MicrobiologyBytes

Aedes (Stegomyia) aegypti (Linneaus) is an important vector of dengue and other arboviruses. Despite its limited flight dispersal capability, its close association with humans and its desiccation-resistant eggs have facilitated many long distance dispersal events within and between continents, allowing it to expand its range globally from its origin in Africa. Its global emergence and resurgence can be attributed to factors including urbanisation, transportation, changes in human movement, and behaviour, resulting in dengue running second to malaria in terms of human morbidity and mortality. Global historical collections and laboratory experiments on this well studied vector have suggested its distribution is limited by the 10°C winter isotherm, while a more recent and complex stochastic population dynamics model analysis suggests the temperature’s limiting value to be more towards the 15°C yearly isotherm. While historical surveys in Australia have indicated that Ae. aegypti occurred over much of the continent, its range has receded from Western Australia, the Northern Territory and New South Wales (NSW) over the last 50 years. It is now only found in Queensland, although recent incursions into the Northern Territory have required costly eradication strategies. The significant reduction in vector distribution has been attributed to a combination of events including the introduction of reticulated water, which reduced the domestic water storage requirements of households that had provided stable larval sites, as well as the removal of the railway-based water storage containers hypothesised as being responsible for the long distance dispersal.

“Drought-proofing” Australia’s urban regions by installing large domestic water tanks may enable the dengue mosquito Ae. aegypti to regain its foothold across the country and expand its range of possible infections. A new paper challenges the common assumption that climate change will drive the spread of this mosquito, suggesting instead that the real driver is human behavior. The study combines current and forecasted climate change conditions with historical epidemics to reveal the risk of dengue infections in all capital cities around Australia by 2050. Researchers developed and critically assessed their models to project the distribution of the mosquito in 2030 and 2050. Currently, dengue fever occurs in Queensland only. However, the implementation of new water tanks, combined with already warm summer temperatures, could enable the mosquito to re-emerge and further its current reach. Dengue risks will not be driven directly by warmer temperatures or changes in rainfall patterns. Australian summers already provide ideal conditions for dengue transmission around the country, but the introduction of government-subsidized water storage devices now adds the ideal breeding ground for the dengue mosquito to re-emerge. While research is properly focused on the impact of anthropogenic climate change, this study highlights the need to look also at our responses to those changes and the outcomes they generate. The current dengue fever epidemic in far north Queensland is approaching 1,000 reported cases over the summer of 2008-2009.

Australia’s Dengue Risk Driven by Human Adaptation to Climate Change. 2009 PLoS Negl Trop Dis 3(5): e429
The reduced rainfall in southeast Australia has placed this region’s urban and rural communities on escalating water restrictions, with anthropogenic climate change forecasts suggesting that this drying trend will continue. To mitigate the stress this may place on domestic water supply, governments have encouraged the installation of large domestic water tanks in towns and cities throughout this region. These prospective stable mosquito larval sites create the possibility of the reintroduction of Ae. aegypti from Queensland, where it remains endemic, back into New South Wales and other populated centres in Australia, along with the associated emerging and re-emerging dengue risk if the virus was to be introduced. Having collated the known distribution of Ae. aegypti in Australia, we built distributional models using a genetic algorithm to project Ae. aegypti’s distribution under today’s climate and under climate change scenarios for 2030 and 2050 and compared the outputs to published theoretical temperature limits. Incongruence identified between the models and theoretical temperature limits highlighted the difficulty of using point occurrence data to study a species whose distribution is mediated more by human activity than by climate. Synthesis of this data with dengue transmission climate limits in Australia derived from historical dengue epidemics suggested that a proliferation of domestic water storage tanks in Australia could result in another range expansion of Ae. aegypti which would present a risk of dengue transmission in most major cities during their warm summer months. In the debate of the role climate change will play in the future range of dengue in Australia, we conclude that the increased risk of an Ae. aegypti range expansion in Australia would be due not directly to climate change but rather to human adaptation to the current and forecasted regional drying through the installation of large domestic water storing containers. The expansion of this efficient dengue vector presents both an emerging and re-emerging disease risk to Australia. Therefore, if the installation and maintenance of domestic water storage tanks is not tightly controlled, Ae. aegypti could expand its range again and cohabit with the majority of Australia’s population, presenting a high potential dengue transmission risk during warm summers.

Related:

• Pathogenesis and prevention of dengue virus infection
• Dengue Virus
• Clustering of dengue virus infections
• Dissecting the Cell Entry Pathway of Dengue Virus
• A new Arenavirus is responsible for deaths of three transplant recipients in Australia

The spread of dengue (DEN) worldwide combined with an increased severity of the DEN-associated clinical outcomes has made this mosquito-borne virus of great global public health importance. This article reviews the latest findings in the field of DEN pathogenesis, vector control and vaccine development. Atypical clinical manifestations as well as a shift in the age of DEN-infected patients have forced a revisit of the current definitions and classifications of the DEN-associated diseases. The antibody-mediated internalization of DEN virus triggers intracellular pathways that further enhance viral replication and output. A correlation between the human leukocyte antigen system and disease severity was found. The development of tetravalent DEN vaccine candidates is reaching its final stages, though the antibody levels required to protect against the four DEN serotypes are still unknown. Efforts have been devoted to identify the host target cells that are permissive or resistant to DEN virus replication and to decipher the downstream events that would result in either disease enhancement or protection. Strong emphasis has recently been made on genetic polymorphisms that confer DEN protection versus susceptibility to severe DEN. The increasing knowledge gained from basic science should help to better design effective DEN vaccines.

Pathogenesis and prevention of dengue virus infection: state-of-the-art. Curr Opin Infect Dis. Mar 2nd 2009

Related:

• Dengue Virus
• How mosquitoes survive dengue virus infection
• Clustering of dengue virus infections
• Dissecting the Cell Entry Pathway of Dengue Virus

Dengue fever and Dengue hemorrhagic fever have emerged as some of the most important mosquito-borne virus diseases in the tropics. No effective vaccine or antiviral drug therapy is currently available against Dengue viruses and the mechanisms of pathogenesis in Dengue infections remain elusive. Recently, virus-induced apoptosis mediated by the Unfolded Protein Response (UPR, or ER stress response) has been hypothesised to represent a crucial pathogenic event in virus infection. In Dengue virus infection, cells elicit an UPR which is observed at the level of translation attenuation (seen by phosphorylation of eIF2alpha) and activation of other specific pathways. Interestingly, specific serotypes of Dengue virus modulate the UPR with different selectivity. Perturbation of the UPR by preventing the dephosphorylation of the translation initiation factor eIF2alpha considerably alters virus infectivity. This report provides evidence that Dengue infection induces and regulates the three branches of the UPR signaling cascades. This is a basis for understanding of Dengue virus regulation and conditions beneficial to virus infection. Furthermore, modulators of UPR such as Salubrinal that inhibit Dengue replication may open up avenues for anti-viral therapy.

Dengue virus serotype infection specifies the activation of the unfolded protein response
Virology Journal 2007 4: 91