Seite - 76 - in Biodiversity and Health in the Face of Climate Change

76 4.3.3 Host Diversity Mosquitoes and ticks feed on a wide range of hosts, and their pathogens circulate in diverse animal species. Lyme disease is caused by Borrelia burgdorferi, which is transmitted to humans through the bite of Ixodes ticks. For Borrelia burgdorferi sensu lato, a broad range of reservoir hosts have been identified, for instance nine small mammals, seven medium-sized mammals and 16 bird species in Europe (Gern et  al. 1998), and eight small mammals in the USA (summarised in Salkeld et  al. 2008). As another example, the Zika virus is known to circulate in monkey and wild mammal populations in Africa, and has been detected in domestic sheep, goats, horses, cows, ducks, rodents, bats, orangutans and carabaos in Indonesia and Pakistan (Vorou 2016). However, large knowledge gaps on the diversity of reservoir hosts for mosquito- and tick-borne pathogens in old and new areas of distribution still exist (Baráková et  al.  2018; Hashiguchi et  al. 2018). Certainly, these gaps will never be closed given the complex dynamics of adaptations between pathogens, vectors and hosts. The conceptual model of the dilution effect in Lyme disease (Ostfeld and Keesing 2012) is the textbook example about how biodiversity on the level of the host spe- cies can directly influence the transmission of an arthropod-transmitted disease. Humans are aberrant hosts for the pathogen, because the pathogens cannot replicate in humans. In a natural cycle of Borrelia burgdorferi, the bacteria are maintained by small mammals and birds (reservoir hosts). The dilution effect model in the Lyme- disease system states that the relative abundance of host individuals should be evenly distributed across host species to decrease the potential for an encounter of the tick with the most competent reservoir host. This model also applies to other vector-host systems in which a generalist vector uses many host species of which only a few are competent reservoir hosts (Swaddle and Calos 2008; Civitello et  al. 2015). The smaller the risk for a host to become infected and thereby maintain and release pathogens, the more species-diverse and abundant is the host community (Civitello et  al. 2013; Levi et  al. 2016). However, this dilution effect is still under academic debate (e.g. Civitello et  al. 2015 and Salkeld et  al. 2015), and there is an urgent need for further research. 4.4 How to  Manage VBDs? Different public health measures exist, ranging from epidemic control through vec- tor control (of mosquitoes, bugs, flies, fleas) and eradication of diseases through vaccination, case treatment and breeding site elimination (WHO 2017b). Vector control tools target specific life stages of arthropod vectors and are of a chemical (e.g. pyrethroid insecticides), biological (e.g. Wolbachia bacteria) or transgenic nature (e.g. genetically modified mosquitoes). Chemical insecticides are addition- ally used for preventive measures such as insecticide-treated bed nets, R. Müller et al.