Lassa fever - Nigeria

1 May 2023

Situation at a glance

Nigeria is currently experiencing a large outbreak of Lassa fever, with 4702 suspected cases, five probable cases, and 877 confirmed cases between epidemiological weeks 1 and 15 of 2023 (week ending 16 April). Among confirmed cases, there have been 152 deaths (CFR 17%). Lassa fever is endemic in Nigeria and parts of West Africa where the multimammate rat, the main reservoir of the Lassa virus, is common.

Responding to the current outbreak is challenging due to the need to respond to multiple emergencies simultaneously.

The symptoms of Lassa fever vary widely, and diagnosis can be difficult. Most cases (∼ 80%) are asymptomatic or mild. Still, the infection can result in severe illness and multiple organ dysfunction with or without haemorrhage. Laboratory testing is therefore needed to confirm the diagnosis.

Laboratory-confirmed cases have been reported in states bordering Cameroon (Adamawa, Benue, Cross Rivers, and Taraba) and states bordering Benin (Oyo and Niger). The overall regional and global risks are considered low because the primary mode of transmission of Lassa fever is through contact with food or household items contaminated with rat excreta. The rate of human-to-human transmission is low.

Description of the situation

Nigeria is experiencing a large outbreak of Lassa fever, with 4702 suspected cases, five probable cases, and 877 confirmed cases between epidemiological weeks 1 and 15 of 2023 (week ending 16 April). Among confirmed cases, there have been 152 deaths (CFR 17%). This is a 20% increase in confirmed cases in comparison with those reported during the same period in 2022 (733).

Cases have been reported from 101 local government areas (LGAs), in 26 out of 36 states, including the Federal Capital Territory (FCT). A high proportion of confirmed cases (72%) are concentrated in three states: Ondo (32%), Edo (29%) and Bauchi (11%). Laboratory-confirmed cases have been reported in states bordering Cameroon (Adamawa, Benue, Cross Rivers, and Taraba) and states bordering Benin (Oyo and Niger).

Epidemiology of disease

Lassa fever is an acute viral haemorrhagic disease caused by the Lassa virus. It is primarily transmitted to humans either through direct contact with infected Mastomys rodents, or through food or household items contaminated with the urine or faeces of infected rodents. Consequently, the virus is transmitted to humans through cuts and scratches or inhaled via dust particles in the air. Human-to-human transmission can occur through direct contact with an infected person's blood or bodily fluids, but this is rare. Lassa fever can spread in healthcare settings without early recognition and treatment and without adequate infection prevention and control (IPC) measures.

Most cases (∼ 80%) are asymptomatic or mild, but the virus can cause severe disease in the remaining 20% of patients, sometimes associated with multiple organ dysfunction with or without haemorrhage. Lassa fever is difficult to diagnose without proper laboratory tests because it can present with various symptoms, ranging from no symptoms to multiple organ failure and death. 

The case fatality rate is usually between 1% and 15% in patients hospitalized with severe disease. The disease is particularly severe in late pregnancy, with maternal death and/or fetal loss occurring in the third trimester in over 80% of cases. Early supportive care with rehydration and symptomatic treatment is critical and improves survival. The antiviral ribavirin has been used to treat Lassa fever but its efficacy is unproven. There is currently no vaccine that protects against Lassa fever.

The disease was first identified in Nigeria in Borno State in 1969 and is endemic in Nigeria. Transmission occurs throughout the year. However, large seasonal outbreaks occur during the dry season, typically from December to April. 

During epidemiological weeks 1 to 15 of 2023, 42 cases were reported among healthcare workers across 11 states. In week 15 of 2023 (10-16 April), 141 new suspected cases, nine new confirmed cases, and one new confirmed death were reported (CFR 11%).  

Responding to the current outbreak is challenging due to the need to respond to multiple emergencies simultaneously. These include outbreaks of COVID-19, diphtheria, meningitis, measles, and cholera, and ongoing humanitarian crises. In addition, the country is dealing with security challenges that affect the timeliness of the response.

