Meningitis in Africa

In the developed world, meningococcal meningitis is mostly a ‘sporadic’ disease. About 10% of us carry Neisseria meningitis as resident bacteria at the back of our throats. If we’re very unlucky, a few cells may find their way into the blood and circulate to the membranes lining the brain- the meninges– and establish a breeding population.

When this happens, it’s bad news.

Although readily treatable with antibiotics, mortality rates from meningococcal meningitis are relatively high, even in western countries. Thankfully it’s rare. Occasionally, short-lived outbreaks of the disease occur in situations where groups of people live in close proximity and share respiratory or throat secretions (intentionally or otherwise). These infrequent disease clusters typically occur in schools or universities, but don’t usually affect more than a handful of people at a time.

In some parts of the world, however, meningoccal meningitis behaves very differently. The worst affected of these regions is the so-called meningitis belt of sub Saharan Africa, where the disease can occur in devastating seasonal waves which sweep through the population. The worst ever epidemic— in 1996— killed around 25,000 people. The 2009 season was a bad one, too; despite vigorous control efforts there were over 1500 deaths.

Why does the disease occur in epidemics in Africa, but not in Europe or North America— and why are these epidemics seasonal? And what can we do about it? These are interesting questions if you have interests in both health and the Earth sciences. If you haven’t, stop reading now.

The short answer to the first question is shown in the Nasa Terra satellite image above: the Harmattan wind. This is a trade wind, which blows across the Sahara and the Sahel- the dry belt of grassland and scrub to the south of the desert- from northeast to southwest in the early part of the dry season, between November and March. The trade winds are returning low-level air streams at the base of the northern Hadley cell, and are therefore fundamental features of the planet’s atmospheric circulation. In the tropics, the Earth’s atmosphere convects in loops like a giant lava lamp, with hot rising air in the equatorial ‘doldrums’ (intertropical convergence zone), then spreading polewards at high level, before cooling and descending at the ‘subtropical ridge’ high pressure zones. In the northern hemisphere winter, the subtropical ridge sits over the Sahara desert. The southward returning flow which completes the convection loop is displaced westwards by the rotating Earth (the Coriolis effect), giving rise to historically significant breezes. ‘Harmattan’ describes the inland African portion of this airflow, and apparently derives from an Arabic word for ‘forbidden thing’.

As the image shows, the Harmattan picks up sand from Sahara. Sometimes this is carried all the way across the Atlantic, with speculative impacts on a huge range of Earth processes: rainfall from cloud seeding, destruction of coral reefs in the Caribbean and Florida, and even mineral fertilisation of the Amazon rainforest.

Unfortunately the dry, sand-carrying Harmattan also has an irritant effect on human throats, which may increase the risk of meningitis in meningococcal carriers. During the cold Sahelian nights, people huddle closely in their homes, which probably facilitates transmission of the bacteria and increases the at-risk population— and also introduces a further respiratory irritant: smoke from the indoor wood fires used for cooking and heating.

The climatic link between prevailing winds and seasonal meningitis has been suspected since the 1960s, after a group of West African nations first asked themselves the above questions, and requested the World Health Organisation (WHO) to investigate. Recent work has elaborated and quantified the link: in a series of papers published between 2004 and 2008, a team of French researchers explored the positive correlations between Harmattan wind speeds and the severity and timing of meningitis outbreaks later in the dry season. This discovery holds the tantalising promise of a climate-related ‘early warning index’, which could be used for epidemic response planning.

Like most current climate/health links, the connection between the Harmattan and meningitis is subtle, and very much modifiable by human actions. Social factors, such as living arrangements and the use of indoor fires for cooking and heating, certainly play a significant role in the epidemiology of the disease. Reducing poverty, as always, will probably be the most effective way of reducing mortality.

For now, the mainstay of meningitis control in Africa is immunisation— but the current polysaccharide vaccine has significant limitations. While it is effective in preventing meningococcal disease, it doesn’t prevent asymptomatic meningococcal carriage, and because it only provides protection for five years— and only in older children and adults— it will never be able to provide the lasting ‘herd immunity’ effect that makes most routine childhood immunisations so effective. For these reasons it is generally considered unsuitable for population-wide prevention purposes in Africa, but it can be— and is— used ‘reactively’ to try to contain an epidemic in its early stages. An early warning system based on Harmattan wind speed could therefore make a real difference— provided that effective planning, adequate disease surveillance, sufficient funds and resources for distribution of vaccine are also in place.

The development of a conjugate vaccine for the African serogroup A meningitis strain is a recent and encouraging development, which could ultimately make the ‘belt immune to the worst meningococcal effects of the Harmattan. Conjugate vaccines should generate much longer protection, are suitable for young children, and prevent meningococcal carriage as well as the disease, allowing herd immunity to come into play. According to the WHO, this new vaccine will be introduced on a population-wide basis in Burkina Faso for the first time later this year. This comes more than ten years after a conjugate group C meningococcus vaccine was introduced as part of the routine childhood vaccination schedule in the UK. Interestingly, the UK conjugate vaccine was originally tested as a combination (‘bivalent’) group C and group A vaccine before the ‘A’ was eliminated as unnecessary (and more expensive) for the UK setting. Anyone who’s worked in the perennially cash-strapped British NHS can see how and why this decision was made— but it’s meant that the African meningitis belt has had to wait another decade before getting an effective vaccine of its own.

So it goes.

Further reading:
WHO factsheet on meningococcal meningits
WHO’s meningitis vaccination program for Africa
Hadley cells
Climate and meningitis: Sultan et al’s 2005 paper
And a link to their 2008 paper

Update 21/3/10
This video uses google earth to illustrate the meningitis/climate link. It’s good, if you’ve got a decent broadband connection.


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