Lassa fever

Overview 

Lassa Fever (LF) is an acute viral haemorrhagic fever (VHF) caused by the Lassa virus, which is known to be endemic in parts of West Africa including Benin, Ghana, Guinea, Liberia, Mali, Nigeria, and Sierra Leone, but likely exists in other West African countries as well.

Disease epidemiology

In endemic countries, LF cases typically peak during the dry season from November to May. However, transmission also occurs all year round with sporadic cases being reported, but below epidemic thresholds. About 100,000 to 300,000 cases occur per year in West Africa, with an estimated 5,000 deaths per year. The overall case-fatality rate for LF is around 1% to 2% in Africa but can reach up to 20% in severe hospitalised cases.

With continued circulation of the Lassa virus in endemic countries, sporadic exportation events to non-endemic countries have been reported.

Pathogen(s)

Lassa virus is an arenavirus and its natural reservoir is the Mastomys natalensis, commonly known as the multi-mammate rat, which is not native outside Africa.

Transmission

Transmission can occur through direct contact with infected rodents or inhalation of aerosol from rodent excretions. Human-to-human transmission can occur through direct exposure to blood or other bodily fluids from an infected person, or through sexual transmission.

Incubation period: Typically 10 days; range is 2 to 21 days.

Infectious period: Infected individuals are not contagious during the incubation period and become infectious once they begin to develop symptoms.

Clinical features

Infection is mild in 80% of cases (low grade fever, malaise, headache). In 20% of patients, more prominent symptoms and severe disease may develop.

Initial symptoms include:

• Gradual onset of fever

• Malaise

• Sore throat with or without oral ulcerations

• Headache

• Back/chest/joint pain

• Nausea

• Vomiting

• Diarrhoea

• Cough

Severe features include:

• Pharyngitis

• Respiratory and gastrointestinal symptoms

• Conjunctivitis

• Facial and neck edema

• Pulmonary oedema

• Bleeding

• Shock

• Encephalitis

• Multi-organ failure

Risk factors

Risk factors include:

• Exposure to rats, rodent droppings, or food contaminated by rodent droppings.

• Unprotected exposure to blood and body fluids from infected cases or contaminated environment.

Diagnosis

Clinical diagnosis in acute infection is primarily by detection of Lassa virus by polymerase chain reaction (PCR) in blood specimens and/or virus isolation from throat swab or urine specimen. The level of viremia generally declines after the sixth day of illness, but in severe cases may persist till mortality.

Diagnostic tests for Lassa virus infections should be performed only after consultation with MOH and the National Public Health Laboratory (NPHL) and samples can be sent to NPHL for diagnosis. The laboratory should contact NPHL for further assistance.

Treatment and management

The initial clinical manifestations of LF are non-specific and may be similar to other diseases such as the other VHFs, malaria, rickettsial infections, enteric fever, and influenza. Pending diagnostic evaluation for LF and other VHFs, and diagnostic evaluation for other similar illness, empirical treatment (e.g., for malaria, enteric fever, rickettsial disease) should be considered, according to local guidelines.

Treatment for LF involves supportive care and the use of ribavirin. Early initiation of ribavirin is recommended for confirmed cases, with preference for intravenous administration over oral if possible. It is important to note that ribavirin is teratogenic, so patients need to be carefully counselled about its side effects and its impact on conception plans for six months. Female patients of reproductive age should be screened for pregnancy prior to its use and should be advised to avoid pregnancy for six months after treatment with ribavirin. Pregnancy should be avoided for six months in women partners of male patients who are treated with ribavirin. Patients with confirmed LF will be transferred for further management at the High-Level Isolation Unit (HLIU) at the National Centre for Infectious Diseases (NCID).

Precaution, prevention and control

General Advice

Patients who are under investigation for LF, pending transfer to the HLIU in NCID, should be isolated in a single-bedded isolation room, and be placed on strict contact and respiratory precautions. Entry of non-essential staff to the patient’s room should be restricted, a register of all staff who enter the patient’s room should be maintained and visitors should not be allowed.

All staff who come into direct contact with a suspected or confirmed LF case or their bodily fluids, including (but not limited to) healthcare workers, persons responsible for transporting patients (e.g., ambulance staff), and cleaners should apply extra infection control measures and wear enhanced personal protective equipment (PPE), comprising:

• Disposable fluid-resistant hood to cover the head and neck areas (staff with long hair may wish to use head cover prior to wearing hood)

• Eye protection gear (e.g., disposable downward face shields secured at forehead or goggles)

• Fluid-repellent N95 mask

• Inner and outer disposable fluid-resistant (AAMI 4) gown (should extend to below knee)

• 12 - inch double nitrile or latex gloves certified for healthcare use (extended cuffs should reach up to mid forearm)

• Disposable fluid resistant boot covers (should extend up to knee-height)

Contacts of persons with LF or healthcare workers with unprotected exposure to an infected patient or their body fluids are at risk of acquiring disease. Such exposures include exposure of mucous membranes or broken skin to blood/body fluids (including respiratory secretions), without the proper PPE, or needle stick injury.

Persons with skin or mucousal exposure to body fluids from a suspect LF case should immediately wash the affected skin surfaces with soap and water. Mucous membranes (e.g. conjunctiva) should be irrigated with copious amounts of water or eyewash solution.

Staff attending to suspect LF case should be closely monitored (e.g. twice daily temperature monitoring), even if they wear the appropriate PPE. Persons with unprotected exposure should seek advice from an infectious disease expert and report the exposure to MOH immediately. A log of all staff attending to the patient should be maintained.

MOH will conduct contact tracing for close contacts and take appropriate follow-up actions, such as quarantine, phone surveillance, or self-monitoring. Contacts may also be referred to NCID for assessment and post-exposure prophylaxis with oral ribavirin.

Notification

LF is an emerging infectious disease and is legally notifiable in Singapore. All suspected cases of LF should be reported to MOH immediately and instructions will be provided on further management of these cases. MOH will arrange for transfer of suspected cases to NCID on a case-by-case basis. Confirmed LF cases will be managed at the HLIU in NCID.

Who should notify:

• Medical practitioners

• Laboratories

When to notify:

• On clinical suspicion

Suspect case definition:

• Persons with fever and/or viral syndrome (e.g., headache, vomiting, fatigue or cough); and

• Onset of symptoms within 21 days of:

  • Travel history to an endemic area (West Africa, mainly in Benin, Ghana, Guinea, Liberia, Mali, Sierra Leone, and Nigeria)

  • Travel history to an area with an ongoing LF outbreak [refer to World Health Organization (WHO)]

  • Contact with a patient with suspect or confirmed LF

• On laboratory confirmation of positive PCR or virus isolation

How to notify:

• Please refer to the Infectious Disease Notification for more information.

Notification timeline:

• Immediately

Resources

Get LF situation updates from WHO.