B-Cell Infiltration in the Respiratory Mucosa of Turkeys Exposed to Subtype C Avian Metapneumovirus

Pathological and phylogenetic characteristics of fowl AOAV-1 and H5 isolated from naturally infected Meleagris Gallopavo

BACKGROUND

In this study, we investigated the prevalence of respiratory viruses in four Hybrid Converter Turkey (Meleagris gallopavo) farms in Egypt. The infected birds displayed severe respiratory signs, accompanied by high mortality rates, suggesting viral infections. Five representative samples from each farm were pooled and tested for H5 & H9 subtypes of avian influenza viruses (AIVs), Avian Orthoavulavirus-1 (AOAV-1), and turkey rhinotracheitis (TRT) using real-time RT-PCR and conventional RT-PCR. Representative tissue samples from positive cases were subjected to histopathology and immunohistochemistry (IHC).

RESULTS

The PCR techniques confirmed the presence of AOAV-1 and H5 AIV genes, while none of the tested samples were positive for H9 or TRT. Microscopic examination of tissue samples revealed congestion and hemorrhage in the lungs, liver, and intestines with leukocytic infiltration. IHC revealed viral antigens in the lungs, liver, and intestines. Phylogenetic analysis revealed that H5 HA belonged to 2.3.4.4b H5 sublineage and AOAV-1 belonged to VII 1.1 genotype.

CONCLUSIONS

The study highlights the need for proper monitoring of hybrid converter breeds for viral diseases, and the importance of vaccination programs to prevent unnecessary losses. To our knowledge, this is the first study that reports the isolation of AOAV-1 and H5Nx viruses from Hybrid Converter Turkeys in Egypt.

Avian Metapneumovirus Infection in Poultry Flocks: A Review of Current Knowledge

Avian metapneumovirus (aMPV) is one of the respiratory viruses that cause global economic losses in poultry production systems. Therefore, it was important to design a comprehensive review article that gives more information about aMPV infection regarding the distribution, susceptibility, transmission, pathogenesis, pathology, diagnosis, and prevention. The aMPV infection is characterized by respiratory and reproductive disorders in turkeys and chickens. The disease condition is turkey rhinotracheitis in turkeys and swollen head syndrome in chickens. Infection with aMPV is associated with worldwide economic losses, especially in complications with other infections or poor environmental conditions. The genus Metapneumovirus is a single-stranded enveloped RNA virus and contains A, B, C, and D subtypes. Meat and egg-type birds are susceptible to aMPV infection. The virus can transmit through aerosol, direct contact, mechanical, and vertical routes. The disease condition is characterized by respiratory manifestations, a decrease in egg production, growth retardation, increasing morbidity rate, and sometimes nervous signs and a high mortality rate, particularly in concurrent infections. Definitive diagnosis of aMPV is based mainly on isolation and identification methods, detection of the viral DNA, as well as seroconversion. Prevention of aMPV infection depends on adopting biosecurity measures and vaccination using inactivated, live attenuated, and recombinant or DNA vaccines.

The influence of maternally derived antibodies on protection against aMPV/A infection in TRT vaccinated turkeys

Avian metapneumoviruses (aMPV) are a causative agent of turkey rhinotracheitis (TRT). Despite vaccination, cases of TRT outbreaks are frequently reported. Considering that there are aMPV-free areas, a part of turkey poults possess (MDA+) or do not possess (MDA-) maternally derived antibodies (MDA) in the first weeks of life, which is the time of TRT vaccination. Study was undertaken to establish the level of protection against homologue aMPV/A infection, in MDA+ and MDA- turkeys, vaccinated against TRT at the 0 or 14th d of life and infected 14 d later. With the use of ELISA test and qPCR techniques, we have established the level of immune system stimulation after the vaccination and how does it correlate with the level of protection against the aMPV infection. Vaccination of MDA+ turkeys (especially at 0 d of life) resulted in weaker IgA production in upper respiratory tract. In addition, we have demonstrated differences in both humoral and cell-mediated immunity stimulation after infection of vaccinated turkeys. Despite these differences, we have shown that all vaccinated birds were protected against the disease which was determined based on the clinical and histopathological scoring, as well as the level of aMPV/A replication and shedding. Nonvaccinated groups of turkeys displayed typical signs of TRT after infection which indicates that MDA alone are incapable of preventing the disease. Differences in TRT course were recorded between different age groups of nonvaccinated birds. Birds infected at the 28th d of life (especially MDA- birds) developed more severe signs, and the level of aMPV replication was higher than that in birds infected on the 14th d of life. Despite the minor role in alleviating TRT course, MDA seems not to interfere with the vaccination efficacy. It is hard to predict whether the observed immune system stimulation differences between MDA+ and MDA- birds after vaccination can influence the outcome of vaccination efficacy under the field conditions.

