Schools and childcare centres have been issued a third health advisory warning them to be alert to a virus that causes severe dehydration in young children and kills hundreds of thousands globally each year. The Department of Health's Centre for Health Protection action comes ahead of the resumption of schools next week after the Christmas holiday. Thomas Tsang Ho-fai, controller of the centre, said there had been a sharp increase in cases of gastroenteritis, with 21 institutional outbreaks - 11 at childcare centres and one at a primary school - reported so far this month, compared with six last month and four in October. One outbreak was found to be due to rotavirus, which alarms the World Health Organisation because globally every year it kills 527,000 children under the age of five. 'For healthy people, rotavirus gastroenteritis is a self-limiting illness,' Dr Tsang said yesterday. 'However, it is occasionally associated with severe dehydration in young children. Immunity after infection is incomplete, but reinfections tend to be less severe than the original infection.' David Sniadack, a medical officer at the WHO's Western Pacific Regional Office, said: 'Rotavirus infections appear to occur in both developed and developing countries.' They accounted for about 40 per cent of hospital admissions due to diarrhoea among under-fives, he said. Although deaths have been minimised by oral rehydration treatment, Dr Sniadack said infections were 'very infectious, with the most common route being faecal-oral route, or just coming into contact with contaminated surfaces'. Symptoms include fever, vomiting and about five to 10 episodes a day of explosive watery diarrhoea. The illness can last three to nine days. A study by paediatricians at Chinese University in 2005 found that the risk of a five-year-old with rotavirus diarrhoea needing to be admitted to hospital was one in 24. Data in the same report, published in the Journal of Infectious Diseases, indicated that '4.6 per cent of all general paediatric admissions to Hospital Authority hospitals in Hong Kong were associated with rotavirus infection'. Dr Tsang said seven outbreaks this month were due to norovirus - outbreaks of which tend to attract international headlines because they often strike passenger cruise ships. He said consultation rates for acute diarrhoea among general practitioners had also increased this month - 33 cases per 1,000 consultations in the week ending December 20, and 35 cases per 1,000 consultations in the week ending December 13 - compared with 19.8 to 27.7 in the preceding four weeks. So far this year there have been five rotavirus outbreaks in childcare centres and primary schools, affecting 39 individuals. There have been 75 norovirus outbreaks, mainly at homes for the elderly and hospitals, affecting 677. The Centre for Health Protection previously sent letters on November 21 and December 19 on the risks of viral gastroenteritis. Acute gastroenteritis caused by rotavirus and norovirus can be found all year round but is more active in the winter. Deadly infection Rotavirus infections kill more than 500,000 children each year Most of those who fall ill do not die. The number of infections globally each year: 111m