More than 1000 runners who competed in an gruelling endurance race in France have fallen ill from animal faeces thought to be in the mud.

A record field of more than 8000 entrants celebrated at the finish line of the Côte d’Azur in Nice last week.

However, the festivities were shortlived for some 30 participants who reported symptoms of diarrhoea, vomiting and fever the following day.

In the days since, that figure has risen to more than 1000 people.

The mystery illness, while still that, is thought to have been caused by harmful bacteria in the mud, with a number of people noting a smell of horse manure at various points in the race.

According to the event's website, there has been no hospitalisations to date.

A participant, pictured left, enjoys a cold beer following the race in France last week. (Source: Twitter/TheMudDay)

The celebrations were shortlived for many. (Source: Twitter/TheMudDay)