Parasites live on or inside their hosts and usually cause fitness loss, sometimes even lead to outbreaks of emerging infectious disease. Understanding how a parasite can infect its hosts (i.e., host specificity of the parasite) is important to both basic studies on disease evolution and the whole ecosystem health.
In this thesis, I use avian haemosporidians, a group of species-rich protozoans as a model system to study host specificity of parasites, because of their high diversity in geographical distributions and host ranges.
Studying host specificity of avian haemosporidians is facing two main challenges. First, it is essential to have a method that accurately defines the species limits of the parasites, which cannot be done by current identification methods. In some cases, a presumed generalist parasite that appears to infect multiple host species may have already adapted separately to different hosts, with the consequences that gene flow is restricted between them, and thus should be considered as reproductively isolated parasite species. In order to detect these cryptic species, reliable molecular markers are required. But genomic sequencing of avian haemosporidians is challenging since they coexist with hosts whose genome size is 50 times larger, causing an excessive amount of contamination in the yielded sequences. Second, the measurement of host specificity is in itself a complicated exercise. To assess host specificity of a parasite, one must consider the parasite’s ability to infect its host and reproduce in the host. The associations between a parasite and its hosts may vary with biotic and abiotic environmental factors, therefore the overall infection patterns of all potential hosts should be analysed, which was not reported in previous haemosporidian studies.
In this thesis, my aim is to answer these two open questions for better understanding host specificity with a set of case studies. In Paper I a cost-effective protocol was developed to obtain multiple nuclear gene sequences throughout the genome of avian haemosporidians from DNA extracted from wild-infected bird blood without any special treatment. Based on analyses of multiple nuclear genes, the phylogenetic structures were investigated respectively for a group of sympatric parasites that present similar morphological characters in Paper II, and allopatric parasites with identical mitochondrial gene sequences in Paper III, to clarify if they should be considered as the same species or several different but cryptic species. I then tested the infection patterns of three generalist parasites in a natural community in Paper IV and found that generalist parasites are not equally adapted to all species in their host ranges but better to a smaller subset of hosts. In Paper V I further investigated the seasonal dynamics of parasite infection patterns and found that infections peaked during the main nesting season in adults and a few weeks later in juveniles, as soon as they started to be captured. Juveniles must become infected already as nestlings, thereafter the infection intensities decrease along the development of their immune systems.
In summary, the findings in this study have provided new insights for further studies on host specificity of avian haemosporidians, with newly developed approaches to define the species limits and to investigate the infection patterns of parasites.
Throughout the history, there have been frequent outbreaks of diseases transmit to humans from animals. But why can some pathogens infect humans while the others cannot?
Avian influenza and west Nile virus can transmit from birds to humans, rabies from dogs or cats; while ape malaria cannot infect humans despite its close relationship to human malaria and that the blood cells of humans are very similar with those of gorillas and chimpanzees. Why can some of the pathogens jump between birds and mammals while others are stuck to only one of many closely related species?
That leads to the concept of host specificity. Based on that, parasites can be divided into two groups, specialist parasites that can only infect one or a few host species, and generalists that can infect many different host species. When we understand the mechanisms of host specificity of parasites, it will enable us to predict the outbreaks of diseases and will therefore be important to the society and ecological health.
In my thesis I used bird blood parasites related to malaria as a model system to study the infections of generalist parasites in their different hosts. First of all, I needed to identify whether these parasites in different hosts belong to the same species. This can be done by comparing their genomic sequences.
A main challenge when studying parasites that lives inside the cells of their hosts is that the collected samples mainly contain materials from the hosts. As the parasites often have simplified structures and small amount of nuclear materials, it is difficult to separate them from the host. To reduce contaminations from the host, I developed a protocol to capture the parasite DNA from the samples and wash away that from the host.
Then I sequenced a group of similar parasites that infect different hosts in different continents. I found that in some cases the parasites with similar microstructures were different on the molecular level and thus should be considered as different species.
In another study I detected three generalist parasites that can infect multiple host species. I tested their infections in different hosts and found significant differences among hosts. The parasites can reach much higher infection levels in a few main host species than in the others. In hosts that were closely related, the infection levels were similar. Birds belonging to the main host species seemed to become infected already in the nest, but then gradually managed to control the parasite and kept the intensity at a stable low level.
The findings in this thesis have provided new approaches for studying the evolution of host-parasite associations and further predict disease outbreaks.