Arboviruses

They are transmitted by insect vectors, hence the name. More than 100 Arboviruses are known to cause human infections. They belong to different families. Of these, the rodent borne viruses are also called Roboviruses.

Arboviruses cause rash, arthralgias, encephalitis and hemorrhagic fevers.

Major Arboviruses

Eastern Equine Encephalitis Virus

*Insect vector is mosquito. Animal reservoirs are wild birds from where humans and horses get infected. Seen during the summer months in Atlantic and Gulf coast states. It presents as a severe encephalitis with fever, nuchal rigidity, headache, seizures, stupor and coma. Long term sequelae are seen even after the patient survives the primary encephalitis. Diagnosis is by detection of rising antibody titres.

Western Equine Encephalitis Virus

Vector is Culex mosquito. Reservoir is wild birds. It is seen in western USA. It causes a milder encephalitis than EEE. Diagnosis is by isolation of virus in culture or detection of antibody in serum.

St. Louis Encephalitis Virus

Vector is Culex mosquito. Animal reservoirs are small wild birds like English sparrows. It is seen in southern, central and western states in urban areas. It causes a moderately severe encephalitis. Detection is by serology.

California Encephalitis Virus

Vector is Aedes mosquito. Reservoir is in rodents. Disease occurs in north and central states. La Crosse Virus is a type of CE Virus that is the most common cause of Arboviral encephalitis in the USA. Disease ranges from mild to severe. Diagnosis is by serology.

Colorado Tick Fever Virus

Vector is Dermacentor tick. Reservoir is small rodents, chipmunks, squirrels etc. It is seen in the Rocky Mountain area among hikers. Disease presents as fever, headache, retro orbital pain and myalgia. Diagnosis is by viral isolation or detection of rising antibody titres.

West Nile Virus

Cases appeared for the first time in 1999 in the USA. Vector is a Culex mosquito. Reservoirs are wild birds. Cases are seen all over the USA but more along the east coast. It presents as a severe encephalitis with muscle weakness mimicking GBS. Diagnosis is made by isolation of the virus in tissue culture or antibody detection in blood or CSF. Viral RNA can be detected by PCR. Nucleic acid probes are used for rapid screening of the virus in blood to prevent blood borne infections.

Yellow Fever Virus

It is not endemic in the USA but can be seen in travellers to tropical Africa and South America. Urban yellow fever is transmitted by Aedes mosquito while jungle form is transmitted by Haemagogus mosquitoes and primarily affects monkeys. Disease presents as sudden onset of fever, headache, myalgia, photophobia followed by jaundice, gastrointestinal hemorrhages, hematemesis, renal failure and shock. It has a high mortality rate. Diagnosis is by viral isolation and antibody detection.

Dengue Virus

It is not endemic in the USA but can be seen in travellers to tropical countries, Southeast Asia and Caribbean. Vector is Aedes aegypti mosquito. It is a Flavivirus. Dengue Virus has 4 serotypes 1 to 4. It primarily presents as sudden onset of fever, headache, malaise, cough followed by debilitating pain in muscles and joints (breakbone fever). Maculopapular rash, thrombocytopenia and leukopenia can occur.

Re-infection with a different serotype causes an exaggerated immune response resulting in life threatening dengue hemorrhagic fever which is characterised by hemorrhages into the skin, gastrointestinal tract and internal organs and DIC. It has been postulated that virus -antibody immune complexes activate complement causing the typical clinical features of antibodies actually increase the entry of virus into monocytes and macrophages resulting in the massive release of cytokines. Diagnosis is made by virus isolation in cell culture or by the demonstration of rising antibody titres by ELISA.