Arthropod-borne viruses (arboviruses)

Arthropod-borne viruses

(arboviruses)

are viruses that can be transmitted to man

by arthropod vectors.

Humans are usually not the natural reservoir for the virus.

Arthropod-borne viruses

(arboviruses)Human pathogens arboviruses are

grouped into three viral families:

1- Togaviridae

2- Bunyaviridae

3- Flaviviridae

ArbovirusesThere are hundreds of arboviruses, many of which are not pathogens to human .Arboviruses are enveloped, with ss-RNA genome.

Family: Togaviridae Enveloped , icosahedral particle .

About 75- nm in diameter .

The viral genome is ss-RNA , with positive polarity .

Enveloped , icosahedral particle .

About 40 -60 nm in diameter .

The viral genome is ss-RNA , with positive polarity .

Flavi = Yellow .

Family: Flaviviridae

Family : Bunyaviridae Enveloped with helical symmetry

About 90 – 100 nm in diameter .

The viral genome consists of three segments of ss- RNA (large , medium and small ) , with negative polarity .

For some genera the S segment is ambisense.

(Ambisense means that some of the genes on the RNA strand are negative sense and others are positive sense.)

Diseases Caused

Fever and rash - this is usually a non-specific illness resembling a number of viral illnesses.

Diseases CausedHaemorrhagic fever - e.g. yellow fever, dengue, Crimean-Congo haemorrhagic fever.

Diseases CausedEncephalitis - e.g. EEE, WEE, St Louis encephalitis, Japanese encephalitis.

1- Arboviruses associated with hemorrhagic fever

1- Dengue virus

Family: Flaviviridae

2- Yellow fever virus

family : Flaviviridae

3- Rift valley fever virus

Family : Bunyaviridae

4- Crimean congo hemorrhagic fever virus

Family : Bunyaviridae

2- Arboviruses associated with encephalitis .

1- West Nile Encephalitis (WNE )

Family : Flaviviridae .

2- Eastern Equine Encephalitis ( EEE )

Family : Togaviridae .

3-Western Equine Encephalitis (WEE )

Family : Togaviridae .

4-Venezuelan Equine Encephalitis (VEE )

Family : Togaviridae

Arboviruses transmission Cycles

Man-Arthropod-Man Cycle

Animal-Arthropod-Man Cycle

Arboviruses transmission Cycles

Transmission Cycles

Man - arthropod -man

e.g. dengue, urban yellow fever.

Reservoir may be in either man or arthropod vector.

In the latter transovarial transmission may take place.

Animal – arthropod - man

e.g. Japanese encephalitis, EEE, WEE, jungle yellow fever.

The reservoir is in an animal.

The virus is maintained in nature in a transmission cycle involving the arthropod vector and animal. Man becomes infected incidentally.

Arthropod Vectors

Mosquitoes

Japanese encephalitis, dengue, yellow fever, St. Louis encephalitis, EEE, WEE, VEE etc.

Ticks

Crimean-Congo haemorrhagic fever,

various tick-borne encephalitides etc.

Sandflies

Sicilian sandfly fever, Rift valley fever.

Animal Reservoirs

In many cases, the actual reservoir is not known. The following animals are implicated as reservoirs

Birds Japanese encephalitis, St Louis encephalitis,

EEE, WEE

Pigs Japanese encephalitis

Monkeys Yellow Fever

Rodents VEE, Russian Spring-Summer encephalitis

Diagnosis

Serology - usually used to make a diagnosis of arbovirus infections.

Culture - a number of cell lines may be used, including mosquito cell lines. However, it is rarely carried out.

Direct detection tests - e.g detection of antigen and nucleic acids are available but there are safety issues.

Prevention

Surveillance - of disease and vector populations

Control of vector - pesticides, elimination of breeding grounds

Prevention

Personal protection - screening of houses, bed nets, insect repellants

Vaccination - available for a number of arboviral infections e.g. Yellow fever, Japanese encephalitis, Russian tick-borne encephalitis

1- Arboviruses associated with hemorrhagic fever

1- Dengue virus

Family: Flaviviridae

2- Yellow fever virus

family : Flaviviridae

3- Rift valley fever virus

Family : Bunyaviridae

4- Crimean congo hemorrhagic fever virus

Family : Bunyaviridae

Crimean Congo hemorrhagic fever

In Africa, Asia and middle east

Vector : Ticks

Vertebrate host : Sheep, goats , cattle, rodents and human

Crimean Congo hemorrhagic fever(Clinical findings)

Most cases are mild or asymptomatic

In severe cases: fever, headache, myalgia, artheralgia, nausea, vomiting, mucosal bleeding and bleeding under the skin

Yellow FeverMainly found in West Africa and S America

Most cases are mild or asymptomatic.

The severe form of the disease may be caused after 3-4 days characterized by fever, myalgia, arthralgia, nausea, vomiting, jaundice, mucosal bleeding, bleeding under the skin, vomiting blood, seizure and coma.

Yellow fever occurs in 2 major forms:

Urban and jungle (sylvatic) yellow fever.

Nonhuman primates are the main reservoir transmitted by forest mosquitoes. Man may become incidentally infected on venturing into jungle areas.

Yellow fever cycle

DengueDengue  is the biggest arbovirus problem in the world today  with over 2 million cases per year. Dengue is found in SE Asia, Africa and the Caribbean and S America.

Dengue cycle

Aedes mosquitoes which reside in water-filled containers

Dengue (clinical findings)

High fever, lymphadenopathy, myalgia, bone and joint pains, headache, and a maculopapular rash.

Severe cases may present with haemorrhagic fever and shock with a mortality of 5-10%.

Rift valley feverGeographical distribution : Africa and Asia

Vector: Sandflies

Vertebrate host :Sheep, goats and cattle , camels and human

Diseases:

1-Most cases are mild or asymptomatic

2- Hemorrhagic form: characterized by fever, headache, myalgia, arthralgia, nausea, mucosal bleeding and bleeding under the skin.

3- Meningoencephalitis

Arboviruses associated with encephalitis .

1-West Nile Encephalitis .

Family: Flaviviridae.

Geog. Dist. : Africa, Asia. Middle East, North America .

Vector : Mosquitoes .

Vertebrate host : Birds, horses, dogs, cats and human .

Diseases :Most cases are mild or asymptomatic .

2- Encephalitis .

Arboviruses associated with Encephalitis

2- Eastern equine encephalitis :

Family : Togaviridae .

Geog. Dist ; America .

Vector : mosquitoes .

Vertebrate host : Birds, horses and human .

Diseases :

1- Most cases are mild or asymptomatic .

2- Encephalitis .

Eastern Equine Encephalitis