The deadly Lassa virus, a relative of Ebola virus, has just been revealed to have ancient roots and to have changed over time. An International team working in the United States and West Africa has been researching the deadly virus and hopes their findings will contribute to new ways to combat it.

What is Lassa Virus?

Lassa virus, also known as Lassa fever, is a viral illness that occurs in West Africa. Named after the town where the first cases occurred, Lassa virus is an acute condition, and endemic in parts of West Africa. Over 5,000 people die from Lassa virus every year, with the number of infections estimated at 100,000 to 300,000. These estimates are crude, however, because surveillance for the disease is not performed consistently.

The virus is spread through contact with the urine and droppings of infected Mastomys natalensis rodents. These rodents are a natural “reservoir” for the virus, and once infected, a human can spread the disease to other humans through blood tissue or secretions. The rodents responsible for the virus often colonize human homes and areas where food is stored, contributing to a fairly efficient spread of Lassa virus from rodents to humans.

Once infected, a human will generally begin to show symptoms within one to three weeks after initial contact. For many patients (80 percent), symptoms are mild and go undiagnosed. These symptoms include general weariness and weakness, a slight fever, and headache. In the remaining 20 percent of individuals though, the disease can progress to more serious symptoms — pain in the chest, back, and abdomen; facial swelling; hemorrhaging; and shock. Some neurological problems have also been known to arise, including tremors, encephalitis, and hearing loss. Death is caused by multiple organ failure.

An antiviral drug has been used successfully with patients and has shown to be most effective when taken during early stages of the illness.

New Information, New Treatments?

The study, published in the journal Cell, utilized a technique called next-generation sequencing to analyze the Lassa virus. Genomes taken from wild Masomyn natalensis and human patients in Sierra Leone and Nigeria were examined to find the origins of the disease.

After studying data, researchers found that some strains of Lassa virus share a common ancestor that can be traced back over a thousand years in present day Nigeria. This information was surprising — Lassa fever was only first identified in 1969.

"This gives us a clear view of how the virus is evolving, which is important to know as we develop vaccines and therapies," said TSRI biologist Kristian G. Andersen, a lead author of the new study, in a press release.

The study found that the virus spread from Nigeria about 400 years ago and moved into Liberia, Guinea, and Sierra Leone. If these countries sound familiar in terms of diseases, it’s because this is the same part of the world where the largest Ebola outbreak has been running rampant since 2013. Ebola is closely related to Lassa; it’s a mutated cousin that has better adapted to mammalian hosts.

The data also shed some light on transmission, showing that most Lassa virus cases are caused by the infections from wild rodents, rather than spreading from human-to-human contact.

"The reason Lassa hasn't yet grown into this huge epidemic is because there is limited transmission between humans," Andersen said. "That's a major difference between Lassa virus and Ebola virus."

The next step for research, according to Andersen, would be understanding how the virus can mutate within an individual host as it is confronted with the immune system.

Source: Andersen K, et al. Clinical Sequencing Uncovers Origins And Evolution Of Lassa Virus. Cell. 2015