Lassa fever is a viral haemorrhagic fever transmitted to humans by rodents. Lassa virus belongs to the family of arenaviruses. The disease is endemic to West Africa, especially in Nigeria, Guinea, Liberia and Sierra Leone. It is observed more frequently during the dry season, from December to March.

The virus’s name refers to the town of Lassa in Nigeria, where the first epidemic was documented in 1969.

The virus is transmitted to humans in two ways:

1. Either by infected rodents (Mastomys natalensis) through direct contact with the animal or its secretions (urine and faeces) or by ingestion or inhalation (food, drink, aerosol, etc.)
2. or from person to person through direct contact by respiratory and sexual means or through contact with contaminated medical equipment.

Most exposed individuals are in rural environments where Mastomys rats are found. Person-to-person transmission is frequent in nosocomial outbreaks in endemic countries. However, it can occur anywhere in the world given the ease of international travel.

• The incubation period varies between 3 and 16 days. People become infectious when they develop the first symptoms (fever, headache, muscle aches and feeling unwell). The clinical symptoms are varied and the difficult differential diagnosis delays the administration of treatment.
• According to a study of patients admitted for Lassa fever in Sierra Leone, the frequency of symptoms was as follows: retrosternal pain (74%), sore throat (60%), back pain (62%), cough (62%), abdominal pain (50%) vomiting (49%), diarrhoea (26%), conjunctivitis (25%), facial oedema (10%) and proteinuria (43%). Continual glandular bleeding has been observed in only 17% of patients.
• In 80% of the cases, the infection is asymptomatic or benign and lasts for one to four weeks.
• One in five infections is associated with severe damage to multiple organs, such as the liver, spleen and kidneys. Other complications are also possible: impairment of the central nervous system (encephalopathy, encephalitis, etc.), pericarditis, moon blindness and orchitis. During recovery, temporary alopecia and gait problems have been described.
• The most frequent complication of the disease is deafness. One-third of patients who survive the disease develop uni- or bilateral deafness, regardless of the severity of the infection. Half of them regain some of their hearing after 3 months.
• Pregnant women are at high risk of miscarriage and death.
• The global mortality rate is 1%. In hospital environments, the mortality of patients with a severe form of the disease can be up to 15%. Death usually occurs within 14 days of the appearance of the first symptoms.

As the symptoms of Lassa fever are so varied and non-specific, clinical diagnosis is often difficult, especially in the early stages of the disease. Lassa fever is difficult to distinguish from other viral haemorrhagic fevers, such as the Ebola virus and from other fever-causing diseases, including malaria, shigellosis, typhoid and yellow fever.

The diagnosis by laboratory tests is carried out by detecting antigens on samples from nose or throat swabs, sputum, urine or stool or by detecting antibodies in the serum.

Treatment consists of early supportive care and intravenous administration of Ribavirin, an antiviral agent, within 6 days. There is currently no vaccine available.

Given that Mastomys rats are very numerous in endemic areas and it is impossible to eliminate them completely, prevention of Lassa fever relies on promoting good ‘community hygiene’ and keeping cats to discourage rodents from entering homes.