West Nile Fever and Encephalitis From Mayoclinic.com

Introduction In the summer of 1999, large numbers of crows began dying in New York City. Health officials eventually discovered the cause, but not before four people died of the same disease — West Nile virus, a mosquito-borne illness that mainly affects birds, humans and horses.

Etiology West Nile virus: A single stranded RNA virus containing an envelope Family flavivirus. Although the exact mechanism of illness is unknown, West Nile virus probably enters the host's bloodstream, multiplies and moves on to the brain, crossing the blood- brain barrier. Once the virus crosses that barrier and infects the brain or its linings, an inflammatory response occurs and symptoms arise.

Virulence Factors Portal of entery –Parenteral rout. Infected mosquito deposits virus under the skin.

Virulence Factors Adhesion –Hemagglutinin binds to target cells – Proteins on the viral envelope called Domain III proteins bind to  V  3 integrin protein on the host cell.

Virulence Factors Evading the Immune System –Virus blocks the production of interferon (IFN)

Virulence Factors Tissue Distruction –Destroys tissue in the central nervous system leading to encephalitis.

Mode of Transmission Amplification of the virus occurs throughout the summer as Culex mosquitoes feed on infected birds. If amplification is significant enough, then bridge vector mosquitoes – (mosquitoes that that bite both humans and birds) become infected in late summer and pose an infection threat to humans. WNV is not transmitted through person to person contact Bridge vector mosquitoes Maintenance vector mosquito

Reservoir and incubation Reservoir Birds (Jays and Crows) Transmission vector Mosquitoes from the genus Culex Incubation period 3-14 days

Signs and Symptoms Most people infected with the West Nile virus have no signs or symptoms. About 20 percent of people develop a mild infection called West Nile fever. Common signs and symptoms of West Nile fever include: –Fever –Headache –Muscle aches –Backache –Lack of appetite –Nausea, vomiting and diarrhea –Skin rash –Swollen lymph glands

Signs and Symptoms cont In less than 1 percent of infected people, the virus causes a more serious neurological infection, –inflammation of the brain (encephalitis) or of the brain and surrounding membranes (meningoencephalitis), and paralysis. Signs and symptoms of these diseases include: High fever Severe headache Stiff neck Disorientation or confusion Stupor or coma Tremors or muscle jerking Signs and symptoms of Parkinson's disease Convulsions Partial paralysis

Risk Factors Among those more likely to develop severe or fatal infections are: –Adults over 50 years old. –People with immune systems weakened by long-term steroid use, chemotherapy drugs or anti-rejection drugs following transplant surgery. –Pregnant women. –People with certain genetic mutations.

Screening and diagnosis Blood sample. –Serological screening for anti-West Nile virus IgM antibodies

Treatment Most people recover from West Nile virus without treatment. Those who develop encephalitis or meningitis may only need supportive therapy with intravenous fluids and pain relievers. Currently, researchers are investigating interferon therapy as a treatment for encephalitis caused by West Nile virus.