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Background: Histomonas meleagridis is a protozoan parasite and the causative agent of histomonosis, an important poultry disease whose significance is underlined by the absence of any treatment and prophylaxis. The recent successful in vitro attenuation of the parasite urges questions about the underlying mechanisms. Results: Whole genome sequence data from a virulent and an attenuated strain originating from the same parental lineage of H. meleagridis were recruited using Oxford Nanopore Technology (ONT) and Illumina platforms, which were combined to generate megabase-sized contigs with high base-level accuracy. Inspecting the genomes for differences identified two substantial deletions within a coding sequence of the attenuated strain. Additionally, one single nucleotide polymorphism (SNP) and indel targeting coding sequences caused the formation of premature stop codons, which resulted in the truncation of two genes in the attenuated strain. Furthermore, the genome of H. meleagridis was used for characterizing protein classes of clinical relevance for parasitic protists. The comparative analysis with the genomes of Trichomonas vaginalis, Tritrichomonas foetus and Entamoeba histolytica identified ~ 2700 lineage-specific gene losses and 9 gene family expansions in the H. meleagridis lineage. Conclusions: Taken as a whole, the obtained data provide the first hints to understand the molecular basis of attenuation in H. meleagridis and constitute a genomics platform for future research on this important poultry pathogen. [ABSTRACT FROM AUTHOR]

In the present work, chickens and turkeys were infected with virulent or attenuated Histomonas rneleagridis to investigate and compare the effect of both isolates on birds. Thereby, histomonads of a clonal culture were propagated in vitro either for a short period of time (21 passages) to preserve virulence or for 295 passages to achieve attenuation. On the first day of life birds of each species were infected with either virulent or attenuated parasites. Throughout the experiment, all birds were examined daily for clinical signs attributable to the infection. Furthermore, the excretion of viable parasites was determined after in vitro reisolation from cloacal swabs. For the investigation of pathological changes of organs a defined number of infected birds were killed on d 4, 7, 10, 14, and 21 postinfection (PT) and necropsy was performed. By this routine, changes in livers and ceca were classified by a scoring system to evaluate the severity of lesions. Samples of cedum, liver, and lung were generated and screened for the presence of parasites by PCR and immunohistochemistry. Turkeys infected with virulent histomonads showed first clinical manifestation of histomonosis on d 10 PT, whereas the remaining birds did not express clinical signs. Positive reisolations of virulent and attenuated histomonads were obtained intermittently from individual chickens and turkeys from d 2 PT until the end of the experiment. Both species of birds displayed lesions in the ceca and the liver following infection with virulent parasites, whereas no changes occurred in birds inoculated with attenuated histomonads. The PCR revealed the dissemination of virulent histomonads in ceca, livers, and lungs of some chickens and turkeys in contrast to attenuated parasites, which were exclusively found in cecal samples. The attenuated isolate of H. meleagridis did not induce clinical signs or pathological changes and offers high safety after infection of chickens and turkeys. Therefore, the in vitro attenuation and the use of avirulent histomonads represent a viable tool for vaccination against histomonosis. [ABSTRACT FROM AUTHOR]