Signs of Snake Bite

Whilst the nature and proportion of the relevant venom toxins (pro- and anti-coagulants, myotoxins and pre- and post-synaptic neurotoxins) will determine the range of effects of Australian snake bites, muscular paralysis is the dominant clinical sign in all serious and fatal cases. The paralysis is usually of the ascending type, first affecting the hindquarters and then progressing to complete paralysis, with death due to respiratory failure.

Although paralysis clearly aids the diagnosis, it is the earlier signs which are especially valuable. For example, in dogs and cats, dilated pupils with absent or reduced pupiliary light reflexes appear to be an early and most important (although variable) sign that significant envenomation has occurred. Only in severely envenomated cases may paralysis occasionally be seen before pupillary dilation. The occurrence of pre-paralytic signs almost invariably indicates the patient has received a lethal dose of venom.

The severity of illness will vary considerably from animal to animal and snake bite can produce diverse clinical signs (see table). Any or all of the following signs may be observed: trembling, vomiting, salivation, defecation, ataxia, dilated pupils, slow or absent pupillary light reflexes, respiratory distress, shallow or abdominal respiration, hindquarter weakness, pallor, jaundice, haemoglobinuria (or myoglobinuria), proteinurea, haematuria, haematemesis, continued bleeding from fang marks or other wounds, and flaccid paralysis progressing to coma or respiratory failure.

The onset of signs may be sudden. The animal may collapse dramatically within a few minutes of the bite, then appear to recover completely over the next thirty minutes and then deteriorate with evidence of systemic poisoning. This sudden collapse in dogs may be due to cor pulmonale and inadequate left ventricular filling caused by transient acute obstruction of the pulmonary circulation and indicates severe envenomation. This effect occurs with brown and tiger snake envenomation and possibly with taipan evnenomation.

Other animals may become ill within minutes of the bite and progressively worsen over the next few hours. In only about 30 per cent of cases can the bite site be found on the animal and there is usually little swelling in the area involved. Sometimes the bite site is found because it is bleeding.

Due to the variation in their clinical features (associated within different size and sensitivity to venom) there are separate pages for dogs, cats, horses and ruminants.

The main early clinical signs in dogs and cats suffering from snake bite

All dogs presented for treatment within one hour of the bite, but the average time for the cats was twenty-one hours. This is consistent with the findings that onset of symptoms of envenomation in dogs (1-6 hours) is faster than the onset for cats (15 hours).

(References: Hill & Campbell 1978 Aust. Vet. J. 54: 437-439; Hill 1979 Aust. Vet. J. , 55: 82-85; Best 1998 Clinical Toxicology, Postgraduate Foundation, pp.231-248)

Contributed by R. Brooks. Last updated: January 2008