Bayou virus (BAYV) is a species of Orthohantavirus comprising enveloped and spherical viruses. It was first identified in 1993 in Louisiana and later confirmed by other investigators. BAYV was recognized as a distinct form of hantavirus disease, now known as hantavirus pulmonary syndrome (HPS). It now represents the second most common hantavirus in the United States behind the Sin Nombre virus. In 1996, the marsh rice rat, which is seen in marshes in the southeast and mountain streams in the northeast, was identified as the natural reservoir of the virus. Although the virus was first identified in Louisiana, it is widespread throughout the Southeastern United States. This hantavirus disease is known as a severe and sometimes fatal respiratory disease, and HPS has a case-rate fatality of almost 50%.
Discovery and identification Early investigations The initial identification of BAYV arose from surveillance of unexplained cases of hantavirus pulmonary syndrome (HPS) in Louisiana. Initial investigations of BAYV were conducted on lung tissue samples from infected individuals, where hantavirus-specific RNA sequences and antibodies were detected through serological techniques. In order to detect HPS antibodies, scientists used a μ-capture enzyme-linked immunosorbent assay (ELISA) and immunoglobulin (Ig) G ELISA with a Sin Nombre virus (SN) nucleocapsid protein as the antigen target. Other significant assays were completed with recombinant-expressed viral N antigens, which were conducted using immunoblot and Western Blot formats and completed as confirmatory tests. The immunoblot assay included five membrane-bound antigens, including SN virus N and G1, recombinant-expressed Seoul virus N, and synthetic peptides of SN N and G1.
Rodent collection and processing
In addition to cases in the Southeastern United States, two BAY virus-HPS cases were seen in Texas in Jefferson County. This study included rodent samples collected using Sherman live traps and used to collect marsh rice rats from Jefferson County and the neighboring Orange County. Heart blood samples from the mice were taken and screened by a recombinant SN virus N antigen ELISA. In addition to the heart blood samples, rodent lung, kidney, and spleen samples were used to make RNA using a standard set of partially nested primers in the viral S segment to complete reverse transcription-polymerase chain reaction (RT-PCR) to analyze the RNA sequence. The PCR products were then used to complete phylogenetic trees from the informative sequences using PAUP 3.1 software.
CDC's role and field studies Following the initial discovery of BAYV in 1993, a large field study was conducted in the Southeastern marshes in the United States, leading to the identification of BAYV in marsh rice rat populations. These results from their field study confirmed an established unique pattern of BAYV confined mainly to the southeastern U.S., which contrasts with Sin Nombre virus, as it is more widespread in the southwestern U.S.
In 1993 the CDC conducted studies on Sin Nombre virus, which caused 11 cases of HPS in Arizona and 18 cases in New Mexico, along with 1-5 cases spread across 12 other states in the western U.S. Through their field studies they found that from western HPS cases in May 1993 to December 1993 cases ranged from ages 12 to 69 years old, with a mean age of 31. They found 23 out of 53 (43%) of individuals infected were female, while 30 out of 53 (57%) were male. Approximately 49% of cases were seen in American Indians, 42% in non-Hispanic whites, 8% in Hispanics, and 2% in non-Hispanic blacks. At the time of the Sin Nombre study, they had not yet identified BAYV as a hantavirus.
The eventual identification of BAYV as a distinct hantavirus species with a different region of infection emphasized the need for the CDC to increase geographically tailored surveillance programs across all areas in the U.S.
Historical cases and geographic expansion Although BAYV was discovered in Louisiana in 1993, retrospective analyses and seroprevalence surveys suggest that HPS cases could be potentially linked to BAYV and may have occurred as early as the 1980s. Though HPS in the U.S. can be attributed to three different hantaviruses (Sin Nombre virus, Seoul virus, and BAYV), the cases linked to the southeastern U.S. indicate a link to BAYV. The analysis of the first 100 U.S. cases identified showed that the disease was distributed in 21 states and had gone unrecognized since at least 1959. HPS outbreaks were seen between 1950-1953 during the Korean War, as more than 3,000 United Nations soldiers fell ill with Korean hemorrhagic fever, a hantavirus disease that sees hundreds of cases per year. The first HPS case in the United States occurred in 1993 in New Mexico, where 33% of cases would be identified. The source of HPS was confirmed when the hantavirus associated with HPS was isolated from a deer mouse in the home of someone confirmed to have HPS. Following virus isolation and PCR conducted on both the lung material from the mice and Vero E6 cell cultures were obtained, and the PCR sequences were found to be identical, confirming the identity of the hantavirus. Historical case studies provide insights into how BAYV may have spread from Louisiana to other southeastern states, mirroring the distribution patterns of marsh rice rats across different wetland habitats.
