Dengue viruses are actively transmitted by Ae aegypti in many countries in the tropical zone throughout the world including Queensland, Australia. The successful control of this species depends on a knowledge of the biology, ecology as well as developing and testing improved surveillance methods.
This thesis is a contribution not only to our knowledge of the biology and ecology of Ae aegypti but also describes the investigations of newer, better or more appropriate indices for improved surveillance with special reference to Queensland. Sampling methods for Ae aegypti immatures in 200 litre drums, survival and development at different temperatures were first tested in the laboratory and then field verifications were carried out. The concepts of key premise and key container were introduced.
The prevalence of container types under various container categories in 3 different regions of northern Queensland have been determined. The productivity of key containers, key container types and other positive container types have been studied. Four major hypotheses and 3 minor ones have been tested. Critical examination of field methods (used in Ae aegypti larval surveys) and Ae aegypti indices have been undertaken. Statistical modelling of house records and container records, cluster analysis of positive houses and positive containers were carried out. Cluster sampling and appropriate sample sizes have been proposed. Potential new indices were investigated and 3 potential new indices have been recommended.
Comparisons of sampling methods for Ae aegypti immatures in 200 litre drums revealed that net sweeping was better than ladle dipping. There was a good Pearson's correlation between the number of Ae aegypti immatures introduced and the mean number caught/sweep and especially so with fourth stage larvae. With sweeping, the coefficient of variation was less when compared to dipping. The number of fourth stage larvae caught from the first sweep with the different numbers of Ae aegypti gave a better correlation than when compared to that for a series of sweeps.
In sampling Ae aegypti immatures in 200 litre drums by net sweeping, it was found that the % recovery for fourth stage was the highest when sampled from the top (compared to the other stages) and was also irrespective of the number introduced. It was revealed that drum water temperature was the most important factor influencing the % recovery by net sweeping. With the decrease in the water temperature (from 30.8 ± l .7°c to 11.8 ± 1.8 °C), Ae aegypti recovery from the top by net sweeping became less after the second sweep. Recovery was also less when rust was absent in the drum water.
Laboratory studies on survival and development of Ae aegypti immatures in relation to temperature indicated that water temperature had a significant effect on the survival of the immature stages to adulthood. Percentage survival was lowest at 15°C, followed by 35°C and highest at 20, 25 and 30°C. At 10°C and 40°C, all immatures died in the first instar. The mean development time ± S.D. from first instar to adult was shortest at 35°C (7.16 ± 0.20 days and longest at 15°C (39.66 ± 2.27 days. At 10°C, it was confirmed that the development of the first stage larvae did not take place at this temperature and that mortality was 100% within 4 weeks. The lower threshold temperature (t) ± S.E. for development was 8.3 + 3.6°C and the thermal constant (k) was 181.2 ± 36.1 day-degrees above the threshold. The sex ratio of adults emerging at 15, 20, 25 and 35°C did not differ significantly from the normal 1:1 ratio. At 30°C, however, significantly more females emerged than males but this could be a biological variation. Adult females and males emerging at 20°C were the largest and the smallest adults emerged at 35°C.
Field surveys from 1988 - 1990 indicated that Breteau Index had increased in Townsville, Charters Towers, Mingela and Ravenswood compared to the 1986 and 1987 field results from the Health Department. The 1986 and 1987 Breteau Index were: North Ward (0.0), Garbutt (0.7), Belgian Gardens (4.0), Mundingburra (4.16), Hermit Park (23.0). The 1989 and 1990 Breteau Index from this present studies were: North Ward (37 in 1989, 35 & 18 for random and block in 1990), Garbutt (25 in 1989, 31 & 10 for random and block in 1990), Belgian Gardens (3.8, 4), Mundingburra (16.7, 26) and Hermit Park (13.6, 30).
