When an Ill Wind Blows

Earlier this year, an epidemic of meningococcal meningitis swept through the African country of Burkina Faso, infecting 19,000 people and killing more than 1,000 in just three months. Meningitis is an infection of the fluid that surrounds a person's brain and spinal cord. The disease is one of the most feared in Africa because it infects quickly and kills at a high rate. Those it doesn't kill often suffer brain damage or deafness.

The incidence and onset of the disease in Africa has long been associated with dry, dusty seasonal winds that blow off the Sahara. Now, IRI scientists and their colleagues at the WHO's Multidisease Surveillance Center in Burkina Faso are hoping to develop climate models to predict the occurrence of meningitis outbreaks and their locations, so that health workers can target the timing of immunizations and other interventions more appropriately.

Burkina Faso lies squarely in the infamous "meningitis belt", an east- west band in Africa's Sahel region that spreads from The Gambia to Ethiopia. The largest epidemic year on record was 1996, when outbreaks in the belt affected more than a quarter of a million people, killed more than 20,000 and left tens of thousands of others with long-term disabilities, according to WHO. Of course, these are conservative estimates, since reporting systems tend to break down during epidemics.

Aiming to prevent another such devastating epidemic, The World Health organization and its operational partners are about to embark on a decade-long, massive immunization campaign using a new, highly effective, conjugate vaccine developed by the the Meningitis Vaccine Project (MVP)for the 350 million inhabitants of the belt. Professor Kader Konde, Director of the Multidisease Surveillance Center in Ouagadougou, Burkina Faso, leads the MVP's vaccine introduction strategy in the region.

Konde recently met with IRI scientists to discuss the challenges of implementing the vaccine program and to explore potential areas of collaboration. The first phase of the vaccine program will target children and young adults in "hyper-endemic" areas, he said, but the challenge is doing this on limited resources and staff. "When we focus on vaccination, all the other activities and caregiving in our health facilities slow or stop because there aren't enough people to do it all," he said.

In collaboration with the Liverpool School of Tropical Medicine, IRI has worked for a number of years to map meningitis risk in the region. IRI's role is to help define the most vulnerable populations, where and when they are most at risk, and to try and understand the key drivers behind the seasonal and year-to-year changes in risk. "Climate has long been understood to be a key determinant of the distribution of epidemic meningitis in Africa," says Madeleine Thomson, who leads IRI's effort in this area. "What we are doing is trying to understand the nature of this relationship and if it can be used to inform control strategies."

An early warning model that can predict changes in epidemic risk could guide the selection of districts for priority deployment of these vaccines, especially since WHO estimates that only 40 million doses can be produced annually, at least in the initial years of the program.

"Because the program has a limited number of vaccines, there is a need to define the high-risk communities first in order to maximize the benefit of the immunizations," says Judy Omumbo, an epidemiologist completing her post-doctoral research at IRI. "This involves stratifying the disease according to the distribution of the population and examining the links between climate data and the evolution and spread of new cases in the population. Knowing when and where the season starts is another big question, because its onset and peak times vary depending on where one lives in the meningitis belt," she says.

Her colleague, IRI climate scientist Sylwia Trzaska is hoping to answer this question by analyzing regional climate data on temperature, humidity, winds, and aerosols such as dust. "We know there is a strong link between meningitis and climate, but we don't know what the threshold levels of the climate variables are that cause onset of the disease," says Trzaska.

"For example, could we one day say, 'The meningitis season will start if the humidity level falls to X, or an area gets 4-5 days where visibility is below X'?" she asks. The purpose of this would be to establish a trigger that could be used as an early-warning signal by health ministries.

Her work has already led to some interesting insights that challenge current thinking on the connection between climate and the disease. Residents of the belt have long associated the onset of the meningitis season with the arrival of the Harmattan--a dry, dusty wind that sweeps down from the Sahara Desert. The prevailing hypothesis was that outbreaks should be stronger and start earlier in the north, since the Harmattan would hit that area first, bringing with it a greater risk of respiratory infection and meningitis. But in examining the data, Trzaska found that the onset of meningitis occurs two to three weeks earlier in the southern part of the belt. "We've found that the southern areas start earlier and end earlier, and that the peak of the meningitis season doesn't correspond to the driest period, but the hottest period," she says.

In fact, she and her colleagues at IRI now think the Harmattan isn't the only air mass determining the distribution of meningitis. "You have the dry, dusty Harmattan sweeping down from the north, and a warm, moist air mass coming off the Atlantic Ocean, which pushes up from the south. Satellite data show very high concentrations of aerosols where these two air masses converge. This is where we think the meningitis cases peak," says Trzaska. The convergence zone of these two winds isn't stationary, and as the season progresses the Harmattan steadily loses ground to the warmer air, which explains how the northern belt peaks after the southern portion, she says. The next step is to use ground-based data to corroborate the satellite measurements, she says.

Relevant Links

BBC's Kill or Cure? Series documentary on meningitis [video]