Lassa Fever: Overview, Treatment and Prevention


Unless you’ve been living under a rock, you’re aware there’s an epidemic of Lassa fever in Nigeria. You might be wondering: What is Lassa fever? How do people get it? What do rats have to do with it?

What is Lassa Fever?

Lassa fever is a viral hemorrhagic fever caused by the Lassa virus. The name is derived from the first recognized cases in Lassa village, Borno State, Nigeria in 1969.

Lassa virus is epidemic in many West African countries between Senegal and the Democratic Republic of Congo, including Nigeria, Liberia, Sierra Leone and Guinea. This means that Lassa fever is usually present and active in the population at any given time, with low mortality rates. There are about 500,000 cases of Lassa fever yearly in West Africa, with about 5,000 deaths. Antibodies against Lassa virus are detectable in 21% of Nigerians; this means that 21% of the population has come into contact with the virus.

When the incidence of Lassa fever increases above the baseline level, as it has in recent times, it becomes an epidemic, with an increased mortality rate of up to 50%. This usually happens in the dry season because of the heat and bush fires that drive the rat vectors out of the bushes and into surrounding homes.

A cute house rat

Not so cute when it makes you sick.

How Is It Transmitted?

Lassa fever is zoonotic i.e. animal-borne. The reservoir of Lassa fever is the house rat, Mastomys natalensis. This rat breeds frequently and produces large numbers of offspring. Mastomys readily colonizes homes and areas where food is stored, and is difficult to eradicate. The infected rat does not get sick or die of the disease.

The infected Mastomys rat excretes the virus in its urine and droppings. Humans are infected by direct contact with these materials. This contact could be by:

  • Ingestion of food items contaminated by rat urine or feces;
  • Touching objects soiled by rat excreta;
  • Exposure of open cuts and sores to rat urine or droppings;
  • Breathing in air contaminated with infected rodent excretion (aerosol);
  • Exposure to rat excreta during trapping and preparation of rats as food.
roasted rats on a tray

Yes, people eat rats.

Person-to-person transmission of Lassa fever may occur after exposure to virus in the blood, tissue, secretions, or excretions of an infected individual. This is most common in health-care settings, where personal protective equipment (PPE) is not available or not used.

What Are The Symptoms And Signs?

The clinical features of Lassa fever usually occur 1-3 weeks after contact with the virus. Most Lassa virus infections (80%) are mild, with slight fever, weakness, and headache. This means that Lassa fever could be misdiagnosed as malaria, typhoid, or many other diseases sharing these nonspecific symptoms.

In 20% of infected individuals, however, the disease may progress to more serious symptoms including severe bleeding from the gums, eyes, or nose, difficulty in breathing, repeated vomiting, facial swelling, pain in the chest, back, and abdomen, and shock. There could also be neurological features like hearing loss, tremors, and encephalitis. Death may occur within two weeks after symptom onset due to multi-organ failure.

The most common complication of Lassa fever is deafness. This may develop in mild as well as in severe cases. Various degrees of deafness occur in approximately one-third of infections, and in many cases hearing loss is permanent.

Approximately 15%-20% of patients hospitalized for Lassa fever die from the illness. However, only 1% of all Lassa virus infections result in death.

When pregnant women are infected with Lassa fever, especially during the third trimester, the death rates are much higher. As many as 30% of infected pregnant women will die, and there is up to 95% rate of fetal death.

Poster saying "no injection or vaccine to prevent lassa fever"Is Lassa Fever Treatable?

Yes. Lassa fever is treated with the antiviral drug ribavirin. It is most effective when given early in the illness, and when given intravenously. The cost of the drug is high in resource-poor environments like West Africa where it is needed most, and this limits its use.

Supportive care is also required, like fluid replacement, oxygen therapy and blood transfusion.
There is currently no vaccine against Lassa virus.


How Can We Prevent Lassa Fever?

Complete elimination of the rodent vector, Mastomys, is impractical, as they are so abundant in epidemic areas.
Prevention and control of the disease involves preventing rats from entering homes and gaining access to food stores. Specific measures include:

  • Maintaining clean households;
  • Storing foodstuff in rodent-proof containers with lids;
  • Disposing of garbage far from the home;
  • Frequent hand washing with soap and water;
  • Trapping, killing and reducing the rodent population;
  • Not eating rats.
roasted rats on a tray

Once more, for emphasis.

Health-care workers should exercise standard infection control measures such as:

  • Basic hand hygiene;
  • Use of personal protective equipment;
  • Safe injection practices;
  • Safe burial practices;


The economic aspect of this disease is of note, with poor hygiene and poor housing contributing to the persistence of Lassa fever. Maintaining good environmental hygiene and food storage practices are essential to controlling Lassa fever. Unlike Ebola (another viral hemorrhagic fever), Lassa fever has a low mortality rate, is treatable, and is easily preventable.


WHO | Lassa Fever Fact Sheet
CDC | Lassa Fever
CDC | Principles of Epidemiology In Public Health Practice
Ogbu O, Ajuluchukwu E, Uneke CJ (2007). “Lassa fever in West African sub-region: an overview”. Journal of vector borne diseases 44 (1): 1–11. PMID 17378212

Unscientific Comments

I keep trying to understand how people eat rats as a delicacy. I’m aware that some people eat rats because of poverty and the fact that the rats are a cheap protein source. During the civil war, people ate rats, geckos, roaches…just to survive. I get survival. What I don’t get is how people see rats as a culinary delight on par with the mainstream chicken or pork.

Have you ever eaten a rat? Why? How did it taste? I really want to know, tell me in the comments.

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