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Histomonosis, or blackhead disease, is a well-known disease in turkeys that can cause high mortality, but outbreaks with lower losses are also observed. The disease is less fatal in chickens but is economically important due to reduced performance and its co-appearance with colibacillosis. The lack of specific prophylactic and therapeutic interventions has led to a re-emergence of the disease in recent years, mainly in turkeys, free-range layers and chicken parent stock. [ABSTRACT FROM AUTHOR]

In the current study, cross-protective immunity induced by a well-defined clonal strain of Histomonas meleagridis, attenuated by prolonged in vitro cultivation against different clonal heterologous isolates of the same parasite was investigated. For this purpose, 86 turkey poults were assigned to groups consisting of 9–10 birds. Birds of four groups were vaccinated on their 1st day of life followed by re-vaccination on their 14th day of life when the remaining turkeys were left untreated. The challenge was performed using four strains of H. meleagridis that were isolated from chickens or turkeys from different outbreaks of histomonosis in Europe and three of them showed diversities in their genome. Hence, every strain used for the challenge was applied to a group of vaccinated and a group of non-vaccinated birds while birds of the negative control group were sham inoculated. Non-vaccinated birds suffered from severe histomonosis due to the challenge with fatalities reaching from 5 to 10 turkeys per group. Vaccinated birds did not contract clinical signs of the disease following challenge and the increase in weight was unaffected compared to birds of the negative control group. A significant difference in lesion scores was recorded between vaccinated and non-vaccinated groups, with very few instances of liver involvement in the former groups. Livers of vaccinated birds that were without recordable macroscopic lesions were also found negative by immunohistochemical investigation. According to the data obtained, the present study demonstrates, for the first time, the cross-protective capability of a tentative vaccine strain of H. meleagridis attenuated in vitro against heterologous virulent isolates of different origin. [ABSTRACT FROM AUTHOR]

Abstract: Histomonosis (blackhead disease or infectious enterohepatitis) caused by the extracellular protozoon parasite Histomonas meleagridis is an important disease of turkeys and a threat to the poultry industry. Due to recent legislation on drug restrictions, research to find new alternatives is an urgent matter in the battle against histomonosis. In the present study, intracloacal inoculation of a low-virulent H. meleagridis strain isolated after serial passages in turkeys clearly demonstrated a reduction of virulence and hence its effectiveness as a vaccine against histomonosis. The low-virulent isolate has been evaluated in a comparative experimental infection study. No mortality nor predominant caecal or liver lesions could be observed in the groups inoculated with 103, 104 or 105 histomonads per bird. Only dilated caeca with a yellow and foamy content could be noticed. Groups inoculated with similar doses of a virulent strain displayed a dose-related pathology and mortality up to 100%. The protective capacity of the strain with reduced virulence could be demonstrated as none of the birds cloacally inoculated with 103, 104 or 105 histomonads died upon challenge with 105 H. meleagridis of the virulent strain. Hereby, 71% of the challenge control group died. Interestingly, no or only very minor pathological lesions in the caeca and liver could be detected after challenge of the birds inoculated with the passaged histomonads. In conclusion, cloacal inoculation of the low-virulent strain obtained after serial backpassages was able to induce protection of turkeys against challenge with a virulent H. meleagridis strain. [Copyright &y& Elsevier]

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]

Background: Histomonosis, caused by the protozoan parasite Histomonas meleagridis, is a severe disease especially in turkeys where it can cause high mortalities. Recently, outbreaks were described in which turkey hens showed no clinical signs despite high mortalities in toms, from which they were separated only by a wire fence. The present study investigated three similar outbreaks of histomonosis whereby in two of them only a few hens were being affected and none in the third. Hens from all flocks were kept until end of production and slaughtered as scheduled. However, in all three cases, the disease progressed in toms reaching nearly 100% within two weeks. Methods: Following diagnosis of the disease, tissue samples were obtained from toms and hens at necropsy. Environmental dust, cloacal swabs and blood were taken on three successive farm visits within compartments of hens and toms and tested by real-time PCR or ELISA. The DNA from a total of 18 samples positive for H. meleagridis was further subjected to conventional PCR utilizing the 18S rRNA primers and sequenced for phylogenetic analysis. Results: All tissue samples and some cloacal swabs were tested positive. Dust samples confirmed the presence of H. meleagridis DNA that spread within entire houses up to 6 weeks after the first clinical signs of histomonosis. Sequence analysis of the 18S rRNA locus demonstrated the presence of the same strain in birds of both sexes within each of the turkey houses. Investigation of serum samples two weeks post-initial diagnosis and prior to euthanasia resulted in antibody detection in 73% of toms and 70% of hens. Until the end of the investigation the number of positive hens per farm increased up to 100% with mean OD-values approaching those noticed in toms prior to euthanasia. Conclusions: For the first time it could be demonstrated that turkey hens kept in the same house as toms became infected during fatal outbreaks in toms. This highlights the value of different diagnostics methods in order to trace the parasite in connection with the host response. The strange phenomenon that only single hens succumb to the diseases despite being infected requires further investigations. [ABSTRACT FROM AUTHOR]