HSV-1, the virus that causes oral herpes, is carried by almost four billion individuals globally. While it usually results in cold sores, researchers have uncovered a potentially severe pathway through which the virus can access the brain — through the nasal passage during oral sexual activity.

Professor Deepak Shukla from the University of Illinois Chicago, who spearheaded the study, described how HSV-1 particles can enter the nasal passages when they come into contact with an infected partner’s saliva or skin during oral sex. Once they penetrate the nasal cavity, the virus can travel directly to the brain, increasing the risk of encephalitis, brain damage, and possibly dementia.

The research, published in the journal *mBio*, also highlighted a crucial human enzyme, **heparanase (HPSE)**, which may exacerbate nasal HSV-1 infections. Normally, heparanase aids in clearing damaged cells, but HSV-1 seems to exploit this enzyme to amplify inflammation within the brain.

“For most individuals, HSV-1 results in occasional cold sores and is not life-threatening,” stated Professor Shukla. “However, if the infection ascends via the nasal route to the brain, the consequences can be far more serious.”

To investigate this hypothesis, Shukla’s team conducted experiments on mice by introducing HSV-1 into their nostrils. Mice with normal levels of heparanase exhibited severe symptoms, such as nasal swelling, breathing difficulties, and ultimately, death. Brain analyses revealed nerve damage, elevated inflammation, and abnormal activity in regions associated with olfaction and virus entry.

Notable behavioral changes were also observed, including memory impairment, anxiety-like behaviors, and balance issues — all manifesting within six months post-infection.

Although herpes simplex encephalitis (HSE) is uncommon — impacting approximately two to four individuals per million — researchers suspect that cases of nasal HSV-1 are likely underreported. The virus may remain latent in the brain, contributing to ailments such as Alzheimer’s disease later in life.

Shukla stressed that the impact can vary considerably based on an individual’s immune resilience and the frequency of outbreaks.

“Our discoveries serve as a cautionary tale,” he remarked. “Unchecked herpes can result in lasting behavioral problems, motor dysfunction, and coordination difficulties. It is crucial to comprehend how the virus operates and the severity it can reach under specific circumstances.”