Dissertation: Rodents amplify, cervids dilute: Host contributions to Lyme disease risk (Kiran)
More than half of the human population in Finland lives in areas where forests, deer and rodents also thrive, creating conditions that support the spread of tick-borne diseases such as Lyme borreliosis. This disease, caused by Borrelia burgdorferi sensu lato bacteria and transmitted by Ixodes ricinus ticks, has increased significantly in Finland in recent decades. Understanding how different wildlife species contribute to Borrelia transmission is essential for predicting and managing disease risk.
The host drives tick infection risk
Rodents, especially bank voles, were found to be key amplification hosts, meaning that areas with more rodents had a higher prevalence of Borrelia infection in ticks. Cervids, on the other hand, increased the total number of ticks but were associated with a lower proportion of infected ticks, indicating a dilution effect where non-competent hosts reduce transmission.
- Rodents clearly enhance the infection hazard by increasing the number of infected ticks, but cervids have the opposite effect, says Doctoral researcher Nosheen Kiran from the University of Jyväskylä.
Even when rodent numbers were experimentally reduced, Borrelia persisted in the environment.
- This means the pathogen can be maintained by multiple hosts, making control more complex, explains Kiran.
The ecological puzzle of Lyme borreliosis
The study found higher Borrelia infection in urban forests, but limited TLR2 genetic variation showed no link to infection, suggesting that ecological factors and other immune genes play a bigger role in shaping disease risk.
Wild host carry diverse Borrelia bacteria
Using Oxford Nanopore MinION sequencing, Kiran’s study found that ticks and bank voles in the wild carry diverse strains of Borrelia afzelii, with multiple infections commonly occurring in the same host.
- Overall, these results deepen our understanding of how rodents and cervids influence Lyme borreliosis risk in Finland and highlight the complexity of disease ecology in northern boreal environments, sums Kiran.
M.Sc Nosheen Kiran defends her doctoral dissertation "Ecology of Borrelia in the boreal forests: the role of rodent abundance and genetics” on Friday November 7, 2025 at 12.00 o’clock in the lecture hall KEM4 on the Ylistönrinne campus at the University of Jyväskylä. Opponent is Professor Annapaola Rizzoli (Fondazione Edmun Mach) and custos is Senior Lecturer Eva Kallio (University of Jyväskylä). The language of the event is English.
The dissertation "Ecology of Borrelia in the boreal forests: the role of rodent abundance and genetics” will be available in the JYX publication archive: https://jyx.jyu.fi/jyx/Record/jyx_123456789_106347?sid=221433465
Further information:
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Doctoral researcher Nosheen Kiran, Kiran, nosheen.n.kiran@jyu.fi
