The development means it will be another two weeks before the quarantine is lifted at the Thornlands practice and horses and staff will have to undergo a third round of testing for the potentially deadly virus.

Biosecurity Queensland Chief Veterinary Officer Ron Glanville today told The Redland Times a stockhorse developed a fever on Wednesday and would be euthanised because its condition had deteriorated.

He said the horse had been kept in a stall near another horse that was put down because of the virus on July 8.

"This was the horse we probably considered to be at most risk," Dr Glanville said.

While the incubation period is longer than the usual 14 days, Dr Glanville said there was a horse in the initial outbreak at Hendra in 1994 that incubated the disease for 16 days.

He said the rarity of the disease meant not a lot was known about it, but this outbreak was giving scientists "a better picture all the time".

"We've actually got three scientists coming up from Melbourne to do the post mortem of this animal," Dr Glanville said.

He said 36 other horses tested at the clinic on Wednesday would have blood samples taken again in a fortnight.

Redlands Veterinary Clinic owner David Lovell said the latest infection was a blow to the business as the quarantine was going to be lifted next week if no other cases were detected.

"It's a disaster. Just when we thought we were on the right track," Dr Lovell said.

This case brings the number of horses to have contracted the virus at the clinic to five.

Three horses had to be put down and one recovered.

Two staff members who were closely involved with the sick horses have also contracted the virus.

A Queensland Health spokeswoman yesterday said vet Ben Cunneen and a senior nurse were in "a stable condition under close medical observation" in the Princess Alexandra Hospital.

About 50 of their colleagues underwent a second round of testing on Tuesday, which had been expected to confirm whether they had contracted the virus.

Queensland Health said in light of the latest infection, some staff members who dealt with the ill horse would have be tested for a third time.

No other staff are showing signs of the virus.

The equine section of the Redlands Veterinary Clinic is operating out of a facility near Beaudesert until the quarantine is lifted.

The small animals section is operating as usual at Thornlands.