BRUSSELS (Reuters) - Belgium and the Netherlands will jointly negotiate the purchase of remedies for rare diseases with pharmaceutical groups, the Belgian health ministry said on Tuesday.

The joint negotiations will focus on diseases which affect fewer than five in 100,000 people, so-called orphan diseases, for which treatments are often very expensive due to the limited market.

"Today, each country negotiates separately with pharmaceutical companies. Together, we represent more patients which allows us to bargain a lower price," Belgian health minister Maggie De Block, a general practitioner herself, said in a statement.

The issue of expensive medicines for rare diseases sparked a debate in the Belgian media two years ago, when the health ministry refused to reimburse the treatment for a seven-year old boy suffering from a rare kidney disease, which cost 9,000 euros ($9,616) every two weeks.

The health ministry eventually agreed to cover the treatment after negotiations with pharmaceutical group Alexion, which produced the drug.

Although regulators have put into place incentives to produce drugs for orphan diseases, such as fast-track regulatory approval, pharmaceutical companies have to recoup sizeable research costs from a much smaller pool of patients, leading to higher prices.

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(Reporting by Robert-Jan Bartunek; editing by Philip Blenkinsop and Louise Heavens)