The Immunological Basis for Vaccination

Vaccination is crucial for health protection of poultry and therefore important to maintaining high production standards. Proper vaccination requires knowledge of the key players of the well-orchestrated immune system of birds, their interdependence and delicate regulation, and, subsequently, possible modes of stimulation through vaccine antigens and adjuvants. The knowledge about the innate and acquired immune systems of birds has increased significantly during the recent years but open questions remain and have to be elucidated further. Despite similarities between avian and mammalian species in their composition of immune cells and modes of activation, important differences exist, including differences in the innate, but also humoral and cell-mediated immunity with respect to, for example, signaling transduction pathways, antigen presentation, and cell repertoires. For a successful vaccination strategy in birds it always has to be considered that genotype and age of the birds at the time point of immunization as well as their microbiota composition may have an impact and may drive the immune reactions into different directions. Recent achievements in the understanding of the concept of trained immunity will contribute to the advancement of current vaccine types helping to improve protection beyond the specificity of an antigen-driven immune response. The fast developments in new omics technologies will provide insights into protective B- and T-cell epitopes involved in cross-protection, which subsequently will lead to the improvement of vaccine efficacy in poultry.

Germline engineering of the chicken genome using CRISPR/Cas9 by in vivo transfection of PGCs

Development of simple and readily adoptable methods to mediate germline engineering of the chicken genome will have many applications in research, agriculture and industrial biotechnology. We report germline targeting of the endogenous chicken Interferon Alpha and Beta Receptor Subunit 1 (IFNAR1) gene by in vivo transgenic expression of the high-fidelity Cas9 (Cas9-HF1) and guide RNAs (gRNAs) in chickens. First, we developed a Tol2 transposon vector carrying Cas9-HF1, IFNAR1-gRNAs (IF-gRNAs) and green fluorescent protein (GFP) transgenes (pTgRCG) and validated in chicken fibroblast DF1 cells. Next, the pTgRCG plasmid was directly injected into the dorsal aorta of embryonic day (ED) 2.5 chicken embryos targeting the circulating primordial germ cells (PGCs). The resulting chimera roosters generated a fully transgenic generation 1 (G1) hen with constitutive expression of Cas9-HF1 and IF-gRNAs (G1_Tol2-Cas9/IF-gRNA). We detected a spectrum of indels at gRNA-targeted loci in the G1_Tol2-Cas9/IF-gRNA hen and the indels were stably inherited by the G2 progeny. Breeding of the G1_Tol2-Cas9/IF-gRNA hen resulted in up to 10% transgene-free heterozygote IFNAR1 mutants, following null-segregation of the Tol2 insert. The method described here will provide new opportunities for genome editing in chicken and other avian species that lack PGC culture.

The Importance of the Bursa of Fabricius, B Cells and T Cells for the Pathogenesis of Marek’s Disease: A Review

The importance of the bursa of Fabricius (BF) for the pathogenesis of Marek’s disease (MD) has been studied since the late 1960’s. In this review, the results of these studies are analyzed in the context of the developing knowledge of the immune system of chickens and the pathogenesis of MD from 1968 to 2022. Based on the available techniques to interfere with the development of the BF, three distinct periods are identified and discussed. During the initial period between 1968 and 1977, the use of neonatal bursectomy, chemical methods and irradiation were the main tools to interfere with the B lymphocyte development. The application of these techniques resulted in contradictory results from no effects to an increase or decrease in MD incidence. Starting in the late 1970’s, the use of bursectomy in 18-day-old embryos led to the development of the “Cornell model” for the pathogenesis of MD, in which the infection of B lymphocytes is an important first step in MD virus (MDV) replication causing the activation of thymus-derived lymphocytes (T cells). Following this model, these activated T cells, but not resting T cells, are susceptible to MDV infection and subsequent transformation. Finally, B-cell knockout chickens lacking the J gene segment of the IgY heavy chain gene were used to further define the role of the BF in the pathogenesis of MD.

Pullorum Disease: Evolution of the Eradication Strategy

The history of pullorum disease is closely intertwined with the history of avian health research and that of the poultry industry. The seriousness of the disease galvanized the attention and brought together, for the first time, the pioneers of poultry health research to work cooperatively on different aspects of the disease. Control of the disease made it possible for intensive poultry production to develop as the basis for the modern poultry industry. During the early 1900s, bacillary white diarrhea (BWD) was a devastating disease of young chickens threatening the developing poultry industry. Dr. Leo F. Rettger isolated and described the bacterial pathogen, Salmonella enterica serotype Pullorum, for the first time in 1900. BWD was renamed pullorum disease in 1929. In subsequent years, Rettger and coworkers were able to reproduce the disease and fulfill Koch's postulates. Rettger et al. also showed that Salmonella Pullorum was vertically transmitted, which was the first time that a pathogen was shown to be vertically transmitted. The development of serologic tests was of crucial importance because it led to the development of effective eradication methods to identify carrier birds and to exclude these birds from the breeder flocks. The negative impact of pullorum disease on the poultry industry ultimately was one of the major reasons that the National Poultry Improvement Plan (NPIP) was developed by scientists, the poultry industry, and the United States Department of Agriculture (USDA). Needless to say, the work of the pioneering researchers formed the basis for the control of the disease. The NPIP started in 1935, with 34 states participating in testing 4 million birds representing 58.2% of the birds hatched. The program rapidly expanded to 47 states by 1948 and tested more than 30 million birds. In 1967, all commercial chicken hatcheries participating in the NPIP were 100% free of pullorum and typhoid disease caused by Salmonella enterica serotype Gallinarum. This historical overview of pullorum disease describes in some detail the progress made, especially during the early years, toward controlling this disease using methodologies that were often very basic but nonetheless effective. One has to admire the ingenuity and persistence of the early researchers leading to their achievements considering the research tools that were available at the time.

Primary Chicken and Duck Endothelial Cells Display a Differential Response to Infection with Highly Pathogenic Avian Influenza Virus

Highly pathogenic avian influenza viruses (HPAIVs) in gallinaceous poultry are associated with viral infection of the endothelium, the induction of a ‘cytokine storm, and severe disease. In contrast, in Pekin ducks, HPAIVs are rarely endothelial tropic, and a cytokine storm is not observed. To date, understanding these species-dependent differences in pathogenesis has been hampered by the absence of a pure culture of duck and chicken endothelial cells. Here, we use our recently established in vitro cultures of duck and chicken aortic endothelial cells to investigate species-dependent differences in the response of endothelial cells to HPAIV H5N1 infection. We demonstrate that chicken and duck endothelial cells display a different transcriptional response to HPAI H5N1 infection in vitro—with chickens displaying a more pro-inflammatory response to infection. As similar observations were recorded following in vitro stimulation with the viral mimetic polyI:C, these findings were not specific to an HPAIV H5N1 infection. However, similar species-dependent differences in the transcriptional response to polyI:C were not observed in avian fibroblasts. Taken together, these data demonstrate that chicken and duck endothelial cells display a different response to HPAIV H5N1 infection, and this may help account for the species-dependent differences observed in inflammation in vivo.

