Above all, once the horse arrives after a long journey take its temperature twice a day for three consecutive days. That figure can be very much higher when journeys have gone wrong as they so often do. So yes, the longer the journey the greater hazard. This may occur because of environmental conditions, technical failure, traffic, or any one of a multitude of factors that can prolong a journey. When this happens the risk of problems is increased.
Shipping fever can exist in mild, moderate or severe forms.
In the mild form it will probably resolve without treatment but the moderate and very certainly the severe forms are life threatening emergencies. Everyone who ships horses needs to be aware of the need to take temperatures twice a day after arrival and call the vet quickly once they recognise that the condition has occurred. In the last few stud seasons here in Australia, the number of Northern Hemisphere stallions coming out for the stud season has increased dramatically. The problems of transporting high value horses over long distances is exactly the same as the problems associated with the transport of horses of any calibre or any sex or any usage over the same distances and that is primarily, shipping fever. Myself and Professor Twink Allen of the University of Cambridge are collaborating on a study of the effects of transport on horses transported long distance for dual hemisphere shuttle stallion purposes.