Epidemiology
Lassa fever is endemic in parts of West Africa, particularly Nigeria, Sierra Leone, Liberia, and Guinea. Cases have also been reported in Ghana, Benin, Togo, Burkina Faso, and the Democratic Republic of the Congo.
The prevalence of antibodies to the virus in the population is reported to be 8% to 52% in Sierra Leone, 4% to 55% in Guinea, and around 21% in Nigeria.[7] There is also evidence of endemicity in the Central African Republic, Mali, Senegal, and other neighbouring countries.[7][8]
The Centers for Disease Control and Prevention (CDC) has estimated the number of Lassa fever cases per year in West Africa to be 100,000 to 300,000, with approximately 5000 deaths.[9] Other studies have reported this figure to be closer to 300,000 to 500,000 cases per year.[1] Unfortunately, estimates are crude as surveillance for cases of the disease is not uniformly performed. In some areas of Sierra Leone and Liberia, there is evidence that 10% to 16% of people admitted to hospitals every year have Lassa fever.[9] People of all ages are susceptible. One prospective cohort study in Nigeria involving 510 patients with confirmed Lassa fever found that a factor independently associated with mortality was age ≥45 years.[10]
In Nigeria, there are outbreaks of Lassa fever nearly every year throughout the country, with peaks between December and February (dry season). A higher than usual peak in cases was observed in the first 3 months of 2020, but incidence has since declined to normal levels. As of 2 October 2022, 6854 suspected cases and 933 confirmed cases have been reported in 2022, with 173 deaths.[8]
In Guinea, the most recent outbreak of Lassa fever occurred between May and December 2021. The outbreak was traced back to a 17-year-old female with haemorrhagic fever in the Guéckédou prefecture in southeast Guinea.[11]
Outbreaks in other areas are sporadic. Cases were confirmed for the first time in the Democratic Republic of Congo and Ghana in 2011. Cases were confirmed in Benin for the first time in 2014, with subsequent outbreaks in 2016 and 2017. An outbreak was reported in Liberia at a United Nations mission in Kakata, Margibi county, in 2014. Cases in Togo have been reported in 2016 and 2017. Isolated cases have also been reported in Burkina Faso (2017) and Côte d’Ivoire. In Sierra Leone, the peak season is the height of the dry season (i.e., February to March) with a smaller peak in December.[12]
There have only been 6 cases of Lassa fever in the US and all have been associated with travel to countries known to have the virus.[9] In the UK until 2022, only 8 confirmed cases of Lassa fever had been imported since 1980, with no evidence of onward transmission from any of these cases.[8] In February 2022, three cases of Lassa fever were confirmed and one death was reported.[13] The initial case was associated with travel to Mali, where Lassa fever is endemic. These were the first UK cases since 2009 and represent the second known case of secondary transmission in Europe. The first documented locally-acquired case outside of Africa occurred in the German federal state of Rhineland-Palatinate in 2016, where an undertaker tested positive after exposure to the corpse of an infected case imported from Togo.[14]
In November 2019, the World Health Organization was informed of two imported cases of Lassa fever in the Netherlands from Sierra Leone. The index case for this outbreak was a Dutch medical doctor who worked in Masanga hospital in the Tonkolili District of Sierra Leone. It is likely that he was exposed to the virus during surgical procedures he performed on two patients (both patients died following surgery). The other case was a health care worker who assisted during the surgery. Contact tracing and monitoring activities have been initiated in Sierra Leone, Uganda, the Netherlands, Germany, and the UK as required.[15]
[Figure caption and citation for the preceding image starts]: Lassa fever outbreak distribution mapCDC [Citation ends].
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