Cytokine Responses in Tracheas from Major Histocompatibility Complex Congenic Chicken Lines with Distinct Susceptibilities to Infectious Bronchitis Virus

The chicken major histocompatibility complex (MHC) B locus has been linked to resistance to infectious diseases. We have previously provided evidence that the MHC congenic chicken lines 331/B2 and 335/B19 differ in susceptibility to infectious bronchitis virus (IBV) strains M41 and ArkDPI in in vivo challenge experiments. Innate immune responses can be difficult to measure in vivo because they are nonspecific and can be triggered by environmental factors. In an attempt to address this issue, we used tracheal organ cultures derived from 331/B2 and 335/B19 birds to study local cytokine production after in vitro challenge with IBV M41. Interferon (IFN)-β, interleukin (IL)-1β, IL-6, and IL-10 gene expression and production were assessed. Tracheal organ cultures derived from 335/B19 birds presented an increased inflammatory response compared to 331/B2. However, it was not possible to discriminate between cytokine responses in IBV-infected and phosphate-buffered saline-treated tracheal organ cultures. Because tracheal processing entails physical damage to the trachea, it is possible that the tracheal organ cultures presented high levels of inflammation regardless of the IBV challenge. To demonstrate the effects of IBV on innate immune responses in the MHC congenic chicken lines, we performed an additional in vivo experiment that focused on cytokine gene expression and production in tracheas up to 60 hr after a challenge with IBV M41. Our results corroborate previous in vivo observations that suggest that detrimental local inflammatory responses in 335/B19 birds might be associated with their susceptibility to IBV and that inflammation does not necessarily lead to the assembly of an appropriate adaptive immune response. This work provides further insight into the increased susceptibility of 335/B19 birds to infectious bronchitis.

Virus-Induced Immunosuppression in Chickens

A healthy immune system is a cornerstone for poultry production. Any factor diminishing the immune responses will affect production parameters and increase cost. There are numerous factors, infectious and noninfectious, causing immunosuppression (IS) in chickens. This paper reviews the three viral diseases that most commonly induce IS or subclinical IS in chickens: Marek's disease virus (MDV), chicken infectious anemia virus (CIAV), and infectious bursal disease virus (IBDV), as well as the interactions among them. MDV-induced IS (MDV-IS) affects both humoral and cellular immune responses. It is very complex, poorly understood, and in many cases underdiagnosed. Vaccination protects against some but not all aspects of MDV-IS. CIAV induces apoptosis of the hemocytoblasts resulting in anemia, hemorrhages, and increased susceptibility to bacterial infections. It also causes apoptosis of thymocytes and dividing T lymphocytes, affecting T helper functions, which are essential for antibody production and cytotoxic T lymphocyte (CTL) functions. Control of CIAV is based on vaccination of breeders and maternal antibodies (MAbs). However, subclinical IS can occur after MAbs wane. IBDV infection affects the innate immune responses during virus replication and humoral immune responses as a consequence of the destruction of B-cell populations. Vaccines with various levels of attenuation are used to control IBDV. Interactions with MAbs and residual virulence of the vaccines need to be considered when designing vaccination plans. The interaction between IBDV, CIAV, and MDV is critical although underestimated in many cases. A proper control of IBDV is a must to have proper humoral immune responses needed to control CIAV. Equally, long-term control of MDV is not possible if chickens are coinfected with CIAV, as CIAV jeopardizes CTL functions critical for MDV control.

A proposed nomenclature for infectious bursal disease virus isolates

Infectious bursal disease virus (IBDV) was initially identified in the USA. For decades, these viruses were not categorized using a typing system because they were considered to be antigenically and pathogenically similar. In the 1980s, a second major serotype, serotype 2, was found in turkeys. Classification of IBDV became more complex with the discovery of antigenic variant strains called "variants" in the United States and a highly virulent strain known as "very virulent" or vvIBDV identified in Europe. To distinguish the IBDV strains identified prior to this time from the antigenic variant viruses, the term "classic viruses" was adopted. Studies over the next three decades produced a wealth of information on the antigenicity, pathogenicity and molecular structure of IBDV isolates. These data made it clear that the descriptive nomenclature used for IBDV was inadequate. For example, not all viruses identified as vvIBDV by genotyping are highly pathogenic; some have reassorted genome segments that result in lower virulence. Furthermore, variant viruses are not an antigenically homogeneous group and the term "classic virus" has been used interchangeably to describe antigenic and pathogenic types of IBDV. These and other issues make the current naming system for strains of IBDV archaic. The lack of uniform testing and standards for antigenicity and pathogenicity makes it difficult to categorize IBDV strains on a global basis. A new nomenclature that includes a genotyping system that can easily be applied worldwide is proposed and serves as a platform to begin discussions on its value to the scientific community.

Lack of evidence that avian oncogenic viruses are infectious for humans: a review

Chickens may be infected with three different oncogenic viruses: avian leukosis virus (ALV), reticuloendotheliosis virus (REV), and Marek's disease herpesvirus (MDV). Several epidemiological studies have suggested a link between these viruses and different types of cancer in people working in poultry processing plants and with multiple sclerosis. In this article, we analyze the epidemiological evidence that these viruses are causative agents for human cancer, followed by description of the relevant key characteristics of ALV, REV, and MDV. Finally, we discuss the biological evidence or lack thereof that avian tumor viruses are involved in the etiology of human cancer and multiple sclerosis (MS). The recent primary epidemiologic articles that we reviewed as examples were only hypothesis-generating studies examining massive numbers of risk factors for associations with various imprecise, non-viral-specific outcomes. The studies lacked precise evidence of exposure to the relevant viruses and the statistical methods failed to adjust for the large risks of false-positive claims. ALV subgroups A-D and J have been eradicated in the United States from the pure lines down to the parent stocks by the breeder companies, which have greatly reduced the incidence of infection in layer flocks and broilers. As a consequence, potential exposure of humans to these viruses has greatly diminished. Infection of humans working in processing plants with ALV-A and ALV-B is unlikely, because broilers are generally resistant to infection with these two subgroups. Moreover, these viruses enter cells by specific receptors present on chicken, but not on mammalian, cells. Infection of mammalian cell cultures or animals with ALV-A, ALV-B, and ALV-J has not been reported. Moreover, humans vaccinated with exogenous or endogenous ALV-contaminated vaccines against yellow fever, measles, and mumps did not become antibody- or virus-positive for ALV. The risks for human infection with REV are similarly limited. First of all, REV also has been eradicated from pure lines down to parent stock by breeder companies in the United States. Broilers can still become infected with REV through infection with fowl pox virus containing REV. However, there is no indication that REV can infect human cells. Low levels of antibodies to ALV and REV in human sera have been reported by a few groups. Absorption of sera with chicken antigens reduced the antibody titers, and there was no clear association with contacts with poultry. Possible cross-reactions with human endogenous or exogenous retroviruses were not considered in these publications. MDV is typically associated with infection of chickens, and almost all experimental data show that MDV cannot infect mammalian cells or animals, including nonhuman primates. One study reports the presence of MDV gD DNA in human sera, but this finding could not be confirmed by another group. A Medline search of the term "gene expression in human cancers" was negative for publications with avian retroviruses or MDV. In conclusion, there is no indication that avian oncogenic viruses are involved in human cancer or MS or even able to infect and replicate in humans.

