Ectoparasite impacts on Gerbillus andersoni allenbyi under natural conditions

To assess ectoparasite impact on individuals and populations of Gerbillus andersoni allenbyi under natural conditions, I addressed the following questions. Do ectoparasites affect their host fitness and, if so, how? Do ectoparasites affect host population level? Does this parasite-host interaction support the traditional concept of parasite evolution towards avirulence? For this purpose, host infestation, infection, survival, haematological indices, and physical condition were recorded. A field experiment which included manipulating host infestation while recording host survival was conducted to determine the causal relations between these variables. G. a. allenbyi was infested by 2 fleas (Synosternus cleopatrae and Stenoponia tripectinata), 5 mesostigmatid mites (Androlaelaps centrocarpus, A. hirsti, A. insculptus, A. marshalli and Hirstionyssus carticulatus), 1 tick (Rhipicephalus sanguineus), and 1 louse (polyplax gerbilli). Ectoparasite burden significantly reduced host survival and red blood cell indices (red cell concentration, haemoglobin concentration and haematocrit). Ectoparasite burden did not significantly affect white blood cell concentration. Gerbils were not infected by haemoparasites or gut helminths which potentially could cause anaemia. The causal relationship between S. cleopatrae burden and host survival was established by manipulation of host infestation. Both ectoparasite removal and initial level of infestation significantly affected host survival. Ectoparasites that caused anaemia were not associated with host physical condition (PC), measured as weight/length3. None of the red blood cell indices was correlated with host PC. These results suggest both that host PC was not affected by ectoparasite burden and that exsanguination leading to anaemia was the main effect of the ectoparasites. Ectoparasite pressure on the host population (based on the ectoparasite effects as estimated by statistical models, combined with dispersion of the infestation within the host population) changed seasonally and was greatest when host density was the highest. A large segment of the gerbil population was affected by ectoparasites during the entire year. An explanation for the evolution of parasite virulence, contrasting parasites that evolve towards increased virulence with parasites that evolve towards avirulence is presented. This classification is primarily based on whether parasite impact is equated with a higher efficiency of host exploitation, or whether it is a 'side effect' of parasite biology.