What are the causes?
Lassa virus is transmitted by the Natal multimammate mouse (Mastomys natalensis), a small rodent commonly found in West Africa. The virus is found in the rodent's urine or feces and can infect humans by direct contact, ingestion of contaminated food or inhalation of contaminated dust particles.
How does the virus spread?
Lassa virus can be transmitted in various ways:
- Directly to humans through contact with the excrement or urine of infected Mastomys rodents; many of these rodents live near or inside housing, and their rate of infection can be as high as 80%.
- Indirectly:
- by consuming contaminated food;
- airborne transmission has not been observed.
Human-to-human transmission is also possible but is less frequent, occurring mainly in healthcare environments without adequate infection control measures, where the virus may be spread through contaminated medical equipment. Sexual transmission and direct contact with contaminated bodily fluids or secretions are also possible.
Lassa virus was named after the Nigerian town where it was first isolated in 1969, in a nurse who fell ill after treating patients and who died from the disease after contaminating two other healthcare workers.
What are the symptoms?
The clinical profile of Lassa fever is variable. The majority of cases are asymptomatic. The incubation period is 2 to 21 days after infection.
If the disease is symptomatic, it gives rise to unspecific clinical signs such as fever, vomiting, nausea, abdominal pain, headache, aching muscles (myalgia), aching joints (arthralgia) and extreme fatigue (asthenia).
In severe cases, the symptoms then get worse, with the emergence of edema, hemorrhagic signs, pericardial and pleural effusion, and more rarely encephalitis.
Death occurs as a result of hypotensive shock (very low blood pressure) and hypovolemic shock (lack of blood or fluid in blood vessels) and kidney and liver failure around 14 days after symptom onset.
Lassa fever is more severe for pregnant women, resulting in frequent maternal death and very often fetal loss.
In patients who survive Lassa fever infection, fever disappears around 10 days after symptom onset, but extreme fatigue, malaise and dizziness can continue for several weeks. Some of these patients develop severe sequelae: temporary or permanent hearing loss in one or both ears, and myocarditis (inflammation and lesions in the heart muscle).
How is Lassa fever diagnosed?
It is difficult to distinguish Lassa fever from other viral hemorrhagic fevers such as Ebola and yellow fever, or from malaria and leptospirosis.
Because the virus is so contagious, tests need to be performed in strict biosafety conditions. The disease is diagnosed in reference laboratories using one of the following methods: serological assays to identify specific antibodies, RT-PCR, which detects the genetic material of the virus, or virus isolation by cell culture.
What treatments are available?
There is currently only one drug used to treat Lassa fever: ribavirin, a broad-spectrum antiviral drug that is active against RNA viruses and particularly used to treat hepatitis C.
Unfortunately, this treatment is not a satisfactory solution to the problem of Lassa fever in endemic countries: its effectiveness has not been widely demonstrated and it needs to be administered very soon after infection. The problem is that the early clinical signs of the illness are similar to those observed for other diseases such as malaria and dysentery, which are widespread in these regions. Lassa virus is therefore often only considered as a possible diagnosis several days after symptom onset, and in the rare cases where ribavirin is available, it is usually administered too late to be effective.
Symptomatic treatment includes rehydration, treatment for pain and management of hemorrhagic complications. Rapid, effective treatment is crucial to improve the patient's chances of survival.
There is currently no vaccine.
How can Lassa fever be prevented?
Prevention of Lassa fever involves good hygiene practices, such as storing food in rodent-proof containers and maintaining a clean household. Care should be taken to avoid contact with rodent excreta.
Raising community awareness of modes of transmission and the adoption of safe food preparation practices are also essential.
In healthcare settings, staff should observe hygiene and infection prevention precautions, including use of personal protective equipment such as masks, gloves, gowns and full-body suits.
Travelers returning from West Africa with fever must be tested for Lassa fever if they have spent time in endemic regions.
How many people are affected?
Each year there are thought to be 100,000 to 300,000 cases of Lassa fever in West Africa, with approximately 5,000 deaths. However, many cases are not reported because of limited access to healthcare and non-specific symptoms in mild cases.
People most at risk are those living in rural areas where Mastomys mice are found. Lassa fever is endemic in Nigeria, Guinea, Liberia and Sierra Leone which are susceptible to regular outbreaks.
September 2024