Passive immunization againstHistomonas meleagridisdoes not protect turkeys from an experimental infection

Vaccination Against Poultry Parasites

The complexity of parasites and their life cycles makes vaccination against parasitic diseases challenging. This review highlights this by discussing vaccination against four relevant parasites of poultry. Coccidia, i.e., Eimeria spp., are the most important parasites in poultry production, causing multiple billions of dollars of damage worldwide. Due to the trend of antibiotic-free broiler production, use of anticoccidia vaccines in broilers is becoming much more important. As of now, only live vaccines are on the market, almost all of which must be produced in birds. In addition, these live vaccines require extra care in the management of flocks to provide adequate protection and prevent the vaccines from causing damage. Considerable efforts to develop recombinant vaccines and related work to understand the immune response against coccidia have not yet resulted in an alternative. Leucozytozoon caulleryi is a blood parasite that is prevalent in East and South Asia. It is the only poultry parasite for which a recombinant vaccine has been developed and brought to market. Histomonas meleagridis causes typhlohepatitis in chickens and turkeys. The systemic immune response after intramuscular vaccination with inactivated parasites is not protective. The parasite can be grown and attenuated in vitro, but only together with bacteria. This and the necessary intracloacal application make the use of live vaccines difficult. So far, there have been no attempts to develop a recombinant vaccine against H. meleagridis. Inactivated vaccines inducing antibodies against the poultry red mite Dermanyssus gallinae have the potential to control infestations with this parasite. Potential antigens for recombinant vaccines have been identified, but the use of whole-mite extracts yields superior results. In conclusion, while every parasite is unique, development of vaccines against them shares common problems, namely the difficulties of propagating them in vitro and the identification of protective antigens that might be used in recombinant vaccines.

Experimental reproduction of histomonosis caused by Histomonas meleagridis genotype 2 in turkeys can be prevented by oral vaccination of day-old birds with a monoxenic genotype 1 vaccine candidate

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.

Histomonosis in Poultry: A Comprehensive Review

Histomonas meleagridis, the etiological agent of histomonosis, is a poultry parasite primarily detrimental to turkeys. Characteristic lesions occur in the liver and ceca, with mortalities in turkey flocks often reaching 80-100%. Chickens and other gallinaceous birds can be susceptible but the disease was primarily considered sub-clinical until recent years. Treating and preventing H. meleagridis infection have become more difficult since 2015, when nitarsone was voluntarily removed from the market, leaving the poultry industry with no approved prophylactics, therapeutics, or vaccines to combat histomonosis. Phytogenic compounds evaluated for chemoprophylaxis of histomonosis have varied results with in vitro and in vivo experiments. Some recent research successes are encouraging for the pursuit of antihistomonal compounds derived from plants. Turkeys and chickens exhibit a level of resistance to re-infection when recovered from H. meleagridis infection, but no commercial vaccines are yet available, despite experimental successes. Safety and stability of live-attenuated isolates have been demonstrated; furthermore, highly efficacious protection has been conferred in experimental settings with administration of these isolates without harming performance. Taken together, these research advancements are encouraging for vaccine development, but further investigation is necessary to evaluate proper administration age, dose, and route. A summary of the published research is provided in this review.

Evaluation of live-attenuated Histomonas meleagridis isolates as vaccine candidates against wild-type challenge

