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

Inhibitory effects of different plant extracts on Histomonas meleagridis in vitro and in vivo in chickens

Histomonosis is a protozoan disease that is caused by Histomonas meleagridis, which can lead to cecal inflammation and liver necrosis in poultry. Since the discontinuation of nitarsone in 2015, there is currently a lack of approved prophylactics for managing histomonosis. In search of potential alternatives, the present investigation evaluated the inhibitory effects of seven commercial plant extracts on the in vitro growth of H. meleagridis. Additionally, the in vivo effectiveness of three of the most promising plant extracts was evaluated. The study demonstrated that all seven plant extracts exhibited antihistomonal properties in vitro. Sodium houttuyfonate, emodin, and osthole displayed the highest effectiveness among these extracts, as evidenced by their respective minimal lethal concentration of 0.25, 0.5, and 1 mg/mL. Subsequently, these three plant extracts were employed in additional in vivo testing. The results indicated that all three extracts could mitigate liver and cecum lesion scores in infected chickens while facilitating a degree of body weight gain following infection compared to the challenged control. The group administered with 0.5 g/kg (feed) sodium houttuyfonate exhibited the most pronounced effect, as evidenced by the absence of a significant difference in weight gain compared to the dimetridazole group (P < 0.05). However, the three plant extracts did not fully restore the body weight of infected chickens to levels comparable with the blank control. The experimental results indicate that sodium houttuyfonate, emodin, and osthole possess properties that contribute to the prevention and treatment of histomonosis, with sodium houttuyfonate demonstrating the most favorable results.

Host-specific targets of Histomonas meleagridis antigens revealed by immunoprecipitation

Histomonas meleagridis, a protozoan parasite responsible for histomonosis (syn. Blackhead disease, histomoniasis), presents an increasing challenge for poultry health, particularly with the ban of licensed prophylactic and treatment options. Recent studies have explored H. meleagridis proteome, exoproteome, and surfaceome, linking molecular data to virulence and in vitro attenuation. Nevertheless, proteins involved in interactions with hosts remain largely unknown. In this study, we conducted immunoproteome analyses to identify key antigens involved in the humoral immune response of the parasite's main hosts, turkeys and chickens. Immunogenic proteins were isolated via immunoprecipitation using sera from chickens and turkeys that were vaccinated with a single attenuated strain and challenged with virulent strains of the protozoan, respectively. Mass spectrometry identified 155 putative H. meleagridis immunogenic proteins, of which 43 were recognized by sera from both hosts. In silico antigenicity screening (VaxElan) identified 33 pan-reactive antigens, with VaxiDL further highlighting 10 as potential vaccine candidates. Comparative analysis revealed host-specific immune responses, with 16 differential immunogenic proteins in chickens (6 specific to virulent and 10 to attenuated preparations) and 19 unique proteins in turkeys, all associated with virulent strains. These results enhance our understanding of H. meleagridis immunogenic protein dynamics and host-pathogen specificities, supporting the development of improved diagnostic tools and potential protective measures against the infection.

Evaluation of an attenuated chicken-origin Histomonas meleagridis vaccine for the prevention of histomonosis in chickens

Introduction

Histomonosis, a protozoan disease caused by Histomonas meleagridis, poses a significant economic burden on domestic poultry in China. To reduce the losses caused by this disease in chickens, an attenuated vaccine was developed by exploiting the diminished virulence of H. meleagridis through successive in vitro passages.

Methods

Four experiments were conducted to evaluate the viability of attenuated H. meleagridis as a potential vaccine candidate. Experiment 1 evaluated the route of infection (oral vs. intracloacal) and dose (5 × 104, 1 × 105, and 2 × 105 H. meleagridis/chicken) using the virulent strain H. meleagridis JSYZ-D10. Experiment 2 evaluated the attenuated effect of the H. meleagridis JSYZ-D168 strain (infection dose: 2 × 105 H. meleagridis/chicken). Experiment 3 evaluated the immunoprotective effect of different immunization doses (5 × 104, 1 × 105, and 2 × 105 H. meleagridis/chicken). Experiment 4 evaluated the immunoprotective effect of different immunization schedules (immunization at 3 days of age; immunization at 14 days of age; two immunizations, one at 3 days of age and one at 14 days of age; immunization and infection dose: 2 × 105 H. meleagridis/chicken).

