MANCHESTER, N.H. (AP) – Cases of whooping cough appear to have doubled in the city in the last six months, according to city health reports.

Meanwhile Londonderry school officials are warning parents to be on the lookout for the disease after state health officials confirmed two cases of whooping cough at the middle school. “I would imagine it’s pretty much spread over the state now,” said Fred Ruszek, Manchester’s public health director.

Lisa Carlson, the city’s medical director, said the reasons behind the rise of whooping cough this winter are unclear. It may be cyclical or it may be due to teenagers and adults whose childhood vaccines wore off, she said. Manchester’s health department hopes it has turned the tide on the disease. After 20 cases earlier this fall and winter, the city was down to one reported case in January.

Whooping cough, also known as pertussis, is caused by a bacteria that can be spread by coughing or sneezing. It typically starts with a low-grade fever and runny nose accompanied by a cough that gets worse over a couple weeks. The illness takes it name from a characteristic dry cough that sometimes ends in a whooping sound. It can be treated with antibiotics. Most youngsters are immunized against whooping cough since it’s required under state law for students entering school.

But some youngster are not immunized for religious or other reasons and the vaccination can wear off on some.

Londonderry school officials on Friday sent out information about what to look for. Pupil Services Director Brian Balke said sick children displaying symptoms for more than one week should be taken to a doctor and kept out of school. Infants, pregnant women in their third trimester and children with compromised immune systems are considered at higher risk, he said.

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Information from: New Hampshire Union Leader, http://www.theunionleader.com

AP-ES-02-04-06 1247EST