Interpopulation variation in the prevalence and intensity of parasitic mite infection in the land snail Arianta arbustorum

The parasitic mite Riccardoella limacum sucks blood in the lung of its host, the land snail Arianta arbustorum. The infection of various host populations was examined in Switzerland. In a lowland snail population, prevalence of infection did not vary among seasons. However, intensity of mite infection in dissected individuals of A. arbustorum was high in autumn, but low in winter and spring when ≤100 mite eggs were found attached to the lung epithelium. A novel, non-invasive parasite screening method was used to estimate the number of mites on living host snails. An analysis of repeatability revealed that 92.9% of the snails were correctly classified as infected or non-infected with this non-invasive method. Prevalence of mite infection was examined in 997 adults of A. arbustorum from 11 natural populations distributed over an altitudinal gradient ranging 335–2360 m. No infected snails were found in 7 populations, while in the remaining 4 populations the prevalence of mite infection ranged 45.8–77.8%. Intensity of infection also differed among the 4 host populations. No geographic pattern in prevalence of infection was found. However, parasitic mites did not occur in snail populations situated at elevations of 1290 m or higher. A possible explanation for this finding could be that the host's hibernation period may be too long at high elevations for mites and their eggs to survive. At low elevations, other factors may affect the presence of R. limacum in the lung of A. arbustorum.