Transmission and reservoir ecology Transmission mechanisms BAYV transmission occurs primarily through aerosolized particles from infected rodents' bodily fluids, particularly urine, feces, and saliva. Humans mostly become infected when they breathe in these particles, especially in enclosed spaces where the virus can become concentrated. This means an individual could come into contact with hantavirus through breathing in hantavirus-contaminated air when cleaning up after rodents, touching contaminated objects to the nose or mouth, being bitten or scratched by infected rodents, or eating hantavirus-contaminated food. Other significant risk factors include living near wetland areas, engaging in agricultural activities, and performing tasks that disturb rodent habitats. The Andes virus, a hantavirus found in South America, has even been found to spread through person-to-person transmission, raising concern about hantavirus evolution in the U.S. A hantavirus infection does not always mean HPS will arise, but the majority of hantaviruses in North, Central, and South America cause HPS.
Reservoir host: The marsh rice rat The marsh rice rat, a semi-aquatic rodent, is the primary reservoir for BAYV. High levels of BAYV antibodies in marsh rice rat populations highlight the virus's endemic presence in these animals, which can carry BAYV asymptomatically throughout their lifespans. However, in the HPS cases in the southwestern U.S., the primary reservoir was deer mice, another semiaquatic rodent found in grasslands. Still, it can also be found in dry areas like deserts or alpine habitats. Adapted to wetland environments, these marsh rice rats are prevalent in marshes, swamps, and coastal wetlands in the southeastern U.S. Aside from wetlands, they can also be found in rural areas in streams, crops, or even dark, moist areas of homes, which is one of the leading causes of human interaction with the hantavirus. Because they are nocturnal and typically burrow and build nests in grassy vegetation, dark and shaded areas next to homes or agricultural crops can become an ideal location for marsh rice rats.
Ecological impacts and habitat requirements The ecology of marsh rice rats and their habitats plays a crucial role in BAYV transmission. As semi-aquatic rodents, these rats thrive in moist, vegetated areas, often near human populations that have encroached on wetland habitats. As mentioned above, these rodents are also nocturnal, so they require a burrowing or overgrown habitat where they can rest undisturbed during the majority of the day. One of the biggest threats to the marsh rice rat is habitat loss, as more and more wetlands are overtaken by development, causing a strain on the marsh rice rat population. Because of this requirement, they can find themselves in tall wetlands, where they are both protected and undisturbed. Seasonal changes, especially flooding and temperature shifts, affect rat population density and, consequently, BAYV transmission rates. The rise in temperature shifts and breeding schedule results in the highest numbers, and BAYV transmission rates occur in the summer months. Marsh rice rats breed many times from March to October, with a gestation period of about 25 days, usually resulting in a litter of 3-5 young. Due to the breeding and gestation time, the population peaks during the summer.
Role of other species in transmission dynamics While marsh rice rats are the primary host, rodent species like deer mice may occasionally harbor BAYV. However, they do not maintain the virus's lifecycle like other hantaviruses. Due to the diversity within wetland ecosystems, ecological studies have explored whether predators like snakes and raptors that feed on marsh rice rats might influence BAYV spread through prey population dynamics. Through these studies, they have found that snakes, birds, and pets cannot carry or be infected by a hantavirus.
Molecular biology and genomic insights Genomic structure of BAYV
BAYV's RNA genome consists of three segments: S (small), M (medium), and L (large). The small segment. (S) of the RNA genome encodes the nucleocapsid proteins (N), which is the most abundant viral protein following infection as the nucleocapsid plays an essential role in intracellular transportation, viral assembly, viral attachment to host cell proteins, and regulating the interferon response in infected cells by inhibiting the IFN-beta promoter. The medium genome segment (M) encodes for the glycoproteins (Gn and Gc), which are involved in binding to the target cell receptor and viral entry and can help modulate immune responses. The large genome segment (L) encodes for an RNA-dependent RNA polymerase, which is essential in carrying out replication and transcription of the genome. Due to each genome segment being critical to the virus's survival and pathogenicity, mutations in any segment could influence its pathogenicity, host specificity, and survival in general.
Mechanisms of viral replication and cell entry Orthohantaviruses targets endothelial cells, primarily within rodent lung tissue, by binding to cellular receptors through its glycoproteins (Gn and Gc). This interaction begins with the surface glycoproteins Gn and Gc, with target endothelial cells or macrophages with Beta-3 integrin on their surface. This binding initiates endocytosis, enabling the virus to replicate within the host cell, and eventually activates the host immune response. This immune response, especially by macrophages and CD8 T cells, may be involved in the pathogenesis that leads to respiratory failure, as the virus will cause the host cell to secrete a wide array of inflammatory cytokines. Activated macrophages secrete proinflammatory cytokines such as TNF-α, interleukin-1 (IL-1), and IL-6. The excess of these cytokines produced by macrophages and activated hantavirus-specific T cells upon antigen recognition on infected pulmonary endothelial cells is probably critical for HPS pathogenesis.
Evolution and genetic variability