legally required when applied to adult rats. However, the Act does not distinguish between manual or mechanically aided cervical dislocation. This differs from agricultural legislative regulation for the protection of livestock. Under the Welfare of Animals at the Time of Killing (England) Regulations (WATOK) (UK Government 2015), cervical dislocation is permitted in poultry up to 5 kg, however the regulation distinguishes between manual and mechanically operated cervical dislocation, and manual operation is further restricted by a lower bodyweight limit (< 3 kg). Importantly, unlike in rodents, attention and scientific research into the effectiveness and humaneness of cervical dislocation of poultry (both manual and mechanical methods) has been subject to scientific interest since 2010, with limited research in other species. It is notable that the scientific basis and motivational drivers for weight restrictions for rodent cervical dislocation are unclear. It can be inferred that there may be potential welfare impacts or operational challenges when cervical dislocation is applied to conscious rodents weighing greater than 150 g, hence requiring the animal to be anaesthetised (i.e. fully unconscious) to negate potential negative animal welfare impacts. The use of sedation (i.e. drug-induced depression of consciousness, ranging from minimal to moderate (Blayney Reference Blayney2012)), could also be an attempt to alleviate potential animal welfare harms, but may also act as a method to overcome challenges in handling and restraining the animal to apply cervical dislocation effectively (i.e. inducing a relaxed more compliant state in the animal (Blayney Reference Blayney2012)). To our knowledge, there is no direct evidence directly comparing the ease and accuracy of cervical dislocation between species to justify the weight restrictions, in addition to the scientific evidence base on which these thresholds originated. Anecdotally, there are examples of practice guidance advising that rodents weighing greater than 150 g should not be subject to cervical dislocation due to heavier cervical musculature (e.g. Leary et al. Reference Leary, Underwood, Anthony, Cartner, Grandin, Greenacre, Gwaltney-Brant, McCrackin, Meyer, Miller, Shearer, Turner, Yanong, Johnson, Patterson-Kane, Pharmaceuticals, Ridge, Underwood, Anthony, Cartner, Grandin, Collins, Greenacre, Gwaltney-Brant, Network, McCrackin, Meyer, Miller, Shearer, State, Turner, Equine, Medicine, Yanong, Johnson, Division, Patterson-Kane, Scientist, Division, Niel, Weary, Hill, Woods, Saint-erne, Stoskopf, Hess, Marshall, Paul-murphy, Frick, Mays, Rhoades, Lenz, Shoemaker, Daniels, Deen, Gilliam, Griffin, Johnson, Kober, Pierdon, Plummer, Reynnells, Webster, Carbone, Flecknell, Friedman, Hickman, Pritchett-corning, Drew, Goldstein, Hartup, Lewbart and Mader2020). However, this information is not evidenced within the guidance itself, and refers to a further guidance document relating to wild bird research (Fair et al. Reference Fair, Paul and Jones1997) and there are no published scientific data showing increased cervical musculature as negatively affecting successful cervical dislocation in rats. Therefore it is possible that these legislative restrictions, may be inadvertently driving a greater dependence on improvised mechanical tools/aids for rats. In contrast, the predominance of manual techniques for mice could reflect both the absence of weight-related constraints and a perception that manual techniques are more immediate and reliable (Clarkson et al. Reference Clarkson, Martin and McKeegan2022). Importantly, our findings suggest that differences in method between rats and mice, may be driven as much by regulatory thresholds and institutional culture as by (limited) scientific evidence. There is a need to re-examine the rationale for existing weight limits and to establish whether they genuinely reflect differences in effectiveness or welfare outcomes. Without such evidence, there is a risk that regulatory frameworks unintentionally promote the use of unvalidated mechanical methods, particularly in rats. This would potentially be to the detriment of animal welfare (e.g. risk of crushing/asphyxiation and/or unnecessary suffering) reported for a range of methods/techniques available for poultry (Sparrey et al. Reference Sparrey, Sandercock, Sparks and Sandilands2014).

