We often talk about the LD50 when discussing venom toxicity. This is a brief explanation of the test, and the reasons why it can’t always be used as an indicator…..
The LD50 is a standard laboratory test recognised by the World Health Organisation, and invloves injecting a certain amount of 20g mice with a certain dosage of a particular snake species’ venom. The mouse LD50 is a figure indicated by the dosage that will kill 50% of the mice injected within a 24 hour period.
These figures are then used to indicate the toxicity value of each particular snake venom.
Therein lies the first of the many flaws with this test.
Based on this, it is clear that these figures cannot be accurate when trying to estimate how lethal a particular bite would be to a human. We are after all not mice!
It is also a known fact that snake venoms vary within a single species. Factors that can influence snake venom are:
- Prey species
In order to truly obtain an accurate comparison between species one would need snakes of the same age,size, gender, and obtain these species from the same locality…Highly unlikely!
Another flaw is the average venom yield. All yields obtained for this test are achieved by milking the snake for its venom. This does not indicate that a snake will inject all the venom in a single bite.
We also know that venomous snakes may bite without injecting any venom. This is known as a “dry” bite. Based on that it is clear that snakes are able to regulate the amount of venom they inject.
The venom yield from a defensive bite from a Black Mamba (Dendroaspis polylepis) taken by surprise in the early morning of a cold winters day, and the venom yield from an angry Mamba that has been cornered, threatened, and is just going into the “blue” in the middle of summer at 14:00.
Which scenario do you think would yield the most venom?
Another flaw, and this is only my opinion, is the use of mice to determine the LD50 of sea snakes.
Quite often you will see the Beaked sea snake (Enhydrina schistosa) listed as the most venomous…What percentage of its diet consists of mice?
Different types of venom and varying toxicity within similar types of venom, have evolved to ensure the survival of any particular species. The toxicity of venom would be better assessed based on the resistance of the target prey.
This test has in the past been dubbed as “Crude and unscientific”
A bit harsh perhaps, but consider this….
Adult mice were unaffected when injected with the venom of the Sydney Funnelweb Spider (Atrax robusuts).
Bear in mind that bites from this spider have resulted in several human fatalities. However, based on the LD50 this spider should be regarded as nothing more than a harmless itsy-bitsy spider.
Jokes aside, the test does not, or rather cannot, cover all variables. It does however provide a baseline comparison between venoms, and does have some merit.
Why should it be important anyway?
A choice of death by Black Mamba or Inland Taipan?
Neither, Thank you.