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% but can reach 15% and above in severe hospitalised cases. During pregnancy, Lassa fever can cause high maternal and foetal mortality, especially late in pregnancy.

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

Pathogen(s)

Lassa virus belongs to the Arenaviridae family and its natural reservoir are rodents in the Mastomys genus, commonly known as the multimammate rat, which is not native outside Africa.

Transmission

Transmission can occur through:

• Exposure to excreta (urine, faeces) of infected rodents.

• Contact with infectious body fluids (blood, urine, vomitus, faeces, secretions, semen, breast milk) and/or organs of infected persons.

• Contact with infected animals or their tissues.

• Contact with contaminated surfaces or materials (e.g., needles and contaminated medical equipment).

Healthcare-associated transmission may occur during patient care, specimen collection, handling or laboratory processing, particularly through needlestick injuries, exposure to infectious blood or body fluids, improper cleaning or disinfection of medical equipment, reuse of needles, or contact with contaminated medical supplies. Appropriate infection prevention and control measures and laboratory biosafety precautions are essential to minimise these risks.

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. They remain infectious as long as the virus is present in the blood or body fluid.

Clinical features

The initial clinical manifestations of LF are non-specific and may be similar to other diseases such as other VHFs, malaria, rickettsial infections, enteric fever, and influenza.

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

• Muscle/chest/joint pain

• Nausea

• Vomiting

• Diarrhoea

• Cough

• Abdominal pain

Severe features include:

• Pharyngitis

• Respiratory and gastrointestinal symptoms

• Conjunctivitis

• Facial and neck oedema

• Pulmonary oedema

• Bleeding

• Shock

• Encephalitis

• Multi-organ failure

LF may lead to varying incidence of hearing loss in both mild and severe cases

Risk factors

Risk factors include:

• Relevant travel to affected countries or regions with ongoing Lassa virus circulation or outbreaks.

• Contact with infected rodents, their excreta (urine, faeces) or food contaminated by rodent droppings.

• Contact with confirmed or suspected cases, or their blood and body fluids.

• Contact with contaminated surfaces or materials (e.g., needles and contaminated medical equipment).

Diagnosis

Clinical diagnosis in acute infection is primarily by detection of Lassa virus by Reverse-transcription Polymerase Chain Reaction (RT-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. Laboratory investigations at the primary care level should not be performed.

Samples can be sent to NPHL for diagnosis. NPHL will provide laboratory support and guidelines for testing, including protocols for specimen transport, testing and reporting.

Treatment and management

There is no specific antiviral treatment approved for LF. Supportive care is the mainstay of treatment (e.g., maintaining fluid and electrolyte balance, management of symptoms and treatment of co-infections (e.g., malaria)).  While the antiviral drug ribavirin has been used in several countries, the drug is associated with side effects and there are some uncertainties about its efficacy. The care would need to be guided by ID specialist, and with multidisciplinary team.  A range of potential treatments including immune therapies and drug therapies are currently being evaluated. Patients with confirmed LF should 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 body 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)

Persons with skin or mucosal 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 CDA immediately. A log of all staff attending to the patient should be maintained.

CDA 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 further management.

Vaccination

There are currently no vaccines to prevent LF.

Notification

LF is an emerging infectious disease and is legally notifiable in Singapore. All suspected cases of LF should be reported to CDA immediately and instructions will be provided on further management of these cases. CDA 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

• On laboratory confirmation

How to notify:

• Please refer to the Infectious Disease Notification (opens in new tab) for more information.

Notification timeline:

• Immediately

Resources

Get LF situation updates from the World Health Organisation (WHO) (opens in new tab).