Akabane virus.

Akabane virus, an arthropod-borne Bunyavirus, is the major cause of epizootics of congenital malformations in ruminants in Australia, Japan, Korea, and Israel, and is suspected to be a cause of sporadic outbreaks elsewhere. Blood-sucking insects, such as biting midges, transmit the virus horizontally to vertebrates. Climatic factors influence the seasonal activity and geographic range of the vector population and, therefore, occurrence of related disease. Inoculated ruminants seroconvert rapidly after a short subclinical viremia. Infection is of consequence only if ruminants are pregnant and not protected by adequate specific neutralizing antibodies. In naive pregnant animals, virus may spread hematogenously to replicate and persist in trophoblastic cells of placental cotyledons and subsequently invade the fetus. A distinct tropism for immature rapidly dividing cells of the fetal central nervous system and skeletal muscle results in direct virus-induced necrotizing encephalomyelitis and polymyositis. If fetuses survive, such injury may manifest as arthrogryposis, hydranencephaly, porencephaly, microencephaly, hydrocephalus, or encephalomyelitis at term. The earlier in gestation that fetal infection occurs, the more severe the lesions, reflecting the large population of vulnerable cells and lack of fetal immunocompetency at earlier stages of pregnancy. Injury during the period of critical cell migration and differentiation in organogenesis may substantially disrupt structural development in target organs. Late gestational infections cause nonsuppurative inflammation in the brain and spinal cord, premature birth, or fetal death with stillbirth or abortion. Affected neonates are nonviable. Control is by vaccination but is not always justified economically. Akabane viral infections must be differentiated from infections with other teratogenic viruses (including related Bunyaviruses), inherited conditions, and maternal intoxications. Diagnosis is made by serology and viral isolation.