Repeated serial in vitro passage of Histomonas meleagridis, the etiological agent of histomoniasis (blackhead) of turkeys, was demonstrated to markedly achieve attenuation and reduction of virulence as compared to the original wild-type isolate. Four experiments were performed to evaluate the route (oral vs. intracloacal) and age (day-of-hatch vs. d 14) for administration of attenuated H. meleagridis isolates as vaccine candidates against homologous or heterologous wild-type challenge. Attenuated H. meleagridis were developed from 2 different strains (Buford strain originating in Georgia; PHL2017 strain originating in Northwest Arkansas). Buford P80a (passage 80, assigned as isolate lineage “a” following repeated passage) was selected as the primary vaccine candidate and was evaluated in Experiments 1–3. Experiment 4 evaluated selected candidates of attenuated PHL2017 (P67, P129) and Buford (P80a, P200a, P138b, P198c) strains against Buford wild-type challenge. As has been demonstrated previously, wild-type H. meleagridis cultures administered orally after 1 day of age were not infective in the current studies, but infection with wild-type cultures could be induced orally at day-of-hatch. Infection was effectively achieved via the intracloacal route at day-of-hatch and in older turkeys (d 21, d 28–29, d 35). Intracloacal inoculation of turkeys with the attenuated passaged isolates as vaccine candidates at d 14 was shown to produce significant (P < 0.05) protection from mortality, reduction in body weight gain, as well as reduction in hepatic and cecal lesions in these experiments following challenge with either the homologous wild-type isolate or from a wild-type strain obtained years later from a geographically disparate area of the United States. Inoculation with the attenuated H. meleagridis isolates at day-of-hatch, either orally or cloacally, did not produce significant protection against subsequent wild-type challenge. While offering significant protection with minimal vaccine-related negative effects, the protection from cloacal vaccine administration was neither significantly robust nor encouraging for industry application using the methods evaluated in the present manuscript since mortalities and lesions were not completely reduced which could thereby potentially allow transmission from residual infection and shedding within a flock.

Comparative investigation of IFN-γ-producing T cells in chickens and turkeys following vaccination and infection with the extracellular parasite Histomonas meleagridis

The re-emerging disease histomonosis is caused by the protozoan parasite Histomonas meleagridis that affects chickens and turkeys. Previously, protection by vaccination with in vitro attenuated H. meleagridis has been demonstrated and an involvement of T cells, potentially by IFN-γ production, was hypothesized. However, comparative studies between chickens and turkeys on H. meleagridis-specific T cells were not conducted yet. This work investigated IFN-γ production within CD4⁺, CD8α⁺ and TCRγδ⁺ (chicken) or CD3ε⁺CD4^-CD8α^- (turkey) T cells of spleen and liver from vaccinated and/or infected birds using clonal cultures of a monoxenic H. meleagridis strain. In infected chickens, re-stimulated splenocytes showed a significant increase of IFN-γ⁺CD4⁺ T cells. Contrariwise, significant increments of IFN-γ-producing cells within all major T-cell subsets of the spleen and liver were found for vaccinated/infected turkeys. This indicates that the vaccine in turkeys causes more intense systemic immune responses whereas in chickens protection might be mainly driven by local immunity.

Cytokine production and phenotype of Histomonas meleagridis-specific T cells in the chicken

The protozoan parasite Histomonas meleagridis is the causative agent of the re-emerging disease histomonosis of chickens and turkeys. Due to the parasite’s extracellular occurrence, a type-2 differentiation of H. meleagridis-specific T cells has been hypothesized. In contrast, a recent study suggested that IFN-γ mRNA⁺ cells are involved in protection against histomonosis. However, the phenotype and cytokine production profile of H. meleagridis-specific T cells still awaits elucidation. In this work, clonal cultures of a virulent monoxenic strain of H. meleagridis were used for infecting chickens to detect IFN-γ protein and IL-13 mRNA by intracellular cytokine staining and PrimeFlow™ RNA Assays, respectively, in CD4⁺ and CD8β⁺ T cells. Infection was confirmed by characteristic pathological changes in the cecum corresponding with H. meleagridis detection by immunohistochemistry and H. meleagridis-specific antibodies in serum. In splenocytes stimulated either with H. meleagridis antigen or PMA/ionomycin, IFN-γ-producing CD4⁺ T cells from infected chickens increased in comparison to cells from non-infected birds 2 weeks and 5 weeks post-infection. Additionally, an increase of IFN-γ-producing CD4⁻CD8β⁻ cells upon H. meleagridis antigen and PMA/ionomycin stimulation was detected. Contrariwise, frequencies of IL-13 mRNA-expressing cells were low even after PMA/ionomycin stimulation and mainly had a CD4⁻CD8β⁻ phenotype. No clear increase of IL-13⁺ cells related to H. meleagridis infection could be found. In summary, these data suggest that H. meleagridis infection induces a type-1 differentiation of CD4⁺ T cells but also of non-CD4⁺ cells. This phenotype could include γδ T cells, which will be addressed in future studies.

