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Lassa Fever in Philadelphia

By Amesh A. Adalja, MD, October 8, 2010

Of the many diseases that can be imported into the United States by travelers, viral hemorrhagic fevers (VHFs) are of particular concern. The hemorrhagic fever viruses, which are classified as category A biological agents, provoke great fear among healthcare workers and the public because they are transmissible and highly lethal. Earlier this year, Lassa fever, one of the VHFs, was diagnosed in a patient in Philadelphia. Case details, summarized below, were just published in the October 2010 issue of Emerging Infectious Diseases.1

Importation from Liberia

The 47 year old male patient had visited Liberia recently, during which time he slept in a rural dwelling that was infested with rodents. Before leaving Liberia to return to the U.S., the patient experienced fever, chills, arthralgias, anorexia, sore throat, diffuse skin tenderness, and mild shortness of breath. He was initially treated with amoxicillin and chloroquine. After 5 days of illness, the patient presented for medical care in the U.S. Notable physical exam findings included fever, splenomegaly, lymphadenopathy, leukopenia, mild thrombocytopenia, and elevations of liver transaminases. He was treated presumptively for malaria until blood smears and antigen testing ruled out that diagnosis.1

Over the ensuing days, the patient developed pharyngitis, exudative tonsillitis, chest pain, and diarrhea, at which point Lassa fever was considered in the differential diagnosis, and contact and airborne precautions were initiated. Ribavirin was not administered, as the patient’s clinical condition was improving. Diagnosis was made on hospital day 5 by PCR.1

The patient defervesced on day 16 of hospitalization and was discharged on day 21 after 2 negative blood PCR results were obtained. Contact investigation revealed no secondary cases.1

Lassa Fever Is Endemic in West Africa

Lassa fever virus is endemic in West Africa. It is spread via contact with the urine of its reservoir host, the Mastomys mouse, or through contact with the bodily secretions of another infected human. The incubation period is usually 7 to 12 days.2

Most Infections Are Subclinical

Only 5% to 10% of those infected experience severe disease. Mortality reaches 15% to 25% among those patients with illness severe enough to require hospitalization.2

Clinical characteristics are varied, and the differential diagnosis may be broad. Common clinical characteristics include: chest pain; sore throat; back pain; nausea, vomiting, diarrhea; and proteinuria.

In endemic areas, the clinical triad of fever, chest pain, and proteinuria has been found to have predictive values of 70% in clinical studies.2

Diagnosis and Treatment

Serology, viral culture, and PCR are used to diagnose Lassa fever with certainty. Along with supportive care, intravenous ribavirin is the mainstay of treatment, with a beneficial effect on survival rates when given within 7 days of illness onset. There is no vaccine available.2

Lassa Fever in the U.S.

This case in Philadelphia is the sixth in the U.S. A 1989 case was diagnosed in a traveler, and the 4 others occurred in patients who were airlifted to the U.S. for care. This recent case illustrates the astute clinician's role in treatment and prevention. The physicians in Philadelphia who identified the illness initiated prompt isolation measures to prevent secondary spread, which has occurred in endemic areas following contact with infectious bodily fluids.1 Nosocomial spread of Lassa fever also has occurred with imported cases; therefore, isolation procedures are recommended.2

Infectious diseases respect no border. As this case highlights, a West African disease can easily appear in any hospital in the U.S. This case of Lassa fever reinforces the need for both clinical vigilance and the ability to manage such cases.


  1. Amorosa V, MacNeil A, McConnell R, Patel A, Dillon KE, Hamilton K, et al. Imported Lassa fever, Pennsylvania, USA, 2010. Emerg Infect Dis. 2010. Accessed October 1, 2010.

  2. Peters CJ. Lymphocytic choriomeningitis virus, Lassa virus, and the South American hemorrhagic fevers. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010.