Vector Transmitted Viruses

Question 1

Western Equine virus

Answer 1

Vector: Mosquito

Pathogenesis:

• Virus enters blood stream
• Local replication in endothelium
• Secondary viremia = seeds CNS
• Replicates in CNS
• Causes direct and inflammatory damage

Major Syndromes:
Encephalitis

Prevention:

• Vector control
• Vaccines from horses, lab workers

Question 2

Eastern Equine virus

Answer 2

Vector: Mosquito

Pathogenesis:

• Virus enters blood stream
• Local replication in endothelium
• Secondary viremia = seeds CNS
• Replicates in CNS
• Causes direct and inflammatory damage

Major Syndromes:
Encephalitis

Prevention:

• Vector control
• Vaccines from horses, lab workers

Question 3

LaCrosse virus

Answer 3

Vector: Mosquito

Pathogenesis:

• Virus enters blood stream
• Local replication in endothelium
• Secondary viremia = seeds CNS
• Replicates in CNS
• Causes direct and inflammatory damage

Major Syndromes:
Encephalitis

Prevention:

• Vector control
• No vaccine

Question 4

West Nile Virus

Answer 4

Vector: Mosquito

Pathogenesis:

• Virus enters blood stream
• Local replication in endothelium
• Secondary viremia = seeds CNS
• Replicates in CNS
• Causes direct and inflammatory damage

Major Syndromes:
Encephalitis
West Nile Fever
Aseptic Meningitis

Prevention:

• Vector control
• Behavior (repellent, etc.)
• No vaccine
• Screen organ donors and blood supply

Question 5

Dengue Virus

Answer 5

Vector: Mosquito

Pathogenesis:
-Replicate in blood vessel endothelium = kills cells, induces cytokines → fever or hypovolemic shock (fluid loss) in severe cases

Major Syndromes:
Febrile illness, hemorrhagic fever

Prevention:

• Vector control
• No vaccine

Question 6

Rabies Virus

Answer 6

Vector:
Bats, skunks, raccoons, foxes (dog transmission virtually eliminated due to vaccination)

Route of transmission:
Animal bites or spelunkers inhaling virus shed in dried bat urine

Pathogenesis:

• Local replication in muscle tissue
• Virus spreads to nerve cells → retrograde travel to CNS
• Replicates in gray matter
• Spreads via nerves to salivary glands and other tissues

Major Syndromes:
Encephalitis

Prevention:

• Vaccine: killed version; used in high risk individuals and those bitten by animals
• Rabies Ig’s used in exposed individuals

Question 7

Sin Nombre Virus

Answer 7

Vector:
Rodents

Route of transmission:
Inhaled in dried rodent urine and feces

Pathogenesis:
Replicates in lungs → acute respiratory distress syndrome with edema and inflammatory infiltrate into lungs

Major Syndromes:
Hantavirus pulmonary syndrome (HPS): mortality rate 76%

Prevention:

• Vector control
• No vaccine

Question 8

Explain “urban cycle” and “sylvan cycle” and be able to cite examples of each.

Answer 8

Urban transmission pattern
o Requires virus cause high titer and prolonged viremia in vertebrate host
o Maintained in urban setting = humans are reservoir species
o Ex: Yellow Fever, Dengue

Sylvan cycle
o Maintained in wild with transmission between some animal and insect vector
• Most common: birds (West Nile) and rodents (Lacroose)
• Usually asymptomatic infection in endemic host
• Sometimes = infection is lethal (Ex: West Nile virus kills crows)

NOTE on animal transmission:
o Requires that virus can replicate in animal host
o Ex: rabies virus replicates in salivary gland = transmitted by bites
o Ex: transmitted through urine or feces = Hantaviruses, Lassa Fever, Ebola, Marburg viruses
o Humans = incidental hosts (infected because at wrong place at wrong time)

Question 9

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 9

Dengue Hemorrhagic Fever
o Occurs during 2nd infection with Dengue virus

4 Dengue serotypes
o Able to cross-react
o BUT only serotype-specific Ab’s are protective → multiple infections possible

If have non-protective Ab → immune enhancement
o Host produces Ab’s against virus that bind but don’t neutralize
o Result: infection of cells with Fc receptors
o Increased formation of immune complexes that precipitate in tissues and organs
o Macrophage infection → releases lots TNFα

DHF = can cause disseminated intravascular coagulation and shock
o Also: arthritis, arthralgias

Question 10

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

Answer 10

Transmission:
Result of bite from infected animal
• Local replication in muscle tissue
Inhalation of aerosolized virus from bat secretions during spelunking
• Local replication in nasopharyngeal mucosa
• Spread to olfactory nerves →CNS

Spread to nerves cells via retrograde transport
o Replicates in CNS gray matter
Spreads via nerves to salivary glands and other tissues

Long incubation period = time to use vaccine and rabies immunoglobulin

Symptoms = from cytolysis of infected cells and inflammation

Pathology = Negri bodies
o Eosinophilic cytoplasmic inclusion body