NSW Arbovirus Surveillance & Vector Monitoring Program

2001 - 2002 Annual Report

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DISCUSSION

The 2000-2001 season saw the first evidence of MVE in NSW since 1974, with the predisposing environmental factors being heavy rainfall in early and late 2000, coupled with activity in central Australia (Doggett et al., 2001). Although the high spring vector mosquito populations that probably initiated the virus activity in inland NSW fell to below average during the summer months, and thus minimised opportunities for transmission of virus to humans (and no human MVE cases were recorded), there remained a concern that the virus might be maintained through localised enzootic cycles or via over-wintering in the mosquito populations for 'reemergence' in the 2001/2002 season. However, no evidence of MVE was detected during the recent season, in either the mosquitoes or sentinel chickens, supporting the theory that MVE is seasonally introduced into southeastern Australia via viraemic birds moving from endemic regions in the north following periods of unusually heavy rainfall (Marshall, 1988). Neither Forbes’ nor the Nicholls’ hypotheses indicated likely MVE activity and rainfall across the inland was well below normal during the summer months. This minimal rainfall, concomitant with the low temperatures, assured that mosquito populations remained low with little subsequent arbovirus activity of any kind. Only around a half to a third of the mosquitoes were collected this season compared to recent years, with one of the lowest virus yields from the inland to date. The human notifications reflected this trend, with relatively few arbovirus infections from the inland.  

The 2000-2001 season also had unprecedented KUN activity across the state, with both chicken seroconversions and virus isolates from the mosquitoes. Despite this intense activity, there was also no evidence for KUN during 2001-2002, which again probably relates to the low mosquito densities.  

Coastal trapping continued to be limited, this year to six locations, with only one site from the south coast. As noted earlier, mosquito populations were average to below average with the dry conditions, up until February when rainfall coupled with spring tides lead to some high collections for that month. Numbers of Ochlerotatus vigilax, the Saltmarsh mosquito, notably rose during this month, although it was often the freshwater mosquitoes that dominated the coastal collections. For example, Ballina yielded high Ochlerotatus multiplex and Ochlerotatus notoscriptus numbers. Densities of Ochlerotatus notoscriptus were well up at Gosford, and the Port Stephens traps collected an abundance of Culex annulirostris, Coquillettidia linealis and Ochlerotatus procax. It is worth noting that several of these species are demonstrated arbovirus vectors or have been found associated with virus activity. In all, the large February collections ensured that the total number of mosquitoes collected over the entire season for the coast was comparable to recent years.  

Batemans Bay was the solitary location where virus isolation was attempted and this was from a single February collection. The three isolates of RR came from only 105 Ochlerotatus vigilax. This carriage rate of around 1 virus infected mosquito in 50 is extraordinarily high, much higher than often observed during epidemics! Fortunately, mosquito numbers during the week of the isolates were well down, almost half the long-term average and there were very few human RR notifications from that entire area health service, only 15, which is also down upon recent years.  

The Sydney collections showed a similar trend to the other coastal localities, with mosquito numbers peaking in late summer/early autumn, while numbers early in the season were not especially abundant. There were no isolates yielded from the mosquitoes and the number of human notifications within the Sydney Region (9BF & 11 RR) was the lowest for many years.  

The mid-north coast continues to be the hot spot of arbovirus activity within NSW and again yielded the highest number of cases for any Area Health Service (AHS), as it has for the last three seasons. The 240 notifications (188BF, 47 RR and 5 unspecified) were double that of next highest AHS (Hunter with 120) and the Mid-North Coast AHS also had the highest notification rate of around 90 cases/100,000 population.  

The major difference in arbovirus activity for the 2001-2002 season was the predominance of BF infections.

Figure 4. Reports of human cases of arbovirus infections by month in NSW, July 1994 - June 2002.

