Introduction 

Neurotropic viral infections, which specifically target the central nervous system (CNS), encompass a wide range of viruses, including poliovirus, herpes simplex virus (HSV), and other members of the enterovirus family. These viruses are capable of causing diseases with varying degrees of neuroinvasiveness and neurovirulence. Polio, caused by the poliovirus, is perhaps the most well-known neurotropic viral infection due to its historical significance and focus on motor neurons, which can lead to paralytic symptoms. Other neurotropic viruses, such as rabies and herpes simplex virus, follow different infection pathways and present distinct neurological outcomes.

Poliovirus, despite its near eradication, remains one of the most well-known neurotropic viruses due to its devastating effects in the 20th century. Comparing its characteristics to other neurotropic viruses can shed light on both its uniqueness and commonalities.

This article will explore the key differences between poliovirus and other neurotropic viral infections, examining their viral pathogenesis, transmission mechanisms, clinical manifestations, and public health impacts. While these viruses share the common characteristic of targeting the nervous system, they differ significantly in terms of their geographic prevalence, associated morbidity, and the immune responses they elicit. A deeper understanding of these distinctions is crucial for improving prevention and treatment strategies for neurotropic viral infections, especially as emerging and re-emerging infections pose new public health challenges.¹ 

Viral pathogenesis 

Poliovirus 

Poliomyelitis, caused by poliovirus, is a highly contagious viral infection primarily affecting the nervous system, leading to paralysis in severe cases. The virus spreads through faecal-oral or respiratory routes, with the risk of neurological damage varying across cases. Global vaccination programs, including inactivated poliovirus vaccines (IPV) and oral poliovirus vaccines (OPV), have been pivotal in reducing polio incidence worldwide. However, continued immunization efforts are essential for achieving complete eradication, especially in regions where the virus remains endemic or vaccine coverage is insufficient.²

Poliovirus, the causative agent of poliomyelitis, primarily targets motor neurons within the central nervous system (CNS), leading to paralysis. After entering the body through the oral route, the virus replicates in the gastrointestinal tract and lymphoid tissues. It then spreads via the bloodstream or retrograde axonal transport to the CNS, where it infects motor neurons in the spinal cord and brainstem. The destruction of these neurons is a major cause of paralysis associated with poliomyelitis. Apoptosis, or programmed cell death, is a key mechanism through which poliovirus induces neuronal damage. This process is regulated by the viral interaction with the cellular receptor CD155, which mediates viral entry into host cells. 

Once inside the neurons, poliovirus induces apoptosis through both intrinsic and extrinsic pathways. Infected cells show characteristic signs of apoptosis, leading to cell death. Interestingly, mutations in CD155 can reduce the extent of apoptosis, potentially allowing the virus to establish persistent infections. This interaction between poliovirus and CD155 is significant not only for viral entry but also for modulating apoptotic responses during infection.³ 

Persistent infections can result in long-term damage, contributing to post-polio syndrome, where patients experience muscle weakness years after the initial infection. The study of poliovirus-induced apoptosis has revealed its critical role in the pathogenesis of poliomyelitis, underscoring the need to understand the molecular mechanisms behind this process to inform treatment strategies. The ability of poliovirus to modulate apoptosis in various cell types through its viral proteins and interactions with host factors highlights its complex pathogenesis.⁴ 

Other neurotropic viruses 

Rabies virus 

Rabies is a nearly always fatal neurotropic viral infection primarily transmitted through the bite of infected animals, such as dogs and bats. The virus spreads through the nervous system, leading to two distinct clinical forms: furious rabies, which is characterized by agitation, hydrophobia, and seizures, and paralytic rabies, which causes progressive paralysis resembling Guillain-Barré syndrome​. The pathogenesis of rabies begins when the rabies virus (RABV) enters the body, most commonly through a bite from an infected animal. The virus is present in the saliva and enters the body via damaged skin or mucosal surfaces. Once inside, the rabies virus targets motor neurons at the neuromuscular junctions, where it binds to the nicotinic acetylcholine receptor. This interaction enables the virus to travel backwards along peripheral nerves towards the central nervous system (CNS) via motor neurons.⁴ 

