An suspected attempt to kill 30 show horses at a well-known stable yard in San Diego, USA by poisoning with oleander leaves has left 3 horses in hospital and 20 more sick.

Bill and Debbie Tomin, owners of Rockridge Farm in Rancho Santa Fe, discovered highly toxic oleander leaves mixed in with sliced apples and carrots when they went to feed the horses at 6am on Thursday 30 July.

Assistant trainer, Theo Robinson, said: “When we opened the stall doors, I knew exactly what it was.”

One horse was already down and others were sickening fast. Oleander causes cardiac arrest in horses if enough is eaten.

A distraught Mrs Tomin told local press: “Someone broke in and tried to kill all our horses. Who would want to do something like this? This was totally planned and is totally sick. They fed them one of the most toxic things a horse can get.”

All 30 horses at the yard are American saddlebred show horses worth over £1.2m. Five are world champions and two are scheduled to compete in the World’s Championship Horse Show in Louisville, Kentucky in three weeks time.

The three horses rushed to San Luis Rey Equine Hospital in Bonsall were given intravenous treatments and the other horses, including two pregnant mares, were fed mineral oil and charcoal to absorb any toxins. They are all under close observation.

It’s believed someone entered the yard by climbing over a large gate between 2-3am on Thursday and planted the mixture without waking sleeping staff on site.

Bill Tomin said he didn’t believe it was anyone in the horse world. He said: “Nobody does this — this is some whack.”

Rockridge Farm is a 10-acre boarding and training facility which has been owned by the Tomins for over 30 years.

The Rancho Santa Fe district of San Diego county is the third most expensive postcode in the USA, according to Forbes magazine, and the most expensive in the state of California.