Immunity to Marek's disease: where are we now?

Marek's disease (MD) in chickens was first described over a century ago and the causative agent of this disease, Marek's disease virus (MDV), was first identified in the 1960's. There has been extensive and intensive research over the last few decades to elucidate the underlying mechanisms of the interactions between the virus and its host. We have also made considerable progress in terms of developing efficacious vaccines against MD. The advent of the chicken genetic map and genome sequence as well as development of approaches for chicken transcriptome and proteome analyses, have greatly facilitated the process of illuminating underlying genetic mechanisms of resistance and susceptibility to disease. However, there are still major gaps in our understanding of MDV pathogenesis and mechanisms of host immunity to the virus and to the neoplastic events caused by this virus. Importantly, vaccines that can disrupt virus transmission in the field are lacking. The current review explores mechanisms of host immunity against Marek's disease and makes an attempt to identify the areas that are lacking in this field.

Role of chicken astrovirus as a causative agent of gout in commercial broilers in India

Several outbreaks of gout were reported in commercial broilers in India during 2011 and 2012, causing up to 40% mortality in the birds. Gross and histopathological observations confirmed gout. Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) analysis from kidney samples of gout-affected birds indicated the presence of chicken astrovirus (CAstV) in 41.7% of cases and a mixed infection of CAstV and avian nephritis virus (ANV) in 36.4% of cases. CAstV isolated from gout-affected kidneys by inoculating embryonated specific pathogen free (SPF) eggs showed dwarfing in embryos and a cytopathic effect in chicken embryo kidney cells. Inoculation of 1-day-old SPF and broiler chicks with CAstVs caused gout and mortality between 4 and 10 days post inoculation. Virus isolation and qRT-PCR analysis showed the presence of only CAstV in inoculated chicks. Sequence analysis of capsid genes indicated a major group of Indian CAstVs that displayed 92.0 to 99.2% intergroup amino acid identity and 83.9 to 90.4% identity with subgroup Bi CAstVs of UK origin. We designated this group Indian Bi. Analysis of the partial polymerase amino acid sequences of our isolates indicated two groups of CAstVs (Indian 1 and 2) that displayed 90.2 to 95.5% amino acid identity between them. We thus report for the first time that, in addition to infectious bronchitis virus and ANV, CAstVs are a causative agent of gout.

A new method for producing transgenic birds via direct in vivo transfection of primordial germ cells

Traditional methods of avian transgenesis involve complex manipulations involving either retroviral infection of blastoderms or the ex vivo manipulation of primordial germ cells (PGCs) followed by injection of the cells back into a recipient embryo. Unlike in mammalian systems, avian embryonic PGCs undergo a migration through the vasculature on their path to the gonad where they become the sperm or ova producing cells. In a development which simplifies the procedure of creating transgenic chickens we have shown that PGCs are directly transfectable in vivo using commonly available transfection reagents. We used Lipofectamine 2000 complexed with Tol2 transposon and transposase plasmids to stably transform PGCs in vivo generating transgenic offspring that express a reporter gene carried in the transposon. The process has been shown to be highly effective and as robust as the other methods used to create germ-line transgenic chickens while substantially reducing time, infrastructure and reagents required. The method described here defines a simple direct approach for transgenic chicken production, allowing researchers without extensive PGC culturing facilities or skills with retroviruses to produce transgenic chickens for wide-ranging applications in research, biotechnology and agriculture.

Chronic Mycoplasma conjunctivitis in house finches: host antibody response and M. gallisepticum VlhA expression

Previous studies have shown that house finch field isolates of Mycoplasma gallisepticum (MG) vary in virulence and ability to induce an antibody response. After experimental inoculation, MG causes persistent, severe disease in a subset of individuals. In this study, we further characterized MG infection using five field isolates, with an emphasis on chronically diseased birds. After experimental inoculation of house finches, MG load was measured by quantitative PCR and anti-MG antibody responses were measured by ELISAs. Birds with chronic disease had significantly higher pathogen loads and antibody responses than did birds without chronic disease. Using a monoclonal antibody (MAb86) specific for a variant of the MG VlhA adhesin and immunodominant surface protein, we show that VlhA expression differs among MG isolates in this study, and that in vivo VlhA changes occur in house finches infected with MG. Overall, our results suggest that chronic MG disease has a strong pathogen-mediated component.

Characterization of chicken thrombocyte responses to Toll-like receptor ligands

Thrombocytes are the avian equivalent to mammalian platelets. In addition to their hemostatic effects, mammalian platelets rely in part on pattern recognition receptors, such as the Toll-like receptors (TLR), to detect the presence of pathogens and signal the release of certain cytokines. Ligands for TLRs include lipopolysaccharide (LPS), which is bound by TLR4, as well as unmethylated CpG DNA motifs, which are bound by TLR9 in mammals and TLR21 in chickens. Similar to mammalian platelets, avian thrombocytes have been shown to express TLR4 and secrete some pro-inflammatory cytokines in response to LPS treatment. However, the full extent of the contributions made by thrombocytes to host immunity has yet to be elucidated. Importantly, the mechanisms by which TLR stimulation may modulate thrombocyte effector functions have not been well characterized. As such, the objective of the present study was to gain further insight into the immunological role of thrombocytes by analyzing their responses to treatment with ligands for TLR4 and TLR21. To this end, we quantified the relative expression of several immune system genes at 1, 3, 8 and 18 hours post-treatment using real-time RT-PCR. Furthermore, production of nitric oxide and phagocytic activity of thrombocytes was measured after their activation with TLR ligands. We found that thrombocytes constitutively express transcripts for both pro- and anti-inflammatory cytokines, in addition to those associated with anti-viral responses and antigen presentation. Moreover, we found that both LPS and CpG oligodeoxynucleotides (ODN) induced robust pro-inflammatory responses in thrombocytes, as characterized by more than 100 fold increase in interleukin (IL)-1β, IL-6 and IL-8 transcripts, while only LPS enhanced nitric oxide production and phagocytic capabilities. Future studies may be aimed at examining the responses of thrombocytes to other TLR ligands.

