Children who experience severe seizures caused by fevers are more likely to suffer epilepsy as adults, researchers have found.
Children can experience seizures caused by fever
US scientists say the infant seizures produce a cannabis-like substance in the brain.
This changes the chemical make-up of the developing brain in a way that increases the risk of developing epilepsy as an adult.
But UK experts said most children who had febrile convulsions would not be affected.
Fever-induced, or febrile seizures, affect three to five per cent of infants and young children.
Researchers from the University of California Irvine College of Medicine studied prolonged febrile seizures in rats so they could understand what was happening in human brains.
They found that the seizures increased the number of receptors in the brain specially primed to detect endocannabinoids, a natural cannabis-like substance which helps transmit specific messages between nerve cells.
The increase was seen in the part of the brain involved in memory which is also the part damaged in adult epilepsy.
One seizure was enough to cause a permanent increase in the number and activity levels of these receptors.
But the seizures also reduce the amount of a second chemical, which controls transmission.
This imbalance means the messaging system is out of control, which the researchers say can which can trigger seizures.
Ivan Soltesz, professor of anatomy and neurology at Irvine, who led the research, said: "These findings show that prolonged seizures in childhood can have highly specific consequences in certain sensitive areas of the brain."
He added: "The significance of these findings is two-fold.
"First, they provide a mechanistic explanation as to why febrile, or fever-induced, seizures in childhood lead to an increased susceptibility to seizures that lasts until adulthood.
"Second, they suggest a new drug that interferes with this occurrence during childhood may significantly decrease the likelihood of future epileptic seizures.
"The components of these new drugs will likely be man-made chemicals distantly related to marijuana."
A spokeswoman for the National Society for Epilepsy told BBC News Online: "Febrile convulsions are quite common and for the majority of people the prognosis will be fine."
She added: "As long as the seizures are not prolonged, they wouldn't necessarily predispose towards epilepsy.
"But if they were, they could cause damage which could then cause problems."
The research is published in the journal Neuron.