More than enough poison to kill an adult was found in murdered Courtney Pieters' blood, the Western Cape High Court heard on Wednesday.
Chief forensic analyst Jacobus van Zyl was the State's first expert witness in the trial against Mortimer Saunders.
Saunders faces charges of premeditated murder and rape, but denies that the toddler's death was planned or that he had sex with her while she was alive.
In his plea explanation, he confessed to the May 2017 murder and to using his fingers to penetrate her after her death.
Saunders said he had given Courtney ant poison to make her sick, before he choked and beat her and used a towel to cover her mouth.
He claimed he had done it because of "ill feelings" between himself and the child's mother, Juanita.
Dosage could have been higher
Saunders – a childhood friend of Courtney's father who lived in the same Elsies River house as the family – said he had also been irritated because the toddler had wanted to watch TV in his room when he wanted to sleep.
Van Zyl analysed samples of the 3-year-old's stomach, kidney, liver, bile and blood, which confirmed the presence of a pesticide.
According to his findings, 0.08mg/litre of carbaryl was found in her blood.
The dosage, however, could have been higher at the time of death, as Van Zyl pointed out that her body was only found nine days after she died.
Depending on the dosage, the chemical could affect the flow of oxygen to the heart, slowing down circulation, and also cause vomiting, tear duct secretions, excessive saliva, tremors, lack of muscle control, paralysis of the diaphragm, slurred speech, convulsions, cyanosis, coma and death.
In adults, 0.5mg/litre is considered toxic, while 0.6mg/litre is lethal.
No confirmation poison was only cause of death
Van Zyl testified he was surprised to find pesticide in the toddler's blood.
"It tells me the person was exposed to a high dose. The person died soon," he said.
Most pesticides metabolise very quickly, but when you die, metabolism stops, Van Zyl explained.
Symptoms could show within 20 minutes to an hour in severe cases, while in mild cases symptoms would take between four and 12 hours to reflect.
While an adult could recover on their own, a child would require urgent medical attention as the main effect the poison had was on breathing, he said.
"The longer you wait, the symptoms get worse."
During cross-examination by Saunders' lawyer advocate Mornay Calitz, Van Zyl said he couldn't confirm if the poison was the only cause of Pieters' death as he had not performed her autopsy.
The trial continues on Thursday.