Lecture 22 – VIRAL ENCEPHALITIS CAUSED BY ARBOVIRUSES (IN THE UNITED STATES)

Summary:

Reading Assignment: (1) Text Chapter 33 (2) Case Study #16: In Cunia, B.A. 1999. Infectious Disease Pearls, Hanley and Belfus, Inc., Philadelphia.

1. The term arbovirus is an acronym for arthropod-borne virus – these viruses are transmitted by arthopods (mosquitoes and ticks). Collective name for a large diverse group of viruses, more than 400. In general, they are named for either the diseases they cause (yellow fever virus), or the place they were first isolated (St. Louis Encephalitis virus).

2. In the United States, most arboviruses are classified in three families – togaviruses, flaviviruses, and bunyaviruses. All three are enveloped viruses with RNA genomes.

Togaviruses – Eastern equine encephalitis virus (EEE) (most severe – 50% mortality rate)

Western equine encephalitis virus (WEE)

Flavivirus – St. Louis encephalitis viruses (SLE) (seen in Texas)

Bunyavirus – California encephalitis virus (CE)

3. Epidemiology:

a. Transmission – viruses are maintained in a vertebrate reservoir in nature and are accidentally transmitted via the bite of infected

mosquitoes to humans (and horses) which are dead-end hosts.

b. Life cycle depends on multiplication in both the vertebrate host (wild birds or small mammals) and the bloodsucking vector. For effective

transmission to occur, the virus must be present in a high enough titer (viremia) in the vertebrate reservoir to be taken up in the small volume of blood ingested during an insect bite. After the virus is taken up by the mosquito, it replicates first in the gut and then spreads to other organs including the salivary glands. After 7-14 days of replication in the mosquito, the saliva of the vector will contain enough virus totransmit an infectious dose. Only the female mosquitoes (which take blood meals) serve as vectors

4. Pathogenesis

The virus is inoculated directly into the bloodstream and then localizes in the vascular endothelium and lymphatic cells of the RES system where it replicates. After a primary viremia, the pathogen spreads to the CNS causing encephalitis - brain hemorrhages and extensive necrosis. How the virus crosses the brain barrier is not known.

5. Diagnosis

a. Clinical diagnosis - based on symptoms and known existence of mosquitoes in the area.

b. Lab Dx. – acute and convalescent antibody titers

6. Treatment and Prevention

a. No antiviral therapy, supportive treatment used.

b. Prevention – personal protection against mosquitoes

c. Vaccines - none