Arboviruses & Roboviruses “Arbo” is derived from arthropod-born viruses they do not cause disease in arthropods. They are enveloped (sensitive) viruses, they need intimate contact or insect bite to be transmitted. No man to man transmission except in yellow fever and dengue. Various animal, rodents and birds are reservoirs of infection.
Roboviruses are not transmitted by arthropods but maintained within the rodent reservoir. Man to man transmission occurs.
Dengue fever Flaviviridae, Arbovirus, icosahedral enveloped (+) ss RNA Replication In the cytoplasm Released by budding
Replication of Dengue virus
Transmission & Pathogenesis of Dengue fever Four serotypes Reservoir probably monkey Vector mosquito; Aedes egypti High Viremia, so man to man transmission occurs Female Aedes aegypti mosquito
I.P.: 5-6 days Epidemiology Dengue fever 1st reported epidemics in In Africa, South East Asia & South America KSA in 2006, 1544 cases (461 children <14 yrs.) Clinical forms 1.Asymptomatic 2.Classic Dengue Fever (Break bone fever ): fever, headache, myalgia, arthralgia, nausea, vomiting, macular rash 3.Dengue Hemorrhagic fever 4.Dengue Shock Syndrome
Dengue hemorrhagic fever Mainly in children Acute onset of fever, hematemesis, melena renal involvement and shock with 50% mortality It needs biosafety level 2
A large subcutaneous hemorrhage on upper arm in Patient with dengue hemorrhagic fever Hemorrhagic conjunctivitis
Pathogenesis of Dengue HF Primary infection with one serotype produces antibodies specific to that serotype. Secondary infection with another serotype is followed by formation of immune complexes between the second serotype and antibodies to the first one. These antibodies not only fail to neutralize the virus but also enhances its ability to infect higher number of monocytes & release of cytokines, & vasoactive mediators leading to DIC, Hemorrhage, shock & death.
Pathogenesis of Dengue HF
Rift Valley fever Bunyaviridae: Helical enveloped (-) ss RNA segmented Single serotype Replication Transmission: 1.mosquito bite 2.contact with tissues, blood or body fluids of infected animals
Epidemiology In Africa, Yemen & KSA 1st reported in Kenya 1931 KSA in , 77 died Epizootics in sheep & cattle in Sudan, Egypt & South Africa. In 1977, an outbreak occurred in Egypt with many deaths in animals and 600 human deaths
Pathogenesis of RFV infection Virus spreads to regional lymph nodes, liver and spleen & infects tissue macrophages Soluble mediators & cytokines recruit more cells, to be infected leading to virus amplification Infection of hepatocytes impairs synthesis of clotting factors Reduced synthesis of albumin by hepatocytes reduced plasma osmotic pressure causing edema. Infected adrenal cortical cells impairs secretion of steroid-synthesizing enzymes leading to hypotension and hypovolemia.
C/P: 1. Mild: most cases, inapparent or flu-like e.g. fever, headache, myalgia, nausea, vomiting. Recovery in 4-7 days 2. Severe: i. Ocular retinopathy ii. Meningoencephalitis iii. Hemorrhagic fever It requires Biosafety level 3
Laboratory Diagnosis of HFVs Direct virus isolation on tissue culture. Detection of viral antigens or antibody by immunofluorescence. Detection of viral nucleic acid by PCR.