Arboviral Encephalitis

When an infected mosquito bites, it injects West Nile virus into the skin's dermis. The virus targets dendritic cells and enters them through receptor-mediated endocytosis. It then replicates locally at the infection site before spreading to regional lymph nodes and eventually entering the bloodstream for systemic distribution.

Arboviruses cross the blood-brain barrier through three main mechanisms. First, they directly infect and compromise endothelial cells, disrupting barrier function. Second, they disrupt tight junctions between endothelial cells, weakening structural integrity. Third, they use a 'Trojan horse' strategy, where infected immune cells transport the virus across the barrier into the central nervous system.

Once inside the central nervous system, West Nile virus replicates in neurons, triggering strong immune responses. This replication releases pro-inflammatory cytokines like IL-6, TNF-α, and interferons. The combined effects of direct viral damage to neurons and this inflammatory environment lead to neuronal death and encephalitis.

West Nile virus cycles primarily between birds as amplifying hosts and mosquitoes as vectors. Humans and other mammals are incidental, dead-end hosts who do not contribute to virus transmission. Infection in humans occurs when an infected mosquito introduces the virus during feeding, but humans cannot transmit the virus back to mosquitoes.

West Nile virus shows a strong preference for neurons in the cerebral cortex, brainstem, and spinal cord. Viral replication in these regions triggers neuroinflammation and neuronal death. The extent of damage to these critical areas determines the severity of neurological symptoms, ranging from mild cognitive impairment to seizures, paralysis, or coma.

Arboviral encephalitis involves inflammation of the brain tissue itself, caused by viral replication in neurons and subsequent immune responses. In contrast, viral meningitis affects the meninges, the membranes surrounding the brain and spinal cord. Both are serious viral infections of the central nervous system but target different anatomical structures.

Most West Nile virus infections are asymptomatic or present as mild febrile illness. However, a small proportion of cases progress to neuroinvasive disease, particularly among immunocompromised individuals and older adults. These populations have reduced immune capacity to control viral replication in the central nervous system.