West Nile Fever and Encephalitis

1. West Nile Fever and Encephalitis From Mayoclinic.com

2. 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.

3. 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.

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

5. Virulence Factors • Adhesion • Hemagglutinin binds to target cells • Proteins on the viral envelope called Domain III proteins bind to aVb3 integrin protein on the host cell.

6. Virulence Factors • Evading the Immune System • Virus blocks the production of interferon (IFN)

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

8. Mode of Transmission Bridge vector mosquitoes Maintenance vector mosquito Bridge vector mosquitoes • 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

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

11. 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

12. 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

13. 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.

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

15. 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.