Unravelling the Immunity of Poultry Against the Extracellular Protozoan Parasite Histomonas meleagridis Is a Cornerstone for Vaccine Development: A Review

The protozoan parasite Histomonas meleagridis is the causative agent of histomonosis in gallinaceous birds, predominantly in turkeys and chickens. Depending on the host species the outcome of the disease can be very severe with high mortality as observed in turkeys, whereas in chickens the mortality rates are generally lower. The disease is known for more than 100 years when in vitro and in vivo investigations started to understand histomonosis and the causative pathogen. For decades histomonosis could be well-controlled by effective drugs for prevention and therapy until the withdrawal of such chemicals for reasons of consumer protection in Europe, the USA and additional countries worldwide. Consequently, research efforts also focused to find new strategies against the disease, resulting in the development of an efficacious live-attenuated vaccine. In addition to efficacy and safety several studies were performed to obtain a deeper understanding of the immune response of the host against H. meleagridis. It could be demonstrated that antibodies accumulate in different parts of the intestine of chickens following infection with H. meleagridis which was much pronounced in the ceca. Furthermore, expression profiles of various cytokines revealed that chickens mounted an effective cecal innate immune response during histomonosis compared to turkeys. Studying the cellular immune response following infection and/or vaccination of host birds showed a limitation of pronounced changes of B cells and T-cell subsets in vaccinated birds in comparison to non-protected birds. Additionally, numbers of lymphocytes including cytotoxic T cells increased in the ceca of diseased turkeys compared to infected chickens suggesting an immunopathological impact on disease pathogenesis. The identification of type 1 and type 2 T-helper (Th) cells in infected and lymphoid organs by in situ hybridization did not show a clear separation of Th cells during infection but revealed a coherence of an increase of interferon (IFN)-γ mRNA positive cells in ceca and protection. The present review not only summarizes the research performed on the immune response of host birds in the course of histomonosis but also highlights the specific features of H. meleagridis as a model organism to study immunological principles of an extracellular organism in birds.

Allocation of Interferon Gamma mRNA Positive Cells in Caecum Hallmarks a Protective Trait Against Histomonosis

Histomonosis is a parasitic disease of gallinaceous birds characterized by necrotic lesions in cacum and liver that usually turns fatal in turkeys while it is less severe in chickens. Vaccination using in vitro attenuated Histomonas meleagridis has been experimentally shown to confer protection against histomonosis. The protective mechanisms that underpin the vaccine-induced immune response are not resolved so far. Therefore, the actual study aimed to evaluate the location and quantitative distribution patterns of signature cytokines of type 1 [interferon gamma (IFN-γ)] or type 2 [interleukin (IL)-13] immune responses in vaccinated or infected hosts. An intergroup and interspecies difference in the spatial and temporal distribution patterns of cytokine mRNA positive cells was evident. Quantification of cells showed a significantly decreased percentage of IFN-γ mRNA positive cells at 4 days post-inoculation (DPI) in caeca of turkeys inoculated exclusively with the attenuated or the virulent inocula, compared to control birds. The decrement was followed by a surge of cells expressing mRNA for IFN-γ or IL-13, reaching a peak of increment at 10 DPI. By contrast, turkeys challenged following vaccination showed a slight increment of cecal IFN-γ mRNA positive cells at 4 DPI after which positive cell counts became comparable to control birds. The increase in infected birds was accompanied by an extensive distribution of positively stained cells up to the muscularis layer of cecal tissue whereas the vaccine group maintained an intact mucosal structure. In chickens, the level of changes of positive cells was generally lower compared to turkeys. However, control chickens were found with a higher percentage of IFN-γ mRNA positive cells in cecum compared to their turkey counterparts indicating a higher resistance to histomonosis, similar to the observation in immunized turkeys. In chickens, it could be shown that the changes of cytokine-positive cells were related to variations of mononuclear cells quantified by immunofluorescence. Furthermore, gene expression measured by reverse transcription quantitative real time PCR confirmed variations in organs between the different groups of both bird species. Overall, it can be concluded that a proportionally increased, yet controlled, allocation of IFN-γ mRNA positive cells in caeca hallmarks a protective trait against histomonosis.

