The 50-year-old infectious diseases medic — who has not been publicly identified — was repatriated on Tuesday from Sierra Leone with a fever and given an experimental drug to try to combat the often-deadly virus.
But since Friday there has been a "progressive aggravation" of the man's condition, the Lazzaro Spallanzani Institute treating him said in a statement.
"He has started to have gastro-intestinal problems (nausea, vomiting and diarrhoea). He has a high fever, currently over 39 degrees Celsius," it said.
The doctor — who became the first Italian to be infected with Ebola while working for an Italian medical association fighting the epidemic in Africa — was not showing signs of bleeding at this stage, however.
Ebola, a disease transmitted through the bodily fluids of infected people, can result in death from uncontrollable bleeding and organ failure. The current Ebola epidemic in west Africa — the worst ever recorded — has so far killed around 5,700 people.