NSW Arbovirus Surveillance & Vector Monitoring Program

1999 - 2000 Annual Report

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WEATHER DATA

Mosquito abundance is dictated principally by rainfall patterns; the more rain, generally the greater the number of mosquitoes. Temperature, tidal flow and irrigation practices are also influential for mosquito breeding. Hence monitoring of environmental parameters, especially rainfall, is a critical component of the Program.

For 1999-2000, the La Niña conditions that developed through 1998 continued into 1999 and resulted in higher than average rainfall across most of the state, from October 1999 to April 2000. Rainfall in the west was especially intense in February, and again in April, resulting in widespread flood conditions. Despite these patterns, coastal regions from November 1999 to February 2000 tended to receive below average rainfall. Temperatures during November to February were consistently well below average in both coastal and inland regions, with areas in the northwest being up to 6⁰C below average in early summer. In contrast, March and April tended to have temperatures 2-3⁰C above average.  

It was noted in the 1998-1999 annual report (Doggett et al., 1999) that there was a chance of MVE activity during 1999-2000, as both the Forbes’  (Forbes, 1978) and the Nicholls’ (Nicholls, 1986) models were predicting possible MVE virus in southeastern Australia. 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 amount of rainfall.  

Forbes' hypothesis refers to rainfall patterns 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 patterns 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 1999) or the first quarter of the current year (January-March 2000) and the last quarter of the current year (October-December 2000), then a MVE outbreak is probable. The quarterly decile figures (Bureau of Meteorology, 1998-2000) covering the current relevant period are in the table below.

 Quarterly Decile figures for the Forbes (1978) catchments.

 As all the catchment basins had decile 7 rainfall in the first quarter of 1999 and the last quarter of 1999, Forbes’ hypothesis was satisfied for the 1999-2000 season. However, Nicholls' hypothesis, which uses the Southern Oscillation (SO) as a tool to indicate possible MVE virus activity, was not satisfied. Nicholls noted a correlation between past outbreaks of MVE and the SO for the autumn, winter and spring period prior to a disease outbreak. For 1999, the winter period was not in line with past MVE outbreaks. Thus, even though one of the models suggested possible MVE activity in southeastern Australia, there was no evidence for MVE in NSW for the 1999-2000 season. Note that Forbes’ hypothesis is still suggesting possible activity for 2000-2001 as all catchment basins in the last quarter of 1999 had rainfall greater than decile 7. The autumn 2000 period according to Nicholls’ hypothesis is also conforming to past MVE outbreaks.

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