Lassa disease was first described in the 1950s but the virus was not identified until 1969 when two missionary nurses died from it in the town of Lassa, Borno state, Nigeria. The virus is a single-stranded RNA virus belonging to the virus family Arenaviridae. Lassa fever is an acute viral hemorrhagic illness of 1-4 weeks duration that occurs in West Africa.
Lassa fever is a zoonotic disease, meaning that humans become infected from contact with infected animals. The host of Lassa virus is a rodent of the genus Mastomys, commonly known as the “multimammate rat.” Mastomys rats infected with Lassa virus do not become ill, but they can shed the virus in their urine and faeces and live in homes and areas where food is stored.
The disease can be contacted by ingestion of foods and drinks contaminated by the saliva, urine and faeces of infected rats. Others include catching and preparing infected rats as food, inhaling tiny particles in the air contaminated with infected rat urine or droppings, and direct contact with a sick person’s blood or body fluids, through mucous membranes, like eyes, nose, or mouth particularly in hospitals lacking adequate infection prevent and control measures. There is no epidemiological evidence supporting airborne spread between humans.
Lassa fever occurs in all age groups and both sexes. Persons at greatest risk are those living in rural areas where Mastomys are usually found, especially in communities with poor sanitation or crowded living conditions. Health workers are at risk if caring for Lassa fever patients in the absence of proper barrier nursing and infection control practices. Families and friends of an infected person, in the course of feeding, holding and caring for them are also at risk.
About 80% of people who become infected with Lassa virus have no symptoms. One in five infections results in severe disease, where the virus affects several organs such as the liver, spleen and kidneys.
The incubation period of Lassa fever ranges from 6-21 days. Within three weeks of coming in contact with the virus, symptoms include fever, headache, chills, diarrhoea, cough, nausea, vomiting, sore throat, abdominal pain, backache, and joint pains. Late symptoms include bleeding from the eyes, ears and nose, bleeding from the mouth and rectum, low blood pressure, eye swelling, swelling of the genitals and rashes all over the body that often contain blood. Protein may be noted in the urine. It could progress to coma, shock and death. Lassa fever is suspected in persons who present with above symptoms with a positive history of being in contact with a suspected or infected person or health worker who had treated either suspected or confirmed infected person. Deafness occurs in 25% of patients who survive the disease. In half of these cases, hearing returns partially after 1-3 months. Transient hair loss and gait disturbance may occur during recovery.
Death usually occurs within 14 days of onset in fatal cases. The disease is especially severe late in pregnancy, with maternal death and/or fetal loss occurring in greater than 80% of cases during the third trimester.
Antiviral drugs can successfully treat Lassa fever. The earlier a person presents, the better the outcome of treatment.
The general public is advised to take note of the following for prevention and control:
1. Avoid contact between rats and human beings.
2. Observe good personal hygiene including hand washing with soap and running water regularly
3. Dispose of your waste properly and clean the environment so that rats are not attracted
4. Store foods in rat proof containers and cook all foods thoroughly before eating.
5. Discourage rodents from entering the house by blocking all possible entry points.
6. Food manufacturers and handlers should not spread food where rats can have access to it.
7. Report any cases of above symptoms or persistent high fever not responding to standard treatment for malaria and typhoid fever to the nearest health centre.
8. All fluids from an infected person are extremely dangerous. Health workers are also advised to be at alert, wear personal protective equipment and observe universal basic precautions.
Early supportive care with rehydration and symptomatic treatment improves survival.
Because the clinical course of the disease is so variable, detection of the disease in affected patients has been difficult. However, when presence of the disease is confirmed in a community, prompt isolation of affected patients, good infection protection and control practices and rigorous contact tracing can stop outbreaks.
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