Practice Essentials
Background
Orbiviruses are one of nine genera in the family Reoviridae. Only four genera within the Reoviridae cause human disease: the orbiviruses, rotaviruses, orthoreoviruses, and coltiviruses. Coltiviruses cause Colorado tick fever. Orbiviruses are distinguished from the orthoreoviruses by their protein structure and arthropod transmission cycles. [1] The genus Orbivirus contains 19 species and at least 130 subspecies.
Orbiviruses are named for their doughnut-shaped capsomeres (orbi means ring in Latin). Structurally, each virion consists of an outer and inner capsid layer that surrounds a core genome of 10 nonenveloped double-stranded RNA segments. Type- and group-specific antigens include the outer VP2 and the core VP7 proteins, respectively. [2] No lipid envelope is present. Overall, they are 70 to 80 nm in diameter.
The orbiviruses are primarily animal pathogens that cause bluetongue disease in sheep, cattle, goats, and wild ungulates; African horse sickness in horses, donkeys, and dogs; and epizootic hemorrhagic deer fever. Many other viruses in this genus infect animals, but the above are the most commonly recognized. Infections may affect fetal development and have been linked to congenital anomalies such as hydrocephalus and arthrogryposis. These are the best-studied orbiviral diseases. Only 7 per 100 orbiviruses are linked to human clinical disease, with only one, Oklahoma tick fever, causing disease within the United States.
Pathophysiology
Orbiviruses are vector-borne pathogens that are transmitted by ticks, mosquitoes, gnats, and midges. Studies on the pathophysiology of orbiviral infections specifically in humans have not been conducted. Data on the pathophysiology of orbiviral infections are derived mainly from studies of animal orbiviruses. According to Palacios et al, the ability to detect them has been hampered by their diversity. They developed a consensus reverse-transcription polymerase chain reaction (RT-PCR) method of targeting the polymerase gene for orbivirus recognition and characterization. [3, 4, 5]
In these cases, the virus enters the cell by endocytosis, the outer capsid is removed, and the core particle undergoes transcription. Viral proteins are synthesized 2 to 14 days after the infection and self-assemble within the cytoplasm. New virions are released and are then capable of infecting more cells, mainly within the lymphoreticular system. Many orbiviruses have a predilection for the vascular endothelial cells, which can lead to disruption in blood flow and subsequent ischemia. Orbiviruses also have a tropism for the nervous system, leading to encephalitis and, possibly, neuropathies.
Epidemiology
Deaths have not been attributed to orbiviral infections in humans. However, severe flulike illness, encephalitis, and polyradiculitis have occurred infrequently in patients with orbiviral infections. Patients with clinical orbiviral infections generally recover with no long-lasting effects.
Current reports do not demonstrate a racial predilection for orbiviral infections; however, no large epidemiologic trials or case series have been performed.
Bites
The patient may give a history of arthropod bites, including ticks, mosquitoes, gnats, and midges. In cases of Oklahoma tick fever or Kemerovo or Lipovnik virus infections, a tick transmits the virus. Orbiviral infections in Africa and South America may be spread via mosquito bites.
Although incubation time from exposure to clinical illness is unknown in humans, it has been estimated at 6 to 9 days in animals.
Based on serologic studies, orbiviral infections often are asymptomatic or present as a mild nonspecific febrile illness for which medical care is not sought. Patients who present for medical evaluation often report fever, myalgia, nausea, vomiting, diarrhea, and confusion. Seizures and, rarely, focal neurologic deficits may also develop.
Physical
Physical findings vary depending on the specific Orbivirus involved; however, the most common physical finding is fever.
Other nonspecific symptoms have been reported, including headache, myalgia, and gastrointestinal problems.
Patients may be encephalopathic or may demonstrate focal neurologic findings.
Cause
Risk factors include travel to an area with known orbiviruses, particularly Texas and Oklahoma. Overseas travel to Russia, Eastern Europe, South America, Central America, and Africa may also lead to orbiviral exposure.
Outdoor activity is the best-known risk factor because disease transmission to humans is via arthropod vectors. The seasonality of the infection relates to the time of arthropod activity.
Treatment
No known specific treatment exists for orbiviral infections. Supportive care is recommended.
