Researchers say measles, which has been successfully controlled in most countries thanks to vaccination, remains a killer disease in Niger, in part because of migration triggered by the rainy season.  VOA's Jessica Berman reports from Washington health workers are calling for better surveillance and cooperation between public health workers and the government to control measles outbreaks.

An international team of researchers says measles epidemics in Niamey, the capital of Niger, decline at the onset of the rainy season, which is followed by sporadic epidemics of unpredictable length and frequency.  The team studied the 20-year history of the ebb and flow of measles epidemics in Niger, in an effort to determine why vaccination campaigns have failed to bring the disease under control. 

Measles is a highly infectious, airborne disease that is transmitted by coughing and sneezing among people living in densely populated areas. 

According to U.S. investigator Matthew Ferrari of Pennsylvania State University, during the dry season large populations of farmers in Niger and other countries that border the Sahara desert move into urban areas for food and water. 

"The cities swell in density," he said.  "Then when the rains begin, everybody moves back out to the hinterlands to pursue agriculture, resulting in lower densities of people, fewer contacts among individuals, and then reduced probability for measles transmissions to occur."

The investigators described their findings in the journal Nature.

Measles vaccination campaigns were successful in North America and England, according to Ferrari, because the number of births and weather conditions remained stable, which made epidemics predictable.  But he says it is hard to immunize with precision in parts of Africa because of migration. 

Measles immunization is a two-dose vaccination, with an initial shot given to children between the ages of six to 12 months, and a second, booster shot, given one month later for full protection.

Ferrari says the first shot gives children 80 percent protection against measles, but most children in Niger do not receive the booster, because they move at the end of the rainy season.

When the disease strikes, investigators found that children in Niger tend to get the disease around the age of two.

"Then what it really means is that we need to step up monitoring annually to identify when these large outbreaks are going to happen and get in and do these large booster campaigns in response to these outbreaks to stave them off before they sort of burn out of control," he added.

Ferrari says he and colleagues at the World Health Organization (WHO) and Doctors Without Borders (MSF) are working with the government of Niger to improve measles surveillance and delivery of vaccine to children.