Aspects of the vector potential of Culex annulirostris Skuse 1889 and other mosquitoes (Diptera: Culicidae) in Queensland, with particular reference to arbovirus transmission at Kowanyama and Charleville

Abundance, age and feeding of Culex annulirostris Skuse were studied at Kowanyama, north Queensland during 11 visits from 1972-1976, and at Charleville on 13 periods from 1974-1976. Virological studies mere also done to evaluate vector potential.Mosquitoes, designated as Culex sp. nr annulirostris or atypical in Annual Reports of the Queensland Institute of Medical Research, with uninterrupted apical dark banding on abdominal sternites V , VI and VII were morphs of Culex annulirostris. Progeny were reared from females of known banding pattern. This polymorphism occurred 4.7-11.8 times more often in males than females. An average of 2% of female Cx annulirostris sampled from 23 localities in Queensland and 2 in the Northern Territory were of this pattern which occurred more in the north than in arid inland centres.At Kowanyama, 104,006 adult female mosquitoes of a probable 47 species were collected. Five new distribution records were added: Anopheles novaguinensis Venhuis, Tripteroides punctolateralis (Theobald), Uranotaenia lateralis Ludlow, Aedes kochi Donitz and Aedes elchoensis Taylor. Culex fatigans Wiedemann was the most common species, because it comprised 88.8% of resting adults. In bait and truck trap collections, Cx annulirostris was consistently more abundant than either Cx fatigans or Anopheles bancroftii Giles and usually comprised 27.1 ± 10.6% (± SD) and 43.5 ± 16.7% of wet (December-March) and dry (April-November) season samples respectively.At Charleville, 71,791 adult females of 20 taxa were taken, which included 4 new distribution records: Mimomyia elegans (Taylor), Aedes quasirubithorax (Theobald) gp, Aedes lineatopennis (Ludlow) and Culex bitaeniorhynchus Giles. Culex annulirostris comprised 56.1-99.0% of mosquitoes collected on each trip. Aedes normanensis (Taylor) was plentiful for short periods after major rainfall. Of 11 species of adults taken by human bait collections in winter, only Cx annulirostris and possibly Anopheles annulipes Walker were numerous, usually near permanent water e.g. Warrego River. In June 1974, immatures of 5 species were collected. At this time, Cx annulirostris completed a full life-cycle in 29-48 days.The physiological age of samples of Cx annulirostris from Kowanyama and Charleville was determined by ovarian tracheation and dilatation methods. Abdominal pigmentation, ovary colour and consistency, body wear, the meconium and parasitic mites were of greater use than ovary size, as determinants of parity. Culex annulirostris (1540 from Kowanyama, 1553 from Charleville) showed only moderate longevity. Gonoactive pars and nullipars were present throughout the year despite low winter temperatures at Charleville.At Kowanyama, Cx annulirostris and most other species fed mainly outdoors except for An bancroftii and Cx fatigans which fed predominantly indoors in darkness but also indoors in light. Mansonia uniformis (Theobald) showed no fixed preference. During the day, Cx annulirostris was collected resting outdoors in grass, trees and on gauze screening.Host preference was determined using pairs of animal-baited stable traps set 1m apart. Anopheles bancroftii, Anopheles amictus Edwards, An annulipes, Anopheles farauti Laveran, Anopheles meraukensis Venhuis, Ma uniformis, Ae normanensis, Aedes vittiger (Skuse), Aedes bancroftianus Edwards, Cx annulirostris and Cx fatigans were abundant enough during 4 trips to Kowanyama and 1 to Charleville for detailed analysis. The host range of numerous other species was extended. Most species exhibited a preference for calf except for An bancroftii and Cx fatigans which preferred man. Culex annulirostris preferred (calf, pig and dog) to (man and kangaroo) to 2 domestic fowls. Preference was similar at both localities.At Kowanyama village, hosts of 6373 blood meals of mosquitoes of more than 20 species were identified by the precipitin test. Culex squamosus (Taylor) was principally ornithophilic (75.6% of meals tested) and 28.7% of Cx fatigans also fed on birds. Uranotaenia albescens Taylor fed almost entirely on amphibians. Other species fed mainly on a broad range of mammals e.g. Cx annulirostris fed on pig, cat, dog, human, cow, horse, marsupial (86.1%), birds (12.9%), and amphibians and reptiles (1.0%). The presence of approximately 100 dogs may divert the feeding of mosquitoes on the 800 humans in the village: 44.6 and 9.5% of Cx annulirostris, 50.5 and 7.0% of An annulipes and 54.0 and 8.8% of Cx fatigans fed on dogs and man respectively. Seasonal shifts in feeding were evaluated using a Feeding Index, based on relative abundance and availability. No seasonal shifts in biting or feeding behaviour were evident. At Charleville, 78.2% of Cx annulirostris collected had fed on mammals. Culex annulirostris was anautogenous and was thought to feed on blood every 3-4 days during summer, and less frequently in winter (every 5-6 days at Kowanyama and 9-10 days at Charleville). Experimental feeding of some or all of 33 Queensland and New Guinean arboviruses demonstrated the high susceptibility of Cx annulirostris to infection: Cx annulirostris and Aedes vigilax (Skuse) permitted multiplication of 20 viruses, Aedes aegypti (Linn.) and Aedes funereus (Theobald) 10, whereas only 2 of 12 tested multiplied in Cx fatigans. Detailed studies were carried out with colonized Ae aegypti as a model on which to base future work with wild-caught Cx annulirostris. Of the 11 viruses tested for infection and transmission efficiency, Ross River and Murray Valley encephalitis (NUE) viruses were transmitted most effectively. With MVE, highly efficient aspects such as 96% infection and 95% transmission and an extrinsic incubation period of 10-12 days, were moderated by the high threshold of infection (the ID50 or dose required to infect 50% was 105.1 infected suckling mouse LD50/mosquito). In a detailed investigation of vector efficiency, a population of Cx annulirostris from Brisbane proved a highly efficient vector of MVE characterized by a low ID50 (102.9LD50/mosquito), total susceptibility to infection, transmission from 7 days and 75-95% transmission by 10 days. More limited investigations suggested that Ae vigilax, Ae lineatopennis and Ae vittiger could also serve as vectors; Ma uniformis was infected easily but 9 infected Cx fatigans failed to transmit MVE 12-14 days after ingestion.Culex annulirostris was the major source of virus isolations: 30 of 42 from Kowanyama included the first isolations of togaviruses during the dry season and or winter; 54 of 100 from Charleville including the first incrimination of Cx annulirostris as the vector of HUE during an epidemic and the discovery of 4 "new" viruses. 41 strains of 9 different viruses were also isolated from Ae normanensis.