The number of cases of Lassa fever in Nigeria has dropped significantly during the first seven months of 2015, according to data from the Nigeria Centre for Disease Control, Nigeria Federal Ministry of Health.

Nigeria map divided by states/United Nations
Nigeria map divided by states/United Nations

Health officials have reported 200 Lassa fever cases to date, compared to 839 cases for the same period in 2014.

In 2014, Lassa fever was a serious problem in 23 of the 36 states of that country.

Ten states have reported Lassa fever this year. The states hit the hardest include Edo (157 cases/4 deaths) and Plateau (22/1).

According to the US Centers for Disease Control and Prevention (CDC), Lassa fever is an acute viral illness that occurs in West Africa. The virus, a member of the virus family Arenaviridae, is a single-stranded RNA virus and is zoonotic, or animal-borne.

Related: Lassa fever: 20 percent of cases could be due to human-to-human transmission, Cambridge study

Lassa fever is a significant cause of morbidity and mortality. While Lassa fever is mild or has no observable symptoms in about 80% of people infected with the virus, the remaining 20% have a severe multisystem disease.

The animal host of Lassa virus is a rodent known as the “multimammate rat” of the genus Mastomys. Humans get infected with Lassa through aerosol or direct contact with excreta from the rodent. Laboratory infections do occur primarily through contaminated needles.

The symptoms of Lassa fever typically occur 1-3 weeks after the patient comes into contact with the virus. These include fever, retrosternal pain (pain behind the chest wall), sore throat, back pain, cough, abdominal pain, vomiting, diarrhea, conjunctivitis, facial swelling, proteinuria (protein in the urine), and mucosal bleeding. Neurological problems have also been described, including hearing loss, tremors, and encephalitis.

Related: Lassa fever virus has ‘very ancient roots’, traced back a millennium to what is now Nigeria

Robert Herriman is a microbiologist and the Editor-in-Chief of Outbreak News Today

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