The role of diseases in mass mortality of wood lemmings (Myopus schisticolor)

Sammanfattning: Like many other rodent species, the wood lemming (Myopus schisticolor) exhibits cycles with large population peaks but unlike most other European rodents, these peaks are frequently followed by mass mortality. It is largely unknown what pathogens wood lemmings carry and if pathogens could affect their population dynamics. Pathogens together with predation, climate and food supply are the main suggested determining factors of population dynamics of small mammals. Here, I tested if pathogens and diseases might be involved in recent mass mortality events of the wood lemming. Therefore, I studied in total 223 wood lemmings from two areas in Värmland and Västerbotten county, Sweden. Samples originated from individuals that were found dead during population outbreaks in 2014 and 2017 together with snap-trapped individuals from 1995-2014. Liver and lung tissue were dissected from each individual and analysed in a laboratory for five pathogens known to be carried by and affecting rodent hosts: Arenavirus (AV), Ljunganvirus (LV), Tick-borne encephalitis virus (TBEV), Cowpox virus, Puumala orthohantavirus (PUUV) and additionally a broad covering Pan-orthohantavirus test. The prevalence of Puumala orthohantavirus in the sampled population was 4.5-9% while the analysis for Pan-orthohantavirus showed positive for 26.9% of the total sample. None of the individuals tested positive for AV, LV, TBEV, or Cowpox. The probability of infection with Pan-orthohantavirus increased with body weight of the individuals but no difference between sexes was found. A spleen-somatic index was used to test the hypothesis that this organ could be used as an indicator of disease, as it is largely connected to the immune system. The probability of infection increased with an increase in spleen index and there was a significantly higher spleen index seen in individuals testing positive for Pan-orthohantavirus. Wood lemmings in Värmland county showed a higher spleen index than animals in Västerbotten and the index was higher in individuals caught in traps than in individuals found dead. The spleen index revealed some interesting patterns, but more research is needed to reveal and understand actual causal relationships between pathogen infections and spleen index. None of the studied pathogens caused the observed mass mortality of wood lemmings. In this study, I was able to study a limited number of pathogens. Hence, I cannot refute the disease hypothesis. Instead, future studies focusing on the disease hypothesis should include other and potentially more fatal pathogens for wood lemmings.