Ecology of co-infection in a tick-borne pathogen

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Many pathogen populations consist of genetically distinct strains. In these systems, the hosts are often infected with multiple strains, a phenomenon referred to as mixed infections or co-infections. In mixed infections, the pathogen strains can interact with each other and these interactions could be cooperative or competitive. In pathogen populations where mixed infections are common, it is critical to study how interactions between strains influence their transmission, frequency, and co-existence. In my previous research, I used the multi-strain tick-borne bacterium, Borrelia afzelii, to study how populations of pathogen strains persist in nature. Using a long-term field study, I showed that the community of strains was stable over a decade, and that some strains were always more common than others. Using experimental infections, I showed that variation in fitness among pathogen strains in the lab explained differences in frequencies among strains in the field. This raises the question: "Why do the common and fit strains not eliminate the rare and weak ones?" To answer this question, we must study interactions between strains across the pathogen life cycle. The long-term goal of my research is to understand the population dynamics of multi-strain pathogens. This NSERC DG contains 3 objectives. Non-transitive competitive interactions (when genotypes cannot be ranked in a hierarchy) are important for biodiversity, but have been ignored in multi-strain pathogens (Objective 1). If pathogens have complex life cycles where strains interact in multiple host types, it is essential to study this complexity (Objective 2). Finally, host immunity is known to influence competition between strains, but less appreciated is that a competent immune system can facilitate co-infection, especially when a late-arriving strain has to invade a host that is already infected with a resident strain (Objective 3). We will use experimental infections with the multi-strain tick-borne bacterium B. burgdorferi to test whether non-transitive competitive interactions between strains in the rodent host (Objective 1) and the tick vector (Objective 2) influence host-to-tick and tick-to-host transmission, respectively, and whether a competent host immune system allows a late-arriving strain to infect a previously infected host (Objective 3). This research will advance our understanding of how interactions between pathogen strains can facilitate their co-existence in natural populations. The outcomes from this research will be important to the fields of evolutionary ecology, microbiology, and the ecology of infectious diseases. From an HQP training perspective, tick-borne diseases are a growing problem for human and animal health in Canada. The HQP trained by this NSERC DG will develop valuable skills and knowledge that will make them highly competitive for jobs in government, industry, and academia.