By Adaobi Rhema Oguejiofor
In Africa, rats are very common mammals that roam about almost everywhere. It is not uncommon to spot a rat in homes, schools, on the streets, hospitals and even places of worship. They have almost become a part and parcel of society. But what happens when these very common animals become carriers of a disease that is very detrimental to the lives of the humans around them? This is the case of Lassa fever.
Lassa fever is an animal-borne, or zoonotic, acute viral haemorrhagic illness caused by the Lassa virus, a member of the Arenavirus family. Humans get infected through contact with food or household items contaminated with rodent faeces. However, human-to-human infections and laboratory transmission can also occur, especially through direct contact with blood and body fluids.
The disease, which is endemic in West Africa, with high prevalence in Nigeria, Sierra Leone, Guinea and Liberia was first identified in 1969 in the town of Lassa, Borno State, Nigeria. Neighbouring countries, according to The Africa Centres for Disease Control and Prevention (Africa CDC), are also at risk as the animal vector for Lassa virus, the “multimammate rat” (Mastomys natalensis) is distributed throughout the West African region.
Although people living in rural areas of West Africa are at the most risk, rare cases have been imported into Britain, the United States, Japan and Canada. In 2010, one imported case from Nigeria was reported in the US. The patient had visited a rural area in Nigeria and developed fever, sore throat, chest pain and diarrhoea without haemorrhagic symptoms.
In 2024, Nigeria recorded over 1,187 confirmed cases across 28 states. Also, the number of Lassa virus infections per year in West Africa as a whole is estimated at 100,000 to 300,000, with approximately 5,000 to 10,000 deaths. The disease also leaves another estimated 30,000 deaf.
Mortality is particularly high in pregnant women and children. The Nigeria Centre for Disease Control and Prevention (NCDC) confirmed 54 cases of Lassa fever in Ondo, Edo, and Bauchi only from December 30, 2024, to January 5, 2025. The NCDC also reported 10 related fatalities, resulting in a Case Fatality Rate of about 18.5 per cent.
Causes and Transmission
Lassa fever is caused by the Lassa virus (LASV). It is carried by Mastomys or multimammate rats, which live in countries in West Africa. These rats can be found living in and around homes, colonizing areas where food is stored and they are also able to live in fields or cleared forests. While Mastomys rats serve as the main reservoir, the Lassa virus has also been isolated from other rodent species, suggesting their potential role as additional sources of infection.
Humans primarily get infected with the Lassa virus through exposure to food or household items that are contaminated with urine or faeces of infected Mastomys rats, or directly via contact with infected rats. The virus can damage a patient’s blood vessels and lower their blood’s ability to clot, causing uncontrolled bleeding.
Although to a much lesser extent, the Lassa virus may also spread between humans through direct contact with blood, urine, faeces or other bodily secretions of a person infected with Lassa fever. Person-to-person transmission may occur primarily in health-care settings, where the virus may also be spread by contaminated medical equipment, such as reused needles.
The virus may also persist in the semen of some males who recovered from the disease for up to a few months. However, to date, no transmission through exposure to infected semen has ever been documented. There is no epidemiological evidence supporting airborne spread between humans.
Symptoms
With Lassa Fever, about 80 per cent of infected people have no symptoms, however, some develop severe multisystem disease and up to 15 per cent die.
The incubation period of Lassa fever ranges from 2–21 days. At the onset, the symptoms of the disease are usually gradual, starting with fever, general weakness headache and malaise. After a few days, the patient may begin to develop sore throat, muscle pain, chest pain, nausea, vomiting, diarrhoea, cough and abdominal pain. As the disease progresses further, in severe cases, there can be an occurrence of facial swelling, fluid in the lung cavity, bleeding from the mouth, nose, vagina or gastrointestinal tract and low blood pressure may develop. Shock, seizures, tremors, disorientation, and coma may be seen in the later stages.
Africa CDC explained that deafness occurs in about 25 per cent of patients who survive the disease and in half of these cases, hearing returns partially after 1-3 months.
Among hospitalized patients, approximately 15 per cent die of Lassa fever disease. In fatal cases, death usually occurs within 14 days of symptom onset. During pregnancy, Lassa fever can cause high maternal and foetal mortality, especially late in pregnancy. In the third trimester, fetal death and maternal death rates can exceed 80 per cent and 30 per cent respectively.
Treatment
Lassa fever is currently treated with the antiviral medication Ribavirin. A patient may also be given other treatments depending on the severity of their condition, including:
Dialysis for kidney damage.
Supplemental oxygen to help with breathing.
IV fluids, medications or supplements to correct electrolyte imbalances.
Patients should also receive early supportive care with rehydration and symptomatic treatment.
Recovery can take a long time and can sometimes lead to other conditions (sequelae). Sudden hearing loss has been reported with varying incidence and up to 25 per cent of recovered patients, both during acute disease and after recovery. In most circumstances, hearing loss is reported to be permanent. Other sequelae including neurological signs, visual impairment, joint pain, transient hair loss and psychological disorders have also been reported to a lesser extent. Therefore, patients who have recovered from Lassa fever need to receive care for sequelae they may experience.
Prevention
To effectively reduce risk one must:
Avoid contact with rodents, especially in areas where Lassa fever is common.
Keep living spaces clean and sanitary, with food items in closed containers to discourage attracting rats and other pests.
Wear protective coverings when caring for someone who has or might have Lassa fever. This includes using a mask, gloves, gowns, non-fogging goggles and boots if accessible.
Lassa fever remains a significant public health challenge in West Africa with the potential for outbreaks and severe consequences. While effective treatments exist, prevention remains very crucial. By raising awareness about the disease, promoting good hygiene practices, and improving rodent control measures, the burden of Lassa fever can be significantly reduced and vulnerable populations protected.