Emerging tick-borne pathogens: on the ecology of multiple infections in ticks and reservoir hosts
Abstract: Most animals will encounter several more or less severe infectious diseases during their lifetime, and simultaneous infections with more than one pathogen, or several different strains of the same pathogen, are common in natural populations. Ticks transmit a wide variety of different pathogens and can also be simultaneously infected with more than one pathogen. The possible interactions between different co-infecting pathogens in animals and ticks are in many cases incompletely known. Here, I investigated the occurrence and distribution of Borrelia afzelii strains in one of their reservoir hosts, the bank vole (Myodes glareolus) and found an aggregation of strains in individual hosts suggesting that there is no strong competition between strains. The infection intensity (amount of bacteria) did not either indicate competition between strain, instead the infection intensity increased with the number of strains in an additive way. Genetically more distant strains were more often found together, indicating that diversity can be sustained by selective pressure favoring dissimilar strains. Furthermore, multiple infections with several strains seem to occur during a limited amount of time in early infections. Besides Borrelia, also other pathogens occur in ticks and rodents. I show that the newly recognized human pathogen ‘Candidatus Neoehrlichia mikurensis’ is widespread and common in rodent populations in Southern Sweden. Furthermore, we show that the prevalence in ticks is around 6% and that co-infection with both ‘Candidatus N. mikurensis’ and B. afzelii occurs in about 2% of ticks in Southern Sweden. The rate of co-infection is significantly higher than expected from a random distribution. The aggregated distribution of these two pathogens in ticks is explained by the aggregated distribution in their reservoir hosts, which in turn can be caused by positive interactions or by variation in general sensitivity in the rodent population. To conclude, positive interactions between both strains and different species of tick-borne pathogens might be more important than previously recognized.
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