This is the first of two articles written by participants in the National School of Tropical Medicine Summer Program at Baylor College of Medicine and Baylor University.
First identified in 1969 in the town of Lassa, Nigeria, Lassa virus (LASV) continues to pose a threat to human health in West Africa, where it remains endemic in countries such as Sierra Leone, Liberia, Guinea, and Nigeria. In West Africa, Lassa virus infects approximately 100,000 to 300,000 people per year, resulting in approximately 5,000 deaths per year. However, according to the Africa CDC (the African equivalent of the CDC), this figure is crude and likely underestimates the incidence of the disease, as case surveillance is not conducted uniformly across West Africa.
Lassa virus is an enveloped, single-stranded RNA virus belonging to the arenavirus family. The virus is transmitted to humans either by the multimammary rat, also known as the “African common rat” or “African soft-furred mouse”, through direct contact with the animal or indirectly through contact with the urine or feces of infected rats. Although rare, human-to-human transmission can occur through direct contact with the blood or bodily fluids of an infected person and can result in epidemics. Human infection with Lassa virus causes Lassa fever. Approximately 80% of infected people have only mild or no symptoms, with the remaining 20% suffering from severe multisystem illness with an overall mortality rate of 1%. In its later stages, Lassa fever can cause organ failure, severe internal bleeding, and shock, with deafness developing as a notable complication in many survivors.
The transmission of Lassa virus through contact with animals highlights the crucial role of “One Health,” which is understanding the interaction between human living conditions and the rodent reservoir, in controlling human disease. In endemic areas, the disease follows a distinct seasonal pattern, with a marked increase in cases from November to April. This seasonal surge is likely related to the reproductive pattern of rats, which have evolved to breed during this dry season due to the lower risk of predation, stable environmental conditions, and reduced competition. However, as food and water become scarcer during the dry months, rodents increasingly enter human dwellings in search of food, increasing human exposure to the virus. The risk of acquiring Lassa fever is significantly higher among people living in resource-poor rural areas where poor sanitation and crowded living conditions are prevalent. Additionally, rural areas that rely on agriculture and livestock farming increase the likelihood of contact with rodents, further increasing the risk of exposure to the virus. Unfortunately, Lassa virus-endemic areas that already have health systems constrained by low resources may not have sufficient public health infrastructure to prevent community-wide outbreaks of Lassa fever.
Currently, there is only one effective medication for Lassa fever: ribavirin, a broad-spectrum antiviral drug. Despite ribavirin’s effectiveness, healthcare providers face many challenges when considering its use for Lassa fever. First, the drug’s effectiveness depends on its early administration. However, the incubation period of the Lassa virus is 6 to 21 days, and the early symptoms of Lassa fever—fatigue, vomiting, abdominal pain, and nausea—are nonspecific, making early diagnosis and intervention difficult. By the time Lassa fever is diagnosed, ribavirin’s window of effectiveness has passed, making it less beneficial. Additionally, ribavirin availability and healthcare infrastructure are scarce in resource-limited endemic settings. Without adequate resources to effectively store and distribute the drug, ribavirin-based regimens are largely ineffective.
Although there is currently no vaccine for Lassa virus, research has shown promising results for a vaccine used in non-human primates. These studies found that the most effective vaccine strategy involved a combination of glycoproteins and full-length nucleoproteins expressed by Lassa virus. These vaccines conferred cell-mediated immunity, primarily involving cytotoxic T lymphocytes. As recently as April 2024, significant progress has been made in the development of a vaccine for Lassa virus. In a trial sponsored by the International AIDS Vaccine Initiative (IAVI), over 600 participants in Nigeria were successfully vaccinated and treated in the first-ever Phase 2 clinical trial of a vaccine for Lassa fever. This vaccine trial represents a monumental advance in public health efforts against Lassa virus in endemic regions of West Africa, providing hope for more effective tools to manage and prevent Lassa fever outbreaks in the future.
In addition to therapeutic advances and preventative vaccines, several public health strategies can be employed to control the spread of the virus and effectively manage outbreaks. First, population control of multimammary rats is crucial to reduce Lassa virus infection in humans. The use of rodent-proof food storage units and rodent traps near and/or around human habitations are two strategies that have been shown to be beneficial in reducing rodent populations near human habitations. Second, community-supported and culturally appropriate public health campaigns that emphasize hygiene practices such as regular handwashing, safe food preparation, and proper waste disposal, as well as educational materials about the potential risks of Lassa fever and behaviors that increase the risk of infection are important public health measures.
Unfortunately, Lassa fever remains a problem in West Africa, highlighting the need for immediate and effective control measures. While recent advances in vaccine development offer hope, robust public health strategies are still needed to protect communities and effectively manage outbreaks until a vaccine is widely available.
By Enrique Martinez
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