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NSW Arbovirus Surveillance & Vector Monitoring Program |  |
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NSW Arbovirus Surveillance & Vector Monitoring Program | |
 2001 - 2002
Annual Report  |
Mosquito abundance is dictated principally by rainfall patterns and irrigation practices in inland regions, while in coastal regions tidal inundation along with rainfall is important. Temperature and/or day-length are often critical in determining the initiation and duration of mosquito activity for species in temperate zones. Hence, the monitoring of environmental parameters, especially rainfall, is a crucial component of the Program.
The end of the La Niņa episode in early 2001 saw drier conditions
prevail with below average rainfall through the following autumn across inland NSW (note
La Niņa = wetter conditions). The winter months and early spring experienced mostly
neutral rainfall activity, being close to average. The Southern Oscillation Index (SOI)
failed to rise substantially in late spring/early summer ensuring that rainfall remained
below average (+ve SOI values = wetter conditions). Early summer 2001-2002 was especially
dry with almost the entire state having below to very much below average rainfall. Average
rainfall was recorded for the autumn months for much of the state, although the northern
tablelands regions had relatively dry conditions.
Similar dry conditions occurred along the coast especially during the
summer months of 2001 – 2002. However in the first week of February, the region from
the mid-north coast to the far south coast had unusually heavy rainfall, which was very
much above average. This rainfall closely followed upon the high spring tides of the last
week of January. For the remainder of the season, rainfall was around normal along the
coast.
Dry conditions usually equate to warmer weather and this held true for
much of 2001. However despite the dry conditions, temperatures were 2-30 below
average, and progressively cooler towards the southwest, for the inland region from late
October 2001 through to March 2002, The coastal strip experienced fewer extremes in
temperatures, which tended to be around average for the season, although the far north
coast did have above average conditions through the summer months.
In the 2000-2001 Annual Report of the NSW Arbovirus Surveillance
Program (Doggett et al., 2001), it was noted
that both the Forbes’ (1978) and the Nicholls’ (1986) models suggested possible
MVE virus in southeastern Australia for that season.
Subsequently there was activity, although largely confined to the northern inland
region. As Forbes’ hypothesis was fulfilled for the 2000-2001 season, this meant that
it was also partially fulfilled for the 2001-2002 season.
To reiterate Forbes’ hypothesis - Forbes associated rainfall
patterns with the 1974 and previous MVE outbreaks, and discussed rainfall in terms of
'decile' values. A decile is a ranking based on historical values. The lowest 10% of all
rainfall values constitute decile 1, the next 10% make up decile 2, and so on up to the
highest 10% of rainfall constituting decile 10. Thus, the higher the decile value, the
greater the rainfall.
Forbes' hypothesis refers to rainfall levels in the catchment basins of
the main river systems of eastern Australia. These include:
· The
Darling River system,
· The
Lachlan/Murrumbidgee/Murray River systems,
· The
Northern Rivers (that lead to the Gulf of Carpentaria), and
· The
North Lake Eyre system.
The hypothesis states that if rainfall levels in these four catchment
basins are equal to or greater than decile 7 for either the last quarter of the previous
year (eg. October-December 2000) or the first quarter of the current year (January-March
2001) and the last quarter of the current year (October-December 2001), then a MVE
outbreak is probable. Rainfall was above decile 7 for all the catchment basins for the
last quarter of 2000 (which satisfied the hypothesis for 2000-2001 as the last quarter of
1999 also had above decile 7 rainfall in all catchments), but not for the first quarter of
2001 or the last quarter of 2001. Thus, Forbes’ hypothesis was not satisfied for the
2001-2002 season.
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| Figure 2. The SO by seasons prior to MVE active years, according to Nicholls (1986). The black bars represent the pre-MVE active seasons. |
Nicholls’ hypothesis uses the Southern Oscillation (SO) as a tool to indicate possible MVE virus activity. He noted a correlation between past outbreaks of MVE and the SO (as measured by atmospheric pressures at Darwin in mm) for the autumn, winter and spring period prior to a disease outbreak. For the autumn, winter and spring periods of 2001, the SO values of 1009.63mm, 1012.93mm and 1010.10mm respectively, were combined outside the range of values for the same period of past MVE active years. Likewise, the summer 2001 – 2002 SO value of 1007.20mm was also much higher than that experienced during MVE active years. Currently, the autumn and winter values according to Nicholls’ for 2002 are 1010.23mm and 1013.8mm, respectively, which are outside the range of values for past MVE years.
