Over the past hundred years, a once-lethal virus transmitted by mosquitoes has evolved to no longer harm humans. Recent studies reveal that the changes in the virus’s ability to infect human cells corresponded with a reduction in illness and fatalities. These discoveries provide crucial insights in virology that may enhance preparations for upcoming outbreaks of other viral diseases.
The narrative of western equine encephalitis and its transition from a fatal disease holds vital lessons about how a pathogen can acquire or lose the capacity to infect humans from animals.
This narrative is encapsulated in recent research from Harvard Medical School, which unveils the mechanisms the western equine encephalitis virus employed to infect humans and links changes in this capability over time to a decrease in sickness and mortality caused by the pathogen.
The findings, published on July 24 in Nature, present significant lessons for public health officials aiming to brace for future viral outbreaks, according to the researchers.
The research journey was filled with unexpected developments, as the scientists noted. The outcomes challenge several foundational assumptions that have been utilized to understand how viruses interact with human cells and the factors causing the rise and fall of outbreaks. This includes the belief that a virus typically targets a single host receptor to invade and infect cells.
“This was a true scientific detective story,” remarked Jonathan Abraham, the senior author of the study and an associate professor of microbiology at Harvard Medical School’s Blavatnik Institute. “The virus continuously surprised us and imparted invaluable lessons on studying viruses.”
The researchers identified the specific proteins on host cells that various strains of the virus used to infect several species, including horses, humans, and birds, over the past century. Their results correlated differences in the virus’s capacity to cause illness in humans and horses with alterations in the viral genome that rendered the virus incapable of targeting proteins present in humans and horses, while still retaining its ability to infect birds and reptiles that act as reservoirs for the virus.
The unexpected variety and fluctuation in the virus’s ability to target host cells emphasize the necessity of studying viruses broadly across time, geography, and host species to track potential outbreaks and monitor emerging infectious threats.
A virus evolves
The main character in this narrative is the western equine encephalitis virus (WEEV), which belongs to a viral family called alphaviruses.
A crucial aspect of comprehending how a virus interacts with a host is understanding the exact pathway it uses to invade cells and establish infection.
WEEV and other alphaviruses typically attach a spike protein to a compatible protein—the receptor—on the surface of a host cell. Upon binding to the receptor, the virus is able to enter the cell. Once inside, it hijacks the cell’s resources for its replication, spread, and survival.
The researchers created non-pathogenic replicas of various viral strains collected from different periods and locations to evaluate their ability to infect host cells in laboratory dishes. They also conducted tests on some strains in mice.
Several dangerous strains of WEEV have been known to cause severe brain inflammation in both horses and humans. Over the years, thousands of horses perished and hundreds of humans fell ill. The case fatality rates for humans peaked at 15 percent in North America in the early to mid-20th century.
Abraham’s team discovered that some of these earlier strains could attach their spike proteins to various types of receptors in animal cells. This finding was unexpected, as the prevailing view in virology maintained that viruses typically target only one type of host cell receptor.
The researchers observed that the strains prevalent during the years of frequent outbreaks could utilize multiple receptors present on the brain cells of both humans and horses, including proteins known as PCDH10 and VLDLR.
Despite the virus continuing to circulate among birds, mosquitoes, and other animals, the last documented human outbreak in the United States occurred in 1987, according to the Centers for Disease Control and Prevention. Since then, only five cases have been reported in the U.S.
In contrast, when the researchers examined more recent strains isolated from mosquitoes in California in 2005, they found that the viral spike protein no longer recognized human receptors but retained its interaction with similar proteins in birds.
From these findings, the researchers propose that the virus evolved, possibly because horses can be vaccinated and are no longer numerous enough in agriculture or transport sectors to effectively amplify the virus. Alternatively, they suggest that the virus may have adapted through antigenic drift—a process where random mutations create small changes in a viral genome that can ultimately alter its interaction with hosts. Regardless of the reason, the researchers noted that subtle changes in the shape of the viral spike proteins affected their ability to bind with cellular receptors.
This alteration in the receptors targeted by the virus is likely a primary factor in the virus “submerging” as a human pathogen in North America, according to the research team. This newfound understanding of the dynamic complexity of viral receptors is vital for comprehending how this virus or similar ones might one day resurface, the scientists stated.
“We need to understand what happens to viruses when they become dormant, to better prepare for when they re-emerge,” expressed Wanyu Li, the lead author and a doctoral student in the virology program at Harvard Medical Sciences.
