Epidemic of blindness in kangaroos - evidence of a viral aetiology

Objective To determine the cause of an epidemic of blindness in kangaroos. Design and procedures Laboratory examinations were made of eyes and brains of a large number of kangaroos using serological, virological, histopatholo gical, electron microscopical, immunohistochemical methods, and PCR with cD NA sequencing. In addition, potential insect viral vectors identified durin g the disease outbreak were examined for specific viral genomic sequences. Sample population For histopathological analysis, 55 apparently blind and 1 8 apparently normal wild kangaroos and wallabies were obtained from New Sou th Wales, Victoria, South Australia, and Western Australia. A total of 437 wild kangaroos and wallabies (including 23 animals with apparent blindness) were examined serologically. Results Orbiviruses of the Wallal and Warrego serogroups were isolated from kangaroos affected with blindness in a major epidemic in south-eastern Aus tralia in 1994 and 1995 and extending to Western Australia in 1995/96. Hist opathological examinations showed severe degeneration and inflammation in t he eyes, and mild inflammation in the brains. In affected retinas, Wallal v irus antigen was detected by immunohistochemical analysis and orbiviruses w ere seen in electron microscopy. There was serological variation in the new ly isolated Wallal virus from archival Wallal virus that had been isolated in northern Australia. There were also variations of up to 20% in genotype sequence from the reference archival virus. Polymerase chain reactions show ed that Wallal virus was present during the epidemic in three species of mi dges, Culicoides austropalpalis, C dycei and C marksi. Wallal virus nucleic acid was also detected by PCR in a paraffin-embedded retina taken from a b lind kangaroo in 1975. Conclusion Wallal virus and perhaps also Warrego virus are the cause of the outbreak of blindness in kangaroos. Other viruses may also be involved, bu t the evidence in this paper indicates a variant of Wallal virus, an orbivi rus transmitted by midges, has the strongest aetiological association, and immunohistochemical analysis implicates ii as the most damaging factor in t he affected eyes.