What is Lassa Fever?

Lassa fever is an acute febrile viral haemorrhagic fever that is transmitted to humans via contact with household items or food that has been contaminated with urine or faeces from Mastomys rats. Lassa virus may also be spread between humans through direct contact with the blood, urine, faeces, or other bodily secretions of a person infected with Lassa fever. It is caused by the Lassa virus, a single-stranded RNA haemorrhagic fever virus from the family Arenaviridae.

It is a zoonotic or animal-borne that is endemic in West African countries such as Nigeria, Ghana, Liberia, etc., especially from common animals like the bush rat in Nigeria. The incubation period for Lassa fever is variable, from six days to six weeks.

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.

Historical Review of Lassa Fever

Lassa fever is endemic in parts of West African countries including Guinea, Liberia, Nigeria, and Sierra Leone, yet other neighbouring countries are at high risk because the Mastomys rat is evenly distributed across the West African region. The estimated number of Lassa fever cases reported in West Africa has been around 100,000 to 550,000, with approximately of over 5,500 deaths. Though, these estimates can’t be verified due to a lack of laboratory facilities for diagnosis, low level of awareness among all individuals, socio-cultural fear of the virus if someone close gets contaminated, and other factors.

The first reported case of Lassa fever was in the town of Lassa, Borno State in Nigeria, in 1969. Subsequent outbreaks occurred in Sierra Leone and Liberia in 1972 and led the CDC to set up a research programme for Lassa fever in Sierra Leone in the mid-1970s, although this was closed down when researchers fled fighting during the civil war.  The mission hospital in Sierra Leone where the Lassa fever programme was based was overtaken by the Rebels.  Throughout the civil war in Sierra Leone (1991-2002) and especially after the 1997 political coup, many health facilities in the country were destroyed and much of the public health infrastructure was damaged.

During the war, some of the international research capacity for Lassa fever moved to neighbouring Guinea, where the disease is also endemic, although the Sierra Leonean doctor Aniru Conteh was instrumental in setting up and managing a specialist Lassa ward at Kenema Government Hospital, with support from the British medical relief organization Merlin.

Epidemics of Lassa fever were reported in Sierra Leone in the 1990s and early 2000s. The civil war and subsequent political instability resulted in widespread displacements of people and the use of sub-par housing materials and temporary accommodation, almost certainly contributing to several serious outbreaks.

Meanwhile in Nigeria, Lassa fever research and clinical care have centered around the development of Irrua Specialist Teaching Hospital, which was declared a Centre of Excellence for the Control and Management of Lassa fever in Nigeria in 2001.

Lassa Fever in Nigeria

Though first described in the 1950s, the virus causing the Lassa disease was properly identified in 1969 after the death of two Nigerian nurses who were caring for an infected obstetrical patient. The virus was named after the town in Nigeria, Lassa in Borno State where the first major cases were recorded.

Lassa fever outbreak in Nigeria is a regular occurrence, especially during the dry season every few years. However, it is becoming a lasting problem with high cases of fatalities.

In the year 2007, The Institute of Lassa Fever Control at Irrua was inaugurated. The objective of the institute is to study and respond to Lassa epidemics more comprehensively. Since the establishment of the institute, outbreaks of Lassa fever have happened annually and lasted for longer periods, suggesting signs that Lassa fever is becoming a year-round disease. The Nigeria Centre for Disease Control (NCDC) was established in 2011 in response to public health emergencies and to improve epidemic preparedness, and is the lead agency in the fight against Lassa fever in Nigeria.

The first most serious outbreak began in December 2011 and was confirmed in early 2012. And within six months of the reported outbreak, 623 suspected cases, including 143 confirmed cases and 93 deaths had been recorded in over 20 states across the Federation.

In 2018, there was an epidemic of Lassa fever in Nigeria where cases were reported across 20 states including the Federal Capital Territory, with most within Edo and Ondo states. This was the largest outbreak of Lassa ever recorded.

Presently, Nigeria is experiencing a large outbreak of Lassa fever, with 5822 suspected cases, 8 probable cases, and 957 confirmed cases between epidemiological weeks 1 and 22 of 2023 (week ending 4^th June). Among confirmed cases, there have been 164 deaths (CFR 17.1%). This is lower than the CFR for the same period in 2022 (20.1%). A total of 47 health workers have been affected (1 new health worker for week 22). 793 patients are currently being managed at various treatment centers across the country.

Cases have been reported from 106 local government areas (LGAs), in 28 out of 36 states, including the Federal Capital Territory (FCT). A high proportion of confirmed cases (73%) are concentrated in three states: Ondo (32%), Edo (29%), and Bauchi (12%). Laboratory-confirmed cases have been reported in states bordering Cameroon (Adamawa, Benue, Cross Rivers, and Taraba) and states bordering Benin (Oyo and Niger).

Lassa fever case management centers are currently operational in four states (Ondo, Edo, Ebonyi, and Anambra states). Unlike before, the healthcare workers in these centers are duly trained in standard infection control and prevention (IPC) as well as in the use of personal protective equipment (PPE). Field teams are carrying out routine investigations on the suspected cases and deaths recorded in some community settings.