The most critically ill horse from the group of 23 saddleback show horses which are believed to have been deliberately poisoned with oleander leaves at a ranch in South California has survived and was due to be released from hospital.

Vet Steven Colburn of Creekside Veterinary Service hospital said: “He’s looking brighter and turning the corner.”

The other two critically ill horses had intravenous treatments and were returned to their home at Rockridge Farm, Rancho Santa Fe, near San Diego, last week.

Other sick horses stayed at the ranch and were treated immediately with mineral oil and activated charcoal, to soak up the poison.

The horses had ingested highly poisonous oleander leaves which had been mixed with chopped apple and carrot and feed to the horses on the night of Thursday 30 July by an intruder to the ranch.

Even small quantities of oleander leaves will induce colic, profuse sweating, abnormal heart function and ultimately cardiac failure.

Mr Colburn said: “Whoever did this knew exactly what he was doing.”

Owners Tom and Debbie Tomin have offered a £6,000 reward and the Humane Society of the United States has offered £1,500 for information leading to the arrest of the culprit.

Five of the total of 30 horses at the ranch are world champions and two are scheduled to compete in the World’s Championship Horse Show in Louisville, Kentucky in three weeks time.