Lassa fever, a disease from the rats. Are there special rats that transmit this disease?


Lassa fever is an acute, viral disease carried by a type of rat that is common in West Africa, and was first discovered in Nigeria, when two missionary nurses became ill with the virus in 1969. Its name is derived from the village of Lassa, where it was first documented.

Lassa fever is a viral infection carried by the multimammate rat Mastomys natalensis (M. natalensis).This is one of the most common rodents in equatorial Africa, found across much of sub-Saharan Africa.

Lassa fever mainly occurs in Sierra Leone, Liberia, Guinea, and Nigeria. However, the Mastomys rat is common in neighboring countries, so these areas are also at risk.


Once a Mastomys rat is infected with the virus, it can excrete the virus in its feces and urine, potentially for the rest of its life.

As a result, the virus can spread easily, especially as the rats breed rapidly and can inhabit human homes.

The most common method of transmission is by consuming or inhaling rat urine or feces. It can also be spread through cuts and open sores.

The rats live in and around human habitation, and they often come into contact with foodstuffs. Sometimes people eat the rats, and the disease can be spread during their preparation.

Person-to-person contact is possible via blood, tissue, secretions or excretions, but not through touch. Sharing needles may spread the virus, and there are some reports of sexual transmission.

Lassa fever can also be passed between patients and staff at poorly equipped hospitals where sterilization and protective clothing is not standard.


An estimated 80 percent of infections do not produce significant symptoms, although there may be a general malaise, headache, and a slight fever.

In the remaining 20 percent of cases, Lassa fever becomes serious.

Symptoms can include:

  • bleeding in the gums, nose, eyes, or elsewhere
  • difficulty breathing
  • a cough
  • swollen airways
  • vomiting and diarrhea, both with blood
  • difficulty swallowing
  • hepatitis
  • swollen face
  • pain in the chest, back, and abdomen
  • shock
  • hearing loss, which may be permanent
  • abnormal heart rhythms
  • high or low blood pressure
  • pericarditis, a swelling of the sac that surrounds the heart
  • tremors
  • encephalitis
  • meningitis
  • seizures

In around 1 percent of all cases, Lassa fever is fatal, and around 15 to 20 percent of all hospitalizations for the disease will end in death.

Death can occur within 2 weeks after the onset of symptoms due to multiple organ failure.

One of the most common complications of Lassa fever is hearing loss, which occurs in around 1 in 3 infections.

This hearing loss varies in degree and is not necessarily related to the severity of the symptoms. Deafness caused by Lassa fever can be permanent and total.

It is particularly dangerous for women in the third trimester of pregnancy. Spontaneous loss of pregnancy occurs in around 95 percent of pregnancies.


The symptoms of Lassa fever vary widely, and diagnosis can be difficult.

Clinically, the disease can resemble other viral hemorrhagic fevers, including the Ebola virus, malaria, and typhoid.

The only definitive tests for Lassa fever are laboratory-based, and the handling of specimens can be hazardous. Only specialized institutions can conduct these tests.

Lassa fever is generally diagnosed by using enzyme-linked immunosorbent serologic assays (ELISA). These detect IgM and IgG antibodies and Lassa antigens.

Reverse transcription-polymerase chain reaction (RT-PCR) can also be used in the early stages of the disease.

Prevention and control

Prevention of Lassa fever relies on promoting good “community hygiene” to discourage rodents from entering homes. Effective measures include storing grain and other foodstuffs in rodent-proof containers, disposing of garbage far from the home, maintaining clean households and keeping cats. Because Mastomys are so abundant in endemic areas, it is not possible to completely eliminate them from the environment. Family members should always be careful to avoid contact with blood and body fluids while caring for sick persons.

In health-care settings, staff should always apply standard infection prevention and control precautions when caring for patients, regardless of their presumed diagnosis. These include basic hand hygiene, respiratory hygiene, use of personal protective equipment (to block splashes or other contact with infected materials), safe injection practices and safe burial practices.

Health-care workers caring for patients with suspected or confirmed Lassa fever should apply extra infection control measures to prevent contact with the patient’s blood and body fluids and contaminated surfaces or materials such as clothing and bedding. When in close contact (within 1 metre) of patients with Lassa fever, health-care workers should wear face protection (a face shield or a medical mask and goggles), a clean, non-sterile long-sleeved gown, and gloves (sterile gloves for some procedures).

Laboratory workers are also at risk. Samples taken from humans and animals for investigation of Lassa virus infection should be handled by trained staff and processed in suitably equipped laboratories under maximum biological containment conditions.

On rare occasions, travellers from areas where Lassa fever is endemic export the disease to other countries. Although malaria, typhoid fever, and many other tropical infections are much more common, the diagnosis of Lassa fever should be considered in febrile patients returning from West Africa, especially if they have had exposures in rural areas or hospitals in countries where Lassa fever is known to be endemic. Health-care workers seeing a patient suspected to have Lassa fever should immediately contact local and national experts for advice and to arrange for laboratory testing.


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