Lassa Fever – United Kingdom of Great Britain and Northern Ireland

On 9 February 2022, WHO was notified by the United Kingdom health authorities of two laboratory confirmed cases and one probable case of Lassa fever. As of 18 February, three cases have been confirmed and one death has been reported. These are the first reported Lassa fever cases in the United Kingdom since 2009, and represents the second known case of secondary transmission of Lassa fever in Europe.

The first case travelled to Mali in late 2021, where Lassa fever is endemic. After returning to the United Kingdom, the individual developed symptoms of fever, fatigue and loose stool, and was subsequently hospitalized. The first case has now recovered. The second and third cases were family members of the first case and did not travel to Mali. Both were admitted to the hospital and the third case has sadly died.

On 8 February, samples from both the first and second cases were laboratory confirmed positive for Lassa virus by PCR. The third case was initially provided care as a probable case of Lassa fever and the diagnosis was confirmed on 9 February.

Incident responses have been activated by health authorities in the United Kingdom to coordinate the clinical and public health management of these cases.

People with exposure to the cases were identified through robust contact tracing activities. Low and high risk contacts are being followed up for 21 days after their last exposure. Post-exposure prophylaxis has been offered to high-risk contacts with the most significant exposures. Systems are in place to assess and manage any contacts who may develop symptoms during the follow up period.

Lassa fever is an acute viral haemorrhagic fever illness that is primarily transmitted to humans via contact with food or household items contaminated with infected Mastomys rats’ urine or faeces. Human-to-human transmission is rare but can occur through direct contact with the blood, urine, faeces, or other bodily secretions, particularly in health care settings. It can cause severe disease in about 20% of patients. The case fatality ratio (CFR) is 1% overall and 15% in severely ill patients. Early recognition and initiation of patient care decrease the CFR and public health impact.

Lassa fever is endemic in Mali and sporadic cases have been exported to Europe from countries in West Africa where the disease is endemic, such as Sierra Leone, Togo, Liberia and Nigeria in recent years. However, the secondary transmission of Lassa fever through human contacts is known to be extremely rare when appropriate infection prevention and control precautions and laboratory biosafety measures are applied. The only secondary transmission of Lassa fever reported in Europe was in 2016 from Germany, when the secondary case performed post-mortem care of a fatal case.

Human-to-human transmission occurs in both community and health care settings, where the virus may be spread by contaminated medical equipment, such as reused needles, as well as direct contact with bodily secretions. Health care workers are at risk if caring for Lassa fever patients in the absence of appropriate infection prevention and control (IPC) practices, although information from recent imported cases in Europe show that secondary transmission of Lassa virus is rare when standard IPC precautions are followed. The possibility of additional secondary cases related to transmission in a health care setting cannot currently be excluded, but is considered very unlikely in the community.

Prevention of Lassa fever in endemic countries relies on community engagement and promoting hygienic conditions to discourage rodents from entering homes. Family members and health care workers should always be careful to avoid contact with blood and body fluids while caring for sick individuals.

In health care settings, staff should consistently implement standard IPC measures when caring for patients to prevent nosocomial infections. Health care workers caring for patients with suspected or confirmed Lassa virus, or handling their clinical specimens, should reinforce standard and contact precautions, comprising appropriate hand hygiene, rational use of personal protective equipment (PPE), environmental cleaning, isolation, safe injections practices and safe burial practices. In order to avoid any direct contact with blood and body fluids and/or splashes onto facial mucosa (eyes, nose, mouth) when providing direct care for a patient with suspected or confirmed Lassa virus, personal protective equipment should include: 1) clean non-sterile gloves; 2) clean, non-sterile fluid resistant gown; and 3) protection of facial mucosa against splashes (mask and eye protection, or a face shield).

Given the nonspecific presentation of viral haemorrhagic fevers, isolation of ill travellers and consistent implementation of standard and contact precautions is key to preventing secondary transmission. When consistently applied, these measures can prevent secondary transmission even if travel history information is not obtained, not immediately available, or the diagnosis of a viral haemorrhagic fever is delayed.

There is currently no approved vaccine. Early supportive care with rehydration and symptomatic treatment improves survival.

Considering the seasonal increases of cases between December and March, countries in West Africa that have endemic Lassa fever are encouraged to strengthen their related surveillance systems to enhance early detection and treatment of cases, and to reduce the case fatality rate, as well as to strengthen cross border collaboration.

• WHO Lassa fever fact sheet
• UK Health Security Agency, Lassa fever cases identified in England, following travel to West Africa

Citable reference: World Health Organization (21 February 2022). Disease Outbreak News; Lassa fever – United Kingdom of Great Britain and Northern Ireland. Available at: https://covid.comesa.int/emergencies/disease-outbreak-news/item/lassa-fever-united-kingdom-of-great-britain-and-northern-ireland