Unlike many other viruses, rabies does not cause viraemia, meaning it does not spread through the bloodstream. Instead, it relies on direct nerve transmission to reach the spinal cord and brain. After entering the CNS, the virus spreads through transneuronal transfer and quickly infects the brainstem, limbic system, and cortical areas, causing the classical neurological symptoms. The furious form is often associated with extensive viral replication in the brain and a stronger immune response, whereas paralytic rabies, characterized by weakness and paralysis, is often associated with viral invasion primarily along motor neurons.⁵

Herpes simplex virus 

Herpes simplex virus (HSV) is a neurotropic virus from the herpesvirus family, known for causing oral and genital ulcers and occasionally leading to more severe conditions such as encephalitis. HSV exists in two main forms, HSV-1 and HSV-2, both of which are capable of establishing lifelong latent infections in the nervous system, with periodic reactivations that result in recurrent outbreaks.

Herpes simplex virus enters the body through mucosal surfaces or broken skin and initially infects epithelial cells. The virus replicates locally, causing vesicular lesions before entering sensory neurons. Through retrograde axonal transport (see above), HSV travels along the axons to the sensory ganglia, where it establishes a lifelong latent infection. During latency, the virus persists in a non-replicating state within the neuron cell nucleus.⁶

Periodic reactivation of HSV can occur due to triggers like stress or immunosuppression, leading to the virus travelling back to the epithelial cells via anterograde transport. This reactivation results in recurrent lesions at the site of the initial infection. While HSV avoids immune detection by residing in neurons, during reactivation it evades the host immune response by interfering with key signalling pathways like interferon production. The ability to cycle between active and latent phases is central to HSV's pathogenesis, contributing to its persistence and frequent recurrences​.⁷ 

Key differences

Viral pathogenesis

Poliovirus primarily affects motor neurons in the spinal cord and brainstem, leading to paralytic poliomyelitis in severe cases. After entering the body through the oral route, the virus replicates in the gut, and in a subset of patients, it enters the bloodstream and subsequently the central nervous system (CNS), causing damage to motor neurons. Unlike HSV and rabies, poliovirus does not establish latency or recurrent infections; it follows an acute course, with symptoms either manifesting within a few days or leading to permanent paralysis.⁷

HSV enters the body through mucosal surfaces or skin and establishes latency in sensory neurons, where it remains dormant for life. The virus can reactivate periodically, causing recurrent lesions. HSV does not typically cause acute motor neuron damage like poliovirus but instead leads to long-term issues such as encephalitis in rare cases. Its ability to evade immune detection during latency and reactivate under certain triggers distinguishes it from polio.⁸

Rabies virus, meanwhile, exhibits a slower pathogenesis. After transmission through an animal bite, the virus travels along peripheral nerves to the CNS, primarily affecting the brainstem and limbic system. Rabies is nearly always fatal once symptoms appear, with severe neurological symptoms such as hydrophobia, paralysis, and agitation. Unlike polio and HSV, rabies has no latency phase and progresses rapidly once it reaches the CNS.⁹

Transmission mechanisms

Poliovirus spreads via the fecal-oral route, particularly in areas with poor sanitation. In contrast, HSV is transmitted through direct contact with infected body fluids, such as saliva or genital secretions, and rabies is transmitted through the bite of an infected animal, usually via saliva.

Clinical outcomes and immune response

Poliovirus primarily causes acute flaccid paralysis, affecting the muscles, while HSV often leads to skin and mucosal lesions and, in rare cases, encephalitis. Rabies, on the other hand, causes severe neurological symptoms, often leading to death. Unlike poliovirus, which can be prevented through vaccination, there is no effective HSV vaccine, though post-exposure prophylaxis can prevent rabies if administered promptly.

Summary 

This article dives into the distinct ways poliovirus, herpes simplex virus (HSV), and rabies virus attack the nervous system, revealing their unique paths of infection, transmission methods, and strikingly different outcomes. From polio’s paralytic punch to rabies’ deadly bite and HSV’s sneaky reactivations, these neurotropic viruses show just how diverse viral threats to the brain can be.