Resolving immune traits against Histomonas meleagridis in poultry, a new Th2 model?

Histomonas meleagridis is a flagellated protozoan parasite and the aetiological agent of histomonosis, a disease that occurs worldwide, mainly in turkeys and chickens. The disease can lead to high mortality but some infections are reported without any clinical symptoms. As a result of the ban on prophylactic and therapeutic drugs in the European Union and the USA due to concerns about possible residues and consumer protection, there is no effective treatment of histomonosis available. Recent investigations at the Clinic for Avian, Reptile and Fish Medicine in Vienna, have shown that chickens and turkeys can be protected against fatal histomonosis by vaccination using in vitro attenuated parasites. Despite this advance, there is a lack of understanding of the host immune reaction following infection with different kinds of histomonads. The present project will thus investigate various hypotheses concerning the immune response of host birds following vaccination and/or challenge with attenuated or virulent parasites. The major questions relate to i) the local immune response of poultry following infection or vaccination, ii) significant changes of T cell populations in birds after inoculation with histomonads and iii) the establishment of an infection model for the type 2 immune pathway in poultry triggered by H. meleagridis. Initially, birds will be experimentally infected to reproduce infection and vaccination, based on a well established model. A variety of immunological and molecular biological methods will be applied to investigate samples from various organs. In combination, the results will provide fundamental new knowledge about the immune mechanisms underlying the protection of poultry against histomonosis. Furthermore, our work will establish an infection model for investigation of the type 2 pathway in poultry, which should be extremely valuable to future studies.

The aim of the project was to comparatively investigate the immune response against histomonosis (syn. histomoniasis or blackhead disease) of experimentally vaccinated and non-vaccinated turkeys and chickens, in order to gain basic knowledge on host-defense mechanisms caused by the parasitic disease. Histomonas meleagridis is a flagellated protozoan parasite and the aetiological agent of histomonosis, a disease that occurs worldwide, mainly in turkeys and chickens. The disease can lead to high mortality in turkeys but is less severe in chickens. As a result of the ban on prophylactic and therapeutic drugs for the use in poultry in several countries due to concerns about possible residues in food, there is no effective treatment of histomonosis left. Recent experiments at the Clinic for Poultry and Fish Medicine, University of Veterinary Medicine Vienna, have shown that chickens and turkeys can be protected against fatal histomonosis, by vaccination using in vitro attenuated parasites. In chickens, production losses can be prevented by vaccination. Despite this advance, there was a major lack of understanding in the host immune reaction following infection with different kinds of histomonads. Therefore, this project was conducted to investigate specific questions concerning the immune response of host birds following vaccination and/or challenge with attenuated or virulent parasites. This includes the local immune response of poultry following infection or vaccination, significant changes of immune cell populations in birds after inoculation with histomonads and characterizing the type of immune pathway triggered by extracellular parasites in poultry. For that, turkeys and chickens were vaccinated and/or infected with virulent histomonads. Caecum, liver and spleen, together with blood were collected on several days post inoculation to determine local and systemic immune responses. By flow cytometry and histological methods specific immune cells were detected, quantified and localized in tissue sections. Additionally, the expression of cytokines was investigated to obtain insights into the immune pathway triggered by H. meleagridis. The results showed that the development of the local immune response against histomonads correlated with the severity of clinical signs. Furthermore, it was observed that the presence of different populations of immune cells in the samples was related to the species and virulence of the pathogen: most important, chickens showed the lowest changes following vaccination whereas turkeys infected with virulent histomonads had extensive infiltrations of leukocytes populations. The expression profiles of immune cells indicated a balanced activation of immune pathways, triggering humoral and cellular responses. In conclusion, findings of the present study provide insights in the immune response during histomonosis also in the context of vaccination. Such results can be seen in broader context and are of relevance for comparatively investigating immune mechanisms caused by extracellular parasites.