This was the first time since notifications began that there were more statewide reports of BF (406) than RR (229), although the overall number of notifications was down this year (Figure 4). The BF activity almost exclusively occurred along the north coast and accounted for around 90% of the cases (361 patients). The Hunter AHS saw the largest increase in cases and was up from the 13 notifications in 2000-2001 to 92 for the recent year, representing a serious epidemic. The Central Coast AHS also had a notable increase, and the North Coast and Mid-North Coast AHS combined had some 250 notifications of BF infections. These two latter regions in 2001 experienced the largest epidemic due to BF in Australia to date, with over 300 cases. Previously, major activity of BF had not occurred in consecutive seasons anywhere in NSW (or in other states) and this discombobulating observation cannot be readily explained. During the 2000-2001 season, there was heavy flooding following record levels of rainfall during March. As this coincided with spring tides, conditions would have been highly favourable for mosquito breeding, especially Ochlerotatus vigilax. This combination of high tides/extreme rainfall leading to large populations of saltwater mosquitoes has been an indicator of BF outbreaks (see Doggett et al., 1999b, for a review of environmental conditions preceding past BF epidemics). For the recent season, the combination of high tides and heavy rainfall also occurred but not to the degree during early 2001, and while large numbers of Ochlerotatus vigilax were collected, although not as many as the preceding season, there were also big numbers of Culex annulirostris, especially from Port Stephens. Perhaps the recent season saw a greater involvement of freshwater mosquitoes in the transmission of BF, rather than the saltwater species, but without any virus isolation from anywhere along the north coast, it is impossible to be certain. The only evidence of the possibility of freshwater mosquito involvement was from a study undertaken in Western Sydney by the Department of Medical Entomology. 

intensive investigation of mosquito and arbovirus activity was conducted at the Comland site (formerly known as ‘ADI’) at Werrington, during February to May, 2002. The rational for this study was the recent activity of RR within the locality (Brokenshire et al., 2000), which is in a well-populated region of Sydney, that the Comland site supported established populations of native animals (including 'kangaroos and emus') that might provide vertebrate reservoirs for arboviruses, and there was a lack of basic ecological knowledge of potential viruses and vectors in the area. While the study failed to detect any RR (perhaps because it was a very 'quiet' RR season statewide), there were several BF isolates and all were from freshwater species (Table 6).

 Table 6. Arboviruses isolated from mosquitoes collected at the Comland site. 

BF = Barmah Forest virus, River, EH = Edge Hill virus, STR = Stratford virus, ? = unknown

Previously, BF had been isolated from Ochlerotatus procax within Sydney, but not from Culex annulirostris (which is the main vector for inland NSW) or from Ochlerotatus notoscriptus. This latter species is a domestic breeding mosquito that has high human contact; the BF isolate is the first record for Australia, and follows recent studies that have demonstrated the species to be a highly competent vector of both BF and RR (Doggett & Russell, 1997). There were some BF notifications from within the region (Dr Kris Hort, Wentworth PHU, pers. comm.) and considering that few saltwater breeding mosquitoes were collected, it was likely that the freshwater species played the major role in the local BF transmission (and this should be considered for the other regions with activity). It is also worth noting from the Comland study that there were several isolates of STR and EH, and both of these viruses have been incriminated in causing clinical disease.

The above continues to highlight the complexity and difficulty in predicting arbovirus activity along coastal NSW based on environmental monitoring alone, without concurrent vector monitoring. For example in 1994-1995, a combination of high tides/heavy rainfall led to the then largest outbreak to date of BF virus (Doggett et al., 1999b) and the mosquito monitoring showed that there was an explosion in the population of the salt marsh mosquito, Ochlerotatus vigilax, and there were many BF isolates from this species. Similar conditions in early 1999 along the north coast of the state also resulted in an arbovirus epidemic, albeit RR, however the epidemiology of the disease suggested that many cases appeared to be further from the coast (although within the coastal strip). The mosquito monitoring prior to and during this period showed that saltwater mosquito species failed to rise in number substantially following the wet conditions. Rather it was freshwater species such as Coquillettidia linealis, Culex annulirostris, Culex orbostiensis and Ochlerotatus multiplex, which became very abundant and several isolates of RR were made from these species (Doggett et al., 1999a). Thus the mosquito monitoring gave an explanation to the observed differences in the RR epidemiology patterns for that year and demonstrates that different outbreaks can involve different mosquitoes. Thus monitoring of environmental conditions alone for predicting arbovirus activity would fail to adequately direct disease control strategies, without intelligence gained from vector monitoring.

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