Cassie C. Jansen

The dengue vector Aedes aegypti: what comes next

Aedes aegypti is the urban vector of dengue viruses worldwide. While climate influences the geographical distribution of this mosquito species, other factors also determine the suitability of the physical environment. Importantly, the close association of A. aegypti with humans and the domestic environment allows this species to persist in regions that may otherwise be unsuitable based on climatic factors alone. We highlight the need to incorporate the impact of the urban environment in attempts to model the potential distribution of A. aegypti and we briefly discuss the potential for future technology to aid management and control of this widespread vector species.

Blood sources of mosquitoes collected from urban and peri-urban environments in eastern Australia with species-specific molecular analysis of avian blood meals

To identify the hosts of mosquitoes collected from urban and peri-urban habitats in eastern Australia, 1,180 blood fed mosquitoes representing 15 species were analyzed by enzyme-linked immunosorbent assay and molecular techniques. Four common and epidemiologically important species could be classified according to their host-feeding patterns: Aedes aegypti was anthropophilic, Ae. vigilax was mammalophilic, Culex quinquefasciatus was ornithophilic, and Cx. annulirostris was opportunistic, readily feeding on birds and mammals. Mitochondrial cytochrome b DNA sequence data showed that more than 75% of avian blood meals identified from Cx. annulirostris collected from Brisbane, Newcastle, and Sydney originated from ducks (Order Anseriformes, Family Anatidae). More than 75% of avian blood meals from Cx. quinquefasciatus from Cairns belonged to one of three Passerine species, namely Sphecotheres vieilloti (figbird), Sturnus tristis (common myna), and Philemon buceroides (helmeted friarbird). This study demonstrates associations between vectors in Australia and vertebrate hosts of endemic and exotic arboviruses.

Vector Competence of Australian Mosquito Species for a North American Strain of West Nile Virus

Since the establishment of West Nile virus (WNV) into the United States, concern has arisen that this virus may also pose a serious threat to Australian biosecurity. The vector competence of 19 Australian mosquito species for a North American strain of WNV was evaluated. Mosquitoes collected from Cairns, Brisbane, and Sydney were exposed to blood containing 10(4.0+/-0.3) cell culture infectious dose(50)/mosquito WNV that was isolated from a crow during the 1999 New York outbreak. Mosquitoes were tested 12-15 days later to determine their infection, dissemination, and transmission rates. A number of Culex spp. demonstrated a high vector competence for this virus, with some populations of Culex annulirostris, the primary Australian Kunjin virus vector, displaying transmission rates up to 84%. Similarly, Cx. quinquefasciatus and Cx. gelidus were highly competent, with infection and transmission rates of >80% and >50%, respectively. Common Aedes spp., including Aedes notoscriptus, Ae. vigilax, and Ae. procax, were moderately susceptible, and some Verrallina spp. and Coquillettidia spp. were relatively refractory to infection. Thus, Australia possesses a number of competent mosquito species that could facilitate local transmission of WNV, should it be introduced.

The proposed release of the yellow fever mosquito, Aedes aegypti containing a naturally occurring strain of Wolbachia pipientis, a question of regulatory responsibility

In 2010 a proposal to release the yellow fever mosquito, Aedes aegypti, containing an intracellular symbiotic bacterium, Wolbachia, as a means of reducing the severity of outbreaks of dengue fever was lodged in Australia. The mosquito was infected with Wolbachia through embryonic microinjection. This proposal uncovered a gap in the regulatory process normally used to assess the release of species into Australia. Firstly, while the association between the mosquito and the bacterium was new, both species naturally occurred in Australia and so legislation governing the introduction of new species into Australia was ruled not relevant. Secondly, the infection of the mosquito with Wolbachia did not involve gene technology and so was not subject to legislation governing the approval of genetically modified organisms. The solution came through the decision to use existing legislation to regulate Wolbachia as a veterinary chemical product. This was a good outcome as it overcame the barrier that a lack of regulatory oversight may have posed to field trials taking place. Furthermore, the approach taken demonstrated a very high level of scrutiny with regard to biosafety. This case is an example of how science is leading to advances that outstrip existing regulatory frameworks. An acceptable regulatory solution has been found, but the novelty of the science is such that the appropriateness of the regulatory process now needs to be reviewed to ensure that it is no more onerous for both the proponents and the regulators than it needs to be.

Emerging tropical diseases in Australia. Part 4. Mosquitoborne diseases.

Abstract Mosquito-borne diseases continue to be a serious public-health concern in Australia. Endemic alphaviruses (including Ross River and Barmah Forest viruses) account for the majority of the arboviral notifications, while some flaviviruses (Murray Valley encephalitis, Japanese encephalitis and Kunjin viruses) cause occasional outbreaks of encephalitis. Dengue epidemics are increasing in frequency in northern Queensland, with the largest outbreak in 50 years occurring during the 2008–2009 wet season. Of great concern are the threats posed by the importation of exotic arboviruses, such as West Nile, chikungunya and Rift Valley fever viruses, the introduction of exotic vectors, and the potential range expansion of key Australian vectors. Environmental and anthropogenic influences provide additional uncertainty regarding the future impact of mosquito-borne pathogens in Australia. This review discusses the trends, threats and challenges that face the management of mosquito-borne disease in Australia. Topical mosquito-borne pathogens of biosecurity and public-health concern, and the potential impacts of environmental and global trends, are discussed. Finally, a short overview of the public-health response capability in Australia is provided.

Arboviruses isolated from mosquitoes collected from urban and peri-urban areas of eastern Australia.

