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Midges are small mosquitoes that can transmit pathogens to susceptible hosts through their blood-sucking act. They are known as biological vectors that can transmit the bluetongue virus (BTV) and the Schmallenberg virus (SBV) to ruminants, among others. Various vector control measures can be used to curtail the spread of the virus during an epidemic. However, for effective vector measures, it is essential to have profound knowledge of the role of biting midges as vectors, as well as their biology and phenology. For several years, midges were not in the focus of research and there are still considerable gaps in knowledge. Therefore, the present work examines various aspects of biting midges of the genus Culicoides, whose function as vectors of the Schmallenberg virus was already proven at the beginning of the project.
The aim of the first part of this work was to determine the percentage of infected midges in various German areas in order to determine the influence of Culicoides midges within the virus epidemic. For this purpose, samples, collected during 2011 and 2012 as part of monitoring projects, were analysed. Additionally, in early 2013, various farms in southern and eastern regions of Germany, where SBV was considered to be largely absent, were equipped with UV traps. The small number of virus-positive samples did not allow a more precise assessment of the viral spread in culicoid midges. Instead, it revealed the importance to conduct targeted samplings of its vectors during an acute outbreak. Additionally, the presented results and statements made by several animal owners, gave reason to believe, that SBV must have affected the southern and eastern parts of Germany earlier than actually assumed. This would consequently have led to an increased immunity in host animals, which provides a reasonable explanation for the low positive values and is in agrement with the statements made by various farmers.
The second part of this work identifies the conditions and surrounding factors under which acute SBV diseases emerged in ruminants in the cold winter months of 2012/2013. After the diagnosis of several acute SBV infections of sheep in a sheep pen in Mecklenburg-Western Pomerania, culicoid midge activity could be proven. This demonstrates that, suitable conditions for its vectors given, an infection of SBV can also take place during wintertime. A more detailed analysis of the surrounding conditions revealed, that the outdoor temperatures during infection were consistently at values of at least 5-9 ° C for several consecutive days, which enabled the flight and blood-sucking activity of the midges within
the shelter.
Midge activity during wintertime represents a crucial component in understanding how the virus can outlast the cold season. A constant midge presence could lead to a low but permanent infection rate throughout the cold months, enabling a recurrence of the pathogen the following year. Instead, a longer vector-free timeperiod would point to other mechanisms that allow the virus to re-occur in Germany on a yearly basis. Thus, the acute cases of SBV infections in sheep rose the question of critical threshold temperatures, representing the beginning of midge activity. The investigation of several stables sheltering cattle, horse or sheep addressed potential differences between indoor and outdoor activity and whether the type of host animal has an influence on the beginning of the flight. In the third part of this work, a long vector-free period and several differences in the onset of midge activity between different types of host animals could be detected. It could also be illustrated that the progression of the flight began differently depending on the present type of host animal/type of stable. For all cattle stables and the sheep barn the first midge activity was measured indoors, whereas for horses, culicoid midges were found to become active either at the same time or almost simultaneously inside and outside the animal shelters. This suggests that the horse stables do not represent good breeding sites for midges, which might be attributed to husbandry practices. In addition, it was possible to determine specific threshold temperatures for the different types of host animals and for various midge species. Altogether, the late beginning of flight, measured at the beginning of March, was surprising. This raises more questions of alternative mechanisms enabling the virus to outlast the winter months. The documentation of species-specific threshold temperatures can be a useful tool f.i. within automated large stables to keep indoor temperatures under the threshold value in order to postpone the onset of culicoid activity of various vector species. This may help to prevent virus transmissions during winter or to evoke a delay in spring, making it more difficult for the virus to overwinter.
To be able to start instant defense measures during an ongoing virus epidemic, which is transmitted by Culicoides midges, reducing the ground-living midge larvae offers a promising option during the warm season. For targeted vector control measures, it is important to know the breeding sites of culicoid midge species. Therefore, four agriculturally used biotopes were sampled and compared to four biotopes of a forest-dominated area. The results clearly show that meadows per se are not suitable breeding habitats for Culicoides
spp. Only the influence of livestock animals induces their potential as developmental sites. The various biotopes of the forest-dominated region were less subject to anthropogenic influences. Although fewer individual midges were found here, it displayed a higher biodiversity than the agricultural habitats. These results demonstrate once more the potential of forests in regards to the preservation of biodiversity. In Particular, the alder on fen site revealed most midge species and also the highest number of collected specimens among the studied biotopes. That illustrates the high impact of this specific humid type of habitat in respect to species diversity and the need of its perpetuation.
As part of this work, new breeding sites for a variety of culicoid species were identified and assigned to the usually rather short profiles of known Culicoides species. For one part, previous observations of chosen substrates could be consolidated. Furthermore, new breeding substrates were identified. Additionally, information of abiotic factors such as ph-value, soil moisture or organic compound of all sampled breeding substrates obtained from a soil analysis, extended the knowledge about the species-specific choice of breeding habitats and their characteristical traits. The additional knowledge about potential breeding substrates and their soil factors might be useful for future epidemiological modelling approaches. It can also raise the effectiveness and accuracy of targeted vector control measurements during an epidemic outbreak. Therefore, it may indirectly contribute to the preservation of endangered rare species. However, there is still an enormous need for more research before this goal can be fully achieved.