Impact of Lassa Fever and its connection with the environment in endemic communities

This is the first of two posts from participants in Baylor College of Medicine’s National School of Tropical Medicine Summer Institute program with 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, the Lassa virus infects approximately 100,000 to 300,000 individuals a year, leading to an estimated 5,000 deaths annually. According to the Africa CDC (African equivalent of the CDC), however, this number is rudimentary and most likely underreports the incidence of the disease as surveillance for cases is not uniformly performed throughout West Africa.

The Lassa virus is an enveloped, single-stranded RNA virus, belonging to the family of arenaviruses. The virus is transmitted to humans by either the multimammate rat, also known as the “common African rat” or the “African soft-furred mouse,” via direct contact with the animal or indirectly through contact with infected rat urine or feces. Although rare, human-to-human transmission can occur through direct contact with an infected person’s blood or bodily fluids and can lead to outbreaks. Human infection with Lassa virus causes Lassa fever. About 80% of those infected will experience mild to no symptoms, with the remaining 20% experiencing a severe multisystem disease with an overall fatality rate of 1%. In its late stages, Lassa fever may cause organ failure, severe internal bleeding and shock, with deafness developing as a notable complication in many survivors.

Transmission of Lassa virus through contact with animals emphasizes the critical role of “One Health,” understanding the interaction between human living conditions and the rodent reservoir, in the control of human disease. In endemic areas, the disease follows a distinct seasonal pattern, with a clear increase in cases from November to April. This seasonal surge is likely linked to the breeding pattern of the rats, who have evolved to breed during this dry season due to lower risk of predation, stable environmental conditions and reduced competition. However, as food and water become scarcer during the dry months, the rodents increasingly enter human dwellings in search of sustenance, increasing human exposure to the virus. The risk of contracting Lassa fever is significantly higher among individuals living in rural, low resource areas where poor sanitation and crowded living conditions are prevalent. Moreover, rural areas that rely on agriculture and livestock increase the likelihood of rodent contact, further elevating the risk of exposure to the virus. Unfortunately, Lassa virus endemic areas that already have healthcare systems restricted by low resources may not have sufficient public health infrastructure to prevent community wide Lassa fever outbreaks.

Currently, there is only one medication with efficacy against Lassa fever: Ribavirin, a broad-spectrum antiviral drug. Despite the efficacy of ribavirin, healthcare providers face many hurdles when considering its use against Lassa fever. First, the drug’s effectiveness is contingent on early administration. However, the incubation period of Lassa virus is 6–21 days and early symptoms of Lassa Fever – fatigue, vomiting, abdominal pain and nausea – are non-specific, making early diagnosis and early interventions challenging. By the time Lassa Fever is diagnosed, the effectiveness window for ribavirin has passed, rendering it less beneficial. Additionally, ribavirin availability and healthcare infrastructure is scarce in resource-limited, endemic regions. Without the proper resources to store and distribute the drug efficiently, treatment regimens with ribavirin are largely ineffective.

Although no vaccine currently exists for Lassa Virus, research has shown promising results for a vaccine used in nonhuman primates. These studies found the most effective vaccine strategy involved a combination of full-length glycoproteins and nucleoproteins expressed by the Lassa Virus. These vaccines conferred cell-mediated immunity, primarily involving cytotoxic T lymphocytes. As recently as April 2024, significant advances have been made in developing a vaccine against the Lassa Virus. In a trial sponsored by the IAVI (International AIDS Vaccine Initiative), over 600 participants in Nigeria have been successfully vaccinated and treated in the first-ever phase 2 clinical trial of a Lassa fever vaccine. This vaccine trial serves as a monumental step forward in the public health efforts against Lassa virus is endemic regions in West Africa, signaling hope for more effective tools to manage and prevent Lassa fever outbreaks in the future.

In addition to advancing therapeutics and preventive vaccines, several public health strategies can be employed to control the spread of the virus and manage outbreaks effectively. First, population control of the multimammate rat is crucial for reducing Lassa virus infection in humans. Use of rodent-proof storage units for food and rodent traps near and/or around households are two strategies that have proven beneficial in reducing rodent populations near human dwellings. Second, community supported, culturally appropriate healthcare campaigns that emphasize hygiene practices such as regular handwashing, safe food preparation and proper waste disposal and 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 continues to be a challenge in West Africa today, emphasizing the need for immediate and effective control measures. While the recent advancements in vaccine development offer hope, implementing robust public health strategies remains necessary to protect communities and manage outbreaks effectively until a vaccine can be widely available.

By Enrique Martinez