Lassa fever - an acute viral infection - belongs to a family of zoonotic diseases referred to as viral haemorrhagic fevers (VHFs) which can cause a haemorrhagic syndrome in humans.  

Lassa fever was first identified in Lassa, Nigeria in 1969.  A member of the Arenaviridae virus family, it is endemic in those countries in West Africa washed by the Gulf of Guinea including Guinea, Liberia, Sierra Leone, Nigeria and Benin. It is contracted by exposure to the excreta of infected rodents (the primary host is Mastomys natalensis - the Natal multimammate mouse). Secondary person to person transmission can occur through exposure to body fluids of infected persons.

Eighty percent of patients who develop Lassa fever have either minor or no symptoms. However, 20% of patients develop severe, multisystem disease with: fever, retrosternal pain, pharyngitis, myalgia (especially felt in the thoraco-lumbar region), cough, abdominal pain, vomiting, diarrhoea, conjunctivitis, facial oedema, proteinuria, and mucosal bleeding. Lassa fever is associated with occasional epidemics, during which the case-fatality rate can be as high as 50%. The incubation period of Lassa fever ranges from 6–21 days.  As the symptoms of Lassa fever are quite varied and nonspecific, clinical diagnosis can be challenging.

To date in 2016, three cases of Lassa fever have been reported in Europe; two imported cases and one secondary case infected in Europe. These cases highlight the need for clinicians in Europe to remain vigilant about VHFs such as Lassa fever in returned travellers.

Two cases were reported in Germany in March. The first case was in a healthcare worker who had been medically evacuated to Germany from Togo for treatment of suspected malaria on 25 February 2016 (1). The area of Togo where the case was working borders Benin, where there is on ongoing outbreak of Lassa fever (2, 3). The patient died on 26 February from multi-organ failure. Lassa fever was confirmed post-mortem on 9 March after an autopsy showed signs suggestive of haemorrhagic disease. A secondary case was reported by Germany in a funeral home worker who had contact with the body of the primary case on 3 March. The secondary case had been placed under voluntary home quarantine since the diagnosis in the primary case on 9 March. Lassa fever was confirmed after the case developed symptoms on 15 March. Public Health authorities in Germany undertook surveillance of contacts of both cases. No further secondary cases occurred.  

An unrelated case was reported by Sweden on 1 April 2016 (4). The case had travelled to northern Liberia where she was reported to have had exposure to rodents. The case returned to Sweden on 2 March, developed symptoms about one week later, and was hospitalised on 17 March. Lassa fever was confirmed on 1 April by Sweden’s Public Health Agency laboratory. Swedish authorities undertook contact tracing and surveillance of 75 contacts of the case. No secondary cases have been reported to date.

These latest cases bring the number of imported cases of Lassa fever to Europe since 2000 to 10 (1). The recent secondary case is the first symptomatic case infected in Europe. A previous secondary case infected in Germany had serological evidence of infection but remained asymptomatic (5).

The delayed diagnosis of the two recent imported cases in Europe highlight the need for clinicians to be vigilant about the possibility of Lassa fever in a sick traveller who has recently returned from West Africa. The case of secondary transmission within Europe stresses the importance of observing appropriate infection prevention and control measures. The Management of Viral Haemorrhagic Fevers in Ireland provides guidance on the initial clinical assessment and management, the required infection prevention control measures, and the Public Health management of a suspected case of VHF (6).

Travellers to West Africa also need to be aware of the risk of exposure and should avoid exposure to rodents and consumption of food or drink which could have been contaminated by rodent excreta. Aid-workers travelling to these regions should be aware of the risk of exposure and ensure appropriate personal protective measures when caring for patients with possible haemorrhagic fever.

 Eve Robinson, Paul McKeown, HPSC

 References

1.         European Centre for Disease Prevention and Control. Lassa fever in Nigeria, Benin, Togo, Germany and USA – 23 March 2016. Stockholm2016.

2.         World Health Organization. Emergencies preparedness, response; Disease Outbreak News; Lassa fever - Togo: World Health Organization; 2016 Available here. 

3.         World Health Organization. Emergencies preparedness, response; Disease outbreak news; Lassa fever – Benin 2016 Available here. 

4.         World Health Organization. Emergencies preparedness, response; Disease Outbreak News; Lassa fever - Sweden 2016 Available here.

5.         Haas WH, Breuer T, Pfaff G, Schmitz H, Kohler P, Asper M, et al. Imported Lassa fever in Germany: surveillance and management of contact persons. Clin Infect Dis. 2003;36(10):1254-8.

6.         Health Protection Surveillance Centre. The Management of Viral Haemorrhagic Fevers in Ireland, Report of the Scientific Advisory Committee of the Health Protection Surveillance Centre. Dublin2012.