Lassa virus

Scientific summary

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What is it?

Lassa virus belongs to the Arenaviridae family and causes Lassa fever, also known as Lassa haemorrhagic fever (LHF). It is a haemorrhagic illness that occurs between one and three weeks after infection.

Lassa fever is a zoonotic disease, meaning it passes from animals to humans. The natural host of Lassa virus is the rodent Mastomys natalensis, otherwise known as the Natal multimammate mouse or rat.

The virus is spread when a person comes into contact with items contaminated with the rodent’s urine or faeces – for example, by handling objects, eating or through open wounds. It can also be inhaled.

Lassa virus can pass from person to person via bodily fluids, and can spread in healthcare settings if suitable precautions are not taken.

Where does it occur?

Lassa virus was identified in 1969 after the death of two missionary nuns in Nigeria. Lassa fever occurs regularly (is endemic) in parts of West Africa, including, Sierra Leone, Nigeria, Guinea, Liberia, Mali Benin and Ghana. It causes more than 300,000 cases of illness and 3000 deaths each year.

Various outbreaks were reported in West Africa in 2016. In March 2016, a healthcare worker evacuated from Togo to Germany died and was subsequently diagnosed with Lassa fever. A funeral home employee who handled the corpse then contracted the disease, but recovered. In Sweden, a 72-year-old woman was diagnosed with Lassa fever following a six-week trip to Liberia. She was later discharged.

In 2018, Nigeria experienced an unusually large increase in Lassa fever cases which led the WHO to declare it an outbreak. From 1 January to 14 October 2018, there has been a total of 536 confirmed cases, with 137 deaths in confirmed cases and 16 in probable cases – giving a case fatality rate of 25.6%. A recent study identified that the spread of disease was largely a result of ongoing cross-species transmission from local rodent populations, rather than via human-human transmission.

What does it do?

Around 80 per cent of patients with Lassa fever have no symptoms. Those that do can experience mild fever or headache. More serious symptoms include:

  • vomiting
  • swelling of the face
  • pain in the chest, back and abdomen
  • bleeding from body parts, including the eyes and nose.

Patients can be given rehydration therapy and supportive care. An antiviral drug ribavirin has been used successfully in some cases.

One per cent of infections are fatal, usually within two weeks of symptoms beginning. Around 15 to 20 per cent of people hospitalised with Lassa fever die from it. A common complication in survivors of Lassa fever is hearing loss.

How do we currently prevent infections?

People are advised to avoid contact with Mastomys rodents, including by keeping homes clean and storing food away from rodents. Healthcare workers should follow specific infection control methods. There is currently no vaccine against Lassa virus.