A 7-year-old boy is brought to the emergency department due to fever, headache, vomiting, and confusion. He has no prior medical conditions and is up to date with immunizations. The patient has not traveled recently, and his parents report no exposure to individuals with similar symptoms or ingestion of anything out of the ordinary. They have recently noticed several dead birds around their neighborhood. Temperature is 39 C (102.2 F). Physical examination shows neck rigidity and right lower extremity weakness with fasciculations. Cerebrospinal fluid analysis shows a leukocyte count of 200/mm³ with 20% neutrophils and 80% lymphocytes. Which of the following is the most likely pathogen responsible for this patient's condition?

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11ec160b_3889_8334_9249_254fd87d5dbc_MD0001_00 West Nile virus is an enveloped RNA virus that is found in warm climates worldwide. The virus replicates extensively within birds and is passed to mosquitos (Culex species) during blood feeding. Viral accumulation in mosquito salivary glands can lead to human transmission. Most infected patients develop low-level viremia that is neutralized within a week by the humoral immune response; these individuals typically remain asymptomatic. However, a minority of patients (particularly the young, elderly, or immunosuppressed) are unable to mount an effective immune response and develop significant clinical manifestations, including: West Nile fever - a self-limited, flu-like illness that is often associated with a maculopapular rash Neuroinvasive disease - the virus is neurotropic and often causes meningitis (eg, fever, headache, neck stiffness), encephalitis (eg, confusion, tremors, seizures), and/or flaccid paralysis (eg, asymmetric weakness of limbs, fasciculations). Cerebrospinal fluid usually shows a lymphocytic pleocytosis. Most patients recover completely with supportive care, but some with neuroinvasive disease have long-term neurologic sequelae or die. (Choice A) Cryptococcus neoformans, a fungus found in bird feces, can cause meningoencephalitis. However, most cases occur in those with severe immunosuppression (eg, advanced AIDS). (Choices B and F) Hemophilus influenzae and Streptococcus pneumoniae are leading causes of acute bacterial meningitis, which is often associated with fever, headache, vomiting, and neck stiffness. However, cerebrospinal fluid usually shows a high neutrophil count. In addition, lower-motor neuron signs such as fasciculations would be atypical. (Choice C) Herpes simplex virus is a leading cause of sporadic encephalitis; it is usually characterized by confusion, focal neurologic deficits (eg, hemiparesis, dysphagia, cranial nerve defects), and seizures. It is not harbored in birds; cerebrospinal fluid usually shows elevated erythrocytes (in addition to lymphocytic pleocytosis); and lower-motor neuron signs (eg, fasciculations) are absent. (Choice D) Nonpolio enteroviruses (eg, Coxsackievirus, echovirus) often cause aseptic (viral) meningitis and can occasionally cause encephalitis. However, these viruses are not associated with bird vectors and do not typically cause lower-motor neuron signs. (Choice E) Polio virus is a common cause of acute, asymmetric flaccid paralysis, but it has been eliminated from the United States. Educational objective: West Nile virus is harbored in birds and transmitted to humans by mosquitos. Most patients are asymptomatic, but the virus can cause an influenza-like illness or neuroinvasive disease with meningitis, encephalitis, and flaccid paralysis.

A 7-Year-Old Boy Is Brought to the Emergency Department Due