Vaccination against histomonosis limits pronounced changes of B cells and T-cell subsets in turkeys and chickens

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.

Histomonosis in poultry: previous and current strategies for prevention and therapy

Histomonosis is a parasitic disease of poultry with worldwide prevalence. The disease can cause morbidity and mortality in chicken and turkey flocks entailing severe economic losses. In the first half of the last century, there was a high demand to control histomonosis as the turkey industry was severely affected by the disease. Consequently, numerous chemical compounds were tested for their efficacy against Histomonas meleagridis with varying outcomes, that are summarized and specified in this review. At the same time, preliminary attempts to protect birds with cultured histomonads indicated the possibility of vaccination. Several years ago antihistomonal drugs were banned in countries with tight regulations on pharmaceuticals in order to comply with the demand of consumer protection. As a consequence, outbreaks of histomonosis in poultry flocks increased and the disease became endemic again. New approaches to prevent and treat histomonosis are, therefore, needed and recently performed studies focused on various areas to combat the disease, from alternative chemotherapeutic substances to plant-derived compounds until vaccination, altogether reviewed here. Considering existing regulations and the overall outcome of experimental studies, it can be concluded that vaccination is very promising, despite the fact that various challenges need to be addressed until the first ever developed vaccine based upon live flagellates in human or bird medicine can be marketed.

Passive immunisation, an old idea revisited: Basic principles and application to modern animal production systems

Immunisation by administration of antibodies (immunoglobulins) has been known for more than one hundred years as a very efficient means of obtaining immediate, short-lived protection against infection and/or against the disease-causing effects of toxins from microbial pathogens and from other sources. Thus, due to its rapid action, passive immunisation is often used to treat disease caused by infection and/or toxin exposure. However immunoglobulins may also be administered prior to exposure to infection and/or toxin, although they will not provide long-lasting protection as is seen with active immunisation (vaccination) in which an immunological memory is established by controlled exposure of the host to the pathogen in question. With multi-factorial infectious diseases in production animals, especially those that have proven hard to control by vaccination, the potential of passive immunisation remains big. This review highlights a number of examples on the use of passive immunisation for the control of infectious disease in the modern production of a range of animals, including pigs, cattle, sheep, goat, poultry and fish. Special emphasis is given on the enablement of passive immunisation strategies in these production systems through low cost and ease of use as well as on the sources, composition and purity of immunoglobulin preparations used and their benefits as compared to current measures, including vaccination (also comprising maternal vaccination), antibiotics and feed additives such as spray-dried plasma. It is concluded that provided highly efficient, relatively low-price immunoglobulin products are available, passive immunisation has a clear role in the modern animal production sector as a means of controlling infectious diseases, importantly with a very low risk of causing development of bacterial resistance, thus constituting a real and widely applicable alternative to antibiotics.

Histomonosis - an existing problem in chicken flocks in Poland

Histomonosis (histomoniasis, blackhead), beside coccidiosis, belongs to the most important parasitic protozoan diseases in poultry. So far Histomonas meleagridis infections with varied mortality rates have been mainly diagnosed in young turkeys. Recently an increasing number of cases have been reported in chicken flocks in Europe resulting in economic losses. It is thought that this situation is predominantly caused by a complete withdrawal of the effective antihistomonals in the EU. Authors listed the selected outbreaks of histomonosis in 10 chicken flocks originated from different farms of 4 regions in Poland: 8 broiler breeder flocks (at mean age of 33 weeks) and 2 commercial layers flocks (at mean age of 38 weeks). This study reported here naturally occurring case of H.meleagridis infection in commercial broiler breeder (BB) flock line ROSS 308 at the age of 16 weeks. We showed acute form of infection with characteristic necrotic foci in the liver, and ulcerative typhilitis. Beside the liver and caeca, the multiple histomonads, lymphoid tissue depletion and heavy destruction in the bursa of Fabricius were observed. Additionally, the absence of systemic diffuse histomonads and lack of Heterakis gallinarum, caecal worm eggs in faecal samples were noted. PCR technique enabled to detect the presence of H.meleagridis genetic material in the investigated tissue samples. Authors indicate that histomonosis can be considered as re-emerging infectious diseases in chicken flocks of intensive production system.

