Emergencies preparedness, response

Epidemic focus

This is a rapid ‘need to know’ spotlight on current infectious disease threats. It is not intended to be an exhaustive list of cases and outbreaks but a focus on reasons for concern about specific infectious diseases currently posing threats to global public health.

The year of the rat? An unusual year for Lassa fever

Hover over the image to explore its interactive elements on Lassa fever. To enlarge the image, click here.


Lassa fever, a viral haemorrhagic fever with symptoms similar to those of Ebola virus disease, is endemic in much of West Africa (particularly Nigeria, Sierra Leone, Guinea and Liberia) and usually sparks a seasonal outbreak from December to March. This year the Lassa fever ‘season’ has lasted longer and generated more cases and more deaths, than expected. Since November 2015, Nigeria, Benin, Sierra Leone, Liberia and Togo have reported more than 300 cases of Lassa fever and 167 deaths. Nigeria has reported most cases: (272 probable and confirmed cases, including 149 deaths) but outbreaks occurred in Benin (54 cases and 28 deaths) Togo (2 cases), Liberia (7 cases, 3 deaths) Sierra Leone (2 cases). There has also been exportation to Europe- to Sweden (1) and Germany (1) with limited secondary transmission (1 case) occurring for the first time in Germany.


This year the ‘Lassa season’ has been longer and has generated more cases, including more severe, fatal cases, than usual. Lassa fever usually kills around 15 percent of those who develop severe cases whereas the case fatality rate being reported this year is above 50 percent.

Nigeria, which is the traditional source of most Lassa cases, has an ongoing outbreak that has affected 23 states, and which extended to new states in mid-April. The high number of cases reported in Nigeria may be partly due to better preparedness, leading to better detection. Nigeria has learned from Ebola and has built better capacity to detect emerging outbreaks, including raising awareness of disease threats through stronger community engagement.

Molecular dating suggests that Lassa virus has been circulating in Nigeria for over a thousand years, and in some other West African countries for hundreds of years. This year, genetic sequencing of the virus causing infections in Togo showed that this virus and one circulating in neighbouring Benin represent a new lineage of the Lassa virus. The virus animal reservoir is a rat species, Mastomys natalensis and the Lassa virus is most commonly passed from rats to humans through contact with rat faeces and urine.

The larger and wider outbreaks being seen in West Africa may be due to increasing urbanization and to climatic conditions favouring the rat, the main source of transmission to humans (85-95 percent of cases). West Africa has enjoyed good rainfall since November 2015, possibly due to the El Nino phenomenon, which has led to good crop yields and plentiful food for rodents.

Human to human transmission occurs in a small percentage of cases but usually only in hospitals or mortuaries. Good infection prevention and control practices in health care facilities are key to preventing the spread of epidemic-prone viruses.


The best ways of preventing human infection are to reduce the rat population and avoid any contact with rat excreta. To avoid contact with rat excreta, people are advised to always wash their hands before handling and eating food; store food in covered containers; cook all foods thoroughly and discourage rats by clearing away any rubbish in or around the house and keeping a cat.