West Nile fever

Disease Description (Causes and Symptoms)

West Nile fever is a zoonotic infectious disease caused by West Nile virus (a virus in the Flavivirus family). It is mainly transmitted when an infected mosquito bites a person. Birds (e.g., crows) are the primary natural hosts (reservoir hosts), while humans and horses are often incidental hosts. The incubation period is usually 2 days to 2 weeks.

Main symptoms

• Mostly mild or asymptomatic.
• Systemic symptoms such as fever, headache, muscle pain, malaise, decreased appetite, and nausea and vomiting.
• A transient skin rash develops on the back, chest, and upper limbs in approximately 50% of cases (resolves within about 1 week).
• Rarely, neurological involvement (West Nile encephalitis/meningitis) may occur, including changes in level of consciousness, seizures, severe muscle weakness, flaccid paralysis, cranial nerve abnormalities, optic neuritis, polyradiculitis, and myelitis. The risk of severe disease is increased in older adults and people who are immunocompromised.

Pathophysiology Summary

• Transmission occurs when an infected mosquito feeds on human blood during the mosquito–bird cycle.
• Humans are often dead-end hosts; although the virus replicates in the body, they do not contribute to widespread transmission.
• Rare cases of transmission through blood transfusion, organ transplantation, breastfeeding, vertical transmission, and laboratory accidents have been reported.

Treatment (Standard Treatment · Precautions)

Treatment

• There is currently no specific antiviral drug or approved human vaccine.
• Most cases receive symptomatic and supportive treatment (fluid and electrolyte replacement and control of pain and fever).
• When the nervous system is involved, intensive treatment similar to that for viral encephalitis is provided (including intensive care unit management and mechanical ventilation when necessary).
• Specialized treatment and a multidisciplinary approach are needed for complications (e.g., myocarditis and severe hepatitis).

Precautions / Self-care

• When symptoms develop, adequate fluid intake and rest are needed to prevent dehydration and complications.
• If neurological symptoms (decreased consciousness, severe weakness, difficulty breathing, seizures, etc.) occur, visit an emergency medical facility immediately.
• If infection is suspected, consult medical professionals regarding activities related to blood/tissue donation or organ donation.
• Use personal protective equipment (PPE) when handling infected birds.
• General contact management for people around the patient (transmission through ordinary daily contact at home is rare) is generally not necessary.

Complications and Prognosis

• Reported complications include myocarditis, pancreatitis, and fulminant hepatitis.
• When the nervous system is involved, the mortality rate ranges from approximately 4–14%, depending on the report. The prognosis is worse in older adults.

Checklist Before an Outpatient Visit (Items to Bring and Example Questions)

Items to bring

• A list of medications currently being taken (including prescription drugs, OTC medicines, herbal medicines, and supplements).
• Recent test results (blood tests, imaging tests, etc.) and any referral letters or medical records received from doctors.
• Notes recording when symptoms began and how they have progressed (when they started, factors that improve or worsen them, the pattern of fever, etc.).
• Recent travel history (including mosquito activity and whether West Nile fever was prevalent in the area), history of outdoor activities (daytime/nighttime/outdoor exposure), recent history of blood transfusion or organ transplantation, and whether you are pregnant or breastfeeding.
• If neurological symptoms are present, arrange for a caregiver to accompany you and be prepared to explain changes in the condition.

Items to confirm during the visit (Items the doctor is likely to ask about)

• The date symptoms began and their progression (fever, rash, headache, muscle weakness, etc.).
• Whether you remember being bitten by a mosquito, and whether you have recently engaged in outdoor activities or visited an endemic area.
• Immunosuppressed status (steroid or anticancer drug use, history of transplantation, etc.) and chronic diseases (diabetes, cardiopulmonary disease, etc.).
• Whether you have recently had a blood transfusion, organ transplantation, pregnancy, or breastfeeding.

Examples of General Questions to Ask the Doctor

• What tests are needed to confirm the diagnosis of this disease? How long will it take to receive the test results?
• What treatment is recommended for my current symptoms? Is hospitalization or intensive care treatment necessary?
• What signs should I report immediately if my symptoms do not improve or worsen?
• What care should I provide at home to reduce the risk of complications?
• Is there a risk of transmitting this disease to other people (family members, newborns, etc.)? When can I resume activities such as donation or blood transfusion?

Examples of Disease-Specific Questions

• Have you received a blood transfusion or organ transplant within the past 2 weeks, or have you visited an area where West Nile fever is prevalent (overseas or in Korea)? (For diagnostic and epidemiological assessment)
• If neurological symptoms (reduced muscle strength, seizures, changes in consciousness, etc.) occur, what emergency measures should be taken? (Related to prognosis and emergency response)