The aim of the Program is to provide an early warning of the presence of Murray Valley Encephalitis (MVE) and Kunjin (KUN) viruses in the state in an effort to reduce the potential for human disease. In addition, it compiles and analyses mosquito and alphavirus (especially Ross River and Barmah Forest virus) data collected over a number of successive years. This will provide a solid base to determine the underlying causes of the seasonal fluctuations in arbovirus activity and the relative abundance of the mosquito vector species with the potential to affect the well-being of human communities. This information can then be used as a basis for modifying existing local and regional vector control programs, and in the creation of new ones.

Background

Arbovirus Surveillance can be divided into two categories: those methods that attempt to predict activity and those that demonstrate viral transmission. Predictive methods include the monitoring of weather patterns, the long-term recording of mosquito abundance, and the isolation of virus from vectors. Monitoring of rainfall patterns, be it short term with rainfall or longer term with the Southern Oscillation, is critical as rainfall is one of the major environmental factors that influences mosquito abundance; generally the more rain, the higher the mosquito numbers. The long-term recording of mosquito abundance can establish baseline mosquito levels for a location (i.e. determine what are normal populations), and this allows the rapid recognition of unusual mosquito activity. The isolation of virus from mosquito vectors can provide the first indication of which arboviruses are circulating in an area. This may lead to the early recognition of outbreaks and be a sign of the potential disease risks to the community. Virus isolation can also identify new virus incursions, lead to the recognition of new virus genotypes and identify new vectors. Information from vector monitoring can also reinforce and strengthen health warnings of potential arbovirus activity.

Methods that demonstrate arboviral transmission include the monitoring of suitable sentinel animals (such as chickens) for the presence of antibodies to particular viruses (e.g. MVE and KUN within NSW) and the recording of human cases of disease. Sentinel animals can be placed into potential ‘hotspots’ of virus activity, and as they are not protected from mosquito bites, may indicate activity in a region before human cases. In general terms, for arbovirus surveillance within NSW, the monitoring of human cases has little direct value, as by the time the virus activity is detected in the human population, often not much can be done to control the virus. Via the other methodologies, the aim of the surveillance program is to recognise both potential and actual virus activity before it impacts greatly on the human population. The recording of human infections does however provide important epidemiological data and can define the locations where surveillance should occur.

These methods of surveillance are listed in order; generally with more rainfall comes more mosquito production. The higher the mosquito production, the greater the probability of enzootic virus activity in the mosquito/host population. The higher the proportion of virus infected hosts and mosquitoes, the greater the probability of transmission and thus the higher the risk to the human population. The NSW Arbovirus Surveillance and Mosquito Monitoring Program undertakes the first four methods of arbovirus surveillance and the results for the 2001-2002 season follow.

MONITORING LOCATIONS

For 2001-2002, mosquito trapping sites were operated at 13 inland, 6 coastal and 6 Sydney locations (Fig 1). Chicken sentinel flocks were placed at 10 sites.

Fig 1. Mosquito trapping locations and Sentinel Chicken sites, 2001-2002.

Back to Contents

top of page