The Modern Myth or Are You A Man Or A Mouse?

Transcription

1 by Brian Bush Australia's Venomous Snakes: The Modern Myth or Are You A Man Or A Mouse? Author's Note: When concerning reptiles, the terms poisonous and venomous require redefining. Poison is toxic if inhaled or ingested. The only poisonous reptiles known in Australia are the Green Turtle and the Hawksbill Turtle. More people have probably died on this earth as a result of consuming turtle flesh than ever has been the case from snakebite in Australia. There are no reported deaths from turtle poisoning here since Venom is generally harmless if ingested but toxic when it comes in contact with the underlying tissue normally protected by the skin. Venomous animals have to break the skin with body-spines, stings or teeth (includes fangs in spiders and centipedes). The only venomous reptiles found in Australia are snakes. For too long we have been telling the world, as well as each other, that Australia's snakes are the most venomous. On what evidence do we make this assumption? Solely on their ability to kill mice! What a joke! The basis for the following discussion and points of view are personal experience, comments heard and questions asked during my time lecturing on venomous animals as well as a perusal of the relevant literature. I might just mention here that I have never lectured to mice! I have heard it said many times by people I believe should know better that this or that Australian snake is "more deadly", "more toxic" or "more venomous" than the Indian Cobra (Naja naja). This is a misrepresentation of the facts and gains little support from the evidence available concerning humans. These types of statements have no place in education if positive results are the goal. All venomous snakes have the potential to be dangerous because of the variable sensitivity between individual people to venoms. However, first a bite has to occur. Australian snakes are inoffensive and very shy. They have had only forty thousand years of human predation to contend with. None of their behaviour has evolved targeting humans. The traditional Aboriginal people only harvested the "quite" nonvenomous snakes, rarely were the "cheeky" venomous species taken. Maybe this is why it is almost impossible to accidentally get bitten by one unless you run around the scrub in bare feet with your eyes shut, and half full on alcohol. On other continents humans have existed for hundreds of thousands of years. Therefore, has the cobra evolved the hood, elevated stance and, in some species, the ability to "spit" venom as a direct response to human predation? What about the snakes that play dead? The Ringhals (Hemachatus haemachatus), one of a few "cobras" capable of spitting, also plays dead. This behaviour would have little effect in deterring a predator looking for a feed, but it may deter a passing human from taking up a club and clobbering the snake into lifeless pulp. There is a widespread misconception that our snakes are very dangerous animals. Many of us do little to dispel this belief, maybe because we like to believe that Australia's snakes are the most dangerous. This is definitely the modern myth! In the past 35 odd years I have met many people who believe tiger snakes have chased them. However, I have yet to meet someone that has been caught by one! I often publicly comment that Australia's snakes are the deadliest in the world if you are a mouse. This is an objective statement supported by the available data. Anyone perusing the literature will have seen reference to the LD 50 using mice to measure relative toxicities of various snake venoms. This is great data if you are a veterinarian specialising in the treatment of snake-bitten mice! I believe it has little relevance when gauging the danger of a particular species to humans. The LD 50 is scientifically indefensible in this regard. The snake currently ranking number one on that list has not been the cause of death to a human in any documented case history that I have seen. No doubt, Australia's medical scientists can take some credit for this. However, the Russell's Viper (Daboia russelli) found from Pakistan to China and Indonesia; the "lance-headed" group of pit-vipers (Bothrops spp.) of South America, the "saw-scaled" vipers (Echis spp.) of northern Africa, the Middle East, India and Sri Lanka are believed responsible for up to 1