Understanding the origin of seasonal epidemics of mycoplasmal conjunctivitis

1. Many host-pathogen systems show regular seasonal oscillations. 2. Seasonal variation in mycoplasmal conjunctivitis prevalence in house finches is an example of such oscillations. 3. An annual pulse of Mycoplasma gallisepticum-naïve juveniles increasing the number of susceptibles, seasonal changes in flocking behaviour increasing transmission rate and a gradual loss of resistance to reinfection with time are sufficient to model the observed seasonal variation in disease prevalence. Nevertheless, experiments are needed to test the underlying mechanisms. 4. We carried out an 18-month experiment with small groups of birds in large aviaries to test two hypotheses. 5. To test the first hypothesis that an influx of naïve juveniles in a group of recovered adults is sufficient to cause an outbreak, we added eight juveniles to a group of 11 adults that had recovered from an earlier infection. In all, three replicates juveniles became infected, but only after some of the adults relapsed. 6. To test the second hypothesis that reintroduction of M. gallisepticum into a multiage group of previously exposed but fully recovered house finches causes a new outbreak, we inoculated two birds in each group in March of the 2nd year. Contrary to what happens in the wild at that time disease prevalence increased rapidly after reintroduction of M. gallisepticum. 7. We conclude that asymptomatic, recovered adults can initiate an epidemic and transmit M. gallisepticum to naïve house finches and that the reintroduction of M. gallisepticum is sufficient to cause a new outbreak, even at a time of the year when mycoplasmal conjunctivitis is low in free-living birds. Date, as such, seems to be less important to explain seasonal variation in conjunctivitis than the presence of naïve juveniles or the introduction on M. gallisepticum. 8. Seasonality in outbreaks is most likely tightly linked to seasonal variation in bird movements and behaviour.

Role of position 627 of PB2 and the multibasic cleavage site of the hemagglutinin in the virulence of H5N1 avian influenza virus in chickens and ducks

Highly pathogenic H5N1 avian influenza viruses have caused major disease outbreaks in domestic and free-living birds with transmission to humans resulting in 59% mortality amongst 564 cases. The mutation of the amino acid at position 627 of the viral polymerase basic-2 protein (PB2) from glutamic acid (E) in avian isolates to lysine (K) in human isolates is frequently found, but it is not known if this change affects the fitness and pathogenicity of the virus in birds. We show here that horizontal transmission of A/Vietnam/1203/2004 H5N1 (VN/1203) virus in chickens and ducks was not affected by the change of K to E at PB2-627. All chickens died between 21 to 48 hours post infection (pi), while 70% of the ducks survived infection. Virus replication was detected in chickens within 12 hours pi and reached peak titers in spleen, lung and brain between 18 to 24 hours for both viruses. Viral antigen in chickens was predominantly in the endothelium, while in ducks it was present in multiple cell types, including neurons, myocardium, skeletal muscle and connective tissues. Virus replicated to a high titer in chicken thrombocytes and caused upregulation of TLR3 and several cell adhesion molecules, which may explain the rapid virus dissemination and location of viral antigen in endothelium. Virus replication in ducks reached peak values between 2 and 4 days pi in spleen, lung and brain tissues and in contrast to infection in chickens, thrombocytes were not involved. In addition, infection of chickens with low pathogenic VN/1203 caused neuropathology, with E at position PB2-627 causing significantly higher infection rates than K, indicating that it enhances virulence in chickens.

Characterization of the host response in systemic isosporosis (atoxoplasmosis) in a colony of captive American goldfinches (Spinus tristis) and house sparrows (Passer domesticus)

Systemic isosporosis, also known as atoxoplasmosis, is a common parasitic disease of passerines. Infection is thought to be endemic in wild birds with fulminant, fatal disease occurring under the influence of stress, concurrent infections, or immunosuppression. Here, we describe the histologic and immunohistochemical characteristics of the cellular infiltrate occurring in captive colonies of American goldfinches and house sparrows. Necropsies were performed on 9 birds, and histologic examination was performed on the intestines of 7 additional birds. Lesions were most severe in the proximal small intestines. Histologically, the changes ranged from variably intense infiltrates of lymphocytes that filled the lamina propria to sheets of large, atypical cells that expanded and obliterated normal mucosal epithelium and invaded through the wall of the intestine and into the ceolomic cavity. Both the smaller lymphocytes and large atypical cells were immunoreactive for CD3. Intracellular parasites consistent with Isospora were detected in the large atypical cells, but they were more easily detectable in the more differentiated lymphocytes. Polymerase chain reaction and virus isolation performed on tissues from 7 birds were negative for retroviruses and herpesvirus. The immunohistochemical results of this study and the destructive nature of the cellular infiltrate suggest that the lesion represents T-cell lymphoma. In birds, lymphomas are most often associated with herpes and retroviruses; the absence of these viruses suggests that the parasite initiated neoplastic transformation. Though much work needs to be done to prove the transformative nature of the lesions, these preliminary results suggest that passerine birds may be susceptible to parasite-associated lymphomas.

Role of Marek's disease herpesvirus in the induction of tumours in Japanese quail (Coturnix coturnix japonica) by methylcholanthrene

The QT35 cell line, established from 20-methylcholanthrene (MCA)-induced tumours in Japanese quail, is positive for Marek's disease virus (MDV), and therefore we examined whether MDV is important for the development of MCA-induced tumours. Japanese quail were inoculated with the JM16 strain of MDV at 1 or 3 days of age or left uninoculated. At 3 weeks of age, quail were injected in the breast muscle with 4 mg MCA in corn oil or corn oil alone. Quail were observed for tumours three times/week and at post mortem at 11 to 12 weeks of age. MDV DNA was detected by polymerase chain reaction (PCR) in spleens of 14/20 birds inoculated with JM16+corn oil and of 53/71 birds inoculated with JM16+MCA. Interestingly, 1/74 quail was positive in the MCA group alone for MDV DNA. Tumours were collected for histopathology, cell line development, and PCR and reverse transcriptase-PCR for the presence of MDV. Tumours developed in 38/83 MCA-treated and 32/85 JM16+MCA-treated quail. Fibrosarcomas without metastasis were the only tumours observed in the MCA-treated quail, while quail treated with JM16 and MCA developed undifferentiated tumours, fibrosarcomas, lymphosarcomas or combinations with or without metastasis. One out of 20 quail receiving JM16 alone developed a lymphosarcoma. Cell line development was not influenced by JM16. Tumours from MCA-treated quail were negative for MDV, while 19/29 were positive in the JM16+MCA group. MDV transcripts were present in 13/18 tumours examined in the JM16+MCA group. In conclusion, MDV did not affect tumour development but did influence tumour aggression and histological type.

Common garden experiment reveals pathogen isolate but no host genetic diversity effect on the dynamics of an emerging wildlife disease

Host genetic diversity can mediate pathogen resistance within and among populations. Here we test whether the lower prevalence of Mycoplasmal conjunctivitis in native North American house finch populations results from greater resistance to the causative agent, Mycoplasma gallisepticum (MG), than introduced, recently-bottlenecked populations that lack genetic diversity. In a common garden experiment, we challenged wild-caught western (native) and eastern (introduced) North American finches with a representative eastern or western MG isolate. Although introduced finches in our study had lower neutral genetic diversity than native finches, we found no support for a population-level genetic diversity effect on host resistance. Instead we detected strong support for isolate differences: the MG isolate circulating in western house finch populations produced lower virulence, but higher pathogen loads, in both native and introduced hosts. Our results indicate that contemporary differences in host genetic diversity likely do not explain the lower conjunctivitis prevalence in native house finches, but isolate-level differences in virulence may play an important role.

