A study conducted in Senegal is casting doubt on the effectiveness of bednets treated to fight malaria.

The research suggests that malaria-transmitting mosquitoes quickly build up resistance to the insecticide used to coat the nets.

In an article published Thursday in a British journal The Lancet Dr. Jean-Francois Trape of the Development Research Institute in Dakar called the finding a “great concern.”

Trape led a team that followed 500 people in the village of Dielmo before and after they began using bednets treated with the chemical.

The scientists noted that after the bednets were introduced in August of 2008, the number of malaria cases dropped.

However, after about two years, malaria cases rose sharply, in some cases to even higher levels than before the bednets were used.

The researchers have called for new strategies to be “urgently defined and implemented” to address the problem of mosquito resistance to insecticide.

The U.N. World Health Organization says malaria killed nearly 800,000 people in 2009.

About 90 percent of the deaths occurred in Africa, with the vast majority of those deaths being among children under the age of five.