Epidemiology

· Lassa fever occurs in Guinea, Liberia, Sierra Leone and regions of Nigeria.

· It is transmitted to humans from wild rodents (the multimammate rat, Mastomys natelensis). Lassa infection in rodents persists and the virus is shed throughout the life of the animal. Disease transmission is primarily through direct or indirect contact with excreta of infected rodents deposited on surfaces such as floors or beds, or in food or water.

· Person-to-person and laboratory infections occur, especially in the hospital environment, by direct contact with blood (including inoculation with contaminated needles), pharyngeal (throat) secretions or urine of a patient, or by sexual contact. Person-to-person spread may occur during the acute phase of fever when the virus is present in the throat.

· The virus may be excreted in the urine of patients for three to nine weeks from the onset of illness. Lassa virus can be transmitted via semen for up to three months.

· All age groups are susceptible to Lassa infection. The incubation period is 6-21 days.

Control The main methods of control are isolation of cases, disinfection, surveillance of contacts and rodent control.

· Isolation: Hospital transmission has occurred when inadequate infection control measures were practised. Therefore, strict barrier isolation of cases in a hospital room and strict procedures for handling of body fluids and excreta should be maintained.

· Disinfection: Patient's excreta, sputum, blood and all objects with which the patient has had contact, including laboratory equipment used to carry out tests on blood, should be disinfected with 0.5% sodium hypochlorite solution or 0.5% phenol with detergent, and, as far as possible, by appropriate heating methods such as autoclaving, incineration or boiling.

Laboratory tests should be carried out in special high-containment facilities; if there is no such facility, specimen handling should be kept to a minimum and performed only by experienced technicians using all available precautions such as gloves and biosafety cabinets. When appropriate, serum may be heat-inactivated at 60°C (140°F) for one hour. Thorough terminal disinfection with 0.5% sodium hypochlorite solution or a phenolic compound is adequate; formaldehyde fumigation can be considered.

· Surveillance of contacts: Identify all close contacts (people living with, caring for, testing laboratory specimens from or having non-casual contact with the patient) in the three weeks after the onset of illness. Close surveillance of contacts should be established by conducting body temperature checks at least two times daily for three weeks after last exposure. In case of temperature greater than 38.3°C (101°F), hospitalize immediately in isolation facilities. The place of residence of the patient during the three weeks prior to onset should be determined and a search initiated for unreported or undiagnosed cases.

Prophylaxis with ribavirin is recommended by some workers for close contacts, but there is little information about its efficacy. No vaccine is currently available.

· Rodent control: The ideal method of prevention in endemic areas is to prevent contact between rodents and humans. However, Mastomys rodents are found widely in Africa with the Lassa virus having been identified in the above-mentioned West African countries. Closely related viruses are found in Zimbabwe and Mozambique.

This species of rodent lives in close proximity to man and humans can be infected by inhalation of tiny droplets (aerosols) of virus-laden rodent excreta, by ingestion of contaminated foods or water, or through breaks in the skin.

Exposure may occur in the home or during occupational activities such as agricultural work or mining. As mentioned above, hospital infections and person-to-person transmission occur, but the number of these cases is small in comparison to the number of community-acquired infections resulting from contact with rodents.

To the extent possible, people in endemic areas should restrict entry of rats into their dwelling, isolate food supplies from rodents, eliminate habitats for rats and minimize activities that produce aerosols containing rodent excreta.