Fundamentally, cervical dislocation, whether applied manually or with the use of a mechanical aid, aims to cause intervertebral cervical separation of the vertebral column, causing extensive damage to the spinal cord and surrounding tissues, whilst disrupting blood flow to the brain (ideally through damage/severance of the carotid arteries in the neck) resulting in death via cerebral ischaemia (Cartner et al. Reference Cartner, Barlow and Ness2007; Suckow et al. Reference Suckow, Stevens and Wilson2012; Clarkson et al. Reference Clarkson, Martin and McKeegan2022). Whilst this mechanism for inducing death via cervical dislocation is broadly agreed across the scientific community, the required actions (i.e. physical manipulation of the neck etc.) are far from established. As there is little or no scientific literature focusing on these actions in rodents, evidence from other species could provide valuable insight. In poultry, there is general agreement that cervical dislocation should involve a rapid stretch and twist action, resulting in more extensive damage to the neck, spinal cord and surrounding tissues and rapid disruption of blood flow to the brain, leading to a faster onset of unconsciousness and death (Mikeska & Klemm Reference Mikeska and Klemm1975; Gregory & Wotton Reference Gregory and Wotton1990; Cartner et al. Reference Cartner, Barlow and Ness2007; Carbone Reference Carbone2011; Kongara et al. Reference Kongara, McIlhone, Kells and Johnson2014). In comparison, cervical dislocation which only involved one component (i.e. stretch or twist alone), resulted in less extensive trauma, extended onset to unconsciousness and is therefore considered less humane with animal welfare outcomes similarly poor to those observed for decapitation (Mikeska & Klemm Reference Mikeska and Klemm1975; Gregory & Wotton Reference Gregory and Wotton1986; van Rijn et al. Reference van Rijn, Krijnen, Menting-Hermeling and Coenen2011; Martin et al. Reference Martin, Sandilands, Sparrey, Baker, Dixon and McKeegan2019). As highlighted by the results of this survey, there is a broad range in techniques and tools/aids used to cause cervical dislocation in both rats and mice (see Figures 3–8). For both rats and mice, in the case of mechanical cervical dislocation, over 50% of respondents expressed a technical need to apply “a forceful push downwards at the cervical region of the spine”, irrespective of the tool/aid used. Based on evidence from other species (e.g. Gregory & Wotton Reference Gregory and Wotton1990; Holson Reference Holson1992; Close et al. Reference Close, Banister, Baumans, Bernoth, Bromage, Bunyan, Erhardt, Flecknell, Gregory, Hackbarth, Morton and Warwick1997; Bates Reference Bates2010; Erasmus et al. Reference Erasmus, Turner, Nykamp and Widowski2010, Martin et al. Reference Martin, McKeegan, Sparrey and Sandilands2016), this poses a potential animal welfare concern, as such an action alone could be considered more akin to crushing the neck and more likely to cause prolonged suffering through delayed onset of unconsciosuness as well as death via asphyxia. As such, any technique considered to primarily cause crushing injury to induce cervical dislocation is prohibited for poultry in agricultural settings in both Great Britain and Europe (European Council [EC] 2009; UK Government 2015), highlighting the greater risk of suffering if applied to conscious animals, including rodents. In contrast, neither regulation governing the killing of animals used for scientific purposes in Great Britain or Europe refer to crushing risks, nor define the mode of action for cervical dislocation in any species for which cervical dislocation is permitted (Home Office 1986; European Parliament and the Council of the European Union 2010). This survey, along with other recent communications (Davidge et al. Reference Davidge, Bulat and Vernet2025; Ferreira Reference Ferreira2025), highlights the requirement for scientific exploration relevant to laboratory rodents to encourage potential reform to existing guidance and legislation.

A striking finding of this survey was the widespread reliance on improvised tools for mechanical cervical dislocation in both mice and rats. Respondents listed items such as pens, scissors, metal rods, cage scrapers and even spanners — none of which have been designed, validated, or standardised for the purpose of killing animals. This survey did not explore the reasons for specific improvised tools being used or selected. This widespread reliance may indicate attempts to refine killing practices at a local level. To our knowledge, there are currently no commercially available, purpose-designed tools for rodent cervical dislocation which have been scientifically validated as ‘humane’, despite the method being both common and widely permitted under legislation (Clarkson et al. Reference Clarkson, Martin and McKeegan2022). The use of improvised tools, such as those identified in this survey, raises serious potential welfare concerns. Firstly, these items are not engineered to apply force in a controlled or consistent manner, meaning that the risk of inaccurate dislocation (e.g. location of dislocation, incomplete/partial dislocation, crushing injury etc) is substantial (Cartner et al. Reference Cartner, Barlow and Ness2007; Carbone et al. Reference Carbone, Carbone, Yi, Bauer, Lindstrom, Parker, Austin, Seo, Gandhi and Wilkerson2012). Additionally, as these tools were not designed with cervical dislocation in mind, the tools themselves may compromise execution of the technique, for example, pens may break during use, scissors may slip, and blunt rods or scrapers may induce crush injuries rather than dislocation. This increases the likelihood of thoracic as opposed to cervical separation, which has been documented in previous studies (Cartner et al. Reference Cartner, Barlow and Ness2007; Carbone et al. Reference Carbone, Carbone, Yi, Bauer, Lindstrom, Parker, Austin, Seo, Gandhi and Wilkerson2012) and is likely to prolong the time to unconsciousness and death (as denoted in poultry; Martin et al. Reference Martin, Sandilands, Sparrey, Baker, Dixon and McKeegan2019). The shape and size of tools used to apply cervical dislocation is likely to be a critical component of their efficacy and humaneness. Relying on scientific data from poultry as none exists for rodents, the size of the ‘edge’ employed to push down on the neck to provide the force required to induce intervertebral dislocations, is essential, and variability in tool performance was linked to age/size of birds, as well as operator technique (Erasmus et al. Reference Erasmus, Turner, Nykamp and Widowski2010; Martin et al. Reference Martin, McKeegan, Sparrey and Sandilands2017, Reference Martin, Sandercock, Sandilands, Sparrey, Baker, Sparks and McKeegan2018a,Reference Martin, Sandilands, Sparrey, Baker and McKeeganb; Woolcott et al. Reference Woolcott, Torrey, Turner, Chalmers, Levison, Schwean-Lardner and Widowski2018; Bandara et al. Reference Bandara, Torrey, Turner, zur Linden, Schwean-Lardner and Widowski2023; Ripplinger et al. Reference Ripplinger, Crespo, Pullin, Carnaccini, Nelson, Trindade, Reichelt and Pairis-Garcia2024). Given the optimal application of cervical dislocation requires intervertebral dislocation at either the atlanto-occipital joint (C0–C1) or the atlanto-axial joint (C1–C2) (Carbone et al. Reference Carbone, Carbone, Yi, Bauer, Lindstrom, Parker, Austin, Seo, Gandhi and Wilkerson2012), being able to target such locations to apply the tool is essential. In mice, the cervical vertebrae are < 1 mm in diameter, typically with the area from the base of the skull to C3 being < 2.5 mm, therefore, any ‘edge’ with a diameter of greater than 1 mm is unlikely to result in intervertebral cervical dislocations above vertebra C3 whilst avoiding unnecessary crushing injury (Babak Kalantar Reference Babak Kalantar2013; Smorgick & Fischgrund Reference Smorgick and Fischgrund2013; Bader et al. Reference Bader, Meyer-Kühling, Günther, Breithaupt, Rautenschlein and Gruber2014; Martin et al. Reference Martin, McKeegan, Sparrey and Sandilands2017, Reference Martin, Sandilands, Sparrey, Baker and McKeegan2018b; Bandara et al. Reference Bandara, Torrey, Turner, zur Linden, Schwean-Lardner and Widowski2023; Nkurikiyumukiza et al. Reference Nkurikiyumukiza, Buteera and El-Sharkawi2024). Secondly, manual and many of the mechanical techniques using these improvised tools may rely on additional actions, such as tail pulling, introducing further failure risks, as well as the potential for unnecessary injuries to the animal (e.g. direct injuries to the tail from pulling (Machholz et al. Reference Machholz, Mulder, Ruiz, Corning and Pritchett-Corning2012; Gencturk & Unal Reference Gencturk and Unal2024; Davidge et al. Reference Davidge, Bulat and Vernet2025)).

