The redback spider is the main spider in Australia that can cause signs of envenomation in dogs and cats. There is considerable species variation in susceptibility to envenomation. The guinea pig, horse and camel are very susceptible while the dog is relatively resistant to the effects of the venom. It is seldom lethal in dogs unless a considerable number of bites are inflicted, as would most likely only occur under experimental conditions. The susceptibility of the cat probably lies between that of guinea pigs and dogs.
The clinical signs in animals are primarily variable disturbances of the autonomic nervous system and muscular paresis. There is initial pain reaction at the bite site which might cause chewing, followed by restlessness with abnormal posture due to pain, muscle tremor and patchy paralysis, tachycardia, hypertension, hyperaesthesia, hyperaemia of the skin and weight loss.
The dominant or additional features in pets which have been noted are as follows:
Dogs
- Gastro-intestinal disturbances (vomiting, diarrhoea)
- Generalised tremors or clonic contractions of skeletal muscle
- Rigidity of abdominal wall, irregular breathing due to muscular contractions of thorax and abdomen
- Malaise, paresis, prostration
- Recovery in previously healthy dogs
Cats
- Hyperexcitability, salivation
- Muscle fasiculation, mucoid ropey or frothy saliva with sporadic protrusion of the tongue and ataxia
However, diagnosis in animals is very difficult unless the bite has been witnessed and the culprit found and identified. No specific investigation aids the diagnosis and there are no methods to detect the venom.
Redback venom acts specifically at nerve endings to cause depletion from synaptic vesicles at motor nerve endings, the depletion of transmitter from "purinergic" nerves and release of catecholamines from adrenergic nerve endings. This gives rise to the sometimes striking sympathetic effects such as the vasoconstriction which results in hypertension.
Use of antivenom during the early acute stages of envenomation will effectively and quickly abolish all effects. Results may be less dramatic if used after the paralysis stage or during the convalescent stage. Animals may respond without antivenom but this can take weeks so the rapid easing of their distress with antivenom should be considered.
Redback antivenom is probably not normally required as a life saving measure in dogs, but its use may be considered in cases where redback envenomation is suspected in order to relieve the immediate discomfort or where a pre-existing condition, such as cardiac dysfunction, is present.
It is reported that cats, being more susceptible to redback venom, do receive antivenom from veterinarians and respond dramatically in minutes with significant lessening of signs.
(References: Brown, R.A. 1980 Control & Therapy No. 988; Wilson, P. 1996 Control & Therapy No. 3882)
Contributed by R. Brooks. Last updated: January 2008