In the present studies in 1990, the Breteau Indices in the 5 areas inspected in Charters Towers ranged from 22.2 to 103.8 compared to 18 in 1983. For Mingela and Ravenswood, the Breteau Index was very high in 1990, being 163.6 and 60 respectively (previous data were not available for comparison). A total of 1,349 premises were inspected in Townsville (1989 and 1990), Charters Towers (1990), Mingela and Ravenswood (1990) in northern Queensland. A total of 223 premises (16.5%) were positive with 451 positive containers. Out of these 223 positive premises, 46 were key premises (premises with ≥3 positive containers) which contributed to 224 Ae aegypti positive containers (49.7%). Although key premises comprised only 3.4% of all the premises inspected, they were responsible for approx. 50% of all the Ae aegypti positive containers during the survey in northern Queensland. One percent of the premises contained 50% of the larvae and 16% of the premises contained 100% of the larvae. Similarly, 2% of the premises contributed to 50% of the total Ae aegypti adults caught. A total of 1,050 adult resting mosquitoes were caught during the 1990 field surveys, with Ae aegypti (50.2%), Ae notoscriptus (17.0%, Ae vigilax (15.7%), Cx quinquefasciatus (7.6%) and other species (4.0%).
According to the results from the statistical modelling of House Records using GLIM, the chance of finding Ae aegypti positive containers are 2.5 times greater in an untidy yard (adjusted model) and 2.6 times greater in a shady premise (adjusted model) irrespective of whether the house is well-maintained or dilapidated. The chance of a positive house still being a positive house the next year is 3.22 times greater than a negative house becoming positive the next year.
The 5 most common positive container types found during the field studies were tyres (15.1%), buckets (13.7%), plant pot bases (13.3%), rain water tanks (8.4%) and ice cream containers (7.5%). Although kitchen items were common (9.3%), they comprised miscellaneous items of varying capacities. Townsville had increased garden accoutrements and disposable items and fewer rain water tanks, but rural towns had more water storage containers. Buckets were common in both Townsville and Charters Towers but not in rural Mingela and Ravenswood. On the otherhand, tyres were common to all the three regions (city, provincial and rural). The variability of the random and block sampling approaches methods for house to house survey in Townsville and Charters Towers suggested that greater sample size was needed. It is suggested that in view of the patchiness of the distribution of the positive houses and the aggregations of positive containers in key premises, cluster sampling would be more appropriate. Cluster analysis of blocks of houses did not reveal any significant clustering (p > 0.05) of positive houses but clustering of positive containers in key premises were found in all the regions. Therefore, cluster sampling was recommended in sampling houses for Ae aegypti larval survey.
It has been determined that certain rain water tanks and well(s) produced high numbers of Ae aegypti immatures and adults and thereby Indicating that key containers exist. The total number of Ae aegypti larvae in large water storage containers were determined by using the mark-release-recapture technique. The mean Ae aegypti immatures/positive container type varied from region to region and bore only a weak relationship to water volume. The mean immatures/ container type was quite stable where water volume was about the same e.g. plant pot bases. In rain water tank or wells, the numbers of immature Ae aegypti approx. equalled that for 30 positive tyres or 40 positive buckets. Out of 451 Ae aegypti positive containers, Ae aegypti was the sole species in 270 positive containers (59.9%) and was found co-breeding with Ae notoscriptus in 138 containers. Ae notoscriptus was sampled alone in 65 containers.
In Townsville, the % contribution of Ae aegypti fourth stage larvae in miscellaneous positive container types was 39.2%. This indicated that rubbish or disposable items were a problem and that more attention should be given to rubbish collections. In Townsville, except for a highly infested rain water tank and a well, there were many breeding sites e.g. garden accoutrements with moderate to low Ae aegypti fourth stage larval population and thus more difficult to control. In Charters Towers, rain water tanks produced the highest number of Ae aegypti fourth stage larvae (27.0%), followed by buckets (18.1%) and tyres (17.2%); the combined contribution of Ae aegypti fourth stage larvae by miscellaneous items was high (25.0%). In Mingela and Ravenswood, rain water tanks contributed 71.5% of the fourth stage larvae and tyres 18.2% giving a combined contribution of 89.7%.
Among the indices used by the World Health Organization, Breteau Index correlated positively with both the average number of Ae aegypti larvae collected/premise (Larval Density Index) and the adult density (Adult Density Index). Out of 7 potential new indices investigated, 3 were recommended for use with special reference to northern Queensland. They are: Adult Productivity Index based on an arithmetic mean, Adult Density Index (adult density) and Premise Condition Index based on house, yard and shade). Computer simulation was carried out to determine the coefficients of variation for the various indices.
The relationship of the Larval Density Index and the various indices were determined to supplement the World Health Organization Density Figure, but the actual meaning of these in terms of risk of transmission of dengue awaits further studies.