Public health response

The Nigeria Centre for Disease Control and Prevention (NCDC) and health authorities of affected states are leading the response to the outbreak. An NCDC Emergency Operations Center (EOC) was activated on 28 January 2023 and Rapid Response Teams (RRTs) have been deployed to the states of Bauchi, Benue, Ebonyi, Edo, Ondo, and Taraba.  

  • Response activities include:
  • Surveillance: detection, monitoring of cases and outbreaks, and laboratory testing;
  • Case management;
  • Prevention measures: public education on personal hygiene, food safety, and rodent control in homes and workplaces;
  • Control measures: administration of antiviral drugs to confirmed cases, supportive care, and contact tracing to identify people who may have been exposed to the virus. 

To improve the response to Lassa fever outbreaks in Nigeria, the NCDC has developed a national action plan, with WHO and other partners, to prevent and control Lassa fever outbreaks. This plan focuses on strengthening surveillance activities, improving and expanding national laboratory capacity, increasing public awareness, and improving the availability of treatment and care for patients. 

WHO risk assessment

Lassa fever is an acute viral hemorrhagic fever endemic to West Africa. Confirmed outbreaks and sporadic cases have been reported in Benin, Burkina Faso, Cote d’Ivoire, Guinea, Ghana, Liberia, Mali, Sierra Leone, Togo and Nigeria. In some regions, Mastomys rodents are consumed as food.

 Although Nigeria is an endemic country for Lassa fever and has developed capacity to manage Lassa fever outbreaks, the current overall risk at the national level is considered high due to several factors:

  • An increase in the number of confirmed cases compared with the same period in previous epidemic seasons;
  • Reduced national capacity due to multiple ongoing health emergencies that have overwhelmed the country's health system and limited access to medical resources;
  • Shortages of personal protective equipment (PPE), resulting in a high risk of healthcare-associated infections;
  • Delays in sample transport for laboratory testing in several states, delaying confirmation of diagnosis and linkage to treatment and care;
  • Inadequate health infrastructure, poor sanitation and the continued presence of disease-carrying rodents; and
  • Lack of awareness and education about Lassa fever among the public and healthcare workers, likely leading to further delays in diagnosis and treatment, increasing the risk of transmission and potential deaths. 

The overall regional and global risks are considered low because the primary mode of transmission of Lassa fever is zoonotic and the rate of human-to-human transmission is low. 

WHO advice

Prevention of Lassa fever requires the promotion of good community hygiene to prevent rodents from entering homes. Effective measures include storing grain and other food in rodent-proof containers, disposing of garbage away from homes, keeping homes clean, and keeping cats in the home. 

In healthcare settings, staff should always follow standard precautions for the prevention and control of healthcare-associated infections when caring for patients, regardless of the presumed diagnosis. These precautions include hand hygiene, respiratory hygiene, personal protective equipment against splashes or other contact with contaminated materials, injection safety, and safe burial rites. 

Healthcare workers caring for suspected or confirmed cases of Lassa fever should take additional infection control measures to avoid contact with the patient's blood or body fluids and contaminated surfaces or materials such as clothing and bedding. When in close contact with patients (within one metre), healthcare workers should wear: face protection (face shield or surgical mask and goggles); a clean, non-sterile, long-sleeved gown; and gloves (sterile for certain medical procedures).

No vaccine for Lassa fever is currently available for use in humans. The antiviral drug ribavirin may be an effective treatment for Lassa fever if given early on in the course of the illness, but data are inconsistent. There is no evidence to support the role of ribavirin as post-exposure prophylactic treatment for Lassa fever. 

WHO continues to advise all countries where Lassa fever is endemic to improve early case detection and treatment to reduce fatalities. 

WHO does not recommend any restrictions on travel or trade to or from Nigeria based on the information in this report.

Further information

Citable reference: World Health Organization (1 May 2023). Disease Outbreak News; Lassa Fever – Nigeria. Available at https://www.who.int/emergencies/disease-outbreak-news/item/2023-DON463