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Histomonosis (syn. blackhead disease) is caused by the protozoan parasite Histomonas meleagridis and can result in high mortality in turkey flocks, a situation driven by the limitation of prophylactic and therapeutic interventions. Multi-locus sequence typing confirmed the existence of two genotypes, with the vast majority of reported histomonosis outbreaks being caused by genotype 1 in contrast to only a few detections of genotype 2. For the first time, genotype 2 of H. meleagridis was successfully isolated from an outbreak of histomonosis in a flock of 5-week-old turkeys and a clonal culture was established. Using this culture, an experimental infection was performed in naïve turkeys. The animal trial reflected the observations from the field outbreak and coincided with a previously reported case of histomonosis caused by genotype 2, albeit no mortality was observed in the infected birds whereas 17.1% mortality was noticed in the field outbreak from appearance of disease until slaughter. Post mortem investigations demonstrated that lesions were restricted to the caeca in the field outbreak and the experimental trial. In parallel with the experimental reproduction of pathological changes, an oral vaccination of day-old turkeys with a monoxenic genotype 1 vaccine was carried out to determine efficacy against a genotype 2 challenge. Successful vaccine uptake was characterized by the presence of the vaccine in the caeca determined by qPCR and immunohistochemistry (IHC). Excretion of the vaccine strain was confirmed prior challenge, with the majority of birds developing antibodies. The new monoxenic vaccine was able to minimize lesions in the caeca demonstrating heterologous protection. No parasites were detected in the liver by IHC in any of the vaccinated birds, compared to non-vaccinated animals. However, in 6 out of 17 birds of the vaccinated group a positive signal was obtained by real time PCR from liver samples with 2 positives being typeable by conventional PCR as genotype 2. Overall, H. meleagridis genotype 2 infection was successfully reproduced. Experimental vaccination with a genetically distantly related genotype 1 was able to reduce lesions, supporting protection by a recently developed vaccine candidate as an efficacious prophylactic strategy. [ABSTRACT FROM AUTHOR]

The protozoan parasite Histomonas meleagridis is the causative agent of histomonosis in gallinaceous birds. In turkeys, the disease can result in high mortality due to severe inflammation and necrosis in caecum and liver, whereas in chickens the disease is less severe. Recently, experimental vaccination was shown to protect chickens and turkeys against histomonosis but dynamics in the cellular immune response are not yet demonstrated. In the present work, different groups of birds of both species were vaccinated with attenuated, and/or infected with virulent histomonads. Flow cytometry was applied at different days post inoculation to analyse the absolute number of T-cell subsets and B cells in caecum, liver, spleen and blood, in order to monitor changes in these major lymphocyte subsets. In addition, in chicken samples total white blood cells were investigated. Infected turkeys showed a significant decrease of T cells in the caecum within one week post infection compared to control birds, whereas vaccination showed delayed changes. The challenge of vaccinated turkeys led to a significant increase of all investigated lymphocytes in the blood already at 4 DPI, indicating an effective and fast recall response of the primed immune system. In the caecum of chickens, changes of B cells, CD4 + and CD8α + T cells were much less pronounced than in turkeys, however, mostly caused by virulent histomonads. Analyses of whole blood in non-vaccinated but infected chickens revealed increasing numbers of monocytes/macrophages on all sampling days, whereas a decrease of heterophils was observed directly after challenge, suggesting recruitment of this cell population to the local site of infection. Our results showed that virulent histomonads caused more severe changes in the distribution of lymphocyte subsets in turkeys compared to chickens. Moreover, vaccination with attenuated histomonads resulted in less pronounced alterations in both species, even after challenge. [ABSTRACT FROM AUTHOR]

Highlights: [•] Reversion to virulence and residual pathogenicity of in vitro attenuated Histomonas meleagridis were investigated. [•] No adverse clinical signs or mortality in chickens and turkeys following infection with attenuated histomonads. [•] No increase in number and severity of lesions within 5 cycles of backpassaging. [•] Lesion scores caused by the virulent strain were statistically different to those induced by attenuated histomonads. [•] In vivo passaging of attenuated Histomonas meleagridis does not foster reversion to virulence in chickens or turkeys. [Copyright &y& Elsevier]

Abstract: Histomonas meleagridis is a flagellated protozoan parasite and the aetiological agent of histomonosis (histomoniasis or blackhead disease), a severe disease in poultry with very limited options for treatment. In the present investigation an inactivated vaccine, based on destroyed parasites and administered by intramuscular injection, failed to induce an efficient protection against a severe challenge. By cloacal infection of 14-day-old turkeys with cloned protozoa passaged in vitro for 95, 215 or 295 times, respectively, severe attenuation could be demonstrated as none of the infected birds died. Following infection with one of the higher passages, the birds resisted the challenge with virulent parasites. Using this approach no mortality due to histomonosis could be recorded in all of the 42 birds subjected to vaccination, either through direct infection or kept as in-contact birds, whereas all of the control animals died. Pathological lesions in the liver and caeca were only noticed in a few birds. The absence of parasitic DNA in the liver of vaccinated birds confirmed the effect of vaccination. For the first time vaccination of turkeys is reported to induce a solid protection against a severe challenge, both based on a clonal culture of Histomonas meleagridis. [Copyright &y& Elsevier]