A new study proves for the first time that a common viral infection can trigger the brain damage that causes Parkinson’s, and the destruction continues long after the virus is gone

Parkinson’s disease has been studied in laboratories for decades, but almost everything researchers know about it has been learned through methods that cannot replicate how the disease actually begins in people. The standard approach involves injecting animals with powerful neurotoxins that rapidly destroy dopamine-producing brain cells, producing Parkinson’s-like symptoms within days. It is useful for studying what happens after the damage occurs. It tells researchers almost nothing about why the damage occurs in the first place, or what set the process in motion years before a person notices their first tremor.

A new study from Texas A&M University has changed that by proving something researchers have theorized for decades but never been able to demonstrate directly: a natural viral infection, one that does not require any toxic chemicals and does not require any genetic modification, can trigger the exact sequence of brain damage that defines Parkinson’s disease, and the damage continues long after the virus itself has been eliminated from the body.

What the experiment showed

The research team, led by Candice Brinkmeyer-Langford of the Texas A&M School of Public Health, infected mice with Theiler’s murine encephalomyelitis virus, known as TMEV, a pathogen that occurs naturally in mouse populations and poses no toxicity risk. Within one week, the virus had infected the dopamine-producing neurons in the substantia nigra, the specific brain region whose gradual death defines Parkinson’s disease in humans. By the end of the first month, those neurons were gone.

The team then tracked the animals for twenty weeks, measuring three distinct outcomes. First, they administered a dopamine-mimicking drug that produces a distinctive movement pattern in animals whose dopamine systems are intact. In infected animals, the pattern was absent, confirming that the virus had caused genuine dopamine neuron loss rather than a temporary disruption. Second, they used a standardized coordination test called the pole test, which measures how quickly and smoothly an animal can navigate a vertical surface. Infected animals were consistently slower than controls at every time point measured, including at week twenty, the final assessment.

Third, they analyzed the animals’ walking patterns using a specialized treadmill capable of measuring more than 100 distinct factors involved in gait, balance, and motor function. The infected animals showed the same physical weakness and walking abnormalities seen in human Parkinson’s patients.

By week twenty, the virus had long since been cleared from the animals’ bodies. The brain damage it caused had not.

The hit-and-run theory

The finding provides the first direct experimental evidence for what researchers call the hit-and-run theory of neurodegeneration. The theory holds that a virus can enter the brain, cause a brief acute infection, be eliminated entirely by the immune system, and leave behind no detectable trace of itself, while simultaneously triggering a slow inflammatory process that continues destroying neurons for years or decades after the infection is resolved.

The mechanism involves the brain’s resident immune cells, which can be reprogrammed by a viral infection into a state of chronic low-level activation. In this state, they continue generating inflammatory signals long after the original threat is gone. Over time, this background inflammation degrades the delicate environment that dopamine neurons require to survive. The loss is gradual enough that the brain compensates for years. By the time physical symptoms appear, typically in a person’s sixties or seventies, the neuron loss may have been underway for a decade or more.

“The toxic-exposure models are useful for studying Parkinson’s, but not all people who are exposed to chemicals go on to develop Parkinson’s, so these models cannot show all the ways a disease as complex as Parkinson’s actually begins or develops over time in people,” said Brinkmeyer-Langford.

Why the existing models are not enough

The toxin models that have dominated Parkinson’s research were designed to produce symptoms efficiently, not to replicate disease origins. They work by introducing a chemical that kills dopamine neurons rapidly and reliably, allowing researchers to study the downstream consequences of that cell death within a compressed timeframe. They have produced decades of valuable data about what happens in the Parkinson’s brain after significant neuronal loss.

What they cannot do is show what happens before that loss, in the period when the brain has already been set on a trajectory toward Parkinson’s but before any symptoms have emerged. They cannot reveal which early immune signals mark the transition from a silent inflammatory process to active neurodegeneration. They cannot identify the window during which an intervention might interrupt the progression before it becomes irreversible. And because they use toxins rather than biological triggers, they cannot be used to test whether treatments that target the immune response to viral infection might prevent the disease from developing at all.

The TMEV model opens all of these questions simultaneously. Because the disease is triggered by a real infection rather than a chemical injection, it follows a timeline more similar to the human disease. Because the virus is then cleared by the immune system, researchers can study the phase in which damage is occurring without the confounding presence of an active pathogen.

The genetic piece

Brinkmeyer-Langford is careful to note that a viral infection alone is not a Parkinson’s sentence. The same team had previously used the TMEV model to demonstrate a viral contribution to ALS, and their broader research program has consistently emphasized that neurodegeneration requires both an environmental trigger and a genetic susceptibility for the damage to take hold.

The Epstein-Barr virus illustrates the same principle. It infects the vast majority of adults worldwide. In most people it causes mononucleosis and then becomes latent. In a subset of genetically susceptible individuals, decades later, it has been linked to multiple sclerosis and certain cancers. The virus is the same in every case. The outcome depends on what the host’s immune system does with it.

Parkinson’s almost certainly works the same way. A virus may be a necessary contributor in some cases, but it is not sufficient on its own. Future research using the TMEV model will attempt to identify which genetic variants make an individual’s dopamine neurons vulnerable to this specific kind of viral inflammatory damage, and which immune pathways are doing the most harm during the silent phase between infection and symptom onset.

What comes next

The Texas A&M team has outlined the next stages of research that this new model makes possible. They plan to run the TMEV model directly alongside the existing toxin models to identify what the viral model reveals that chemical induction misses. They will search for early blood-based biomarkers that could identify individuals in the pre-symptomatic phase of viral-triggered neurodegeneration, before dopamine neurons are lost beyond the threshold where movement problems appear. And they will map the specific immune signaling loops through which a cleared viral infection continues influencing the brain’s cellular environment.

The urgency of that research is not abstract. Parkinson’s affects more than 10 million people worldwide, making it the second most common neurodegenerative disease after dementia. As the global population ages, the number of cases is expected to rise substantially. The diseases that destroy the most neurological function in the most people are the ones whose earliest phases remain the most invisible, and the most scientifically inaccessible.

A model that lets researchers watch the process from its actual beginning, rather than from its catastrophic middle, changes what kinds of questions can be asked and what kinds of interventions can be designed. The virus is gone. The research it made possible is just beginning.

Source

Candice Brinkmeyer-Langford, Tae Wook Kang, Rahul Srinivasan, C. Jane Welsh. “Theiler’s murine encephalomyelitis virus as the infectious agent for a virally induced mouse model of Parkinson’s disease.” Brain, Behavior, and Immunity-Health, 2026.
DOI: 10.1016/j.bbih.2026.101230