Virology IX Vector Transmitted Viruses

Question 1

1,2. Describe the vectors, reservoirs and route of transmission for the following viruses: Western and Eastern Equine Encephalitis, LaCrosse Encephalitis, West Nile. Describe the pathogenesis, major syndromes and preventive measures associated with the viruses listed.

Answer 1

virus introduced by mosquito replicates locally in endothelial cells, secondary viremia occurs, resulting in seeding of nervous system where the virus replicates in neurons and meninges

symptoms are cause by cell death and host inflammation, more hospitalizations in those over 50 yo

Question 2

1,2. Describe the vectors, reservoirs and route of transmission for the following virus: Dengue. Describe the pathogenesis, major syndromes and preventive measures associated with the viruses listed.

Answer 2

Hemorrhagic fevers caused by direct cell damage to the vascular endothelium and damage caused by the host immune response

Question 3

1,2. Describe the vectors, reservoirs and route of transmission for the following virus: Rabies and Sin Nombre. Describe the pathogenesis, major syndromes and preventive measures associated with the viruses listed.

Answer 3

Hantavirus pulmonary syndrome is caused by Sin Nombre Virus; diagnosis by serology

shed virus is inhaled in dried rodent urine and feces, the target organ is the lungs and major symptoms are due to pulmonary edema (mechanism is unknown, there is little inflammation, necrosis or cytopathic effect)

mortality is 76%

Question 4

3. Explain “urban cycle” and be able to cite and example.

Answer 4

if the endemic hosts of a virus are humans, the virus can be maintained through an urban cycle that occurs in cities, towns or villages (Yellow Fever and Dengue)

Question 5

3. Explain “sylvan cycle” and be able to cite an example of each.

Answer 5

viruses maintained in the wild, with transmission occurring between some animal and the insect vector; birds (west nile), small rodents (Lacrosse Encephalitis) are the most common
for some viruses, they can survive in infected insect eggs that are laid in the late fall and then hatch in spring, exist in hibernating small animals, asymptomatic animal hosts with chronic infection

(cycle does not occur in cities)

Question 6

4. Describe the role of the immune system in the pathogenesis of Dengue Hemorrhagic Fever. Describe why this does not happen with other viruses listed in this outline.

Answer 6

Dengue Hemorrhagic Fever can be lethal and is seen in people who have had a primary Dengue infection with one of four serotypes; upon infection with a different serotype, enhancing antibodies increase pathogenesis, resulting in hypovolemic shock ; increased severity if DHF results in formation of immune complexes and blood vessel damage due to complement activation and infection of macrophages

immune enhancement occurs when the host produces antibodies that bind but do not neutralize

Question 7

5. Describe how rabies virus is transmitted and spreads in the host.

Answer 7

rabies is predominantly transmitted by animal bites but spelunkers are thought to be infected by inhalation of virus shed in dried bat urine (dx by brain tissue IHC)

there is local replication in muscle tissue and the virus spreads to nerve cells, traveling retrograde transport to the CNS and replicates in gray matter, virus can spread via nerves to salivary gland and other tissues (cytopathic effect: negri body)

symptoms are due to cytolysis of infected cells and inflammation

rabies long incubation period allows time for vaccine and rabies immunoglobulin, disease is lethal once symptoms develop

Question 8

Contrast transmission of virus via mosquito v. tick.

Answer 8

virus is shed in mosquito saliva and in tick excrement

Question 9

List 6 vector borne viruses that are endemic to the US.

Answer 9

Western and Eastern Equine Encephalitis, St. Louis Encephalitis, California group of Bunyaviruses (Lacross Encephalitis), Colorado Tick Fever and West Nile Virus

all cause encephalitis as the primary clinical manifestation

Question 10

List 3 notable vector borne viruses that are important for traveler precautions.

Answer 10

Venezuelan Equine Encephalitis
Dengue Fever (Hemorrhagic Fever)

Yellow Fever (Hemorrhagic Fever), high fever, black bloody vomitus and bleeding due to disruption of clotting factors leading to bleeding

Question 11

What are the four predominant syndromes caused by viruses transmitted by animals?

Answer 11

encephalitis, fever/uremic syndrome (Hantavirus), pulmonary syndrome (Hantavirus), and Hemorrhagic fever

Question 12

What does it mean to be an incidental host?

Answer 12

virus is maintained in an endemic host (arthropod/animal) and humans serve as dead end hosts (cannot transmit virus) and humans do no serve as a reservoir of infection (viremia is low titer and transient)

Question 13

Describe the process of infection by infected arthropod.

Answer 13

arboviruses are introduced directly into the blood by an infected arthropod, the virus replicates in the reticuloendothelial system and vascular endothelium

Question 14

What is the mechanism of pathogenesis for Lassa and Ebola viruses?

Answer 14

cause severe hemorrhage due to viral cytopathic effect in vascular endothelium, i.e., Ebola envelope glycoprotein is toxic to vascular endothelial cells (unlikely to involve adaptive immune pathology)

not diagnosis only at reference centers or CDC (BL-4)

Question 15

Describe basic treatments for encephalitis, hemorrhagic fever and available antivirals.

Answer 15

most tx. is supportive
encephalitis: measure to reduce brain swelling and ventilation support (no steroids)

hemorrhagic fevers: dealing with shock and loss of blood volume, ventilation support

antivirals ribavirin and interferon have activity against some viruses; passive immunization and option with Ebola and Rabies (immune globulin)