Description:

Bites and stings from animals, venomous and non-venomous, cause an unknown number of injuries per year, but statistics from a few key groups of venomous animals indicate that there are millions of cases annually, with at least 125,000 deaths. While in most cases of venomous animal injury, the primary problem is direct venom toxicity effects, there may also be significant local tissue injury and non-venomous animals will principally cause direct trauma. Because of the global magnitude of human injury, morbidity and mortality from venomous animal bites and stings, this area will be dealt with in some detail, even though it encompasses more than just primary physical trauma. Venomous animals Venomous animals are found in most groups or classes of the Animal Kingdom and in most habitats, both terrestrial and marine, reflecting the selective advantage venom may bestow, in both acquiring prey and deterring predators. In this section of the chapter, the types of animals, the types of venoms, clinical effects and general comments on management will be covered. This is followed by a more detailed look at individual types of animals and their effects on humans. Overview of venomous animals Of the approximately 26 Phyla of animals, at least 6 contain species that use venom or internal poison, as either pure defense, or both for offence and defense (Figure 1). A few groups, however, account for the vast majority of cases of human envenoming or poisoning by animals: • Venomous snakes – >125,000 deaths/year. • Scorpions – approximately 5,000 deaths/year. • Stinging insects – hundreds of deaths/year due to anaphylactic reactions to venom. • Puffer fish – several hundred deaths/year. • Jellyfish – possibly scores of deaths/year. • Spiders – perhaps 10-50 deaths/year. • Stinging fish – perhaps 1-10 deaths/year. • Venomous molluscs – perhaps 1-10 deaths/year. Taxonomy considerations Fundamental to the understanding of trauma from venomous animals is a knowledge of the taxonomy of these animals, for without a reliable way of identifying an animal, it will not be possible to accurately record cause and effect, essential in elucidating epidemiology, etiology, pathophysiology, and clinical management. It is beyond the scope of this chapter to detail the taxonomy of all venomous animals. A simplified scheme is outlined in Figure 1. Overview of venoms Venoms are nearly always complex mixtures of varied biologically active substances (toxins) which may work independently or synergystically and each of which may have one or more quite distinct target sites and actions. In many venoms a single component or group of closely related components may be responsible for most or all major effects in envenomed humans, but in other venoms, particularly snake venoms, a multitude of diverse components may each cause distinct major effects, resulting in a complex, multisystem disease process. Venoms have evolved principally because they benefit the venomous animal, giving it some competitive advantage over related non-venomous species. In many species the venom has evolved from digestive juices, especially enzymes, the venom gland being a highly evolved digestive gland. It is not surprising therefore that many venomous animals use their venom principally to aid digestion, explaining many of the rather unpleasant effects on envenomed humans. At some point in evolution some venomous animals have evolved venom with rather different effects, designed to assist acquiring prey or as a defense against predators. It is this latter group of venom toxins that often cause the major systemic effects of envenoming in humans. Often these toxins may have evolved from digestive enzymes, but their principal action is not related to enzymic activity. Indeed, there may be no significant residual enzymic activity, despite considerable sequence homology with the original enzyme. Classic examples of this are t