Since June, there have been four apparently pre-meditated attacks on horses at his stables, near Marlborough, Wiltshire.
In the first attack a filly, Rioja, was cut under the tail with almost surgical precision - leaving a two-inch deep, four-inch long cut across its genitals. The weapon used was either a scalpel or craft knife.
A month later a second unnamed racehorse was attacked in an almost identical manner, but this time it was cut from left to right.
In September a three-month-old colt, Mio Brujo, received a nine-inch wound, two inches deep, again across the genitals.
Earlier this month Rioja was attacked again. This time it was slashed under the belly, developing a life-threatening infection from which it has yet to recover.
Mr Witheford is known in horseracing circles as the "horse whisperer" for his empathic relationship with animals, travelling all over the country rehabilitating problem horses for their owners. He believes he knows what is going on.
"It is part of some kind of ritual. The body of a horse is similar to a man's. The blood is at the same temperature. Whoever is doing this is taking blood for some ritual," he said.
It is estimated that as many as 80 horses are attacked each year, with many of the attacks centred on the sex organs. In September, just across the border in Hampshire, two ponies were shot with a crossbow. In Kent last month, two horses had their tales cut off and hung on trees. One of the animals bled to death.
Reports of horse-slashing surface periodically - normally starting and finishing abruptly with no explanation. While some doubt the macabre phenomenon, believing it to be self-inflicted or accidental, there appears to be mounting evidence to the contrary.
In 2003 police investigated 12 incidents in a triangle of villages in Derbyshire and South Yorkshire. The perpetrators left "calling cards" of sticks and branches.
In the 1990s, Hampshire Police investigated between 20-30 attacks as part of an 18-month inquiry into equine attacks.
But catching the slashers has proved problematic. Neighbours of the Marlborough stables have reported seeing a white Ford Transit van in the vicinity of the paddock where all three horses have been slashed. A man in his 50s, wearing a flat cap, was spotted getting out of the van before it drove off, prompting fears the slashers may be working in a team.
"These people are sick," Mr Witheford said. "Horses will forgive but they won't forget. They have been left not just physically damaged but also mentally scarred."