A fourth horse trained by Danny O’Brien has returned an elevated Cobalt reading, reports Racing Victoria.

Racing Victoria (RV) Stewards have informed O’Brien that the urine sample taken fromBullpit after winning the Jeep 55 Second Challenge Heat over 955m at Moonee Valley on December 19 last year contained Cobalt at a concentration above the threshold.

The sample was tested at the Chem Centre in Western Australia and was then confirmed by the Hong Kong Jockey Club Laboratory. Racing Analytical Services Limited (RASL) confirmed the positive swab on Tuesday.

O’Brien is already under investigation by RV Stewards after his gallopers Bondeiger,Caravan Rolls On and De Little Engine returned elevated Cobalt readings above the threshold during last year’s Spring Carnival.

Cobalt is an essential mineral nutrient which affects multiple body systems and is toxic at high dosages. Racing Victoria imposed the threshold of 200 micrograms per litre in urine on April 14, 2014 as a local rule, which was then made a nationwide Rule of Racing by the Australian Racing Board on January 1, this year.

Fellow Victorian based trainers Peter Moody, Mark Kavanagh and the training partnership of Lee and Shannon Hope are also under investigation for elevated Cobalt readings above the threshold.