Arboviruses

Objectives Overview What are Arboviruses? How are they transmitted? Yellow fever Dengue fever Other arboviruses

OVERVIEW

Definitions Arthropod : invertebrate animal having an exoskeleton, a segmented body, and jointed appendages. Vector : is any agent (person, animal or microorganism) that carries and transmits an infectious pathogen into another living organism.

Arthropod-borne diseases The diseases which require a vector and it is the arthropods. Ricketsia ricketsii, Borrelia burgdorferi, Yersinia pestis, Francisella tularensis (Arboviruses) - Dengue, West Nile, Encephalitic viruses - Malaria, Dracunculiasis, Tape worms

Arboviruses The term ARBO is an abbreviation of "ARthropod BOrne". A class of viruses transmitted to humans by arthropods such as mosquitoes and ticks. Symptoms of arbovirus infection generally occur 3–15 days after exposure to the virus and last 3 or 4 days. Common clinical features of infection are fever, headache, and malaise, but encephalitis and hemorrhagic fever may also occur. All arboviruses have an RNA genome, and most have a lipid- containing envelope and consequently are inactivated by ether or sodium deoxycholate.

 Togaviridae Genus Alphavirus  Flaviviridae Genus Flavivirus  Bunyaviridae Genus Bunyavirus  Reoviridae Genus Orbivirus  Rhabdoviridae Genus Vesiculovirus  Arenaviridae Genus Arenavirus  Nodaviridae Current taxonomic status of some arboviruses Approximately 80 arboviruses known to cause human disease

Morphology - enveloped icosahedral nucleocapsid (exception: Reovirus have a naked double-layered capsid).

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 vector - 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. Both cycles may be seen with some arboviruses such as yellow fever.

10 Man-Arthropod-Man Cycle

11 Animal-Arthropod-Man Cycle

Yellow Fever

Definition Is an acute systemic illness - a hemorrhagic fever - caused by the Flavivirus. The infection is most common in areas of Africa and South America, affecting travelers to and residents of those areas.

There are 2 classifications of yellow fever: Jungle (Sylvatic) Yellow Fever: spread by jungle mosquitoes and mainly affects monkeys. Jungle mosquitoes bite infected monkeys, bite and infect man. (on going to the jungle) Urban Yellow Fever: spread by Aedes aegypti mosquito between humans. * Urban yellow fever is most likely to occur in settlements close to the jungle, where infected monkeys and mosquitoes live. Urban yellow fever is extremely rare in the Americas.

Pathogenesis Incubation period : 3 -6 days then 5-50% of infected people develop disease. Starts with transient viraemia with primary viral multiplication in lymph nodes; manifested as a nonspecific 1-3 days febrile illness, followed by a brief remission. Then secondary multiplication occurs in liver, spleen, kidneys, heart and bone marrow with much tissue damage. This is manifested by a "toxic" syndrome, hemorrhagic phenomena, jaundice and DIC

Clinical Features

Diagnosis A) Suspected case: A case with acute onset of fever then jaundice within 2 weeks of onset of the 1 st symptom. Hemorrhagic symptoms develop late in the disease. B) Confirmed case: A suspected case that shows either: -Epidemiological criteria -Laboratory criteria: Cultivation of the virus from blood or tissue. Antigen detection by immuno-fluorescence (IF) or immuno- histochemistry. Viral RNA detection by RT-PCR. Specific antibody detection.

Prevention 1) Eradication/control of Aedes Aegypti mosquitoes 2) Vaccination: (most effective) 17 D strain: – Live attenuated vaccine. – Very effective, safe, long lasting protection but labile and expensive to administer. French Dakar Vaccine: – Live attenuated brain tissue- derived vaccine inoculated by skin scratch. – More stable and cheap to administer. – Occasionally causes CNS complications.

Dengue Fever

Definition An acute febrile arboviral disease, endemic in south Asia. Epidemiology -Dengue is the biggest arbovirus problem in the world today with over 2 million cases per year. -Dengue is found in Tropics, particularly coastal, urban areas. (SE Asia, Africa and the Caribbean and S America)

Clinical Manifestation Human infections arise from a human-mosquitoes- human cycle Classically, dengue presents with a 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%. (Dengue haemorrhagic fever or Dengue shock syndrome.)

Diagnosis: Serology No specific antiviral therapy is available Prevention: -Vector control -Protection of man from the bites of mosquitoes -Live attenuated vaccines (being tried in Thailand with encouraging results)

Summary of dengue fever

Other Arboviruses ArbovirusIPSymptomsComplications Case fatality rate Vector(s)Primary host(s)host Geographic distribution Japanese encephalitis virus 5–15 days Asymptomatic in most cases; fever, headache, fatigue, nausea, and vomiting Encephalitis, seizures, paralysis, coma, and long- term brain damage 20-30% in encephalitis cases CulexCulexmosquitoes, esp Culex tritaeniorhynchusCulex tritaeniorhynchus Domestic pigsDomestic pigsand wading birdswading birds Southeast and East Asia Rift Valley fever virus 2–6 days Fever, headache,myalgia a nd liver abnormalities Hemorrhagic fever, meningoencephali tis 1% in humans; in pregnant livestock, 100% fatality rate for fetuses Culex tritaeniorhynchusCulex tritaeniorhynchusan d Aedes vexansAedes vexans Micropteropus pusillusMicropteropus pusillus andHipposideros abaeHipposideros abae Eastern, Southern, and Western Africa Tick-borne encephalitis virus 7–14 days Fever, headache, muscle pain, nausea, vomiting, meningitis, and encephalitis Paralysis and long- term brain damage 1-2% Ixodes scapularisIxodes scapularis,Ixodes ricinus, and Ixodes persulcatusIxodes ricinusIxodes persulcatus Small rodents Eastern Europe and Southern Russia West Nile virus 2–15 days Asymptomatic in most cases; fever, headache, fatigue, nausea, vomiting, rash Swollen lymph nodes, meningitis, encephalitis, acute flaccid paralysisacute flaccid paralysis 3-15% in severe cases CulexCulexmosquitoesPasserinePasserinebirds North America, Europe, West and Central Asia, Oceania, and Africa