Nigeria has reported at least 144 cases of polio since April 11 this year, down from 217 cases during the same period in 2004, the World Health Organization (WHO) said Wednesday.

"Of concern is that 18 states ... have detected 144 wild poliovirus cases in the low transmission season," the WHO in a weekly report.

Yet transmission of polio in the country is becoming more focused as 68 percent of the cases are concentrated in six states in northwestern Nigeria, it added.

The WHO said three previous rounds of immunization campaigns aimed at eradicating polio this year were effectively implemented but "inadequate micro-planning at the ward level" had led to missed households and children.

"If transmission is to be interrupted in 2005, the quality of immunization activities must improve in the low transmission season," it said.

The next round of immunization days will take place August 6-9 this year.

Nigeria currently accounts for three-quarters of the new polio infection worldwide, and has exported the crippling disease to some 10 previously polio-free African countries.

Source: Xinhua