Histomonas meleagridis--new insights into an old pathogen

The protozoan flagellate Histomonas meleagridis is the etiological agent of histomonosis, first described in 1893. It is a fastidious disease in turkeys, with pathological lesions in the caeca and liver, sometimes with high mortality. In chickens the disease is less fatal and lesions are often confined to the caeca. The disease was well controlled by applying nitroimidazoles and nitrofurans for therapy or prophylaxis. Since their introduction into the market in the middle of the previous century, research nearly ceased as outbreaks of histomonosis occurred only very rarely. With the ban of these drugs in the last two decades in North America, the European Union and elsewhere, in combination with the changes in animal husbandry, the disease re-emerged. Consequently, research programs were set up in various places focusing on different features of the parasite and the disease. For the first time studies were performed to elucidate the molecular repertoire of the parasite. In addition, research has been started to investigate the parasite's interaction with its host. New diagnostic methods and tools were developed and tested with samples obtained from field outbreaks or experimental infections. Some of these studies aimed to clarify the introduction of the protozoan parasite into a flock and the transmission between birds. Finally, a strong focus was placed on research concentrated on the development of new treatment and prophylactic strategies, urgently needed to combat the disease. This review aims to summarize recent research activities and place them into context with older literature.

Vaccination against histomonosis prevents a drop in egg production in layers following challenge

The effect of attenuated Histomonas meleagridis on pullets was investigated and the protection of vaccinated adult laying hens against a severe challenge was studied in the same experimental setting. Four groups of 25 pullets were set up at 18 weeks of life and birds in two groups were vaccinated with in vitro-attenuated H. meleagridis. Chickens in two groups (vaccinated and non-vaccinated) were challenged 5 weeks later with virulent histomonads, while the remaining groups were retained until termination of the study 11 weeks post vaccination. Vaccination of pullets did not have any impact on their subsequent performance. Egg production of non-vaccinated but challenged birds dropped significantly (P ≤ 0.05) between 2 and 4 weeks post challenge (p.c.) to 58.7%, compared with 90% in control chickens. At 4 weeks p.c., the drop in egg production in vaccinated and challenged birds was significantly lower (P=0.02) than in non-protected layers. Pathological changes were found only in challenged birds 2 and 6 weeks p.c. Several non-vaccinated birds showed severe lesions in the caeca with sporadic involvement of the liver and atrophy of the reproductive tract. Vaccination prior to challenge reduced the incidence of pathological findings. For the first time, vaccination of pullets with in vitro-attenuated histomonads could be shown to be an effective and safe prophylactic tool to prevent a severe drop in egg production of commercial layers following experimental infection.

Experimental infection of chickens with Histomonas meleagridis confirms the presence of antibodies in different parts of the intestine

Specific pathogen-free chickens were infected with a clonal culture of the protozoan parasite, Histomonas meleagridis. Severe lesions were found within the caeca of all birds euthanized at 7 and 14 days post-infection (d.p.i.). Following necropsy of birds, intestinal samples were taken to establish ex vivo tissue cultures to determine the IgG, IgA and IgM antibody levels in the supernatants before and after incubation with a recently established ELISA. Presence of antibodies was also determined in the sera and first optical density values for IgG above the cut-off were detected at 14 d.p.i. IgA levels remained low in the serum with a small peak 4 weeks p.i., a phenomenon also found for IgM. The intestinal tissue samples showed very strong immunological reactions in the parasitized caeca with an initial peak of IgM, high levels of IgG and a continuous increase of IgA. In the duodena and jejuna, IgA values reached similar high levels as those obtained in the caeca, whereas IgG and IgM increased only slightly.