Swimming pools have been placed on alert in Andalucia as a vile social media trend is once again making the rounds in Spain.

The so-called ‘brown challenge’ involves deliberately defecating in a swimming pool and sharing or encouraging the act online as a prank.

While some participants treat it as a joke, experts warn it can force pools to close, trigger lengthy disinfection procedures and expose swimmers to serious health risks.

The prank caused widespread disruption last summer, when more than 300 swimming pools across Spain were affected.

One of the most high-profile cases occurred at the municipal pool in Tanos, Cantabria, which was forced to close for the second time in a week because of the viral trend.

A sports club in Sopela, in the Basque Country, also temporarily shut its pool after a similar incident.

The challenge first gained attention in 2018 but has resurfaced almost every summer since, with its latest return causing concern among pool managers, lifeguards and local authorities.

Health experts stress that the issue goes far beyond inconvenience. Human faeces can contain millions of microorganisms, including bacteria, viruses and parasites capable of causing illness.

Dr Manuel Viso previously warned that children are particularly vulnerable because they often swallow small amounts of pool water while swimming.

People with cuts or open wounds may also be at greater risk of infection if the water becomes contaminated.

Contrary to popular belief, chlorine does not instantly eliminate faecal contamination. Pools must usually be closed while operators follow strict cleaning and disinfection protocols before they can safely reopen.

Authorities say the consequences are not only financial, with closures disrupting facilities during the busiest weeks of the year, but could also have legal implications if public health is put at risk.