Several cyclists, including riders due to start the Giro d’Italia on Friday, fell ill after a Belgian one-day race, with cow manure on the roads suspected to be the cause.

Three Lotto-Intermarché riders suffered from abdominal pain, diarrhoea, fever and vomiting, and were briefly taken to hospital, the team said from Bulgaria, where the Giro begins on Friday.

Arnaud De Lie, the winner of the Famenne Ardenne Classic on Sunday and who is expected to be the Belgian team’s leader at the Giro, initially showed no symptoms before feeling nauseous during the flight to Bulgaria.

“He’s not feeling well, but his participation in the Giro is not compromised at this stage,” the team said of De Lie. Only five of Lotto’s eight riders were able to attend the race presentation on Wednesday.

According to Belgian broadcaster Sporza, other teams such as Alpecin have also been affected, while Lotto’s sporting director Maxime Bouet said: “Half the peloton is ill.”

Lotto said the riders may have been contaminated by cow manure on the Ardennes course, with wet roads causing excrement to be splashed on to riders.

Although the exact cause has not yet been confirmed, campylobacter – a type of bacteria responsible for gastrointestinal infections – is suspected to be the source.

In Brussels, local authorities said vandals damaged a commemorative stone slab made in honour of Belgian cycling great Eddy Merckx. The monument depicts the face of five-time Tour de France winner Merckx, now 80 and still one of Belgium’s most beloved sporting figures.

“This Wednesday morning, residents were shocked to discover that the monument in tribute to Eddy Merckx, located in Bouvreuils Square, had been vandalised,” the mayor of Woluwe-Saint-Pierre, Benoît Cerexhe, said in a statement, adding that he was “outraged”.

“Who could want to attack an athlete, a symbol of our country? Nothing justifies such an act.”

An official in the mayor’s office said CCTV footage in the area would be analysed to try to identify the perpetrators and the monument – inaugurated in 2019 when the Tour de France began in the city – would be repaired without delay.