A scientist and her young son have been left critically injured after they were attacked by a giraffe at a wildlife reserve in South Africa.

Dr Katy Williams, a 35-year-old American, and her three-year-old son, Finn, are in a critical but stable condition. An operation was performed on Finn during the early hours of Tuesday to release pressure on his brain caused by injuries sustained in the incident.

The family were attacked at about 6pm on Monday by a female giraffe 150 metres from their home in Blyde Wildlife Estate, in the country’s north-east.

Williams’ British husband, Sam, 36, was returning from a trail run on the estate when he found his wife and son being trampled by the giraffe. He was able to intervene and chase the giraffe away, according to a statement given by Marina Botha, the family’s lawyer.

Riaan Cilliers, the manager of the estate, said: “We are all in shock about this very sad incident and we ensure the family that they are in our prayers.”

Cilliers said the giraffe in question had a two-month-old calf, which could have had an influence on her behaviour.

Facebook Twitter Pinterest Sam Williams described the attack as ‘an unfortunate act of nature’. Photograph: Handout

The Williamses are both scientists with post-PhD experience in animal related fields. Sam Williams described the incident as “an unfortunate act of nature” in which the giraffe saw his wife and son as a threat to her calf.

The injured pair were initially treated at the scene by medics from the nearby town of Hoedspruit. They were then airlifted by the emergency medical service ER24 to Busamed Modderfontein hospital in Johannesburg.

Botha’s statement said they were transferred to a local airfield and the medical team “worked relentlessly to stabilise mother and son while waiting for two helicopters to arrive from ER24”. The helicopters arrived later in the evening and Finn was the first to be airlifted, followed shortly by his mother in a second helicopter.

“Dr Sam Williams has conveyed his gratitude to all emergency personal, ER24, the medical team of Busamed Modderfontein hospital, the management team and residents of Blyde Wildlife Estate as well as the community of Hoedspruit for their assistance and support in this difficult time for him and his family,” the statement added.

• This article was amended on 10 September 2018 to make clear that Katy Williams is a scientist in her own right.