In a study looking at Bluetongue Virus and African horse sickness virus, polyanionic aromatic compound aurintricarboxylic acid (ATA) was trialed as it has been shown to have broad-spectrum antiviral activity. It was shown to successfully prevent replication in vitro; however, it was not protective in vivo. More research will need to be pursued to determine if ATA is a treatment option. [28]
Background
Orbiviruses are one of nine genera in the family Reoviridae. Only four genera within the Reoviridae cause human disease: the orbiviruses, rotaviruses, orthoreoviruses, and coltiviruses. Coltiviruses cause Colorado tick fever. Orbiviruses are distinguished from the orthoreoviruses by their protein structure and arthropod transmission cycles. [1] The genus Orbivirus contains 19 species and at least 130 subspecies.
Orbiviruses are named for their doughnut-shaped capsomeres (orbi means ring in Latin). Structurally, each virion consists of an outer and inner capsid layer that surrounds a core genome of 10 nonenveloped double-stranded RNA segments. Type- and group-specific antigens include the outer VP2 and the core VP7 proteins, respectively. [2] No lipid envelope is present. Overall, they are 70 to 80 nm in diameter.
The orbiviruses are primarily animal pathogens that cause bluetongue disease in sheep, cattle, goats, and wild ungulates; African horse sickness in horses, donkeys, and dogs; and epizootic hemorrhagic deer fever. Many other viruses in this genus infect animals, but the above are the most commonly recognized. Infections may affect fetal development and have been linked to congenital anomalies such as hydrocephalus and arthrogryposis. These are the best-studied orbiviral diseases. Only 7 per 100 orbiviruses are linked to human clinical disease, with only one, Oklahoma tick fever, causing disease within the United States.
Pathophysiology
Orbiviruses are vector-borne pathogens that are transmitted by ticks, mosquitoes, gnats, and midges. Studies on the pathophysiology of orbiviral infections specifically in humans have not been conducted. Data on the pathophysiology of orbiviral infections are derived mainly from studies of animal orbiviruses. According to Palacios et al, the ability to detect them has been hampered by their diversity. They developed a consensus reverse-transcription polymerase chain reaction (RT-PCR) method of targeting the polymerase gene for orbivirus recognition and characterization. [3, 4, 5]
In these cases, the virus enters the cell by endocytosis, the outer capsid is removed, and the core particle undergoes transcription. Viral proteins are synthesized 2-14 days after the infection and self-assemble within the cytoplasm. New virions are released and are then capable of infecting more cells, mainly within the lymphoreticular system. Many orbiviruses have a predilection for the vascular endothelial cells, which can lead to disruption in blood flow and subsequent ischemia. Orbiviruses also have a tropism for the nervous system, leading to encephalitis and, possibly, neuropathies.
Epidemiology
Frequency
United States
Within humans, Oklahoma tick fever is a tick-borne febrile illness that may be caused by an orbivirus in the Kemerovo group. The exact frequency is unknown, but clinically recognized disease is uncommon. Two serologically diagnosed human cases have been reported in Oklahoma and Texas. These cases involved elevated levels of antibodies to Six Gun City and Lipovnik viruses, which are members of the Kemerovo serogroup. Viral isolation was not performed; therefore, the precise identity of the cases is not definitively known.
International
Internationally, only infrequent cases of clinical illness in humans have been reported in Russia, Eastern Europe, Africa, and South and Central America. To date, fewer than 50 cases have been described in the literature.
Mortality/Morbidity
Deaths have not been attributed to orbiviral infections in humans. However, severe flulike illness, encephalitis, and polyradiculitis have occurred infrequently in patients with orbiviral infections. Patients with clinical orbiviral infections generally recover with no long-lasting effects.
Race
Current reports do not demonstrate a racial predilection for orbiviral infections; however, no large epidemiologic trials or case series have been performed. Further clarification of this point requires more clinical cases with in-depth epidemiological investigations.
Sex
In the few human cases of orbiviral infections, incidence does not seem to be associated with sex.
Age
All age groups may be infected with orbiviruses. However, seroprevalence studies suggest that most infections occur in childhood.
Breed
Worldwide, more than 100 orbiviruses affect animals. These diseases include bluetongue disease in sheep, cattle, goats, and wild ungulates; African horse sickness in horses, donkeys, and dogs [6] ; and epizootic hemorrhagic deer fever.