2 50,000 deaths each year (Swaroop & Grab, 1954) and do not rank a mention. Maybe there has been no comparison made yet, or maybe they have venoms that are more dangerous to humans than to mice. Number 24 on that list (Broad et al., 1979) is the Olive Whip Snake (Demansia olivacea). I believe it would have Buckley's chance of killing any human except maybe a decrepit old snake handler hypersensitive after years of continual exposure to venoms. All the LD 50 does is compare the different venoms' toxicities on mice and is, therefore, biased towards mammal-specific feeders. To be more relevant each specific venom must have the LD 50 criterion applied using a variety of animals such as fish, frogs and birds. Even then, the data collected would not reflect the toxicities of the respective venoms on humans. Mouse tissue is 50 times less responsive to Sydney Funnelweb Spider (Atrax robustus) venom than tissue from humans or monkeys (Underhill, 1988: 170). Kellaway (1934: 678) found that adult mice were unaffected by A. robustus venom. A worker could mistakenly judge this spider to be harmless when examining the LD 50 toxicity of its venom on mice! Remember also that Australia's snakes rarely envenom when biting defensively. Envenomation occurs in less than 1 in 10 bites (Sutherland & King, 1991: 1), or is it that a minority of people bitten experience systematic envenomation due to a hypersensitive reaction to the venom? Many of Australia's venomous snakes such as Pseudonaja spp. do not even wait for their venom to immobilise prey. Instead, using constriction to restrain it, they often swallow it alive. I am of the opinion its primary purpose is digestion. I can't help wondering, why has there been this preoccupation with the emphasis on exaggerating the danger of Australia's venomous snakes? Could it be that many of the researchers involved are government funded. They would have a vested interest in obtaining results that would most suit a favourable decision on continued funding. It is probable that more people die each year in Australia from horse riding related accidents than snakebite. We do not go around hitting horses on the head with a shovel! Those educators specialising in reptiles must definitely do all in their power to improve the snake's image. Rather than promote the negatives, emphasise the positives! Figure 2 is from Sutherland (1992 & 1994), media reports and other sources to It presents the causes of snakebite believed to have resulted in 30 deaths in the past 18 years in Australia. An increased awareness of snakes through education and appropriate footwear could have reduced this by 16. A further reduction of 13.3% may have been possible if it was illegal to kill snakes. Four of the above victims may have still been alive today. The fact is that while the relevant wildlife authorities allow the killing of snakes they are directly contributing to the frequency of snakebite and therefore fatalities from this. Both professional and amateur herpetologists must encourage a more positive attitude towards Australian snakes. This means less emphasis on the irrelevant laboratory tests of venom toxicity. Those in a position of authority perpetuate many myths unknowingly. For example, some diving instructors continue to incorrectly refer to the "small mouth and rear fangs" in sea snakes. They often comment that sea snakes can only bite between the fingers or on the ear lobes. This is far from the truth (see Limpus, 1987 Pg 198). Zimmerman (1988) relates an experience where a Stokes Sea Snake (Astrotia stokesii) bites both a camera's strobe arm and a diving flipper thrust towards it. However, there are no documented fatalities in Australia from sea snake bite. I once had an article refereed and returned with the comment that "... Australia's elapids have grooved fangs". Along most of the fang's length this "groove" is a closed seam, only being opened towards the tip where it does not detract significantly from the fang's function or effectiveness. Many people make an erroneous distinction between tiger snakes' (Notechis spp.) fangs and Taipans' (Oxyuranus scutellatus), saying the former differ by being grooved. The only distinction is the length, in both species they are effectively hollow. On another occasion and another referee a problem with semantics arose in reference to the words "dangerously venomous". The comment came back. "...delete dangerously, you already have venomous". These terms are far from synonymous! We strive for objectivity but when dealing with snakes, subjectivity commonly comes to the fore. Many believe the widespread fear of these animals is a result of teaching but, after working with aborigines, I think that in many of us our fear is instinctive. On top of this we have the added burden of the way snakes have been 2

3 portrayed historically. Moses, in the Book of Genesis, symbolised evil as a serpent. As snakes were the bad guys already he wasn't treading on any toes. The mythical Medusa and Cleopatra do little to improve the image. To increase people's awareness and improve the snake's image in Australia much more positive information must be available in any educational session. Maybe this will encourage people's want to conserve them. I am of the opinion that changes to the relevant Policies and Acts allowing the killing of snakes need to occur to place more responsibility on the person to justify the killing. The respective wildlife authorities have only limited resources for the education needed to bring about this type of change. However, both amateur and professional herps, with our altruism are ready to go. Tread carefully though, do not fall into the trap of perpetuating the myth just because someone else did. Ponder On This One of a government's responsibilities is to protect its people. In allowing the killing of venomous snakes it contributes towards snakebite resulting in injury and death. Therefore it may be negligent in its responsibility by continuing to endorse open seasons or similar. The LD 50 Described The LD 50 is a standard laboratory test with set guidelines laid down by the World Health Organisation. With the toxicity test in snakes, individual mice within a sample receive equivalent quantities of venom (each mouse is of the same weight). Each sample receives less venom than the previous. The LD 50 is the amount of venom administered to each mouse to cause 50% of the sample to die. The lower the LD 50 the higher the toxicity is on mice!. For the limitations and problems with this type of test see White (1987). The Answers To Some Common Questions Australia's most venomous (yield) snake is the King Brown (Pseudechis australis). Believed involved in very few fatalities. The most toxic snake venom on mice (of the species tested) is the Inland Taipan (Oxyuranus microlepidotus). See Figure 3. Australia's deadliest snakes are the brownsnakes (Pseudonaja spp.). Believed involved in 18 of the past 28 deaths attributed to snakebite. See Figure 4. The world's deadliest snake, based on documented deaths, is probably the Saw-scaled Viper (Echis carinatus) especially in Sri Lanka. The deaths of nearly fifty people per million from snakebite occur there each year. Today in Australia we have 0.13/million deaths each year. See Figure 1. The most misrepresented data by the Australian herpetological community is the list of snakes showing their lethality on mice compiled by Broad et al (1979). Often presented as if it directly relates to humans. It does not. Acknowledgment I thank Pat Garland for his critical appraisal and constructive comments on this article, much of which comes from the heart not the head. References BROAD, A.J., SUTHERLAND, S.K. & COULTER, A.R The lethality in mice of dangerous Australian and other snake venom. Toxicon 17: KELLAWAY, C.H A note on the venom of the Sydney Funnelweb Spider Atrax robustus Med. J. Aust.. 1: LIMPUS, C Sea Snakes. Pp In Toxic Plants & Animals, a Guide for Australia.. (Eds. J. Covacevich, P. Davie & J. Pearn) Queensland Museum. MUNRO, D Chambers World Gazetteer. 5th Edit. Chambers & Cambridge, International. PARRISH, H.M Analysis of 460 fatalities from venomous animals in the United States. Amer. J. Med. Sc. (Feb). SPAWLS, S. & BRANCH, B The Dangerous Snakes of Africa. SUTHERLAND, S.K Deaths from snake bite in Australia Med. J. Aust. Vol. 157: SUTHERLAND, S.K Snakebite deaths in Australia and a management update. Med. J. Aust. 3