Abstract To determine the presence of arboviruses in mosquito populations from major urban areas of eastern Australia, a total of 67,825 mosquitoes, representing ∼60 species, was collected and tested from Cairns, Brisbane, and Sydney between January 2005 and April 2008. Mosquito pools were screened by inoculation onto mosquito cell cultures and the detection of viral antigen using a panel of flavivirus and alphavirus monoclonal antibodies in an enzyme-linked immunosorbent assay. Suspect positive samples were confirmed using virus-specific real-time reverse transcriptase–polymerase chain reaction assays. No flaviviruses were detected, but 2 alphaviruses were isolated from mosquito pools collected from Cairns, with 1 Barmah Forest virus isolate from a pool of 100 Aedes vigilax and 1 Ross River virus isolate from a pool of 83 Verrallina carmenti. In addition, a single Aedes alternans collected from Sydney yielded an isolate most similar to Stretch Lagoon virus, a newly described virus from the genus Orbivirus. These results suggest that during the study, arboviruses were circulating at a low level in the areas sampled. The findings from this study will promote public health awareness of the risk of arboviruses in urban areas, leading to more informative public health campaigns to safeguard the Australian public.

A novel insect-specific flavivirus replicates only in Aedes-derived cells and persists at high prevalence in wild Aedes vigilax populations in Sydney, Australia

To date, insect-specific flaviviruses (ISFs) have only been isolated from mosquitoes and increasing evidence suggests that ISFs may affect the transmission of pathogenic flaviviruses. To investigate the diversity and prevalence of ISFs in Australian mosquitoes, samples from various regions were screened for flaviviruses by ELISA and RT-PCR. Thirty-eight pools of Aedes vigilax from Sydney in 2007 yielded isolates of a novel flavivirus, named Parramatta River virus (PaRV). Sequencing of the viral RNA genome revealed it was closely related to Hanko virus with 62.3% nucleotide identity over the open reading frame. PaRV failed to grow in vertebrate cells, with only Aedes-derived mosquito cell lines permissive to replication, suggesting a narrow host range. 2014 collections revealed that PaRV had persisted in A. vigilax populations in Sydney, with 88% of pools positive. Further investigations into its mode of transmission and potential to influence vector competence of A. vigilax for pathogenic viruses are warranted.

Emerging tropical diseases in Australia. Part 1. Leptospirosis.

Abstract Human leptospirosis is a zoonotic disease of global importance that causes significant morbidity and mortality, particularly in developing nations. In this review, the history, epidemiology, transmission, clinical presentation and treatment of this disease, and its impact in Australia, are discussed. Central to this review is the delineation of diagnostic methods for the disease and the challenges that this disease presents for both the clinician and diagnostic laboratory. This information should furnish clinicians with an updated tool to help overcome a number of problems associated with the diagnosis of leptospirosis.

Expectoration of flaviviruses during sugar feeding by mosquitoes (Diptera : Culicidae)

Abstract Biological transmission of arboviruses to a vertebrate host occurs when virions are expelled along with saliva during blood feeding by a hematophagous arthropod. We undertook experiments to determine whether mosquitoes expectorate flaviviruses in their saliva while sugar feeding. Batches of Culex annulirostris Skuse and Culex gelidus Theobald (Diptera: Culicidae) were orally infected with Japanese encephalitis (family Flaviviridae, genus Flavivirus, JEV), Kunjin (family Flaviviridae, genus Flavivirus, KUNV; a subtype of West Nile virus), and Murray Valley encephalitis (family Flaviviridae, genus Flavivirus, MVEV) viruses. After a 7-d extrinsic incubation, these mosquitoes were offered sucrose meals via cotton pledgets, which were removed daily and processed for viral RNA by using real-time TaqMan reverse transcriptase-polymerase chain reaction (RT-PCR) assays. JEV, MVEV, and KUNV RNA was detected in all pledgets removed from batches of Cx. gelidus on days 7–14 postexposure. In contrast, detection rates were variable for Cx. annulirostris, with KUNV detected in 0.3 M sucrose pledgets on all days postexposure, and JEV and MVEV detected on 57 and 50% of days postexposure, respectively. Higher concentrations of sucrose in the pledget did not increase virus detection rates. When individual JEV-infected Cx. gelidus were exposed to the sucrose pledget, 73% of mosquitoes expectorated virus with titers that were detectable by TaqMan RT-PCR. These results clearly show that flaviviruses are expectorated by infected mosquitoes during the process of sugar feeding on artificial pledgets. Potential applications of the method for arboviral bioassays and field surveillance are discussed.

Emerging tropical diseases in Australia. Part 5. Hendra virus

Abstract Hendra virus (HeV) was first isolated in 1994, from a disease outbreak involving at least 21 horses and two humans in the Brisbane suburb of Hendra, Australia. The affected horses and humans all developed a severe but unidentified respiratory disease that resulted in the deaths of one of the human cases and the deaths or putting down of 14 of the horses. The virus, isolated by culture from a horse and the kidney of the fatal human case, was initially characterised as a new member of the genus Morbillivirus in the family Paramyxoviridae. Comparative sequence analysis of part of the matrix protein gene of the virus and the discovery that the virus had an exceptionally large genome subsequently led to HeV being assigned to a new genus, Henipavirus, along with Nipah virus (a newly emergent virus in pigs). The regular outbreaks of HeV‐related disease that have occurred in Australia since 1994 have all been characterised by acute respiratory and neurological manifestations, with high levels of morbidity and mortality in the affected horses and humans. The modes of transmission of HeV remain largely unknown. Although fruit bats have been identified as natural hosts of the virus, direct bat–horse, bat–human or human–human transmission has not been reported. Human infection can occur via exposure to infectious urine, saliva or nasopharyngeal fluid from horses. The treatment options and efficacy are very limited and no vaccine exists. Reports on the outbreaks of HeV in Australia are collated in this review and the available data on the biology, transmission and detection of the pathogen are summarized and discussed.