[Parasite-host relations of fleas and rodents]

Not infrequently lice can reach great abundance on rodents. The distribution of lice on animals is uneven. The infection with lice depends not only on the physiological and ecological properties of the host but also on the environmental conditions. The distribution of lice on rodents can be affected by the development of transport communications. If a population of rodents is parasitized by two species of specific lice, they belong to different genera. On single animals only one species of lice occurs as a rule; if two species occur, then one of them is dominant. In this way decreases the interspecific competition between lice. Seasonal and age changes in lice infection are associated with those in host's biology and behaviour under certain climatic conditions. It is clearly displayed in rodents breeding once a year. The infection with lice increase with the rise in rodents abundance. Along with the confirmation of the general rule of poor infection with specific parasites at the borders of the species distribution a case of high infection with lice at the border of the host's distribution was noted. In addition to specific lice, alien species are recorded on small mammals which appear due to exchange of parasites between hosts. Examples are given all conquering new hosts by lice; as a result close species of lice can parasitize phylogenetically distant hosts.