IFNγ Production Profile in Turkeys of Different Immunological Status after TRT Vaccination

Introduction

Despite vaccination against avian metapneumoviruses (aMPV), cases of turkey rhinotracheitis (TRT) caused by aMPV field strains are frequently reported. Differences have been shown in the level of immune system stimulation after aMPV vaccination between turkeys that do and do not possess specific anti-aMPV maternally derived antibodies (MDA). The article describes the influence of MDA on the production of IFNγ in the spleen of aMPV-vaccinated turkeys.

Material and Methods

MDA+ or MDA- turkeys were vaccinated against TRT after hatching or on the 14^th day of life. Spleen samples were collected 3, 7, and 14 days post vaccination for mononuclear cell isolation. Real-time PCR, flow cytometry, and the enzyme-linked immunospot assay were used to evaluate the levels of IFNγ gene expression, production, and secretion by cells within the spleen samples.

Results

Increased IFNγ gene expression was noticed after vaccination only in birds that did not possess MDA or possessed MDA at relatively low level (MDA+ birds vaccinated at 14 DOL). In all birds, an increased percentage of T lymphocytes producing IFNγ was recorded. The proportion of anti-aMPV IFNγ-secreting cells was increased only in MDA- birds.

Conclusion

Besides having a protective role, MDA are known to interfere with vaccination efficacy. The analysis of our results confirms that MDA can decrease the level of immune system stimulation after aMPV vaccination of turkeys.

Three-step Anti-aMPV IgA Expression Profile Evaluation in Turkeys of Different Immunological Status after TRT Vaccination

Maternally derived antibodies (MDA) don not protect turkeys against rhinotracheitis (TRT) but high MDA influences upper respiratory tract (URT) immunity stimulation after avian Metapneumovirus (aMPV) vaccination. Humoral immunity can not be considered as an indicator of protection against TRT, but specific antibodies inhibit aMPV replication and alleviate the course of TRT. Scarce reports indicate the role of IgA in protection against TRT. The aim of our study was to investigate the impact of MDA on stimulation, antigen specificity acquisition of B lymphocytes, and the production of specific IgA after TRT vaccination of turkeys.

The results of our study indicate that MDA on the day of TRT vaccination causes disturbances at different levels of specific humoral immunity expression including antigen specificity acquisition of B IgA+ lymphocytes as well as production and secretion of IgA. Vaccine immunity against aMPV associated with sIgA is well expressed in birds not possessing MDA on the day of TRT vaccination, whereas it is inhibited in MDA+ birds.

These results corroborate our previous findings and indicate that MDA could be responsible for TRT vaccination failure. These findings could explain the observed frequency of TRT field outbreaks despite aMPV vaccination of turkey flocks.

Development of vaccine-induced immunity against TRT in turkeys depends remarkably on the level of maternal antibodies and the age of birds on the day of vaccination

BACKGROUND

Avian Metapneumovirus (aMPV) infections are a huge economical issue for the poultry industry worldwide. Although maternal antibodies do not protect turkey poults against turkey rhinotracheitis (TRT), almost no studies have been conducted so far regarding the impact of these antibodies on vaccine induced immunity development against aMPV infection. We conducted four experiments on commercial turkeys aimed at comparing local humoral and cell mediated immune response of maternally delivered anti-aMPV antibody positive (MDA+; Experiment I and II) and negative (MDA-; Experiment III and IV) turkeys following vaccination with an attenuated live aMPV subtype A vaccine at the day of hatch (Experiment I and III) or at two weeks of age (Experiment II and IV).

RESULTS

Regardless of the birds' age, vaccination of MDA- turkeys resulted in strong stimulation of CD8(+) T lymphocytes in the Harderian gland and tracheal mucosa, whereas vaccination of MDA+ birds stimulated mainly CD4(+) T cells in those structures. An increase in the level of anti-aMPV IgY antibodies was noted in the serum (but not in tracheal washings) as early as 7 days after vaccination, but only in birds possessing low levels (MDA+ birds vaccinated at 2 weeks of age) or no maternal anti-aMPV antibodies at the time of vaccination. In MDA+ turkeys vaccinated at hatch, the decrease in serum levels of maternal anti-aMPV antibodies proceeded faster (in comparison to control group), which, together with faster viral clearance, indicates that maternal antibodies can inhibit vaccine virus replication and influence the development of vaccine-induced immunity.

CONCLUSION

This study provides the first documented evidence that the frequency of TRT outbreaks in the field and/or failure of TRT vaccination could be correlated with differences in the immunological status and/or age of vaccinated turkeys.

The pathogenicity of avian metapneumovirus subtype C wild bird isolates in domestic turkeys

Background

Avian metapneumovirus subtype C (aMPV/C) causes severe upper respiratory disease in turkeys. Previous report revealed the presence of aMPV/C in wild birds in the southeast regions of the U.S.

Methods

In this study, aMPV/C positive oral swabs from American coots (AC) and Canada geese (CG) were passaged three times in the respiratory tract of specific pathogen free (SPF) turkeys and used as aMPV/C P3 virus isolates in subsequent studies.

