A fever that primarily hits toddlers sharing swimming pools is sweeping across Japan, according to the National Institute of Infectious Diseases.

The institute said it has received reports of around 41,500 patients between January and late June from around 3,000 designated pediatric clinics. Patients have been reported in all 47 prefectures, it said.

The institute warned in May that this summer could see the biggest outbreak of the fever in a decade.

The latest figures represent a rise of more than 1.7 times the level in 2004, when a 10-year high was marked.

Based on estimates, the actual number of patients, including those not covered by the designated clinics, could be around 10 times greater, the institute said.

Since the start of the year, the average number of patients with pharyngoconjunctival fever, commonly called "pool fever" in Japan, per clinic has exceeded the corresponding number in 2004 in almost every week, the institute said.

In the latest reporting week of June 26 to July 2, an average of 1.28 persons were reported with the fever per clinic, outpacing the 0.87 in the corresponding period of 2004.

More than 70 percent of the patients were 5 years old or under, and 90 percent were age 9 or under.

An institute official advised people to take such precautions as washing hands, gargling and not sharing towels.

Major symptoms of the fever include temperatures of around 39 degrees, headaches, fatigue and sore eyes and throat. It typically clears up in three to five days, but in some cases may cause severe complications such as respiratory problems.