A patient is being treated in hospital for Lassa fever, a health watchdog said today.

The patient travelled in Nigeria before returning to the UK and is now being treated at the high security infectious diseases unit at the Royal Free Hospital in north London.

The Health Protection Agency (HPA) said the patient, whose gender is not being disclosed, represented an isolated case and there was no risk to the general public.

Lassa fever is caused by the Lassa virus and is endemic in Nigeria, Sierra Leone, Liberia, Guinea and the Central African Republic.

Hundreds of thousands of people are infected in these countries each year and isolated cases have been seen in Europe and the US.

Infection also occurs in other African countries including Mali, Senegal and the Democratic Republic of Congo.

Around eight in 10 people infected with Lassa virus develop mild or no symptoms, but in 20% of cases people will have severe illness.

Symptoms include fever, headache, sore throat, a cough, nausea, vomiting, diarrhoea and muscle pain.

Around 2 per cent of all patients die from the illness and death rates are particularly high in women in the final months of pregnancy.

A common complication of Lassa fever is deafness.

The patient being treated returned from Nigeria on January 6 and was initially admitted to Homerton University Hospital in east London.

He or she was then transferred to the Hospital for Tropical Diseases at University College Hospital.

Staff from the Homerton carried out testing for a wide range of common infections associated with travel to Africa, all of which came back negative.

Samples were then referred to HPA officials who Lassa fever was diagnosed on January 22.

Dr Dilys Morgan, a Lassa fever expert at the HPA, said: "This is an isolated case of Lassa fever in a traveller who has recently returned from Nigeria.

"It is important to stress that there is no risk to the general public from this patient.

"Lassa fever is an infection that is found in West Africa and is seen rarely in this country in those who have travelled to parts of the world where it is common.

"The infection is not easily spread to others and then only by direct contact with bodily fluids.

"The usual incubation period for Lassa fever is seven to 10 days.

"We are working closely with Homerton University Hospital and the Hospital for Tropical Diseases, University College Hospital to identify any members of staff who may have been involved in the care of this patient, and who came into contact with their bodily fluids.

"These people will be provided with information about Lassa fever and asked to get in contact with us should they develop any symptoms. Patients and visitors to the hospital are not at risk."

The Lassa virus is spread by infected rats through their urine and droppings.