Paul Tapner's Badminton Horse Trials winner Inonothing fractured a patella during the cross-country at the World Equestrian Games at the weekend.
The society says it has, for years, been encouraging officials overseeing eventing to develop and implement safety measures in the cross-country phase of eventing to protect horses.
It noted that although no horses died during the cross country competition, injuries to three horses "illustrate the need for greater safeguards".
Four horses suffered falls which resulted in disqualification; three sustained non-fatal injuries.
The society said its staff witnessed several of the incidents, and met with FEI and US Equestrian Federation officials to discuss them.
"While improvements have been made and we're relieved no horses perished, the international eventing community must continue to put the welfare of the horse first in the sport, ahead of the goals and pursuits of individual competitors, or the success of national eventing teams," said Keith Dane, director of equine protection for the society.
"Riders should be encouraged to always hold the safety of their horses paramount, and officials should be trained and empowered to disqualify entrants who appear to be riding dangerously out of control."
In a meeting with the society, eventing officials described the changes in recent years to make the sport safer for horses and riders, and acknowledged that even further improvements were needed, the society said.
Recent advancements include the use of obstacles that break away when a horse hits it with enough force (reducing the chance of fatal rotational falls) and the use of warnings or disqualifications for riders who display reckless riding on the course.
The society said organisers viewed the disqualification rate at the event as a hopeful sign that new, tougher rules are causing entries to be eliminated from competition before potential accidents happen, thus reducing the number of injuries.
Nineteen of 79 entries - 24 per cent - failed to complete the course due to refusals, penalties or falls.
It said the recent changes, "which encourage responsible stewardship and hold exhibitors and officials more accountable for horses' safety", are gaining wider acceptance among the majority of international delegations represented in the sport.