BUNYAVIRIDAE

1. Classification and general characteristics

   Bunyaviruses are enveloped viruses approximately 90 - 105 nm diameter. There are 2 glycoproteins G₁ and G₂. G₁ is involved with attachment and possesses hemagglutinating activity. G₂ is involved with fusion. The genome consists of 3 circular segments of - ss RNA arranged in helical configuration with the nucleocapsid.

   There are over 250 recognized bunyaviruses. Most of the bunyaviruses are arthropod borne (mosquitoes, ticks, sandflies, culicoides, etc.) and cause persistent, nonlethal, lifelong, infections in their natural wild animal host.

   The family is divided into 4 genera based on antigenic differences (SN, HI and CF).

   • Bunyavirus: Representative viruses include - La Crosse fever virus; Akabane virus; and Cache valley virus.
   • Phlebovirus: Representative viruses include - Rift valley fever virus; (zoonotic); sandfly fever virus.
   • Nairovirus: Representative viruses include - Nairobi sheep disease, Crimean hemorrhagic fever (zoonotic).
   • Hantavirus: Hemorrahagic fever viruses - zoonotic (These viruses are shed in the urine and saliva of persistently infected rats.)

2. Viruses of veterinary interest
   1. Akabane virus - The virus is currently found in Australia and Asia. It is not currently present in the USA. The virus affects cattle (epidemic). Common manifestations include: abortion; and congenital anomalies such as hydrocephalus; hydraencephaly; microencephaly; and arthrogryposis. Infects sheeps, goats, camels, horses, zebras and wild ruminants - inapparent infection. Arthropod borne infection. Infected mosquito biting pregnant cows (no clinical signs), reaches fetus via maternal circulation and causes death and abortion (survivors develop hydraenocephaly and arthrogyposis. Diagnosis is by clinical, pathologic and epidemiologic observations, confirmed by serology (paired serum samples). Inactivated vaccines was found to be safe and efficacious.
   2. B. Cache valley virus - Currently in the US. It causes abortion in sheep and congenital anomalies.
   3. Rift valley fever virus - This virus causes a severe systemic disease in sheep and goats (epidemics in Africa) resulting in mortality rates that range from 20-90%. Cattle are also affected but less severely. Clinical manifestations include high fever, nasal discharge, bloody diarrhea (enteritis), and abortion that frequently approaches the 90-100% rate. Encephalitis occurs in some animals. Common post mortem lesions included: focal areas of necrosis in the liver, and subserosa hemorrhages throughout the gastrointestinal tract. Rift valley fever virus is a typical arbovirus (transovarial transmission).

      The virus is transmitted by mosquitoes (Aedes spp.). The high viremias that occur in infected animals facilitates transmission by both mosquitoes and biting flies.

      The virus also affects man. It is mostly debilitating but death can occur in 0.1% of cases. Clinical manifestations include fever, vomiting, diarrhea which may take the form of hemorrhagic enteritis and occasionally meningoencephalitis and exudative retinitis.

      Diagnosis - virus isolation and serology (paired serum samples). Transmission - mosquitoes, fomites, direct contact. Attenuated live-virus vaccine (not used in pregnant animals because of abortion). Use of inactivated vaccines solved this problem. Vector control - use of insecticides.

   4. La Crosse fever virus - This virus belongs to the California serogroup of the Bunyavirus genus. It causes encephalitis in humans up to approximately 20 years of age. It is most severe in young children. Some individuals that recover from encephalitis will develop a form of epilepsy. Others will have cognitive impairments while others may have permanent paresis.

      The virus is transmitted by Aedes triseriatus. A woodland mosquito that deposits its eggs in tree holes. The increasing number of tire dumps is making a major contribution to the A. triaseriatus population.

   5. Nairobi Sheep Disease - causes disease in sheep and goats in Africa and India. Transmitted by all stages of the tick (Rhipicephalus appendiculatus) - transovarial infection. Affected animals may die within a few days or pregnant animals abort. Recovered animals are usually immune. Vaccines and vector control (dipping) are effective.
   6. Crimean - Congo Hemorrhagic fever - a zoonotic disease. Extends from China through central Asia to India, Pakistan, Afghanistan, Iran, Iraq, other Persian Gulf countries, Middle East, eastern Europe, to most saharan and sub-saharan Africa. Transmission is via tick (Hyalomma) - transovarial transmission. No evidence of clinical disease in animals, (sheep, cattle, and goats). Prevention is by vector control.
   7. Hantavirus - Hemorrhagic fever (zoonotic). Rodent host reservoirs - inapparent infection and persistent shedding. Causes severe to mild disease depending on rodent/virus combination.