Geoff Isbister, University of Newcastle
The idea that Australia is home to many of the most deadly snakes in the world is based on animal research from the 1970s that looked at the effect of 25 venoms on mice. While not entirely untrue, the claim is also not quite right.
A more accurate statement might be that Australian snakes are the best mouse killers in the world: they’re able to kill the most mice with the smallest amount of venom. While that’s clearly bad news for mice, how does it translate into human risk?
The occurrence and severity of a snake bite depends on a complex interaction between snake behaviour, venom toxicity and human behaviour. Significant factors include how toxic the venom is; how much of it is injected by the snake; and how humans encounter and interact with snakes.
Australian snakes have very toxic venoms but inject tiny amounts at a time because most have short fangs. The only evidence of a brown snake bite may be a small scratch, for instance, but the venom is so toxic that it quickly results in the person’s blood failing to clot, which puts them at risk of bleeding to death.
Mulga snakes (King Brown) can deliver larger amounts of venom, but have one of the less toxic venoms of dangerous Australian snakes.
Historically, tiger snakes and death adders were responsible for most deaths. They’re widely distributed throughout Australia and their bites cause paralysis.
Before the advent of modern intensive care, paralysis was – more often than not – fatal. But with the development of antivenom in the 1930s and 1950s, and machines that can breathe for people, paralysis from snakebite has become uncommon.
Taipans also cause paralysis, but are a rare cause of snakebite in Australia (in contrast to Papua New Guinea where they cause much havoc).
In modern times, brown snake bites have become more common and now cause the majority of such deaths in Australia. This group of snakes appears to have thrived despite human invasion and the destruction of natural habitats. Brown snakes are now the most common cause of severe snake envenoming in Australia, according to the Australian Snakebite Project.
They cause the majority of the one to five deaths from snakebites each year, usually from early collapse and cardiac arrest. Unfortunately, antivenom is unlikely to help these people because cardiac arrest happens within 30 minutes of the bite. Early basic life support from bystanders is most important for snake bites because this can keep someone alive until they’re transported to hospital.
Severe snake envenoming is actually quite rare in Australia, with only about 100 cases each year. After brown snakes, red-bellied black snake bites are the next most common, but they rarely cause severe envenoming and occur only in eastern Australia.
Tiger snakes, which continue to account for a significant number of bites in southern Australia, are one of three snakes found in Tasmania and account for almost all serious snake bites in Victoria. They cause all three major types of toxicity: coagulopathy (making a person’s blood unable to clot), neurotoxicity (paralysis) and myotoxicity (muscle damage).
Snake bites are treated with antivenom, which needs to be given as soon as possible after a bite to be effective. The Australian Snakebite Project has demonstrated that only one vial of antivenom is required to treat all cases of snake envenoming.
But many of the effects of snake envenoming are irreversible in the short term (muscle damage, for instance, and paralysis), so antivenom won’t help for these. Instead, treatment in intensive care will support the patient while the body repairs. This is why antivenom needs to be given early.
Using antivenom comes with the risk of an allergic reaction, so it’s important that only people with envenoming be treated. Recent research measuring snake venom enzymes in blood appears to identify envenoming early. It is hoped that development of bedside testing of these enzymes will improve early recognition.
Although the effects of venom are reasonably well understood, why they cause severe toxicity in humans remains unclear. After all, we are not prey for snakes; small reptiles (such as skinks) or small mammals (such as marsupial rats) are their primary targets.
The toxicity we see in humans, such as venom’s clotting effects that commonly occur with brown snake, tiger snake and taipan bites, is most likely a chance occurrence. This idea is supported by recent research that shows many animals, including rodents and skinks, are highly resistant to the clotting effect of snake venom. But they’re highly susceptible to the neurotoxic effects of snake venoms.
In most other parts of the world, vipers, which have much larger fangs, are much more common. They inject ten or more times as much venom as Australian snakes, but have less toxic venoms. The other major difference is that vipers can cause local skin and tissue damage and, in some cases, this can lead to amputation. Unlike the human impact of Australian snakes, viper envenoming is a huge public health issue worldwide.
This article is part of our series Deadly Australia. Stay tuned for more pieces on the topic in the coming days.
Geoff Isbister, Director, Clinical Toxicology Research Group, University of Newcastle
This article was originally published on The Conversation. Read the original article.