This article is from the WSSF 2015 AFRMA Rat & Mouse Tales news-magazine.
Foam rubber toys or materials are not recommended to use around rats. A pet rat owner had given their two 4-month-old female pets a foam rubber cat ball toy which they of course tore up. Two months later one of the rats was sluggish with a bloated belly that had developed in 24 hours. Exploratory surgery found she had a fluid-filled stomach with a 1 cm x 0.5 cm piece of foam causing an obstruction in her duodenum. They thought the foam was sitting in the stomach for a month or more before moving into the intestines. Just before surgery the rat had green fluid around her mouth. After she was anesthetized, a large amount of green/black fluid came out of her nose and mouth which they think she aspirated and caused her to die.
From Debbie Ducummon’s The Rat Report #153 February 2005.