Results

The results showed that the intracloacal route of infection was more effective and stable compared to the oral route. The pathogenicity of the JSYZ-D168 H. meleagridis strain was significantly reduced compared to the original virulent strain. Chickens vaccinated by intracloacal immunization at a dose of 2 × 105 H. meleagridis/chicken on day 14 provided effective protection against a virulent strain challenge, significantly resulting in increased body weight and reduced lesions in the cecum and liver within 28 days post-immunization (p < 0.05). Poor immunoprotection was obtained either when the immunization dose was 1 × 105 H. meleagridis/chicken or when the immunization program was a single immunization at 3 days of age only.

Discussion

In conclusion, the administration of a vaccine provides a measurable degree of protection against the detrimental effects induced by H. meleagridis, thus warranting its endorsement in clinical settings.

Tissue cytokines in chickens from lines selected for high or low humoral antibody responses, given supplemental Limosilactobacillus reuteri and challenged with Histomonas meleagridis

Histomonas meleagridis, a protozoan parasite, induces blackhead disease (histomoniasis) in poultry. During hatching, chicks from lines divergently selected for high (HAS) and low (LAS) antibody responses to sheep red blood cells were divided into two groups, each of HAS and LAS, and placed in pens with wood shavings as litter. Feed and water were allowed ad libitum. Half of the chicks from each line had Limosilactobacillus reuteri (L. reuteri) inoculated to their drinking water. On day 18, all chicks were given a transcloacal inoculation of 100,000 H. meleagridis cells. Then, 10 days later, they were euthanized, followed by collection of tissues from the brain, cecal tonsil, ceca, liver, thymus, and spleen for qPCR analyses of cytokines involved in immunological development. Changes in cytokine expressions were most numerous in the cecal tonsil, ceca, and liver. In the absence of a functional medication for control of histomoniasis, L. reuteri and/or its secretory product, reuterin, might serve, in some genetic populations, as a means to reduce the impact of histomoniasis in chickens. The data demonstrate that L. reuteri treatment had tissue specificity between the two genetic lines, in which the effects were targeted primarily toward the cecal tonsil, ceca, and liver, which are the primary tissue targets of the parasite (H. meleagridis), as well as the thymus and spleen. However, interactions among main effects reflect that responses to inflammatory markers observed in tissues for one genetic line may not be observed in another.

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.

Research note: Lateral transmission of Histomonas meleagridis in turkey poults raised on floor pens

Histomoniasis is caused by the protozoa Histomonas meleagridis (HM) that are laterally transmitted among birds leading to high mortality in commercial flocks. This study tested an HM infection model assessing the lateral transmission of HM in turkey poults raised on floor pens. Day (d)-old female turkey poults (n = 320) were individually wing-tagged and allocated to one of four treatment groups (4 floor pens/group and 20 poults/pen) based on the percentage of poults inoculated with HM: 1) 10% (HM10); 2) 20% (HM20); 3) 30% (HM30); and 4) 40% (HM40). On d 9, seeder poults intracloacally received a 1 mL inoculum/bird containing ∼80,000 histomonads. Poults were individually weighed on d 0, 9, and 25 and feed intake recorded on per pen basis. On d 25, all birds were euthanized by cervical dislocation and ceca and liver were evaluated for HM lesions. Data were analyzed using JMP (Pro16) and significance (P ≤ 0.05) between treatments were determined by LSD test. Mortality was 7.63%, 12.5%, 21.58%, and 20.59%, while transmission rates from inoculated to non-inoculated birds were 62.5%, 57.5%, 92.43%, and 78.75% in HM10, HM20, HM30, and HM40 groups, respectively. Average daily feed intake was proportionally reduced with the increasing number of inoculated poults from HM10 to HM40. Average daily gain was significantly lower in HM30 and HM40 poults compared to those in HM10 and HM20 during the postchallenge period (d 10–25). Therefore, we herein report the successful lateral transmission of HM among turkey poults raised on floor pens. This research model closely resembles commercial field conditions and affords a much-needed platform for conducting relevant basic and applied research on histomoniasis in poultry.