Pathogenicity of a quail (Coturnix coturnix japonica)-derived Marek's disease virus rescued from the QT35 cell line

The QT35 cell line was established in 1977 from methylcholanthrene-induced tumors in Japanese quail. It was later shown that at least some of the QT35 cell lines were latently infected with Marek's disease (MD) virus (MDV). An MDV-like herpesvirus, named quail MDV (QMDV), was isolated from QT35 cells in 2000 by Yamaguchi et al. To determine the pathogenicity of QMDV, we inoculated 10-day-old specific-pathogen-free chickens with QMDV JM (virulent), RB-1B (very virulent), or 584A (very virulent plus). In addition, we inoculated 5-day-old Japanese quail with QMDV, JM, or RB-1B. QMDV is pathogenic in chickens with a tumor incidence comparable to JM. QMDV also caused MD in three out of 18 infected Japanese quail. In conclusion, QMDV is a virulent MDV, and its presence in QT35 cells has implications for the use of QT35 cells for vaccine production.

Chicken anemia virus

Chicken anemia virus (CAV), the only member of the genus Gyrovirus of the Circoviridae, is a ubiquitous pathogen of chickens and has a worldwide distribution. CAV shares some similarities with Torque teno virus (TTV) and Torque teno mini virus (TTMV) such as coding for a protein inducing apoptosis and a protein with a dual-specificity phosphatase. In contrast to TTV, the genome of CAV is highly conserved. Another important difference is that CAV can be isolated in cell culture. CAV produces a single polycistronic messenger RNA (mRNA), which is translated into three proteins. The promoter-enhancer region has four direct repeats resembling estrogen response elements. Transcription is enhanced by estrogen and repressed by at least two other transcription factors, one of which is COUP-TF1. A remarkable feature of CAV is that the virus can remain latent in gonadal tissues in the presence or absence of virus-neutralizing antibodies. In contrast to TTV, CAV can cause clinical disease and subclinical immunosuppression especially affecting CD8+ T lymphocytes. Clinical disease is associated with infection in newly hatched chicks lacking maternal antibodies or older chickens with a compromised humoral immune response.

Negative modulation of the chicken infectious anemia virus promoter by COUP-TF1 and an E box-like element at the transcription start site binding δEF1

Expression of enhanced green fluorescent protein (EGFP) under control of the promoter-enhancer of chicken infectious anemia virus (CAV) is increased in an oestrogen receptor-enhanced cell line when treated with oestrogen and the promoter-enhancer binds unidentified proteins that recognize a consensus oestrogen response element (ERE). Co-transfection assays with the CAV promoter and the nuclear receptor chicken ovalbumin upstream promoter transcription factor 1 (COUP-TF1) showed that expression of EGFP was decreased by 50 to 60 % in DF-1 and LMH cells. The CAV promoter that included sequences at and downstream of the transcription start point had less expression than a short promoter construct. Mutation of a putative E box at this site restored expression levels. Electromobility shift assays showed that the transcription regulator delta-EF1 (δEF1) binds to this E box region. These findings indicate that the CAV promoter activity can be affected directly or indirectly by COUP-TF1 andδEF1.

Pathogenesis of rotavirus infection in turkey poults

The pathogenesis of rotavirus infection was examined after experimental infection of conventional and specific-pathogen-free (SPF) turkey poults. In six experiments birds were exposed to turkey rotavirus isolates Tu-1 or TU-2 or the chicken isolate Ch-1 at 7, 10 or 42 days of age. Poults were examined between 1 and 24 days post-infection (dpi) for diarrhoea, gross and histopathologic lesions, virus excretion in the intestinal tract, viral antigen in intestinal epithelial cells, and the development of serum antibodies. Between 2 and 5 dpi watery droppings were observed in conjunction with remarkable paleness of the intestinal tract which was grossly observable. Maximum viral replication occurred between 2 and 5 dpi, during which period viral antigen could be demonstrated in the epithelial cells of the duodenum, jejunum, ileum and colon. Sporadically, virus antigen-positive cells were seen in the cecum. As early as 4 to 5 dpi rotavirus antibodies could be detected by indirect immunofluorescence assays. Remarkable leukocyte infiltration of the lamina propria, vacuolation of the epithelial cells and scalloping of the villous surface at the tips were observed in the intestine of infected birds. Infection with rotavirus caused a significant impairment at 2 and 4 dpi of absorption of D-xylose from the intestinal tract.

Characterisation of two highly oncogenic strains of Marek's disease virus

The RB-1B and ALA-8 strains of Marek's disease (MD) virus, which were isolated from chickens with MD and which had been vaccinated with the herpesvirus of turkeys (HVT), were evaluated for their oncogenic potential in genetically susceptible (P-line) and resistant (N-line, PDRC) chickens. RB-1B and ALA-8 were both highly oncogenic, causing a high incidence of MD in both susceptible and resistant birds. Vaccination of P-line birds with SB-1 or HVT did not protect satisfactorily against RB-1B. However, a bivalent vaccine consisting of SB-1 and HVT enhanced protection significantly. HVT alone, and the bivalent vaccine, protected PDRC and N-line chickens well against RB-1B, but SB-1 was less protective in PDRC birds. HVT protected equally well against challenge with ALA-8 and the standard JM-10 strain. Differences in the pathogenesis of viral infection could not be detected among ALA-8, RB-1B and JM-10 between 4-7 days post-infection (d.p.i.). However, after d.p.i. 12 RB-1B caused significantly higher levels of viral internal antigen and virus isolation rates than did JM-10 in the same genetic strain. Prior vaccination prevented the expression of ALA-8 at 5 and 20 d.p.i., but not that of RB-1B. Pathogenetic events such as expression of VIA or level of virus infection appeared to be directly related to the level of protection observed in challenged birds.

Vaccination of village chickens in The Gambia against Newcastle disease using the heat-resistant, food-pelleted V4 vaccine

The Australian non-pathogenic, heat-resistant V4 strain of Newcastle disease virus (NDV) in food-pellet form was used on a single occasion to vaccinate village poultry in The Gambia. The response of the chickens to the vaccine virus was monitored with the haemagglutination inhibition (HI) test. Pre-vaccination HI tests showed that the majority of chickens tested did not have antibodies to NDV. At 4 and 12 weeks post-vaccination, vaccinated chickens showed 30 and 48% seroconversion, respectively. The HI titres were indicative of protection, but challenge experiments were not undertaken. Only a low percentage of the control groups were antibody positive at these times.

Intestinal IgA response and immunity to rotavirus infection in normal and antibody-deficient chickens

Rotavirus inoculation by oesophageal cannulation resulted in subclinical infection without decreasing intestinal D-xylose absorption in both intact and embryonally bursectomised, antibody-deficient (EBx) 8-week-old specific-pathogen-free chickens. In intact chickens, rotavirus-specific IgM, IgG and IgA responses were detected in serum, while the intestinal antibody response consisted almost entirely of IgA Serum IgG and intestinal IgA levels were increased for at least 70 days following a single inoculation with the virus. Intact chickens recovered from a primary rotavirus infection between 4 and 14 days post inoculation (dpi) and developed resistance to homotypic challenge between 14 and 28 dpi. These responses were only slightly delayed in EBx birds, which recovered from primary infection between 8 and 28 dpi and developed resistance between 14 and 42 dpi. This suggested that the intestinal IgA response in chickens participated in both recovery from and resistance to rotavirus infection, but that it was not the only mediator of recovery and resistance.

