A large number of Orbivirus serotypes have been recognised and numerous vectors are responsible for its transmission, primarily biting midges (Culicoides spp). Transmission can occur through bull semen. Transplacental transmission has been reported in cattle in Europe.
Clinical signs in cattle are rare and presentations usually involve fever, conjunctivitis, dyspnea, ptyalism, salivation and joint stiffness. The characteristic swollen blue tongue is due to lesions of nasal mucosa, nasal discharge.
Examination of the affected animals sometimes reveals oral vesicles and ulcers and ulcerative dermatitis. Abortion has been recorded in pregnant cows. Birth defects occur in calves such as hydranencephaly or porencephaly, resulting in ataxia and blindness.
A diagnosis is based on presenting clinical signs, prevalence of the orbivirus in the region and ancilliary testing. Ancillary tests include histopathological analysis of postmortem samples, ELISA and PCR tests.
There is no specific treatment for cattle with bluetongue, although novel antiviral agents have been developed recently. Supportive feeding, fluids and administration of broad-spectrum antimicrobials may assist recovery.
Prevention of outbreaks must address control of insect vectors.
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