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 {{WBRQuestion
|QuestionAuthor= {{YD}} (Reviewed by  {{YD}})
|ExamType=USMLE Step 1
|MainCategory=Microbiology
|SubCategory=Infectious Disease
|MainCategory=Microbiology
|SubCategory=Infectious Disease
|MainCategory=Microbiology
|SubCategory=Infectious Disease
|MainCategory=Microbiology
|MainCategory=Microbiology
|MainCategory=Microbiology
|SubCategory=Infectious Disease
|MainCategory=Microbiology
|SubCategory=Infectious Disease
|MainCategory=Microbiology
|SubCategory=Infectious Disease
|MainCategory=Microbiology
|SubCategory=Infectious Disease
|MainCategory=Microbiology
|MainCategory=Microbiology
|SubCategory=Infectious Disease
|Prompt=During an outbreak of an infectious disease, a 30-year-old man presents to the emergency department with fever for the past 2 days. He complains that he has diffuse body aches with myalgia and loss of appetite. Further history-taking reveals that the patient was recently bitten by a ''Culex'' mosquito. The patient's blood pressure is 110/84 mmHg, heart rate is 102/min, and temperature is 38.4 °C (101.12 °F). Physical examination, including neurological exam, is unremarkable. The physician suspects the patient might be infected by an agent that has also infected bird species in the same district. Which of the following best characterizes the infectious agent responsible for this patient's condition?
|Explanation=West Nile virus (WNV) is an enveloped positive-sense ssRNA [[neurotropic]] virus that is considered a member of the [[Japanese encephalitis]] serocomplex. It belongs to the family [[Flaviviridae]]. The natural reservoir of the virus is mainly birds, but it is usually transmitted by ''[[Culex]]'' mosquito bites to humans and other animals, and less commonly transmitted from human to human by [[blood transfusion]]s or [[tissue transplantation]]. 

WNV infection is considered a spectrum of clinical disease that may have an asymptomatic course, a mild "[[West Nile fever]]" characterized by fever and constitutional symptoms similar to the clinical presentation of the patient in the vignette, or a more severe "[[neuroinvasive disease]]" that includes severe [[neurological deficits]]. If left untreated, the virus usually self-resolves among [[immunocompetent]] patients, but may progress to a complicated course among the elderly, [[immunosuppressed]] patients, or those with malignancies, advanced [[cardiovascular]], and [[renal]] disease. Diagnosis is often made by [[serological testing]], [[plaque reduction neutralization test]] ([[PRNT]]), [[reverse transcription polymerase chain reaction]] ([[RT-PCR]]), [[immunofluoresence]], or [[immunohistochemistry]]. 

Management is generally aimed at [[supportive care]] only, but antiviral pharmacologic therapy has been frequently administered in neuroinvasive cases. Universal screening for WNV is not recommended, but screening donors of blood products and tissue transplants for WNV using nucleic acid testing (NAT) is mandatory. [[Prognosis]] is excellent among patients with mild disease, but the disease may cause permanent neurological impairment or even death if neuroinvasive disease develops.
|AnswerA=Enveloped single-stranded positive-sense RNA virus
|AnswerAExp=West Nile virus is an enveloped single-stranded positive-sense RNA virus.
|AnswerB=Enveloped single-stranded negative-sense RNA virus
|AnswerBExp=West Nile virus is not an enveloped single-stranded negative-sense RNA virus. Examples of enveloped single-stranded negative-sense RNA virus include [[influenza virus]], [[parainfluenza virus]], [[RSV]], [[mumps]], [[measles]], and [[rabies]].
|AnswerC=Non-enveloped single-stranded positive-sense RNA virus
|AnswerCExp=West Nile virus is not a non-enveloped single-stranded positive-sense RNA virus. Examples of non-enveloped single-stranded positive-sense RNA viruses include [[norovirus]], [[HEV]], and [[picornavirus]]es.
|AnswerD=Non-enveloped single-stranded negative-sense RNA virus
|AnswerDExp=West Nile virus is not a non-enveloped single-stranded negative-sense RNA virus. Most single-stranded negative-sense RNA virus are enveloped, such as [[influenza virus]], [[parainfluenza virus]], [[RSV]], [[mumps]], [[measles]], and [[rabies]].
|AnswerE=Non-enveloped double-stranded RNA virus
|AnswerEExp=West Nile virus is not a non-enveloped double-stranded RNA virus. Examples of non-enveloped double-stranded RNA virus include [[reovirus]]es, such as [[coltivirus]] and [[rotavirus]].
|EducationalObjectives=West Nile virus (WNV) is an enveloped positive-sense ssRNA neurotropic virus.  The natural reservoir of the virus is mainly birds, but it is usually transmitted by ''Culex'' mosquito bites to humans and other animals. Clinical presentation may range from an asymptomatic course or a mild West Nile fever to a severe neuroinvasive disease.
|References=Peterson LR, Brault AC, Nasci RS. West Nile virus: review of the literature. JAMA. 2013;310(3):308-15<br>
First aid 2014 page 161
|RightAnswer=A
|WBRKeyword=West nile virus, West Nile virus, WNV, Outbreak, Virus, Flaviviridae, Flavivirus, Flaviriruses, Fever, Infection, Enveloped, Single stranded, Single-stranded, Single, Stranded, RNA, Positive, Sense, Positive-sense, Positive sense, Structure, Bird, Birds, Mosquito, Culex, Transmission, Neurotropic, Neuroinvasive
|Approved=No
}}