The contribution of feathers in the spread of chicken anemia virus

Chicken anemia virus (CAV) spreads vertically and horizontally, however, the process is mostly still obscure. To further clarify the horizontal CAV spread, we examined the contribution of feathers. We demonstrated that CAV could be amplified from DNA purified from feather shafts of experimentally infected chicks, and the process efficacy was evaluated by comparing the amplification of DNA purified from feather shafts and lymphoid organs of CAV-experimentally infected chicks. DNA from feathers was found as an efficient source for CAV detection. Further, to substantiate whether CAV reaches the feather shafts passively via the blood, or intrinsically, causing histopathological changes, the feather follicle tissues were examined for CAV-induced lesions. Specific histological changes were found, however, immunohistochemistry failed to detect viral proteins. To determine whether the feather shafts are a source of infective virus, they were homogenized and used to infect 1-day-old chicks via the mucosal entries (eyes, nose and oropharynx). That infection mode simulates the natural route of horizontal infection in commercial poultry houses. We demonstrated the CAV-infection by serology, virology and pathology, showing that feather shafts carry infectious CAV either on their surface or within their feather pulp, and concluded that feathers contribute to the horizontal CAV dissemination.

Interleukin-6 expression after infectious bronchitis virus infection in chickens

Viral infections in chickens pose a major health threat to the poultry industry. Infectious bronchitis virus (IBV) usually causes respiratory disease; however, the disease severity is influenced by the genotype of the chicken and the IBV strain involved. Nephropathogenic strains of IBV, such as the Australian T strain, can cause high mortalities due to kidney failure characterized by mononuclear cell infiltration and inflammation. In a previous study, a line of specific pathogen-free chickens, the S-line, was shown to be susceptible to high mortalities from IBV infection. The cause of these high mortalities is unknown but it is suspected that differential cytokine expression may play a role. With this in mind, we decided to study the role of the proinflammatory cytokine interleukin (IL)-6 during infection to determine its contribution to nephritis and influence on disease susceptibility. To investigate this, we infected the susceptible S-line and the more disease-resilient HWL line with the T strain of IBV and measured their cytokine response levels. In both lines of birds, IL-6 mRNA levels were elevated in the kidneys at 4 d postinfection. However, in S-line chickens, these levels were 20 times higher than those in the HWL chickens. In addition, S-line birds also showed three times higher serum IL-6 levels than HWL birds after IBV infection. These findings suggest that IL-6 may play a role in IBV-induced nephritis and may open an avenue to develop alternative strategies, such as the use of antiinflammatory cytokines, to overcome the nephropathogenic effects of IBV.

Isolation and molecular characterization of a new Muscovy duck parvovirus from Muscovy ducks in the USA

Between 1997 and 1999 several cases of a new disease in Muscovy ducks were reported in Pennsylvania, USA. The cases were characterized by locomotor dysfunction, weakness, recumbency, 40 to 60% morbidity and 10 to 40% mortality. The most characteristic microscopic lesions were moderate to severe degenerative rhabodomyopathy. In order to characterize the aetiological agent, virus isolation was attempted from the spleen, liver, heart, skeletal muscle and intestine by inoculation of 14-day-old Muscovy duck embryos with tissue homogenates. Deaths occurred on the second egg passage and parvoviruses were isolated by serial passage of allantoic fluid from dead embryos and then in Muscovy duck embryo fibroblast (MDEF) cultures. Parvovirus particles were observed in allantoic fluids and supernatants of MDEF cultures by transmission electron microscopy. Two genomic fragments, comprising 1108 nucleotides of the right open reading frame that codes for the structural viral proteins 1, 2 and 3, were amplified by polymerase chain reaction from one of the isolates, Muscovy duck parvovirus (MDPV)/PSU-31010. Comparison of this fragment with available sequences of other MDPV and related goose parvovirus (GPV) isolates showed that it had only 84.5% sequence identity with other MDPV isolates and 84.6% identity with the GPV isolates. This region shares over 99% identity among previously sequenced MDPV isolates and 95% identity among the related GPV isolates. This suggests that MDPV/PSU-31010 is divergent from all other sequenced MDPV and GPV isolates, and may represent a new group of avian parvoviruses.

Animal vaccination and the evolution of viral pathogens

Despite reducing disease, vaccination rarely protects against infection and many pathogens persist within vaccinated animal populations. Circulation of viral pathogens within vaccinated populations may favour the development of vaccine resistance with implications for the evolution of virus pathogenicity and the emergence of variant viruses. The high rate of mutations during replication of ribonucleic acid (RNA) viruses is conducive to the development of escape mutants. In vaccinated cattle, unusual mutations have been found in the major antigenic site of foot and mouth disease virus, which is also involved in receptor recognition. Likewise, atypical changes have been detected in the immunodominant region of bovine respiratory syncytial virus. Large deoxyribonucleic acid (DNA) viruses are able to recombine, generating new genotypes, as shown by the potential of glycoprotein E-negative vaccine strains of bovine herpesvirus-1 to recombine with wild-type strains. Marek's disease virus is often quoted as an example of vaccine-induced change in pathogenicity. The reasons for this increase in virulence have not been elucidated and possible explanations are discussed.

Genetic Analysis of a Duck Circovirus Detected in Commercial Pekin Ducks in New York

The genetic organization of the duck circovirus (DuCV) 33753-52 detected in commercial Pekin duck flocks from Long Island, NY, is described. The nucleotide sequence of virus 33753-52 exhibited high similarity with DuCVs previously detected in Germany and Hungary. It is possible that this DuCV from New York shares the same ancestor with the European counterparts. The virus 33753-52 exhibited genetic features characteristic of other circoviruses, such as the presence of two major open reading frames (rep and cap), two intergenic regions, one stem-loop structure, four intergenic direct repeats, and the conserved motifs for the rolling circle replication and for the dNTP binding domain in the Rep protein. This report is the first report of the presence of DuCV in commercial Pekin duck farms in the United States. The clinical and pathologic significance of DuCV in the duck farms located on Long Island needs to be clarified. DuCv was detected in culled birds, due to low body development, leg deformities, or arthritis. Staphylococcus aureus and Riemerella anatipestifer serotype 4 were isolated from some of the DuCV-positive birds. The apparent low prevalence of the virus suggests that at this time, this infection is not a significant problem for the duck industry in New York. However, the immunosuppressive properties of this virus need to be clarified as well as its role as a predisposing agent for other diseases.