For instance, determining if dangerous variants of the pathogen survive in isolated insect populations, or if the virus has acquired the ability to infect other animals, could serve as valuable warning signals for potential resurgence of diseases believed to be eradicated.
A virus’s intricate behavior
Through their experiments, the researchers found that certain historical WEEV strains exhibited unexpected behaviors.
The team employed the eastern equine encephalitis virus—a more lethal relative of WEEV—as a control for some experiments. In one test, they discovered that an old strain of WEEV could utilize the same receptor as the eastern virus, a feat that newer WEEV strains could not achieve. They also identified various WEEV strains utilizing different receptors. Some strains could bind to avian receptor proteins but not those found in human or equine cells.
These findings serve as a significant reminder that viruses exist within a dynamic system and that viruses themselves are also dynamic, displaying subtle yet crucial differences across time and geographical regions. This notion was notably highlighted by the rapidly evolving SARS-CoV-2 virus responsible for the COVID-19 pandemic, as noted by the researchers.
“It was a wake-up call,” Abraham remarked. “It tells us that studying just one strain of a virus doesn’t provide the complete understanding. Viruses might appear simple, but they are quite complex and constantly evolving.”
Utilizing lessons for pandemic readiness
Typically in virology, researchers often evaluate a restricted range of viral strains. These latest findings to truly grasp the complexities of the virus.
“There is a wealth of biological knowledge awaiting discovery through investigating the variety within these intricate systems,” stated Abraham. He also emphasized the critical need to examine as much viral diversity as possible to prepare for potential outbreaks.
Numerous viruses thrive in the insects and animals that inhabit our surroundings, according to Abraham. For example, the Powassan virus, which is prevalent in New England, can sometimes resurface, resulting in severe or life-altering illnesses.
Abraham mentioned that there are multiple factors that could explain these re-emergences. Are there various strains of Powassan with different levels of danger? Could environmental shifts or evolutionary changes in the virus itself play a role in triggering new outbreaks? By examining these elements and the wide range of viral diversity, researchers can enhance their ability to predict and guard against outbreaks.
In an unexpected development, while Abraham and his team were carrying out their experiments, a resurgence of the WEEV virus was reported in South America, a region that had previously seen a significant drop in cases. It appears that the viral strains in South and North America are genetically different; furthermore, the South American variant of the virus does not survive long enough for it to be transferred effectively between continents by migratory birds. Nonetheless, Abraham pointed out that the recent outbreak in South America highlights the need for constant vigilance and an improved scientific grasp of these unpredictable, evolving viruses.
“The return of WEEV took us all by surprise,” remarked Li. “Now that we know its cellular host receptors, we have the necessary tools to delve into the molecular factors behind WEEV’s resurgence.”
Abraham and his team are currently studying the strains linked to the recent outbreak in South America.
“A minor change in the viral genome, an increase in rainy season intensity fostering mosquito populations, or shifts in human habitation or working patterns could potentially spark an outbreak,” Abraham warned. “The more knowledge we acquire, the better equipped we will be to safeguard ourselves.”
Authorship, funding, disclosures
The additional contributors included ChieYu Lin, Himanish Basu, Jessica Oros, Tierra Buck, Praju V. Anekal, Jesse Plung, Xiaoyi Fan, Wesley Hanson, Haley Varnum, Adrienne Wells, Colin J. Mann, Laurentia V. Tjang, Pan Yang, Brooke M. So Yoen Choi, Isaac M. Chiu, Vesna Brusic, Paula Montero Llopis, Joshua M. Boeckers, and Hisashi Umemori from Harvard Medical School, along with Jessica A. Plante, Rachel A. Mitchell, Divya P. Shinde, Jordyn L. Walker, Scott C. Weaver, and Kenneth S. Plante from the University of Texas Medical Branch.
This research was funded by the Burroughs Wellcome Fund Investigators in the Pathogenesis of Infectious Disease Awards, the Vallee Scholar Award, the Smith Family Foundation Odyssey Award, the Charles E.W. Grinnell Trust Award, NIH grant R01 AI182377, a G. Harold and Leila Y. Mathers Foundation Award, NIH grant T32AI700245, T32GM144273, NIH grant R24 AI120942, the Jackson-Wijaya Fund, NIH T32 CA009216-40, and NIH grant R01 MH125162.
The authors extend their thanks to the Micron (Microscopy Resources on the North Quad) Core, the Molecular Electron Microscopy Core Facility, and the Immunology Flow Cytometry Facility at Harvard Medical School for their assistance and support. They also acknowledge Grace H. Raphael at the University of Texas Medical Branch for her contributions to experiments involving authentic viruses.
