What is Lassa Fever?

By Liji Thomas, MD

Lassa fever is a viral hemorrhagic fever caused by an RNA virus of the arenavirus family. It is most common in a large region of West Africa, and is named after the Nigerian town where it was first discovered by Western medical professionals. It is animal-borne, and the vector is a species of rat named Mastomys natalensis. Besides Nigeria it is endemic in Guinea, Liberia, and Sierra Leone. It is also found sporadically in neighboring countries due to the distribution of the rat’s habitat over the whole region.

The mortality due to Lassa fever is about 5000 deaths a year, out of 100,000 to 300,000 cases. The death rate depends on the severity of illness. Up to 20% of hospitalized patients die of the infection, but the mortality shoots up to 50% during an epidemic. In contrast, about 1% of all infected cases die.  

The virus is shed in the urine and feces of infected rats for possibly the whole lifetime of the animal. This rat is widespread throughout the forests and grasslands of West and Central Africa. It is also common inside homes and food storage areas, leading to a high frequency of transmission to humans.

Routes of infection

The infection may be acquired through:

  • Contact between broken skin or mucous membranes and rat excreta
  • Inhalation of the aerosol, as occurs during the sweeping of an area where the droppings are present
  • Ingestion of food contaminated with rat urine or feces
  • The use of the rat as food, with both preparation and eating of the rodent being potential routes of infection
  • Person to person contact via sweat, blood, feces, urine or other body fluids
  • Contaminated needles, or exposure to an infected aerosol, in healthcare settings.

The symptoms take up to 3 weeks to manifest, and are so mild as to be ignored in about 80% of cases. They start with a low fever, tiredness, malaise, body aches, a sore throat and headache. There may be nausea and vomiting, or diarrhea.

In more serious cases, high fever develops, along with symptoms such as:

  • Bleeding from the eyes, the nose or the gums (which accounts for its classification as a hemorrhagic fever)
  • Appearance of ulcers, or small vesicles, on the palate and throat, with patches of white or yellow exudate
  • Breathing difficulties
  • Swelling of the face
  • Severe pain in the back, the chest or the abdomen
  • Effusion from the pleura or the peritoneum
  • Shock
  • A few patients also have nervous symptoms such as deafness or tremors

Death occurs within 2 weeks in seriously sick patients, and is due to the failure of multiple organs. Death is preceded by the occurrence of widespread bleeding into the skin, the mucosa and internal organs.

Complications of the disease include:

  • Deafness (in about a fourth of affected people), which is permanent in about a third of patients
  • Spontaneous abortions
  • Fetal death (in up to 95% of pregnant women with Lassa fever)
  • Maternal death when infection occurs during the third trimester of pregnancy  

Diagnosis and treatment

Diagnosis is by:

  • Serologic tests, namely, the ELISA for IgM and IgG antibodies, and for the virus antigen
  • Virus culture is another technique, requiring a week to 10 days
  • Tissue staining with specific antibody-linked stains can show the virus on post-mortem
  • A research technique used in lab diagnosis is the use of RT-PCR

Treatment with Ribavirin is quite effective, but needs to be given in the earlier stages of infection, in the first 6 days.

Supportive treatment with oxygen, intravenous fluids, antibiotics for other complicating infections and respiratory support are all vital in maximizing the chances of recovery.

Risk factors

Exposure to a large population of infected rats or humans is an effective means of virus transmission.


To prevent rat-human transmission, contact between the rats and humans should be minimized:

  • Use rat-proof containers for food storage
  • Avoid using rats as food
  • Avoid attracting rats to the house by cleanliness and healthy waste disposal practices
  • Use rodent-control measures such as traps in and around the house

The infected person sheds the virus in their urine for up to 9 weeks following infection, and in semen for up to 3 months. Sexual contact of any sort should be avoided during this period.

In the healthcare system, the use of barrier nursing precautions is very effective in limiting the spread of infection:

  • Use of gown, gloves, mask and cap
  • Sterilizing all equipment used for care of the patient
  • Careful segregation of biologically hazardous waste
  • Isolation of patients till recovery is well advanced


Last Updated: Sep 8, 2015

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