The most severely affected countries are in the region known as the Manu River Union (Liberia, Sierra Leone and Guinea) and Nigeria (all marked red on the map below). Outbreaks are common in these countries.
It is important to note that the areas of Lassa fever endemicity are not limited by political boundaries, but rather by geographical areas, as shown here.
There is evidence of Lassa virus infection in other African countries (in blue on the map below), demonstrated either by sporadic human cases, or by rodent or human sero-prevalence studies. It should be noted however that data are often sparse and under-diagnosis in these countries is likely. A recent paper describes detection of Lassa virus in rodents in southern Mali (Emerg Infect Dis 2010), and a human case of Lassa fever acquired in Mali occurred in London in 2009 (Eurosurveillance 2009)
Imported UK cases
Eight cases have been imported into the UK since 1980. All cases have derived from either Sierra Leone or Nigeria, except for a single case in February 2009 which had been acquired in Mali. World-wide, imported cases are very rare and occur almost exclusively in persons with high risk occupations such as medical or other aid workers. The risk to tourists is considered to be very low.
Last reviewed: 19 April 2012