West Nile virus

Culex species mosquito borne flavavirus transmitted primarily among birds with humans serving as incidental hosts.

Horses are accidental hosts.

Has become endemic in the contiguous United States since its discovery in 1999.

One of the most widespread arboviruses in the world.

Prevalent in South Africa and the United States.

Has been observed throughout the continental US and is by far the most common domestic cause of arboviral infection.

After amplification in birds, West Nile virus is transmitted to humans by mosquitoes, with increased incidence during the warmer months of the summer and early fall.

Climate change potentially will result in an increase in the geographic and temporal range of West Nile virus infections.

Has produced three largest arboviral invasive disease outbreaks ever recorded in the US with nearly 3000 cases each year in 2002, 2003, and 2012.

Virus transmitted primarily by the bite of Culex pipiens complex of mosquitoes.

Culex pipiens, a medium sized brown mosquito- the common house mosquito.

Culex pipiens subspecies pipiens found in northern US and subspecies quinquefasciatus in the southern half of the US. and Culex tarsalis found in many areas of the plains and western states.

Culex lay eggs in stagnant water, and upon hatching adults tend to stay nearby.

West Nile virus survives in the environment through a transmission cycle in which mosquitoes develop infection from feeding on infected birds.

West Nile virus spreads to the mosquitoes salivary glands and is transmitted when the mosquito injects its infected saliva into other, uninfected birds.

Rarely has been transmitted through blood transfusion or solid organ transplantion.

Incubation period of 3-14 days.

About 20% of people who are infected with the West Nile virus develop a fever and flu like symptoms, with fatigue and weakness sometimes persisting for weeks or months.

Most human infections are asymptomatic.

Approximately 25% of patients reports symptoms.

Less than 1% of patients with West Nile infections develop Neuroinvasive disease, usually meningitis, encephalitis, or acute flaccid paralysis.

Older patients and individuals with comorbidities or are immunocompromised are atbmost risk for severe disease.

Symptoms tend to be nonspecific and can include acute onset of fever, chills, malaise, headache, vomiting, diarrhea, and myalgias.

Nonspecific febrile illness occurs in 20% of those infected.

Encephalitis and meningitis are potentially fatal complications, but these strike less than 1% of those who are infected.

People older than 50 years are at the highest risk for neurologic disease and death from west Nile virus infection.

Children can become seriously ill with West Nile virus.

Infected birds develop West Nile virus illness involving the circulation of large amounts of virus in the bloodstream for several days that readily infect the next biting mosquitoes.

Infected mosquitoes can bite mammals, including humans and infect them with the virus.

West Nile virus is maintained in a bird-mosquito- transmission cycle and has been detected and 65 different mosquito species and 326 bird species in the United States.

Most birds and mammals can be infected with mosquito borne West Nile virus but only Blue Jays, crows, horses and humans die from the infection.

The American Robin contributes most to maintain the transumission cycle, as it has several broods per season and hatchlings are more susceptible to West Nile virus infection than are adult birds.

An enzootic system between mosquitoes and birds which maintain and amplify the virus.

Mammals and humans are dead-end hosts.

Rare episodes of person-person transmission through a transfusion of blood and blood products, organ transportation, pregnancy, and breast-feeding have been reported.

Single-stranded RNA virus of the genus Flavivirus.

Prone to mutation and adaptation.

Endemic in Asia, Africa and the Middle East.

Spread mainly through the migration of birds, associated with the largest outbreaks of arbovirus encephalitis in the U.S.

Leading cause of arboviral neuroinvasive disease in the US.

May be transmitted via blood and blood products.

Has been transmitted through blood transfusion, organ transplantation, breast-feeding, laboratory injuries and from mother to fetus.

Causes seasonal epidemics of febrile illness and neurologic disease.

The leading cause of arboviral encephalitis in the US.

Neurological manifestations increases with age, with the highest incidence among individuals over the age of 70 years.

CDC estimates 54,000 people infected in 2009, of whom 10,000 had neuroinvasive disease,

A notifiable disease.

Reported incidence in 2009 0.13 per 100,000 population.

Most infected patients will not become clinically ill and some will develop debilitating or deadly central nervous system disease.

Infections typically begin in summer when viral levels in birds and mosquitoes reach critical threshold, and decline with temperature drops in September.

Epidemics are usually noted to be preceded by appearance of dead crows or blue jays.

Infection followed by lifelong immunity.

Risk of severe disease increases with age and comorbidities.

Incubation period from mosquito bite to symptoms 2-14 days.

Approximately 80-90% of infected individuals remain a symptomatic , and includes essentially all healthy children and young adults.

May present as a flulike illness with fever, malaise, headache, back pain, myalgias, anorexia, rash, meningitis, encephalitis, poliomyelitis-like syndrome polyradiculopathy or meningoencephalomyelitis.

0.6% of infected patients will progress to neuroinvasive disease that causes meningoencephaltis and possible death.

Without a neuroinvasive disease manifestation, the infection generally has a benign course.

Neuroinvasive disease appears more commonly among older adults, with the highest incidence in patients over 70 years of age and among immunocompromise persons.

Neuroinvasive infections can manifest as meningitis with fever, headache nuchal rigidity, and photophobia, encephalitis with impaired mental status, or meningoencephalitis.

Some patients have acute flaccid paralysis, with inflammation of the anterior horn of the spinal cord and focal loss of interior-hor neurons.

The mortality rate among patients with neuroinvasive disease can range from 2-20%.

Neurologic symptoms may last from months to years.

Serologic surveys indicate that approximately 20% of patients with West Nile virus develop symptoms.

1 in 150 infected patients develop neurological disease, manifested by inflammation of the brain and nervous system.

The risk of neurological disease increases with age and is higher among organ transplant recipients.

Many patients who survive neurological disease have long-term disability.

Resistant to acyclovir.

Asymptomatic blood donors while viremic, may transmit disease particularly to the immunosuppressed, elderly, and infants.

No specific antiviral treatment is available presently, no specific antiviral treatment is available presently, and a higher mortality has been shown among patients who receive ribavirin.

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