Expression of Marek's disease virus phosphorylated polypeptide pp38 produces splice variants and enhances metabolic activity

The phosphorylated polypeptide (pp)38 of oncogenic Marek's disease (MD) herpesvirus (MDV) is expressed during lytic infections in vivo and in vitro, but its functions have not been fully elucidated. The quail cell line QT-35, latently infected with MDV, was used to generate QTP32 in which pp38 is expressed under control of a tetracycline controlled promoter to examine possible functions of pp38. Induction of pp38 did not influence late MDV genes expression, but it enhanced mitochondrial dehydrogenase activity significantly. Two new pp38 splice variants were found in induced QTP32 cells, in additional in vitro systems and MDV-infected chickens. Differential expression of full-length pp38 and splice variants suggests that the splice variants are important during latency and perhaps transformation. Polypeptides of 40 and 20kDa were detected by Western blot using monoclonal antibody H19. These polypeptides were also produced in DF-1 cells transfected with a pp38 construct in which the splice acceptor sites had been mutated. Our results add important new information to the role of pp38 in the pathogenesis of MD. The data suggest that pp38 and the two newly described splice variants may influence metabolic activity, which may have important consequences for the understanding of latency and tumor development.

Classification of Marek's disease viruses according to pathotype: philosophy and methodology

The concept of pathotype in Marek's disease (MD) probably dates from the recognition of a more virulent form of the disease in the late 1950s (Benton & Cover, 1957). Distinctions between MD virus strains were further expanded with the description of the vv pathotype in the early 1980s and of the vv+ pathotype in the 1990s. Pathotype designations reflect important biological properties that correlate with the break-through of vaccinal immunity in the field. However, pathotyping methods applied by various laboratories have not been uniform, preventing critical comparison of results. Better uniformity of pathotyping procedures is desirable.The Avian Disease and Oncology Laboratory (ADOL) method is based on induction of lymphoproliferative lesions in vaccinated chickens. This method has been used to pathotype more than 45 isolates and is the basis for the current pathotype classification of MD virus strains. Its limitations include requirements for a specific type of chickens (15x7 ab+), large numbers of animals, and a statistical method to compare lesion responses to those of JM/102W and Md5 control strains. Because of these limitations, it has not been and is not likely to be used in other laboratories. Comparability in pathotyping can be improved by the comparison of field isolates with standard prototype strains such as JM/102W, Md5 and 648A (American Type Culture Collection) or their equivalents. Data may be generated by different in vivo procedures that measure tumour induction, neurological disease (both neoplastic and non-neoplastic lesions), or solely non-neoplastic criteria (such as lymphoid organ weights or virus replication). Methods based on neoplastic criteria, especially when generated in MD-immunized chickens, will probably correlate most closely with that of the ADOL method and be most relevant to evolution of MD virus in the field. Based on data from several trials, a modification of the ADOL method that utilizes fewer chickens and can be conducted with commercial specific pathogen free strains is proposed. The modified method is based on "best fit" comparisons with prototype strains, and is expected to provide results generally comparable with the original method. A variety of other alternative criteria (see earlier) are also evaluated both for primary pathotyping and as adjuncts to other pathotyping methods. Advantages and disadvantages of alternative methods are presented.

Isolation of Marek's disease virus: revisited

Splenocytes from chickens infected with low-passage stocks of Marek's disease virus (MDV) RB-1B, a very virulent (vv) strain and vv+ RK-1 were used to compare the efficacy of chick kidney cells (CKC), chicken embryo fibroblasts (CEF) and chicken embryo kidney cells (CEKC) for virus isolation. CKC were superior to CEF and CEKC. MDV foci were present at 4 days post infection in CKC but not until 6 days post infection in CEF or CEKC. Virus yield was higher in CKC than in CEF or CEKC at 6 days post infection. Passage of RB-1B in CKC yielded a significantly higher virus increase than with CEF or CEKC. The same was true for RK-1 comparing CKC with CEKC. Interestingly, RK-1-infected CEF were negative or had very low number of foci in passage 1, but virus yield increased 500-fold to 600-fold on passage in CKC, CEF, and CEKC. Recommendations on procedures for successful virus isolation are provided.

Attenuation of Marek's disease virus by deletion of open reading frame RLORF4 but not RLORF5a

Marek's disease (MD) in chickens is caused by the alphaherpesvirus MD virus (MDV) and is characterized by the development of lymphoblastoid tumors in multiple organs. The recent identification and cloning of RLORF4 and the finding that four of six attenuated strains of MDV contained deletions within RLORF4 suggested that it is involved in the attenuation process of MDV. To assess the role of RLORF4 in MD pathogenesis, its coding sequence was deleted in the pRB-1B bacterial artificial chromosome clone. Additionally, RLORF5a was deleted separately to examine its importance for oncogenesis. The sizes of plaques produced by MDV reconstituted from pRB-1BdeltaRLORF5a (rRB-1BdeltaRLORF5a) were similar to those produced by the parental pRB-1B virus (rRB-1B). In contrast, virus reconstituted from pRB-1BDeltaRLORF4 (rRB-1BdeltaRLORF4) produced significantly larger plaques. Replication of the latter virus in cultured cells was higher than that of rRB-1B or rRB-1BdeltaRLORF5a using quantitative PCR (qPCR) assays. In vivo, both deletion mutants and rRB-1B replicated at comparable levels at 4, 7, and 10 days postinoculation (p.i.), as determined by virus isolation and qPCR assays. At 14 days p.i., the number of PFU of virus isolated from chickens infected with rRB-1BdeltaRLORF4 was comparable to that from chickens infected with highly attenuated RB-1B and significantly lower than that from rRB-1B-infected birds. The number of tumors and kinetics of tumor production in chickens infected with rRB-1BdeltaRLORF5a were similar to those of P2a chickens infected with rRB-1B. In stark contrast, none of the chickens inoculated with rRB-1BdeltaRLORF4 died up to 13 weeks p.i.; however, two chickens had tumors at the termination of the experiment. The data indicate that RLORF4 is involved in attenuation of MDV, although the function of RLORF4 is still unknown.

Positive and negative regulation of chicken anemia virus transcription

Chicken anemia virus (CAV) is a small circular single-stranded DNA virus with a single promoter-enhancer region containing four consensus cyclic AMP response element sequences (AGCTCA), which are similar to the estrogen response element (ERE) consensus half-sites (A)GGTCA. These sequences are arranged as direct repeats, an arrangement that can be recognized by members of the nuclear receptor superfamily. Transient-transfection assays which use a short CAV promoter construct that ended at the transcription start site and drive expression of enhanced green fluorescent protein (EGFP) showed high basal activity in DF-1, LMH, LMH/2A, and primary theca and granulosa cells. The estrogen receptor-enhanced cell line, LMH/2A, had significantly greater expression than LMH cells, and this expression was significantly increased with estrogen treatment. A long promoter construct which included GGTCA-like sequences downstream of the first CAV protein translation start site was found to have significantly less EGFP expression in DF-1 cells than the short promoter, which was largely due to decreased RNA transcription. DNA-protein binding assays indicated that proteins recognizing a consensus ERE palindrome also bind GGTCA-like sequences in the CAV promoter. Estrogen receptor and other members of the nuclear receptor superfamily may provide a mechanism to regulate CAV activity in situations of low virus copy number.

