Lassa Fever - Fact Sheet for Health Professionals

(Images from CDC, NCID, Special Pathogens Branch)

Lassa Fever

Lassa fever is an acute viral haemorrhagic fever (VHF) caused by Lassa virus, a member of the arenavirus family. The disease was first described in the 1950s, and the virus was identified in 1969, when two missionary nurses died from it in the town of Lassa in Nigeria. It is known to be endemic in parts of West Africa, namely Guinea, Liberia, Sierra Leone, and Nigeria. There is some evidence of endemicity in the Central African Republic, Mali, Senegal and other neighbouring countries (see maps). In 2009 a confirmed case from Mali was imported into the UK, and in 2011 a case was reported for the first time in the Democratic Republic of the Congo.

Clinical symptoms

Infection is mild or asymptomatic in 80% of cases, but can cause severe illness and is fatal in approximately 1-3% of patients. The incubation period for disease is usually between 7 and 10 days, although a range of 3-21 days has been reported in some cases.

Early disease

• The onset of illness is insidious, with fever and shivering accompanied by malaise, headache, generalised aching and a sore throat. This may be accompanied by nausea, vomiting, diarrhoea or cough.
• There may be patches of white or yellowish exudate and occasionally small vesicles or shallow ulcers on the tonsils and pharynx and this is an important diagnostic feature.
• As the illness progresses the body temperature may rise to 41ºC with daily fluctuations of 2-3ºC. Fever is very variable, occurring constantly or in peaks, and lasting on average for 16 days; extremes of 6-30 days have been reported.

Late/severe disease

Severe attacks are characterised by extreme lethargy and exhaustion, disproportionate to the level of fever. During the second week of illness there may be oedema of the head and neck, encephalopathy, pleural effusion, and ascites. Renal and circulatory failure may occur, aggravated by vomiting and diarrhoea. In the severest cases bleeding into the skin, mucosae and deeper tissues occurs, usually leading to death.

In non-fatal cases the fever subsides and the patient's condition improves rapidly although tiredness may persist for several weeks. There is usually a leucopaenia although a high polymorphonuclear leucocytosis is encountered occasionally. Late complications include sensorineural deafness in around 25% of patients, persisting for life in around a third of those affected.

Infection is fatal in around 15% of hospitalised patients. Lassa fever is particularly severe in pregnant women in the third trimester; the foetus dies in about 95% of cases. Symptoms in children are similar to those in adults, but infant infection can result in "swollen baby syndrome" with oedema, abdominal distension, bleeding and often death.

Diagnosis

Clinical diagnosis of Lassa fever is difficult, and it can be confused with other infections such as severe malaria, typhoid fever, and other viral haemorrhagic fevers.

Highly specialised laboratory facilities are required for a definitive diagnosis, and samples must be handled with extreme care to prevent transmission. Lassa fever can be diagnosed using RT-PCR for nucleic acid detection, virus isolation or antibody detection methods. ( Laboratory services)

Transmission

Lassa virus is present in wild multimammate rats ( Mastomys species), which shed the virus in their urine and droppings. These are common in rural areas of tropical Africa, and often live in or around homes. Mastomys breed very frequently and produce large numbers of offspring. Once infected, rodents continue to shed virus throughout their life.

Transmission of Lassa virus to humans normally occurs through contamination of broken skin or mucous membranes via direct or indirect contact with infected rodent excreta, on floors, home surfaces, in food or water. Transmission is also possible where rodents are caught and consumed as food.

Person to person transmission occurs through exchange of infected bodily fluids, such as blood, saliva, urine or semen. This can happen either in the laboratory, in a healthcare setting, or via sexual or other close contact. Transmission to close contacts usually only occurs while the patient has symptoms, and the virus is present in the throat. However, virus can be excreted in the patient's urine for between 3 and 9 weeks after the onset of illness, and may be transmitted via semen for up to three months.

Risk groups

People living in endemic areas of West Africa with high populations of rodents are most at risk of Lassa fever. Imported cases rarely occur elsewhere in the world. Such cases are almost exclusively in persons who have worked in endemic areas in high risk occupations such as medical or other aid workers. The risk to tourists is considered to be very low. Imported Lassa fever is extremely rare in the UK; only 8 confirmed cases of Lassa fever have been imported since 1980, and there has been no evidence of onward transmission from any of these cases.

People in close contact with the bodily fluids of infected patients are at risk of transmission, however this is rare and medical staff are at low risk if adequate protective measures are taken.

Guidelines for Management and Control

In the UK, there are formal Guidelines for the management and control of viral haemorrhagic fevers. The document is available here

Treatment

Treatment with the antiviral drug ribavirin is most effective when started within the first 6 days of illness, and should be given intravenously for 6 days. However, this drug does not prevent deafness associated with Lassa fever. Supportive care such as fluid replacement, blood transfusion or other appropriate measures is also essential.
 

Prevention and Control

There is no vaccine, although research is underway to produce one.

In endemic areas infection can be prevented by rodent control, and avoiding contact with rodents or their excreta including storage of food in rat proof containers etc.

Person to person spread is prevented by taking appropriate measures to avoid contact with bodily fluids of an infected patient. In healthcare settings this requires the use of special barrier nursing procedures or VHF isolation precautions, which involve isolating infected patients, and staff wearing protective clothing for contact with the patient. In Britain, patients with Lassa fever are cared for in a high security infectious disease unit.

All objects with which the patient has had contact should be disinfected with sodium hypochlorite solution containing 1000ppm available chlorine, or 0.5% phenol with detergent, and if possible by autoclaving, incineration or boiling. Overt contamination with blood or body fluids should be treated with freshly prepared sodium hypochlorite solution containing 10,000ppm available chlorine.

Once the patient has recovered they are only infectious via semen and urine. Sexual intercourse must be avoided for 3 months.

Contact surveillance

Those persons who have been in close contact with an infectious patient must be identified within three weeks of the patient's onset of illness. As a precaution, contacts deemed to be at high risk should be monitored by taking their temperature daily for 21 days after their last exposure to the patient. If a high temperature (above 38ºC/101ºF) develops the contact should stay at home and inform the person in charge of their monitoring immediately. Further details of these procedures are available from the Duty Doctor at HPA Colindale.

Last reviewed: 19 April 2012