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from Halls Creek in the East Kimberley region and Derby in West Kimberley in 1960 had demonstrated that subclinical infections with both MVE and Kunjin viruses had occurred in the human population (Stanley and Choo, 1961; 1964), there had been no reported cases of Australian encephalitis in Western Australia or in the Northern Territory. Unfortunately no baseline studies were undertaken on either mosquito densities or virus incidence before the completion of stage one of the irrigation project; indeed no studies were initiated until completion of stage two, the construction of the Ord River dam. While the Ord River irrigation area undoubtedly had enormous and profound effects on the ecology of the region, most of the evidence for increases in mosquito densities and waterbird populations is circumstantial. The climate in the Kimberley and adjacent areas of the Northern Territory comprises a relatively short (four month) monsoonal wet season during which heavy rainfall events occur and the major rivers extend across vast floodplains, and a very dry ‘dry’ season during which most of the country becomes arid and, in the latter half, even large rivers cease to flow. Results from studies at various locations, such as Billiluna and Halls Creek, suggest that MVE virus is occasionally epizootic in many arid areas of the Kimberley. It is probable, therefore, that the area in which the Ord River irrigation area was established was similar and, consequently, that prior to the irrigation scheme being implemented, MVE was also epizootic. Since 1972, our studies in the Ord River irrigation area and elsewhere in the Kimberley region on virus isolations from mosquitoes, on serological investigations of humans, animals and sentinel chickens, and on human cases of Australian encephalitis, have clearly shown that MVE virus is now enzootic in the Ord River area and probably in other foci such as the Derby and Broome areas of the West Kimberley region. Elsewhere, in arid areas of the Kimberley and in the Pilbara, MVE virus is epizootic and virus activity is probably initiated either by virus reactivation from desiccation-resistant mosquito eggs or by introduction through viraemic vertebrate hosts. The situation in the Northern Territory is less clear as insufficient data have been accumulated. However, it is probable that MVE is enzootic in the wetlands in the north of the Northern Territory, but epizootic in the more arid areas further south extending east from the Kimberley border. Since 1978 there has been a substantial increase in the number of cases of Australian encephalitis throughout the Kimberley and Northern Territory that cannot be ascribed to either an increase in population or a heightened awareness among clinicians. Thus, although based largely on circumstantial evidence, we believe that the Ord River Irrigation Area has had a profound effect on MVE virus activity and indeed has resulted in the virus becoming enzootic in the area. We also believe that this large, stable enzootic focus has provided the source for regular epizootic incursions to other areas of the Kimberley and adjacent arid areas of the Northern Territory, and to the Pilbara, and has probably established smaller enzootic foci in the West Kimberley. As virus can persist in desiccation-resistant mosquito eggs, it is probable that most areas of the Kimberley and adjacent areas of the
DOI link for from Halls Creek in the East Kimberley region and Derby in West Kimberley in 1960 had demonstrated that subclinical infections with both MVE and Kunjin viruses had occurred in the human population (Stanley and Choo, 1961; 1964), there had been no reported cases of Australian encephalitis in Western Australia or in the Northern Territory. Unfortunately no baseline studies were undertaken on either mosquito densities or virus incidence before the completion of stage one of the irrigation project; indeed no studies were initiated until completion of stage two, the construction of the Ord River dam. While the Ord River irrigation area undoubtedly had enormous and profound effects on the ecology of the region, most of the evidence for increases in mosquito densities and waterbird populations is circumstantial. The climate in the Kimberley and adjacent areas of the Northern Territory comprises a relatively short (four month) monsoonal wet season during which heavy rainfall events occur and the major rivers extend across vast floodplains, and a very dry ‘dry’ season during which most of the country becomes arid and, in the latter half, even large rivers cease to flow. Results from studies at various locations, such as Billiluna and Halls Creek, suggest that MVE virus is occasionally epizootic in many arid areas of the Kimberley. It is probable, therefore, that the area in which the Ord River irrigation area was established was similar and, consequently, that prior to the irrigation scheme being implemented, MVE was also epizootic. Since 1972, our studies in the Ord River irrigation area and elsewhere in the Kimberley region on virus isolations from