Pro-inflammatory Responses in Chicken Spleen and Brain Tissues after Infection with Very Virulent Plus Marek's Disease Virus

In chickens infected with virulent (v) or very virulent (vv) Marek's disease (MD) virus (MDV) strains, small to moderate increases in plasma nitric oxide (NO) levels are seen, respectively, whereas very virulent plus (vv+) strains induce very high levels in vivo. The data presented in this report show that chickens presenting with clinical neurological disease following infection with the vv+ RK-1 strain have significantly higher in vivo NO levels compared to RK-1-infected non-symptomatic chickens. Using quantitative real-time PCR (qPCR) assays, DNA was used to measure MDV copy numbers in the spleen and brain of P2a (MD-susceptible) and N2a (MD-resistant) chickens following infection with the JM-16 (v) or RK-1 (vv+) strains. RNA was used to measure inducible NO synthase (iNOS), interferon-gamma (IFN-gamma), interleukin (IL)-1beta, IL-6, and IL-8 mRNA levels, in addition to MDV-specific mRNA expression using quantitative RT-PCR (qRT-PCR) assays. Viral DNA loads were found to be considerably higher in RK-1-infected chickens than JM-16-infected chickens at most time points in both organs, with viral copy numbers being two to four logs lower in the brain. Large increases in iNOS, IFN-alpha, IL-1beta, IL-6, and IL-8 were seen in the brains of RK-1-infected chickens. These data strongly support the hypothesis that pro-inflammatory responses, including high levels of iNOS/NO, IFN-alpha, and pro-inflammatory cytokine expression in the chicken brain, may play a major role in the neurological diseases associated with vv+MDV strains.

Detection of chicken anemia virus in the gonads and in the progeny of broiler breeder hens with high neutralizing antibody titers

Previous evidence for the presence of chicken anemia virus (CAV) in the gonads of immune specific-pathogen-free chickens raised the question whether this occurs also in commercial breeders. The presence of CAV was investigated by nested PCR in the gonads and spleens of hens from two 55- and 59-week-old, CAV-vaccinated (flocks 2 and 3), and two 48- and 31-week-old non-vaccinated broiler breeder flocks (flocks 1 and 4). In addition, lymphoid tissues of 20-day-old embryos from these hens were also investigated for the presence of CAV. CAV was detected in the gonads and of 5/6 and 11/22 of the vaccinated hens and in some hens also in the spleen alone. Embryos from 7/8 and 5/18 of these hens were positive. In the non-vaccinated flocks, CAV was detected in the gonads of 11/34 and 10/10 hens in flocks 1 and 4, respectively. In addition, 11 birds in flock 1 had positive spleens. CAV DNA was detected in 3/11 and 2/10 of their embryos. CAV-positive gonads and embryos were detected in samples from hens with moderate as well as high VN antibody titers. Vaccinated chickens positive for CAV in the gonads and in their embryos had VN titers ranging from >1:512 to <1:2048. In non-vaccinated chickens, the VN titers of CAV positive chickens ranged from 1:128 to 1:4096. These results demonstrate that CAV genome can remain present in the gonads of hens in commercial broiler breeder flocks even in the presence of high neutralizing antibody titers that have been associated with protection against CAV vertical transmission. It also suggests that transmission to the progeny may occur irrespectively of the level of the humoral immune response in the hens.

Chicken Infectious Anemia Virus: An Example of the Ultimate Host–Parasite Relationship

Chicken infectious anemia virus (CIAV) is a resistant and ubiquitous virus of chickens causing disease in young chickens and immunosuppression in all birds. This paper reviews the current knowledge of CIAV with a focus on new findings indicating that immunosuppressive effects have not been fully appreciated, especially as they relate to the development of antigen-specific cytotoxic T cells. A more complete understanding of the immunosuppressive effects of CIAV emphasizes the need for better vaccines, especially for the broiler industry. In addition, a new model is proposed for the control of viral replication in the reproductive tract of specific-pathogen-free chickens, which may be latently infected. This model suggests that virus transcription is controlled by viral enhancer and repressor elements, which are regulated by different hormones. As a consequence, CIAV has a well-adapted relationship with its host, avoiding immune detection, ensuring passage of virus to the next generation, and eliciting limited pathology to the host.

Association between rate of viral genome replication and virulence of Marek's disease herpesvirus strains

The early pathogenesis of Marek's disease virus (MDV) infection is characterized by a lytic infection followed by the induction of latency. Genetically resistant N2a and susceptible P2a chickens were infected with the less virulent JM-16 or the very virulent plus (vv+) RK-1 MDV strains to examine the relationship between virulence and resistance on virus replication during 1-10 days postinfection (dpi) using real-time quantitative polymerase chain reaction (qPCR) and quantitative reverse transcriptase (qRT)-PCR assays. The numbers of copies of the viral DNA or transcripts amplified by these assays were normalized relative to cellular controls and subjected to three-way ANOVA. Viral DNA but not RNA was present in spleens at 1-3 dpi in decreasing quantities, and at 4 dpi, viral DNA started to increase concomitant with the initiation of viral transcription independently of virus strain and genetic resistance. At 6 dpi, JM-16 became latent in resistant N2a and susceptible P2a chickens with low levels of viral transcripts, but transcriptional activity increased in susceptible P2a chickens at 9 and 10 dpi. In contrast, infection with vv+ RK-1 never went into latency in both chicken lines. Viral transcripts were present from 4 to 10 dpi showing a higher and more persistent viral activity that may lead to severe damage to the lymphoid organs resulting in increased immunosuppression and increased incidence of MD. The use of qPCR and qRT-PCR to determine viral DNA load and transcriptional activity may offer an alternative to the current system of pathotyping to characterize new MDV isolates.

Detection of chicken anemia virus DNA in embryonal tissues and eggshell membranes

Chicken infectious anemia virus (CIAV) is a ubiquitous and highly resistant virus of chickens that causes anemia and death in chicks less than 3 wk of age and immunosuppression in chickens older than 3 wk of age. The production of specific-pathogen-free eggs free of CIAV is essential for research and vaccine production. Currently, flocks are screened for CIAV by antibody tests to ensure freedom from CIAV infection. Recent evidence, however, indicates that chickens may carry and vertically transmit CIAV DNA independently of their antibody status. In this study, we tested embryos and eggshell membrane residues by nested polymerase chain reaction (PCR) as a sensitive method of detecting CIAV DNA. CIAV DNA could be detected in the blastodisks and semen obtained from antibody-positive and -negative chickens. Examination of different tissues between 18 and 20 days of incubation indicated that many but not all organs of individual embryos were positive. The lymphoid organs and gonads had the highest incidence of CIAV DNA, which was significantly different (P < 0.05) from the incidence in the liver. Eggshell membrane samples from embryos or newly hatched chicks were an excellent noninvasive source for the detection of CIAV DNA, identifying significantly more positive embryos than did pooled lymphoid organs. The use of dexamethasone injections as a method to improve the detection of carrier birds did not result in an increase of vertical transmission or cause seroconversion in the treated hens. A combination of testing eggshell membrane residues at hatch and periodic testing of blood DNA by nested PCR can be used to identify chickens carrying CIAV DNA and may be used to eradicate carrier birds.