Results

Wild bird P3 isolates showed similar growth characteristics when compared to virulent aMPV/C in chicken embryo fibroblast ( CEF) cell cultures and their glycoprotein G gene sequence was closely related to the G gene of aMPV/C Colorado reference virus. Three-day-old commercial or SPF turkeys were inoculated oculonasally with wild bird aMPV/C P3 isolates. At 5 and 7 days post-inoculation (DPI), severe clinical signs were observed in both of the AC and CG virus-exposed groups. Viral RNA was detected in tracheal swabs by reverse transcriptase polymerase chain reaction (RT-PCR). In addition, immunohistochemistry showed virus replication in the nasal turbinate and trachea. All virus-exposed turkeys developed positive antibody response by 14 DPI.

Conclusions

Our data demonstrate that aMPV/C wild bird isolates induced typical aMPV/C disease in the domestic turkeys.

Pathogenic and immunogenic responses in turkeys following in ovo exposure to avian metapneumovirus subtype C

Commercial turkey eggs, free of antibodies to avian metapneumovirus subtype C (aMPV/C), were inoculated with aMPV/C at embryonation day (ED) 24. There was no detectable effect of virus inoculation on the hatchability of eggs. At 4 days post inoculation (DPI) (the day of hatch (ED 28)) and 9 DPI (5 days after hatch), virus replication was detected by quantitative RT-PCR in the turbinate, trachea and lung but not in the thymus or spleen. Mild histological lesions characterized by lymphoid cell infiltration were evident in the turbinate mucosa. Virus exposure inhibited the mitogenic response of splenocytes and thymocytes and upregulated gene expression of IFN-γ and IL-10 in the turbinate tissue. Turkeys hatching from virus-exposed eggs had aMPV/C-specific IgG in the serum and the lachrymal fluid. At 3 week of age, in ovo immunized turkeys were protected against a challenge with pathogenic aMPV/C.

Local and systemic immune responses following infection of broiler-type chickens with avian Metapneumovirus subtypes A and B

Infections with avian Metapneumovirus (aMPV) are often associated with swollen head syndrome in meat type chickens. Previous studies in turkeys have demonstrated that local humoral and cell-mediated immunity plays a role in aMPV-infection. Previous experimental and field observations indicated that the susceptibility of broilers and their immune reactions to aMPV may differ from turkeys. In the presented study local and systemic immune reactions of broilers were investigated after experimental infections with subtypes A and B aMPV of turkey origin. Both virus subtypes induced a mild respiratory disease. The recovery from respiratory signs correlated with the induction of local and systemic aMPV virus-neutralizing antibodies, which began to rise at 6 days post infection (dpi), when the peak of clinical signs was observed. In a different manner to the virus neutralizing (VN) and IgG-ELISA serum antibody titres, which showed high levels until the end of the experiments between 24 and 28 dpi, the specific IgA-ELISA and VN-antibody levels in tracheal washes decreased by 10 and 14 dpi, respectively, which may explain the recurring aMPV-infections in the field. Ex vivo cultured spleen cells from aMPV-infected broilers released at 3 and 6 dpi higher levels of IFN-γ after stimulation with Concanavalin A as compared to virus-free birds. In agreement with studies in turkeys, aMPV-infected broilers showed a clear CD4+ T cell accumulation in the Harderian gland (HG) at 6 dpi (P<0.05). In contrast to other investigations in turkeys aMPV-infected broilers showed an increase in the number of CD8alpha+ cells at 6 dpi compared to virus-free birds (P<0.05). The numbers of local B cells in the Harderian gland were not affected by the infection. Both aMPV A and B induced up-regulation of interferon (IFN)-γ mRNA-expression in the nasal turbinates, while in the Harderian gland only aMPV-A induced enhanced IFN-γ expression at 3 dpi. The differences in systemic and local T cell and possibly natural killer cell activity in the HG between turkeys and chickens may explain the differences in aMPV-pathogenesis between these two species.

Protection against avian metapneumovirus subtype C in turkeys immunized via the respiratory tract with inactivated virus

Avian metapneumovirus subtype C (aMPV/C) causes a severe upper respiratory tract (URT) infection in turkeys. Turkeys were inoculated oculonasally with inactivated aMPV/C adjuvanted with synthetic double-stranded RNA polyriboinosinic polyribocytidylic acid (Poly IC). Immunized turkeys had elevated numbers of mucosal IgA+ cells in the URT and increased levels of virus-specific IgG and IgA in the lachrymal fluid and IgG in the serum. After 7 or 21 days post immunization, turkeys were challenged oculonasally with pathogenic aMPV/C. Immunized groups were protected against respiratory lesions induced by the challenge virus. Further, the viral copy number of the challenge virus in the URT were significantly lower in the immunized turkeys than in the unimmunized turkeys (P<0.05). These results showed that inactivated aMPV/C administered by the respiratory route induced protective immunity against pathogenic virus challenge.