The mystery of meningitis

It’s every paediatrician’s nightmare. A child arrives in the Emergency Department with a high fever, a rash whose spots don’t go white when you press them under a glass and who turns their head away from the light. The toddler was well until a few hours before and, from the parents’ story, things are going downhill fast. If you get the diagnosis wrong, the child could be dead within hours.

The safest assumption is that meningitis-causing bacteria are coursing though the patient’s bloodstream (septicaemia) and infecting the membranes (meninges) around the brain. The imperative is to get intravenous antibiotics into the child as soon as possible.

Several types of bacteria can cause meningitis but one of the most common is meningococcus. Meningococcal subtypes are identified by single letters, such as A, B, C and Y.

What has been concerning health authorities is the increase in cases of Meningococcus W or MenW.  In Britain eight years ago MenW represented only 1% of meningococcal disease; today it’s 25% and growing. In Australia the number of cases, while small, is also growing. Such is the concern that two states (Western Australia and New South Wales) have started offering immunisation to teenagers.

The anxiety is amplified because MenW affects older age groups, and meningitis is generally a disease doctors expect to see only in children or young adults. So clinicians’ antennae aren’t up when someone older comes into the surgery sick. MenW also doesn’t necessarily present as meningitis. It can cause infected joints, pneumonia and a dangerous condition called epiglottitis, where the flap that seals the upper airway when we swallow (the epiglottis) becomes inflamed and swollen and may suddenly and unpredictably block the airway completely. The death rate from MenW is 13% compared to about 5% for other subtypes. In South America, mortality can be as high as 28%.

The mystery is why MenW is spreading. One theory is the “niche hypothesis” – that meningococcus occupies an ecological niche, like sparrows in the city. If someone  came along and knocked off all the sparrows, common mynahs might take over the sparrows’ nooks and crannies. With meningococci, there are vaccines that target various subtypes. In Australia, for example, every infant receives MenC vaccine in their first year of life. MenC disease is thus now much rarer. So has MenW just muscled in to the vacancy?

It’s a possibility but there may be other factors operating. For instance, after MenC immunisation was introduced into Australia for some reason MenB also started to decline, whereas the niche hypothesis suggests it should have risen.

The spread of meningococcal subtypes is complex, poorly understood and not uniform around the world. In the United States there has been a rise in MenY, and no one is sure why. New Zealand has had a nasty form of MenB, and again no one knows why.

The annual Hajj pilgrimage to Mecca where massive numbers of people from all over the world congregate, mix, then go back home has, in the past, been implicated in MenW spread.

Given the mystery, the main strategy to control meningococcal disease is immunisation. The two most common age groups affected by the bacteria are the under fives and teenagers, many of whom will carry meningococci in their noses without coming down with the disease. Why some people are carriers while others fall sick is another puzzle, probably answered by a combination of good health and good genes.

The death rate from MENW is 13% compared to about 5% for other subtypes. In south america, mortality can be as high as 28%.

With MenC the policy is to get in early and vaccinate infants. With MenW, given the older age groups affected, and the need to get good coverage quickly countries like Britain and Australia have opted to focus on teenagers.

Is there anything the rest of us can do? Meningococcal disease is rare in wealthier nations, so panic or fear are not useful responses. Doctors need to be vigilant and make the diagnosis as early as possible so that the person can be treated effectively and families and close contacts can be protected with prophylactic antibiotics.