|Tjaden, N; Thomas, S M; Fischer, D; Beierkuhnlein, C: Extrinsic incubation period of dengue: Knowledge, backlog and applications of temperature-dependence, PLoS Neglected Tropical Diseases (2013), doi:10.1371/journal.pntd.0002207|
Dengue is generally believed to be one of the most hazardous vector-borne diseases, with over 40% of the world's population being at risk of an infection . While in the past the disease has mainly been observed in the tropical regions, recent studies suggest that, under the pressure of future climate change, new areas even as far north as Europe may become endangered. In fact, in 2010 the first European cases of autochthonous dengue since the epidemic outbreak in Greece in the late 1920ies  were reported from Croatia  and France . Currently Madeira experiences an on-going epidemic of dengue fever with about 2000 cases within two months . When it comes to determine the risk of dengue to occur in a given region, the extrinsic incubation period (EIP) plays an important role. The EIP is commonly defined as “the interval between the acquisition of an infectious agent by a vector and the vector’s ability to transmit the agent to other susceptible vertebrate hosts” . The underlying concept of EIP in the case of dengue is, that after being ingested by a mosquito through a bloodmeal, some time is required for the virus to replicate, escape the midgut and spread through the mosquito’s body until it ultimately reaches the salivary glands, from where it can be passed on to another host during the next bloodmeal. For dengue, the duration of the pathogens EIP is known to be temperature-dependent but very few mechanistic risk models (usually based on the basic reproductive number R0, i.e. the number of secondary cases produced by one primary case in a completely susceptible population ) have taken that into account until now. In fact, most of those models implemented for dengue are using fixed values for the duration of the EIP or rather rough estimates of temperature dependence . This may be due to the fact that experimental studies on this topic are rare and their results may appear to some point inconsistent or even contradictory. However, the implementation of a realistic, temperature-dependent EIP will greatly improve mechanistic dengue modeling: Since it appears as an exponent in the equations used for the determination of R0 and vector capacity [7, 9, 10], even small changes of the EIP can have a large impact on the results of those models. The practical relevance of this issue has been demonstrated for dengue  as well as other vector-borne diseases such as malaria  or bluetongue . In addition, correlative models based on environmental factors and vector distributions (also referred to as climate envelope models or environmental niche models) have to be revised and enhanced. Currently, those models usually focus on the spatial distribution of vector species. But if temperatures are not supporting amplification and establishment of the virus even though the vector is present, risk assessment solely based on vector distributions leads to an over-estimation of areas at risk. Combining such models with information on temperature requirements for the virus derived from the EIP can reduce uncertainty . Here, we give a short overview on the few experimental studies that are explicitly addressing the temperature-dependence of dengue EIP. We analyze the implications of these studies and discuss current uncertainties in modeling dengue risk in face of climate change. We identify methodological challenges and formulate suggestions for the design of future studies from a spatio-ecological point of view.
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