Lassa Fever Risk Limited Outside of West Africa

For patients who considered at risk of either infection, appropriate isolation precautions are very important to prevent spread of the infection to others.

On May 25, 2015, the Centers for Disease Control and Prevention (CDC) confirmed a case of Lassa fever in a New Jersey resident that had returned from West Africa, where the viral disease is common. The individual’s condition worsened over a period of time before he eventually passed away.1

The CDC and New Jersey Department of Health identified 177 potential contacts of the New Jersey patient and monitored them for a period of time, which covered the incubation period for Lassa fever, according to Thomas M. File, Jr., MD, chair of infectious disease division of Summa Health System in Akron, Ohio, professor of internal medicine and chair of the infectious disease section of Northeast Ohio Medical University in Rootstown, Ohio. None contracted the disease, even though 15 potential contacts were classified as having “high-risk” exposure.

There was no transmission of the infection associated with the New Jersey patient. 

“For patients who considered at risk of either infection, appropriate isolation precautions are very important to prevent spread of the infection to others,” said File.

Most Lassa fever infections are mild and go undiagnosed, according to Tina Tan, MD, state epidemiologist of New Jersey. Approximately 20% of those infected go on to have more serious symptoms. Lassa fever is not common in the United States. There have been only a total six cases of Lassa fever in travelers returning to the States since 1969. However, in West Africa, 100,000 to 300,000 cases occur annually, resulting in approximately 5,000 deaths.1

Lassa Fever Transmission Limited Between Humans

Lassa virus is carried by rodents and transmitted to humans through contact with urine or excrement infected rodents. 

“People at greatest risk of Lassa virus infection are those who live in or visit regions in West Africa and have exposure to the multimammate rat. When people swallow or breathe in particles of urine or droppings from the infected rats, they may become infected with the virus,” said Tan.

Transmission between humans is rare but can occur. 

“A person can get Lassa fever from another person who is infected and showing symptoms of the disease. Lassa fever is spread through direct contact with the blood or body fluids of an infected, person that is showing symptoms of the disease,” said Tan. “Casual contact (including skin-to-skin contact without exchange of body fluids) does not spread Lassa fever. People who do not have symptoms are not contagious. It is also spread through exposure to objects, such as needles, that have been contaminated with infected body fluids. Lassa fever is not spread through the air.”

Lassa Fever Differs From Ebola

According to File, there are regional similarities between Lassa fever and Ebola because both occur in the same countries in West Africa. 

“In West Africa, we are seeing a waning of Ebola activity where Lassa fever is occurring. The symptoms associated with both conditions are similar, [and] travelers usually present with fever and constitutional symptoms. In addition, travelers over time can develop hemorrhagic manifestations as the infection may progress to affect the clotting system of the patient,” said File.

Although Lassa fever can produce hemorrhagic symptoms in people who are infected, the disease is not related to Ebola. “In general, Lassa fever is less likely to be deadly than Ebola (approximately 1% death rate versus approximately 70% death rate without treatment) and less likely to be spread from person to person,” said Tan.

Ebola occurs more sporadically, and when we do see it, it occurs in outbreaks. These outbreaks spill over from animal reservoir into human population with subsequent human to-human transmission, according to John S. Schieffelin, MD, assistant professor of clinical medicine and pediatrics and internal medicine in the infectious disease department at the Tulane University School of Medicine in New Orleans, Louisiana. “Lassa fever is a bit different – it is seen year round in West Africa, with frequent rodent to human transmission.”

While treatment for both conditions remains limited, antiviral medications have been tried. According to File, currently there are no specific antiviral treatments for Ebola, however, antiviral therapy with ribavirin has worked for patients with Lassa fever. “Several antiviral medications have been tried with Ebola in a limited number of patients, but it is unclear how effective it has been in eliminating the infection,” says File.


Overall, Tan said that it is critical that health care providers are aware of disease importation and take travel histories when evaluating patients. In addition, providers need to remember to report notifiable conditions to their local health departments so that prompt public health actions can be implemented.


1.     Lassa Fever Reported in NJ Traveler Returning from West Africa. State of New New Jersey Department of Health. Available at: Accessed: July 2, 2015.