This survey also revealed a lack of consensus regarding which additional physical actions are necessary for a successful cervical dislocation. Manual operators typically emphasised a strong push forward or downward, with some citing a lateral twist, whereas operators who utilised a tool/aid (i.e. mechanical) more often stressed a downward push combined with pulling the tail. These divergent views suggest that current practice is highly driven at the institutional level, relying on ‘local processes’ and personal belief/preferences rather than empirical evidence. This was further supported in this survey through highlighting the inconsistent availability and content of SOPs both within and between UK institutions. For mice, over one in five respondents did not know whether an SOP existed at their institution, while for rats, nearly 30% stated that no SOP was in place. These findings suggest a lack of institutional clarity and standardisation, despite cervical dislocation being one of the most common killing methods used in the UK (Clarkson et al. Reference Clarkson, Leach, McKeegan and Martin2023). Previous studies have shown that dislocation frequently occurs at thoracic rather than cervical levels in mice (Cartner et al. Reference Cartner, Barlow and Ness2007; Carbone Reference Carbone2011), but they did not describe the cervical dislocation technique in sufficient detail and/or specific additional actions used. It therefore remains unknown which techniques most reliably achieve high cervical dislocation and rapid unconsciousness in mice.

Our findings highlight a critical knowledge gap: systematic, controlled studies are needed to determine which techniques reliably achieve humane outcomes, and whether different approaches carry risks of unnecessary injury, such as tail damage or thoracic trauma and/or dislocation. The reliance on improvised tools highlights an important gap in knowledge in the required physical actions for humane and effective dislocation in rodents (e.g. stretch and/or twist), needed to inform tool design. Without purpose-built, validated devices/tools, research institutions are potentially placing both personnel and animals in a vulnerable position and inadvertently not aligning with the legal requirement to minimise unnecessary pain, distress and suffering (Home Office 1986). From an ethical standpoint, it is questionable whether it is acceptable for research institutions to sanction the use of equipment not designed for killing, particularly when there are serious potential animal welfare consequences. This finding underscores the urgent need for research into the design, validation, and implementation of dedicated cervical dislocation tools for rodents that are both effective and ergonomic for the user.

Finally, as a self-report survey, this study captures perceptions rather than direct observations of practice. Some questions were answered by individuals who may not routinely perform cervical dislocation on rats, potentially impacting the summarised perceptions reported. In addition, responses were anonymous, meaning that representativeness of the UK sector could not be verified. However, anonymity was essential to encourage responder engagement, given the subject matter is sensitive in nature and the use of animals for scientific purposes is under constant scrutiny from both the public and media. Nevertheless, the survey achieved a large sample size (n = 317) across the UK, across diverse roles and institutions, providing valuable insights into sector-wide practices and uncertainties.