Why are box jellyfish stings sometimes fatal?

Box jellyfish stings can be fatal due to the highly potent venom delivered by nematocysts—specialized capsules containing barbed threads that inject toxins into the victim upon contact.123

In highly toxic species like the Australian box jellyfish (Chironex fleckeri), the venom acts rapidly, potentially causing extreme pain, paralysis, and cardiac arrest within minutes. While the exact microbiological mechanism remains a subject of scientific study, researchers have proposed several theories regarding how these protein-based toxins affect the body:12

• Cellular destruction: Some evidence suggests that proteins called porins rip cells open, releasing potassium and leading to heart failure.1
• Direct cardiac impact: Other studies indicate the venom may directly attack heart muscle.1
• Cholesterol interaction: Some researchers have found that the venom interacts with cholesterol to enter and destroy cells.1

Additionally, some stings can trigger Irukandji syndrome, which may lead to severe muscle cramps, vomiting, and heart failure. The severity of the reaction is largely determined by the amount of skin surface area that comes into contact with the tentacles.1