Learning about microbes from Hajj

by Salman Hameed

Hajj is a gathering of over 2.5 million people in Mecca each year. As a side note, I have always wondered if Muslims who are afraid of crowds (enochlophobia?) perform Hajj. Nevertheless, such a large gathering provides a fantastic opportunity to study large movements of people as well the transfer of germs in crowded areas. Indeed, this information is being used to prepare for London Olympics. From last week's Science (you will need subscription to access the full article):

The field of mass-gathering health has its roots in the hajj, the sacred journey to Mecca that every Muslim is called to undertake at least once. The hajj draws more than 2 million pilgrims every year and in the past has been the epicenter of smallpox, plague, and typhus outbreaks. Now, the biggest problem is Neisseria meningitidis, a deadly bacterium that can cause meningitis and sepsis. It first surfaced in 1987, when pilgrims became infected with a Neisseria variety known as serogroup A. In response, Saudi Arabia required that hajj travelers be vaccinated against the pathogen. In March 2000, pilgrims contracted a new strain, W135, and carried it back to the United States, Africa, Asia, and Europe, touching off small epidemics. Since 2002, vaccination against W135 is mandatory, as well.

Several factors conspire to make the hajj a conduit for microbes. Pilgrims are often quite old before they can afford the journey, “and a lot of people are in poor health, because they want to go before they die,” Khan says. Nearly 200,000 pilgrims arrive from low-income countries with poor health care systems, and many sleep in close quarters in tent cities. Because the timing of the hajj is tied to the lunar calendar, the pilgrimage can coincide with the peak influenza season in the Northern Hemisphere or the meningitis season in sub-Saharan Africa.

But there are some differences with sporting events: 

Several factors may limit the spread of infectious diseases through sporting events. People traveling to these events are relatively young, wealthy, and healthy, which means they're a lot less likely to become infected. In addition, Khan and his group have analyzed travel movements into the host cities for various Olympic Games and did not see the millions of additional visitors that come to the hajj. There was no significant increase in travel to Athens in 2004, for instance, and in 2008, Beijing saw fewer visitors than usual. “A lot of people are just avoiding these cities when they host the Olympics,” Khan says. 

But even at the hajj, the real surprise is sometimes how little diseases actually spread. In 2009, 2.5 million pilgrims converged on Mecca in the middle of the H1N1 influenza pandemic, but fewer than 100 cases of flu were detected. “People are shoulder to shoulder in prayer for days, so we were really surprised,” Khan says. But he emphasizes that Saudi Arabia took extraordinary measures of precaution: screening travelers at airports, isolating the sick and treating them, and immunizing others. 

To help understand the gap between fears and facts, Khan intends to use the London Olympics as a testing ground. He and his team have analyzed travel movements into London in the past 5 years, together with Olympic ticket sales, to estimate where visitors to the 2012 games will come from. By combining this information with infectious disease surveillance data from around the world, they will predict what pathogens to expect; after the games, those predictions will be compared to observations.                     

Read the full article here. For some footage of the Hajj, check out this National Geographic documentary, Inside Mecca.