Introduction

The term asphyxia covers a number of conditions that result in interference with uptake and utilisation of oxygen and elimination of carbon dioxide.

Asphyxia can be caused by:

• Lack of oxygen in the inspired air

• Obstruction of the air passages

• Restriction of chest movements

• Histotoxic anoxia.

Words that are commonly used in asphyxia cases include suffocation, smothering, strangulation, choking, throttling, hanging and crush asphyxia.

General features

Asphyxiation may occur under a variety of circumstances ranging from non-accidental injury in the form of hanging or smothering, through poisoning with cyanide, to death during unsuitable transportation. Additionally, accidental asphyxiation arises in house fires and in farmyard incidents such as postural asphyxiation where a bull, for example, falls in a handling crush and the trachea is occluded as the neck is twisted to the side or under the body. The final cause of death in all of these incidents is considered to be cerebral hypoxia followed by cardiac arrest.

The time taken for unconsciousness to supervene and for death to occur is frequently difficult to assess. Sudden, complete and simultaneous occlusion of both carotids will lead to unconsciousness in a number of seconds. However, this is an unusual event and in most cases of asphyxiation unconsciousness is preceded by a period of intense distress as the animal fights for its life. Struggling during asphyxiation of a conscious animal may lead to bruising on face, legs, hips and spine that should not be confused with other causes of trauma.

Strangulation

Strangulation may be deliberate or accidental. Canine victims of deliberate strangulation may be hanged from a tree branch (Fig. 9.1), a loft hatch or other convenient high point. The dog’s lead, rope, dressing-gown cord, electric flex or wire may be used. Accidental strangulation can occur when a tethered dog jumps over, or on to, a kennel roof or platform and becomes trapped with the tether tight around the neck. In horses and ponies, accidental strangulation by a neck rope or head collar can result from poor animal husbandry practices. Similarly, non-elasticated collars can contribute to strangulation of domestic cats.

Fig. 9.1 Two puppies hanged from a tree by their leads.

(By kind permission of the Royal Society for the Prevention of Cruelty to Animals.)

Externally there may be limited evidence of ligature marks in fatal strangulation. The hair and looseness of the skin around the neck can protect against skin abrasion (Case study 9.1). However, careful examination may reveal areas where there is indentation of hair (Fig. 9.2) or patchy loss of hair or reddening or mild bruising of the skin (Fig. 9.3). Cyanosis may affect the teats, perineum and vulva. Focal intradermal haemorrhages can develop in the skin of the posterior abdomen and immediately cranial to the ventral vulva in dogs that have been hanged. There may be more generalised congestion of the skin of the ventral neck, thorax or abdomen.

Case study 9.1

Fire fighters attended a fire in an apartment. The blaze, which was centred on the bedroom, was extinguished and the remainder of the apartment was searched for victims or other signs of fire. In the kitchen, a middle-aged female dog, ‘Taffy’, was found lying in her bed. She was dead.

The circumstances of the fire raised suspicions of arson and the police requested that the body of the dog be subjected to post-mortem examination to confirm the cause of death, suspected to be smoke inhalation.

The dog was in excellent bodily condition, was clearly well looked after and showed no signs of natural disease. A single subcutaneous bruise, measuring 45 × 30 mm was present on the back of the neck. The lungs, windpipe, larynx, eyes and subcutaneous tissues were extensively congested and evidence of smoke inhalation was absent. This combination of findings suggested that Taffy had died as a result of asphyxiation by ligature rather than smoke inhalation.

During interview by the police, the boyfriend of the dog’s owner admitted hanging Taffy by the cord of a dressing gown from the loft hatch before setting the apartment alight.

Fig. 9.2 The lighter coloured puppy shown in Figure 9.1 has indentation of the hair around the neck (arrows) but no abrasions of skin or neck bruising.

Fig. 9.3 The darker puppy shown in Figure 9.1 had mild bruising and reddening of the skin on the ventral surface and both sides of the neck. It was necessary to clip the hair to appreciate this change.

Subcutaneous bruising may be present over the larynx, at the angle of the jaw (Fig. 9.4) or where the knot in the ligature has tightened against the neck in animals that have been hanged by being dropped from a height. These animals may also show epiglottic congestion, fresh ecchymotic haemorrhages over the laryngeal cartilages or in the glosso-epiglottic fold and congestion of the sclera of one or both eyes. Fracture of the hyoid apparatus may occur in mature animals with mineralised hyoid bones (Fig. 9.5). There appears to be limited published information on the age of mineralisation of the hyoid bones¹ in the various species and breeds, and radiographic examination of the neck in alleged strangulation cases is recommended before the commencement of the necropsy.