Jetsetters are key clues to epidemics
Travellers who bring illness home act as sentinels of disease.
When the first patients with dysentery started trickling into health clinics in Sierra Leone in early 1999, Philippe Guerin wasn't sure what to think. Guerin, a medical epidemiologist, knew that the symptoms he was seeing could be produced by several different pathogens, but resources were slim in the war-torn country, and healthcare workers did not have the facilities to pinpoint the source of the outbreak. As the flow of patients began to swell, healthcare workers collected samples and shipped them to Paris for testing.
About six months later, after three thousand cases of dysentery, and more than a hundred deaths, the results were in. Sierra Leone was in the midst of an outbreak of Shigella, a bacterium that causes bloody diarrhoea and kills more than a million people a year. The strain of Shigella in Sierra Leone, called 'Sd1', was known for its exceptional mobility and aptitude for causing epidemics in tropical regions. But although the strain had been working its way through other developing nations, it hadn't yet been reported in West African countries like Sierra Leone.
"Because of the harsh conditions and the civil war, everything was in place to nest this outbreak," says Guerin, a Paris-based scientific director at Epicentre, a non-profit public-health organization created by Médecins san Frontières. "But we lost at least six months waiting on a diagnosis."
Now Guerin says that West African healthcare workers could have been warned of the Sd1 outbreak if industrialized nations had better disease-surveillance networks to pass on information about their travellers' symptoms.
From Norway to Africa
Guerin and his colleagues then searched systematically through published papers from 1940 to 2002 and surveillance data from 16 European countries for reports of Sd1 from 1990 to 2002. Their results, published in BMC Public Health this month1, showed that surveillance data had picked up the presence of Sd1 in West Africa in 1992 — seven years before the outbreak in Sierra Leone.
The added warning time could have been valuable, says Guerin. Clinics in West African could have prepared to treat cases of dysentery aggressively using the class of antibiotics that would fend off Sd1. "Instead, there was no plan," he says.
Bugs in the system
The World Health Organization estimates that every year, about 580,000 travellers from industrialized nations pick up Shigella during their journeys. Some of those travellers carry it home with them, where they are then diagnosed and treated.
Wealthy travellers wandering through poor nations make an excellent sentinel for disease, notes Kevin Kain, an epidemiologist at the Toronto General Research Institute in Ontario. "Immunologically, they're like a one-year old," he says. "They're a canary in the coal mine."
But although the disease might then be entered into a surveillance network like the Program for Monitoring Emerging Diseases (ProMed) or GeoSentinel, that information is not systematically shared with the developing nations where the disease came from, says Stephen Morse, an epidemiologist at Columbia University in New York.
An added difficulty, notes Morse, is that not all travellers who get a nasty disease are diagnosed properly. Some never turn up at a clinic at all. Kain admits to having treated himself after coming down with a case of dysentery in Tanzania in 1991. He didn't report anything to Tanzanian officials, and returned home healthy. "I was part of the problem," he says.
But Guerin suggests that a single report is enough to sound an alarm, which would trigger a better inspection of travel surveillance networks and data sharing with the country concerned (hopefully without too many false alarms from isolated incidents). There will still be problems, of course, if the source country doesn't have the resources to tackle the outbreak.
- Guerin P.J., et al. BMC Public Health, 7. 8 (2007).