The region's medical officer of health is encouraging parents to take advantage of a free national vaccine for rotavirus.
The vaccine was introduced on July 1.
The virus, which usually lasts between 9 and 19 days, causes vomiting, diarrhoea and stomach pains, and is spread through people not washing their hands after going to the toilet.
The Nelson Marlborough District Health Board medical officer of health Dr Andrew Lindsay said rotavirus was a very common childhood illness that affected almost every child at some stage. It was often mild, but more serious cases could cause severe dehydration which might be be fatal.
According to Ministry of Health statistics, 1 in 5 New Zealand children is treated for rotavirus, and 1 in 43 is hospitalised for the virus.
The most severe cases usually occur in children between 3 months and 2 years.
Breastfeeding children who contracted rotavirus were unable to feed properly and would lose a dangerous amount of fluid, Dr Lindsay said. "It's the littlies that get the most severe illness."
It was possible to catch rotavirus twice, but the virus was usually less severe the second time, Dr Lindsay said.
The Ministry of Health guidelines advise that children should be vaccinated at 6 weeks, 3 months and 5 months. Dr Lindsay said there was a "short window of opportunity for vaccinations" and delaying them could make it significantly more likely that a child would contract the virus.
There were no regional statistics as the virus was not notifiable, meaning that doctors did not have to report cases to the medical officer of health.
However, overseas studies had shown that the rate of rotavirus hospitalisations had dropped drastically after vaccinations were introduced, and Dr Lindsay expected the same would happen in New Zealand.
Vaccinations were a way of tackling the "tip of the iceberg", he said.
Dr Lindsay advised parents to take their children to the GP if they were showing symptoms of rotavirus, and to keep them away from other children.
It was also important that people were aware of the importance of washing their hands, particularly after changing babies' nappies and before handling food.
- The Marlborough Express
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