4 Vol. 163: SUTHERLAND, S.K. & KING, K Management of snake-bite injuries. RFDSA Monograph Series 1: SWAROOP, S. & GRAB, B Snake bite mortality in the world. Bull. of World Health Organisation 10: TRINCA, J.C The treatment of snake bite. Med. J. Aust. 1: UNDERHILL, D Australia's Dangerous Creatures. Readers Digest, Sydney. 368pp. WHITE, J Elapid Snakes: Venom Toxicities and Actions. Pp In Toxic Plants & Animals, a Guide for Australia. (Eds. J. Covacevich, P. Davie & J. Pearn) Queensland Museum. ZIMMERMAN, K.D The question of sea snake aggression. Herpetofauna 18(2): 11. Venom toxicity is only one of many contributing factors determining how dangerous a particular species is. Australia's snakes pale to almost insignificance when compared to the fatalities caused by their counterparts in other parts of the world. The above figures are very conservative for the developing countries sampled, eg, West Africa has recently been found to have 23,000 deaths each year with less than 10% of victims being treated in modern medical facilities (Spawls & Branch, 1995). Many victims probably die in the bush and are not included in any sample. The 23,000 figure may be very conservative! Data for USA from Parrish (1963); Africa, India and South America from Swaroop & Grab (1954); and Australia from Trinca (1963). Populations are from Munro (1988). Accident - unavoidable without exceptional awareness. Herp - herpetologist bitten while handling snake. Kill - bitten while killing snake. Mis ID - bitten while handling venomous snake believing it to be harmless. Trod - bitten after treading on snake.? - insufficient data to determine cause. LETHALITY IN MICE From Broad, Sutherland & Coulter (1979) (In descending order including LD 50 in mg/kg. In first figure venom diluted in saline solution only, 2nd includes the addition of a blood protein, bovine serum albumin) Inland Taipan (Oxyuranus microlepidotus) Eastern Brown Snake (Pseudonaja textilis) Northern Taipan (Oxyuranus scutellatus) Eastern Mainland Tiger Snake (Notechis s. scutatus) Reevesby Island Tiger Snake (Notechis ater niger) Beaked Sea Snake (Enhydrina schistosa) Western Mainland Tiger Snake (Notechis scutatus occidentalis) Chappell Island Tiger Snake (Notechis ater serventyi) Southern Death Adder (Acanthophis antarcticus) Copperhead (Austrelaps superbus) Indian Cobra (Naja naja) Black Mamba (Dendroaspis polylepis) 4

5 Dugite (Pseudonaja a. affinis) Papuan Black Snake (Pseudechis papuanus) Stephens's Banded Snake (Hoplocephalus stephensii) Rough-scaled Snake (Tropidechis carinatus) King Cobra (Ophiophagus hannah) Spotted Black Snake (Pseudechis guttatus) Collett's Snake (Pseudechis colletti) 2.38 King Brown Snake (Pseudechis australis) Red-bellied Black Snake (Pseudechis porphyriacus) Small-eyed Snake (Cryptophis nigrescens) 2.67 Olive Whip Snake (Demansia olivacea) Eastern Diamond-backed Rattlesnake (Crotalus adamanteus) Reprinted from the 1995 Monitor 7 (1): (updated Dec. '97) FIRST AID for SNAKEBITE 1. Apply broad firm pressure bandage over bite and as much of limb as possible. After covering bite continue up the limb and, if sufficient bandage, back down again. 2. Immobilise with splint. On leg splint in straight position. On forearm splint to elbow and support arm in sling. 3. Keep victim still. Bring transport to victim and take to nearest hospital. Note: Do not wash venom from bite site! Medical staff may be able to identify antivenom required from a swab of this. For bites on trunk of body or face apply local pressure only with flat of hand. 5