Lassa Fever

Causative agent

Lassa fever, a zoonotic disease, is an acute viral illness caused by Lassa virus which belongs to the family Arenaviridae. The reservoir host of Lassa virus is the multimammate rat Mastomys natalensis, which are common in West Africa.

Lassa fever was first identified in 1969 in Nigeria and named after the town in Nigeria where the first cases occurred. The disease is endemic in Benin, Ghana, Guinea, Liberia, Mali, Nigeria and Sierra Leone, but probably exists in other west African countries as well.

Clinical features

Symptom onset is usually gradual, starting with fever, general weakness, headache and malaise. After a few days, sore throat, muscle pain, chest pain, nausea, vomiting, diarrhoea, cough and abdominal pain may follow. As the disease progresses, facial swelling, fluid in the lung cavity, bleeding from the mouth, nose, vagina or gastrointestinal tract and low blood pressure may develop in severe cases. Shock, seizures, tremor, disorientation, and coma may be seen in the later stages. Deafness occurs in 25% of patients who survive the disease. In fatal cases, death usually occurs within 14 days of symptom onset due to multi-organ failure. The overall case fatality rate of Lassa fever is 1% but observed case fatality rate among patients hospitalized with severe diseases is 15% and above.

Mode of transmission

Humans typically become infected either through exposure to food or household items that are contaminated with urine or faeces of infected multimammate rat, or direct contact with infected rats. Infection can also occur by breathing in air contaminated with infected urine or droppings. Lassa virus can spread through human-to-human transmission via direct contact, through broken skin or mucous membranes, with the blood, urine, faeces or other bodily secretions of an infected person. The disease can also be transmitted in hospital settings without appropriate infection prevention and control practices.

Incubation period

It ranges from 2 to 21 days.

Management

Currently, there is no licensed vaccine or antiviral treatment approved for Lassa fever. Early intensive supportive care including rehydration and symptomatic treatment can improve survival.

Prevention

There is no licensed vaccine available for Lassa fever. To prevent the infection, the public should adopt the following measures when travelling to affected places:

• Perform hand hygiene frequently, especially before and after touching the mouth, nose or eyes; before eating; after using the toilet, after touching public installations such as handrails or door knobs; or when hands are contaminated by respiratory secretion after coughing or sneezing. Wash hands with liquid soap and water, and rub for at least 20 seconds. Then rinse with water and dry with either a clean cotton towel or a paper towel. If hand washing facilities are not available, or when hands are not visibly soiled, hand hygiene with 70 to 80% alcohol-based handrub is an effective alternative.
• Avoid close contact with feverish or ill persons, and avoid contact with patients’ blood and bodily fluids, and objects contaminated with blood or bodily fluids of patients.
• Avoid contact with rodents.
• Cook food thoroughly before consumption.
• Store food in rodent-proof containers.
• Travellers should seek medical advice promptly if they become ill within 21 days after returning from affected areas and inform doctor of recent travel history.

When taking care for patients with suspected or confirmed Lassa fever, healthcare workers should always apply infection control measures, including the use of appropriate personal protective equipment and avoid contacting patients’ blood and bodily fluids and contaminated surfaces or materials such as clothing and bedding.