TUBERCULOSIS: Human migration

Human migration has had a major effect on the spread of tuberculosis throughout the course of human history. In modern times, geographic barriers have been easily overcome, and mass migration reached unprecedented levels in the latter half of the 20th and beginning of the 21st centuries. Nearly 1 billion—or one of seven—people are migrants. An estimated 740 million are internal migrants and 200 million are international migrants, with most (130 million) moving from one developing country to another and 70 million moving from a developing to a developed country.1 The numbers of people forcibly uprooted by conflict and persecution is thought to have reached 42 million (26 million internally displaced people and 14 million refugees), amid a sharp slowdown in repatriation and more prolonged conflicts resulting in protracted displacements.2
Migrants are disproportionately affected by tuberculosis, a reflection of the high rate of disease in their country of origin due to poverty and made worse by limited health-care and public health infrastructure.3 Migration has significantly affected the epidemiology of tuberculosis in high-income countries in Europe, USA, and Canada, which have a low incidence of tuberculosis but where most cases now occur in migrants (ie, foreign-born individuals); most cases of multidrug-resistant tuberculosis are imported as well. Immigrants from low-income high-incidence countries continue to have high rates of tuberculosis for years to come due to reactivation of latent infection, although the greatest risk of active disease is within the first several years after arrival. Molecular typing has confirmed that tuberculosis in immigrants is usually due to acquisition of infection in their home country and there is little transmission to others in their new country.4, 5
Although most of the attention on migration and tuberculosis is focused on south to north movement, most of the annual 9·3 million new cases of tuberculosis continue to occur in low-income and middle-income countries. Much effort and expense has gone into screening immigrants coming to high-income countries. However, there is limited evidence to guide these measures and thus, not surprisingly, service models for such screening significantly vary within western Europe,6 the USA, and Australia. Most migrants are missed by these screenings, including undocumented migrants who are thought to make up a substantial proportion of cases in some countries but who are hesitant to interact with government and public health officials. In addition to south to north movement, migration can occur within a country's borders, from rural to urban areas. Delays in diagnosis of tuberculosis have been described in rural-to-urban Chinese migrants, who account for an increasing share of cases in Beijing (now one in three cases) but have much worse outcomes and cure rates than do non-migrant urban residents with tuberculosis (eg, 37% vs 91% cure rates).7 Thus irrespective of where and how migration occurs, national tuberculosis programmes must address the challenges posed by migration.
http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(10)60574-0/fulltext