Nearly a third of Waimea College's teachers have been absent as the school deals with a suspected norovirus outbreak.
Principal Larry Ching said today it was a challenge managing the school but he hoped they were over the worst of it.
Public Health staff have been into the school investigating the cause of the vomiting and diarrhoea illness which has mainly affected staff.
Despite checks of the school, including the staffroom, the Medical Officer of Health had found there was no obvious way it had originated, Mr Ching said.
The 1500-student school has 150 staff with nearly 100 of them teachers, and yesterday 30 were absent.
The 30 teachers included 24 on sick leave plus some away on training or with sports teams. Today 21 were absent.
While some who were ill over the weekend have recovered, Public Health has advised a 48-hour stand-down since their last symptoms which means they have been unable to return to class.
Mr Ching said the school had managed to provide classes by bringing in relievers and some teachers doing extra classes during their usual preparation time.
"It's been a bit of a challenge, I would not like to do it for a week," said Mr Ching.
Because there had been only one new case today, from a non-teaching staff, he believes the school is over the worst of the outbreak. "Within one or two days we should be right," he said.
The illness does not appear to have hit many students. Yesterday 50 children were absent which was typical, and of those six or seven had been affected by some kind of tummy bug, he said.
Mr Ching said he would only have considered closing the school if the outbreak had escalated or continued for some time.
A newsletter had been sent out with advice from Public Health describing the symptoms and how to manage the illness and emphasising hand cleaning.
Mr Ching said while the school did a big clean daily, it had doubled its cleaning of the toilet area, taps and handles.
Public Health had taken samples from some staff who were ill yesterday as part of the effort to determine the cause. Mr Ching said it was suspected to be norovirus because of the symptoms. "We need to act to control knowing it is here."
Dr Jill Sherwood, medical officer of health for Nelson Marlborough, said no new staff absences had been reported today.
Ministry of Education head of sector enablement and support Katrina Casey said her office had been notified that 24 teachers were away sick yesterday, but Dr Sherwood said her reports put the number of staff absentees at 35.
The public health service interviewed more than half of these teachers about their symptoms, and also spoke to seven students who were ill.
"Putting the information together supports that the outbreak occurring is most likely norovirus, but we can't prove that," she said.
Dr Sherwood said having seven students sick with vomiting and diarrhoea was not out of the ordinary at Waimea College, saying it looked like the illness was mostly confined to teachers. She said it was unlikely that the gastroenteritis had been introduced via food or water, saying viruses of this type were commonly circulating in the community at this time of year.
"Most likely, a person has been ill and some surfaces have become contaminated."
She said Public Health would continue to investigate the outbreak, but its main focus was on limiting the virus' spread. Closing the school would be an unusual reaction in this circumstance, she said.
Public Health has asked those infected to stay at home until 48 hours after their last episode of vomiting or diarrhoea, and sent messages out to students and staff about the situation. It has requested that the school's cleaners put in extra work with a chlorine-based solution.
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