Lassa fever is an acute viral hemorrhagic illness endemic to West Africa which causes significant annual outbreaks of disease. It was first identified fifty years ago, when two missionary nurses became ill and died in the town of Lassa, Nigeria. Lassa fever is zoonotic, transmitted to humans via contact with food or household items contaminated with rodent urine or faeces. Person-to-person infection can also occur, particularly in hospitals where infection control is poor. 

 

Lassa fever is difficult to diagnose, and surveillance data is limited and disputed. While the current disease burden is underestimated, there are an estimated 300,000 to 500,000 cases and 5,000 related deaths each year. Despite this disease burden, no vaccine for Lassa fever is currently available. One in five infections result in severe disease, where the virus affects several organs, such as the liver, spleen, and kidneys. Among patients hospitalized with severe cases, the fatality rate is 15% and deafness occurs in 25% of patients who survive.

Pregnant women are at particular risk from the disease. Among women who contract Lassa fever during the third trimester of pregnancy, more than 80% of cases result in the death of either mother or child. Death usually occurs within two weeks of the disease first appearing. In many countries where Lassa fever is endemic, women also suffer from limited access to adequate health services, exacerbating the risks they face.