Since road transport is a complex operation made up of several components including handling, loading, unloading, separation from a familiar environment, isolation, confinement, loss of balance, extremes of temperature and humidity, exposure to noxious gases, and deprivation of food and water, it is difficult to judge which of these components is responsible for the behavioural and physiological changes that previous studies have found (Cregier, 1987; Richardson, 2002).
Transport in general is a stressor in horses that can cause tachycardia (Clark et al., 1993; Waran and Cuddeford, 1995), changes in plasma ascorbic acid (Baucus et al., 1990) and serum cortisol concentrations (Stull et al., 2004). A satisfactory transportation environment for a horse provides for thermal comfort (Stull, 1997, 1999; Stull and Rodiek, 2002), physical comfort, minimal disease or maximum health, and behavioural needs. The previous transportation experience and health of the animal, driving conditions and length of the route (Kiley-Worthington, 1991; Ruiz-de-la-Torre et al., 2001), standard of handling/driving (Cregier, 1987; Henderson, 1995; Stull, 1997), transport design, orientation in relation to the direction of travel (Cregier, 1987; Clarke et al., 1993; Smith et al., 1994a,b; Waran et al., 1996; Stull, 1997; Houpt, 1998; Broom et al., 2002), cleanliness, stocking density, environmental factors, and the behaviour of travelling companions can be a potential source of stress during transport (Leadon, 1994; Stull, 1997).
Although it appears that the whole process of transportation can be associated with physiological and biochemical changes in the horse, very little is known about what causes these (Waran and Cuddeford, 1995). There is, relative to reports of the effects of transport on other animals, a shortage of data that relate to the effect of transport on horses. These data are essential for the better guidance of horse owners and trainers, for practicing equine clinicians, and for national and international regulatory authorities (Leadon, 1994).
The first step to minimising stress during transportation is to be able to identify the signs and symptoms of stress in the horse, including observations on appropriate or abnormal behaviour, indicators of sickness, and injuries. Once a stressor is identified, its rapid elimination will assist in termination of the stress response. Proper management of the horse to minimise stress during transport includes space allocation, waste management, sanitation, preventative health programmes, training, and nutrition; all factors controllable by owners and hauliers (Stull, 1997).
Isolation Stress/Separation Anxiety
Herd species are highly social animals and typically exhibit behavioural and physiological stress responses when separated from a familiar environment and/or visually isolated from herd mates, show strong responses to other group members, and fear unfamiliar situations and sounds (Broom et al., 2002), as has been documented for cattle (Hopster and Blokhuis, 1994; Boissy and Le Neindre, 1997; Piller et al., 1999), sheep (Parrott et al., 1988; Parrott, 1990; Baldock and Sibly, 1990; Abdel-Rahman, 2000), red deer (Price et al., 1993) goats (Richardson, 2002) and horses (Kiley-Worthington, 1991; Mal et al., 1991; Jezierski and Gorecka, 1999, 2000; Broom et al., 2002; Strand et al, 2002).
Separation-induced manifestations of stress are usually quantified in terms of heart rate, ACTH, cortisol and β-endorphin responses or behavioural anomalies such as struggling and large increases in vocalisation (Baldock and Sibly, 1990; Boissy and Le Neindre, 1997; Abdel-Rahman, 2000; Broom et al., 2002).