Impact of mixed species infection on trypanosome virulence and transmission
Animal African trypanosomes are protozoan parasites that cause nagana, a devastating disease of livestock in sub-Saharan Africa. It is well documented that strains of one causative agent, Trypanosoma congolense (subtype: Savannah), exhibits strikingly different virulence profiles in the hosts that they infect – measured in terms of host survival - though the molecular basis of these different phenotypes are not known. This parasite is also known to be found in mixed infections with other African trypanosomes, namely T. brucei and T. vivax. Such mixed infections, or coinfections, affect both the host and parasite, and are modulated by a number of factors, including inter-strain or inter-species competition. Limited work has been done to study how such competition may impact upon parasite virulence, or transmission potential to a new host. To this end, we sought to generate genome and transcriptome data for different T. congolense field strains and identified differences in the predicted protein sequences between these lines. Transcriptome data also revealed differentially expressed transcripts between strains of high virulence and those of low virulence. Next, we utilised field- and laboratory studies to investigate the interaction between coinfecting African trypanosomes. Surveys from the field indicated that T. congolense, T. vivax, and T. brucei indeed infect cattle in the same geographical area. This screen, using two different assays, was capable of detecting trypanosome coinfections, although mixed infections between African trypanosome species were not observed. In vivo experiments in mice highlighted the interactions that occur between T. congolense and T. brucei. First, it was shown that a T. congolense field strain (MF1 CL1) could drive T. brucei EATRO 1125 PFR-Ty to become cell-cycle arrested, and pre-adapted for transmission. Secondly, we observed a dynamic cycling in parasitaemia between the two species within the same host over the course of a chronic infection – only one species peaked at a given time. Furthermore, T. brucei EATRO 1125 PFR-Ty that re-emerged after a peak of T. congolense IL3000 parasitaemia comprised of a high proportion of PAD1+ cells. Taken together, these data indicate competition avoidance between these conspecifics. Thirdly, a line of T. brucei that was retrieved from a chronic coinfection, exhibited enhanced virulence when compared to the original line. Mice infected with this chronic line of parasites showed significant weight-loss, and immunofluorescence assays highlighted a reduced capacity to form stumpy cells in this group. These data suggest selection for more virulent cells, which could enhance transmission. Finally, we generated a luciferin-reporter line of T. brucei (AMLuc 4.2), to investigate competition avoidance between this species, and T. congolense IL3000. This newly-generated line of parasites was then used to establish a model for in vivo imaging of trypanosomes in a live host, and the ex vivo imaging of infected organs.