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Lassa fever is a Viral Hemorrhagic Fever (VHF) which is endemic in parts of West Africa (see CDC map above), and is commonly carried by multimammate rats, a local rodent that often likes to enter human dwellings.
Exposure is typically through the urine or dried feces of infected rodents, and roughly 80% who are infected only experience mild symptoms.
The incubation period runs from 10 days to 3 weeks, and the overall mortality rate is believed to be in the 1%-2% range, although it runs much higher (15%-20%) among those sick enough to be hospitalized.
Like many other hemorrhagic fevers, person-to-person transmission may occur with exposure to the blood, tissue, secretions, or excretions of an individual (cite CDC How it spreads), but casual contact is not considered a risk.
In recent years we've seen a small number of exported cases turning up in Europe, the UK, and occasionally the United States, among returning travelers from West Africa. Local transmission is rare, however:
- in 2022 the UK reported a small cluster (see WHO Statement On Lassa Fever (ex Mali) Cluster In the UK) where 2 family members were infected (1 fatally).
- In 2016, Germany saw a cluster as well (see Germany's RKI Statement On Lassa Fever Cluster In Cologne).
Late yesterday the CDC announced the detection of a suspected case of Lassa Fever in recent traveler to Africa who lived in Iowa.
Lassa Fever Suspected in Death of U.S. Traveler Returning from West Africa
STATEMENT
CDC assesses the risk to the general public is extremely low
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The CDC and the Iowa Department of Health are investigating a suspected case of Lassa fever, which was diagnosed today in an Iowa resident who returned to the United States from West Africa early this month. The patient was not sick while traveling so the risk to fellow airline passengers is extremely low. The patient was hospitalized in isolation at the University of Iowa Health Care Medical Center when they died earlier this afternoon. Testing by the Nebraska Laboratory Response Network lab early this morning found the illness was presumptively positive for Lassa fever and confirmatory testing is planned.
Lassa fever is a viral disease common in West Africa, but rarely seen in the United States. In West Africa, Lassa virus is carried by rodents and spread to humans through contact with urine or droppings of infected rodents. In rare cases, it can be transmitted from person to person through direct contact with a sick person's blood or body fluids, through mucous membranes, or through sexual contact. The virus is not spread through casual contact, and patients are not believed to be infectious before symptoms begin. About 100,000 to 300,000 cases of Lassa fever, and 5,000 deaths related to Lassa fever, occur in West Africa each year.
If confirmed, the Iowa suspected case would be the ninth known occurrence since 1969 of Lassa fever in travelers returning to the United States from areas where Lassa fever is commonly found.
CDC is supporting state and local public health officials from Iowa who are working to learn more about how the patient may have contracted Lassa fever. Early information suggests the patient may have had contact with rodents while in West Africa. CDC and other federal agencies have offered federal assistance to Iowa. People with Lassa fever can only transmit the illness when they have active symptoms and CDC is working with state public health officials to identify people who had contact with the patient after the patient's symptoms began. Those identified as close contacts of the patient will be monitored for 21 days.
Additional information will be shared as we learn more about this patient. For more information about Lassa fever, see the CDC website at About Lassa Fever | Lassa Fever | CDC.
