Long-term variation in an occurrence of Rhabditis orbitalis parasitic larvae (Nematoda, Rhabditidae) in the eyes of montane rodents

During a long-term survey (1999–2005) of parasitic larvae of nidicolous nematode Rhabditis orbitalis Sudhaus et Schulte, 1986 in the West Tatra Mts, the nematodes were detected in the eye orbits of snow voles Chionomys nivalis in 1999, 2004 and 2005. In the last year, R. orbitalis was detected also in another vole species Clethrionomys glareolus and Microtus tatricus both in previously studied site and in the Low Tatra Mts, however, it has never appeared at other rodent species. For the first time, an irregular occurrence of the R. orbitalis parasitic larvae has been ascertained in natural conditions of the Slovak mountains. The eye nematodes occurred only during autumn or cold and rather wet summer months, when density of the preferred host Ch. nivalis was relatively low. A hypothesis has been put forward that an occasional appearance of parasitic eye larvae in the life cycle of bacteriophagous R. orbitalis represents a strategy for surviving periods of a scarcity of bacterial food, which could be influenced by a combination of weather conditions, density of host rodents and seasonal abundance of R. orbitalis in rodent nests.