mosquitoes, on serological investigations of humans, animals and sentinel chickens, and on human cases of Australian encephalitis, have clearly shown that MVE virus is now enzootic in the Ord River area and probably in other foci such as the Derby and Broome areas of the West Kimberley region. Elsewhere, in arid areas of the Kimberley and in the Pilbara, MVE virus is epizootic and virus activity is probably initiated either by virus reactivation from desiccation-resistant mosquito eggs or by introduction through viraemic vertebrate hosts. The situation in the Northern Territory is less clear as insufficient data have been accumulated. However, it is probable that MVE is enzootic in the wetlands in the north of the Northern Territory, but epizootic in the more arid areas further south extending east from the Kimberley border. Since 1978 there has been a substantial increase in the number of cases of Australian encephalitis throughout the Kimberley and Northern Territory that cannot be ascribed to either an increase in population or a heightened awareness among clinicians. Thus, although based largely on circumstantial evidence, we believe that the Ord River Irrigation Area has had a profound effect on MVE virus activity and indeed has resulted in the virus becoming enzootic in the area. We also believe that this large, stable enzootic focus has provided the source for regular epizootic incursions to other areas of the Kimberley and adjacent arid areas of the Northern Territory, and to the Pilbara, and has probably established smaller enzootic foci in the West Kimberley. As virus can persist in desiccation-resistant mosquito eggs, it is probable that most areas of the Kimberley and adjacent areas of the
from Halls Creek in the East Kimberley region and Derby in West Kimberley in 1960 had demonstrated that subclinical infections with both MVE and Kunjin viruses had occurred in the human population (Stanley and Choo, 1961; 1964), there had been no reported cases of Australian encephalitis in Western Australia or in the Northern Territory. Unfortunately no baseline studies were undertaken on either mosquito densities or virus incidence before the completion of stage one of the irrigation project; indeed no studies were initiated until completion of stage two, the construction of the Ord River dam. While the Ord River irrigation area undoubtedly had enormous and profound effects on the ecology of the region, most of the evidence for increases in mosquito densities and waterbird populations is circumstantial. The climate in the Kimberley and adjacent areas of the Northern Territory comprises a relatively short (four month) monsoonal wet season during which heavy rainfall events occur and the major rivers extend across vast floodplains, and a very dry ‘dry’ season during which most of the country becomes arid and, in the latter half, even large rivers cease to flow. Results from studies at various locations, such as Billiluna and Halls Creek, suggest that MVE virus is occasionally epizootic in many arid areas of the Kimberley. It is probable, therefore, that the area in which the Ord River irrigation area was established was similar and, consequently, that prior to the irrigation scheme being implemented, MVE was also epizootic. Since 1972, our studies in the Ord River irrigation area and elsewhere in the Kimberley region on virus isolations from mosquitoes, on serological investigations of humans, animals and sentinel chickens, and on human cases of Australian encephalitis, have clearly shown that MVE virus is now enzootic in the Ord River area and probably in other foci such as the Derby and Broome areas of the West Kimberley region. Elsewhere, in arid areas of the Kimberley and in the Pilbara, MVE virus is epizootic and virus activity is probably initiated either by virus reactivation from desiccation-resistant mosquito eggs or by introduction through viraemic vertebrate hosts. The situation in the Northern Territory is less clear as insufficient data have been accumulated. However, it is probable that MVE is enzootic in the wetlands in the north of the Northern Territory, but epizootic in the more arid areas further south extending east from the Kimberley border. Since 1978 there has been a substantial increase in the number of cases of Australian encephalitis throughout the Kimberley and Northern Territory that cannot be ascribed to either an increase in population or a heightened awareness among clinicians. Thus, although based largely on circumstantial evidence, we believe that the Ord River Irrigation Area has had a profound effect on MVE virus activity and indeed has resulted in the virus becoming enzootic in the area. We also believe that this large, stable enzootic focus has provided the source for regular epizootic incursions to other areas of the Kimberley and adjacent arid areas of the Northern Territory, and to the Pilbara, and has probably established smaller enzootic foci in the West Kimberley. As virus can persist in desiccation-resistant mosquito eggs, it is probable that most areas of the Kimberley and adjacent areas of the
ABSTRACT