Introduction

Traps of various designs have been used since humankind learned to hunt. They form an important part of the arsenal of methods employed, worldwide, to control the so-called vermin species, to catch fur-bearing animals and to trap animals for food or the live animal trade. Although trap design has evolved in parallel in different parts of the world, the basic concepts and principles are universal: ensnaring, foot- or leg-hold devices, cages or nets.

Traps and snares may, and often do, injure the victim and in so doing may cause characteristic lesions that are valuable during investigations of illegal killings or other offences relating to trapping. Specific types of damage and, indeed, the absence of certain injuries can provide clues to the events preceding the death or release of the captured animal.

Spring traps

Spring traps have spring-loaded jaws that are released when pressure is applied to a trigger or plate in the centre of the trap (Fig. 11.1). Some of these traps, known as ‘body grip’ traps and ‘humane spring traps’, are designed to kill by closing on the body of small creatures such as rats, squirrels, stoats, weasels, mink and rabbits (Fig. 11.2).

Fig. 11.1 Spring trap in the ‘set’ position with the jaws (arrows) open. The flat metal plate in the centre is the trigger that ‘springs’ the jaws.

Fig. 11.2 Weasel caught around the chest by a body-grip spring trap.

Regulations vary around the world as to which traps may be used, and on their placement, to ensure that ‘non-target’ species (e.g. domestic cats) are not caught accidentally. The restrictions may, for example, require that traps are set in natural or artificial tunnels, that they are checked at specific periods (e.g. every 24 hours) and that any trapped animals are removed. The forensic veterinarian may be asked to determine whether the animal has been in the trap for longer than the maximum allowable period.

The jaws of the trap crush the chest and/or spine resulting in death. Animals caught in these traps show extensive internal injuries including fractured ribs, haemothorax, pulmonary laceration, ruptured liver and fracture of the thoracolumbar spine. The indentation marks of the jaws may be clearly seen across the body.

Use of spring traps for birds of prey

Body-grip spring traps designed for use on small animals may be misused to capture birds of prey. Baited traps are set in the open, in contravention of the regulations, and birds of prey trigger the trap by alighting on the trigger plate. The jaws close on the leg or legs of the bird, trapping, but not killing, the bird. Unable to extricate itself from the trap, the bird eventually dies from shock related to the severity of the leg injuries, dehydration and exhaustion. Any live bird in the trap may ultimately be killed by a blow from a stick or by dislocation of the neck (Fig. 11.3). It is, therefore, good practice to subject any ‘found dead’ bird of prey to a thorough post-mortem examination to determine the cause of death.

Fig. 11.3 Goshawk. Radiography showing separation of cervical vertebrae from head following manual ‘pulling’ of the neck.

Leg lesions in birds caught in spring traps consist of abrasion and tearing of the skin, haemorrhage and fracture (frequently compound) of the tibiotarsal bone (Fig. 11.4). Some birds alight with one leg more extended than the other (e.g. buzzards). Those birds can suffer similar injuries to both legs but the lesions in the more extended leg are higher on that leg than those in the partially flexed leg.

Fig. 11.4 Buzzard. Compound fracture of the tibiotarsal bone caused by spring trap.

In an attempt to mask or hide evidence that might indicate the bird has been caught in a spring trap, the jaws of the trap may be padded to lessen obvious injury to the legs. However, these padded jaws still exert considerable pressure and fracture of the tibiotarsal bone may occur. Additionally, the foot of the trapped leg may become considerably swollen and discoloured if the bird remains in the trap from some hours (Fig. 11.5).

Fig. 11.5 Buzzard. Fracture of the lower limb and marked swelling of the foot following prolonged entrapment in a spring trap.

Spring traps attached and set on the top of a fence-post in an open area with no other perching sites are referred to as ‘pole’ traps. These traps are unselective and trap one or both legs of the alighting bird. Once gripped around the leg(s) and unable to fly away, the exhausted bird may fall from the post to hang suspended by the trapped leg(s). In such cases, the legs may show the injuries described above and haemorrhage may be found in the muscles of the thigh and around the hip joint. Additionally, the head and neck may exhibit considerable congestion. Struggling may cause damage to feathers and bruising over bony prominences.

Leg- and foot-hold traps

These traps, like the body-grip traps, have a trigger and jaws to grip the prey. As the name suggests, leg-hold traps are designed to catch the leg or foot rather than the body. They are used for larger species such as wolves, foxes, coyotes, bobcats, racoons and feral cats.

Considerable efforts have been directed towards studying^1–6 and improving trap design to minimise injury, and some countries and states have taken steps to ban the use of the most injurious and cruel traps.

Soft tissue lesions include oedema (with or without haemorrhage) and laceration of skin and muscle (Fig. 11.6). Muscle injury may be accompanied by damage to tendons and ligaments or dislocation of elbow, hock and toe joints. Over-exertion, caused by attempts to escape from a protracted period of entrapment, can lead to degenerative changes in the muscles (exertional rhabdomyolysis).

Fig. 11.6 Racoon dog. Laceration of the skin and muscle of the forelimb caused by the teeth of a steel-jawed leg-hold trap.

Radiography is an essential part of the examination of animals when there is suspicion of trapping. Fractures may be found in the toes and foot or the long bones further up the limb. These fractures may be simple or compound (Fig. 11.7). Padded traps cause significantly less injury to the target species than steel-jawed traps, but may still result in serious injuries (e.g. limb fractures) to smaller non-target species.

Fig. 11.7 Racoon dog. Laceration and compound fractures of the digits caused by the teeth of a steel-jawed leg-hold trap.

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