Cases of the rat-borne virus detected in the UK have been linked to travel to west Africa and are being closely monitored.
A person in the UK has died after contracting Lassa fever, health officials have said.
Two cases were announced earlier this week with a third individual under investigation. The UK Health Security Agency (UKHSA) confirmed on Friday that the third case was Lassa fever and the patient had died.
The cases are within the same family and are linked to recent travel to west Africa. They are the first cases identified in the UK since 2009.
The patient was being treated by Bedfordshire Hospitals NHS Foundation Trust.
A spokesperson said: "We confirm the sad death of a patient at our trust, who had confirmed Lassa fever. We send our deepest condolences to their family at this difficult time.
"We will continue to support the patient’s family and our staff and are working closely with colleagues from the UK Health Security Agency to undertake a robust contact tracing exercise".
One of the cases has recovered, while the other will receive specialist care at the Royal Free London NHS Foundation Trust.
What is Lassa Fever?
Lassa fever is a viral illness caused by Lassa virus which occurs mostly in countries in west Africa, including Nigeria, Sierra Leone, Mali, Liberia and Guinea. It’s transmitted to humans mainly through food or household items contaminated by infected rats’ urine and faeces.
The most common symptoms include gradual onset of fever, malaise and general weakness, according to the World Health Organisation (WHO), and this can progress to headaches, vomiting and diarrhoea.
Various degrees of deafness also occur in 25 per cent of recovered patients, although for half of this group, it will return after one to three months.
In the most severe cases, facial swelling, fluid in the lungs, bleeding from the mouth, nose, vagina or gastrointestinal tract, and low blood pressure may present as symptoms, according to the WHO.
Is Lassa fever a cause for concern?
"Cases of Lassa fever are rare in the UK and it does not spread easily between people. The overall risk to the public is very low," said Dr Susan Hopkins, Chief Medical Advisor at UKHSA.
In 2019, EU/EEA countries reported just two cases.
The virus can be spread between humans, but only through bodily fluids. Before these recent cases there have only been eight cases of lassa fever imported to the UK since 1980.
Where there are infections though, it can have serious outcomes. Around 1 per cent of cases prove fatal, with death happening within 14 days.
Lassa fever is especially severe late in pregnancy, with maternal death and/or foetal loss occurring in more than 80 per cent of cases during the third trimester. However, 80 per cent of those infected have no symptoms.
Because the symptoms of Lassa fever are so varied and non-specific, clinical diagnosis is often difficult, especially early in the course of the disease. And there is currently no vaccine against it.
While it may not be a cause for concern in Europe, it remains endemic in west Africa and there are 100,000 to 300,000 Lassa fever cases estimated to be diagnosed each year.