Antigenicity of Two Turkey Astrovirus Isolates

The C-terminal end of the capsid protein of Avian Nephritis Virus is antigenic and induces broadly cross-reactive antibodies

Avian nephritis virus (ANV) has been isolated frequently from commercial broilers in many countries. The prevalence and economic impact of ANV however has been difficult to ascertain due to the lack of convenient serological techniques. In this study the full-length and fragments of the ANV capsid protein were expressed in Baculovirus and affinity purified recombinant proteins used for the detection of ANV antibodies in ELISA. The crystal structure of Human Astrovirus (HAstV) was used as a model to determine potential homologous C-terminal antigenic regions in ANV. The rp37 fragment from three ANV strains NSW_3, ANV-1 and ANV-2, and a shorter NSW_3 fragment (rp33) were compared for their ability to detect ANV antibodies in seven reference chicken sera. The ANV-1 rp37 antigen was the most strain specific whereas the NSW_3 rp37 and rp33 antigens detected antibodies in all heterologous sera, including ANV-1 serum. Irrespective of the strain used, the two NSW_3 protein fragments rp37 and rp33 were found to be superior as antigens for ELISA when compared to the full-length capsid protein rp75. An ELISA designed using the NSW_3 rp33 could reliably differentiate between uninfected and infected commercial broiler flocks, as demonstrated by statistically significant differences between the OD values. This study identified an ANV immunogenic region and successfully used recombinant protein expression of this region to detect cross-reactive ANV antibodies. The results of this study facilitate future studies into the epidemiology and importance of ANV infections in commercial poultry.

Avian Astroviruses

Avian astroviruses comprise a diverse group of viruses affecting many avian species and causing enteritis, hepatitis, and nephritis. To date, six different astroviruses have been identified in avian species based on the species of origin and viral genome characteristics: two turkey-origin astroviruses [Turkey Astrovirus type 1 (TAstV-1) and type 2 (TAstV-2)]; two chicken-origin astroviruses [Avian Nephritis Virus (ANV) and Chicken Astrovirus (CAstV)]; and two duck-origin astrovirus [Duck Astrovirus type 1 (DAstV-1) and type 2 (DAstV-2)]. ANV has also been detected in turkeys, ducklings, pigeons, and guinea fowl; and TAstrovirus-2-like viruses have also been found in guinea fowl. Astroviruses are commonly associated with enteric disease syndromes in poultry including runting-stunting syndrome of broilers (RSS), poult enteritis complex or syndrome (PEC or PES), poult enteritis mortality syndrome (PEMS), and enteritis in guinea fowl. The molecular characterization of the different avian astroviruses shows great genetic variability among each type, and this variability influences the ability to detect these viruses by molecular and serological techniques. In this chapter, we review the different aspects related to avian astroviruses, including molecular biology, pathogenesis, diagnosis, and control.

Genetic characterization of astroviruses detected in guinea fowl (Numida meleagris) reveals a distinct genotype and suggests cross-species transmission between turkey and guinea fowl

Astroviruses can infect mammalian and avian species and are often responsible for gastroenteric disease symptoms. In this study, the complete open reading frame (ORF) 2, the 3' end of ORF1b and the corresponding intergenic region of astroviruses identified in farmed guinea fowl (Numida meleagris) were sequenced and genetically analysed. Overall, the genetic sequence of guinea fowl astroviruses was related to turkey astrovirus type 2 (TastV2), although a marked genetic distance was revealed based on ORF2, which might indicate the circulation of a distinct virus genotype and serotype in guinea fowl. Furthermore, the genetic data presented herein suggest that either recombination between different astroviruses infecting distinct hosts or adaptation of a given astrovirus to a new host had occurred. In either case, direct or indirect interspecies transmission of astroviruses is likely to have occurred between turkey and guinea fowl, indicating the ability of viruses belonging to the family Astroviridae to cross species barriers.

Astrovirus infections in humans and animals - molecular biology, genetic diversity, and interspecies transmissions

Astroviruses are small, non-enveloped, positive sense, single-stranded RNA viruses first identified in 1975 in children suffering from diarrhea and then described in a wide variety of animals. To date, the list of animal species susceptible to astrovirus infection has expanded to 22 animal species or families, including domestic, synantropic and wild animals, avian, and mammalian species in the terrestrial and aquatic environments. Astrovirus infections are considered among the most common cause of gastroenteritis in children, second only to rotavirus infections, but in animals their association with enteric diseases is not well documented, with the exception of turkey and mink astrovirus infection. Genetic variability has been described in almost all astrovirus species sufficiently examined infecting mammals and birds; however, antigenic variability has been demonstrated for human astroviruses but is far less investigated in animal viruses. Interestingly, there is an increasing evidence of recombination events occurring in astroviruses, which contributes to increase the genetic variability of this group of viruses. A wide variety of species infected, the evident virus genetic diversity and the occurrence of recombination events indicate or imply either cross-species transmission and subsequent virus adaptation to new hosts or the co-infection of the same host with different astroviruses. This can also favor the emergence of novel astroviruses infecting animals or with a zoonotic potential. After more than 30 years from their first description in humans, there are many exciting streams of research to be explored and intriguing questions that remain to be answered about the relatively under-studied Astroviridae family. In the present work, we will review the existing knowledge concerning astrovirus infections in humans and animals, with particular focus on the molecular biology, interspecies transmission and zoonotic potential of this group of viruses.

Comparison of capsid gene sequences of turkey astrovirus-2 from poult-enteritis-syndrome-affected and apparently healthy turkeys

This study was conducted to determine genetic variations in the capsid gene of turkey astrovirus-2 (TAstV-2) detected in apparently healthy and poult enteritis syndrome (PES)-affected turkeys. Capsid genes of astroviruses obtained from 30 PES-affected and 45 apparently healthy turkey flocks had sequence homologies of 73.4-100% and 72.4-100% at the nucleotide levels, respectively. The analysis of deduced amino acid sequences revealed one amino acid deletion at position 552 in 28 (93.3%) of 30 PES-affected cases. However, there were two deletions (at positions 551 and 552) in 31 (68.9%) of 45 TAstV-2 from apparently healthy flocks. The TAstV-2 (6.7%) from two PES-affected cases had two amino acid insertions each between positions 552 and 553, while TAstV-2 from 14 (31.1%) of 45 healthy flocks had two insertions at the same position. Phylogenetic analysis based on nucleotide sequences revealed that the astroviruses in this study were closely related to most of the previously published TAstV-2 isolates. The sequence homology of TAstV-2 in this study ranged from 70.4% to 99.4% at the nucleotide level with those of previously published TAstV-2 isolates. The variations at the amino acid level in the capsid gene suggest the possibility of the existence of different serotypes of turkey astrovirus. The close relationship of turkey astroviruses from apparently healthy flocks to those from PES-affected cases in capsid gene phylogeny necessitates further studies to compare complete capsid gene sequences from both types of flocks from different geographic areas for better understanding of TAstV circulating in turkeys.

Molecular characterization of avian astroviruses

Astroviruses are frequently associated with enteric diseases in poultry, being isolated from cases of runting-stunting syndrome (RSS) of broiler chickens, poult enteritis complex (PEC), and poult enteritis mortality syndrome (PEMS) of turkeys. Currently, five types of avian astrovirus have been identified: turkey astroviruses 1 and 2 (TAstV-1, TAstV-2), avian nephritis virus (ANV), chicken astrovirus (CAstV) and duck astrovirus (DAstV). The objective of this study was to molecularly characterize the different types of avian astroviruses circulating in commercial poultry. Sequence analysis of a region of ORF2, which encodes the capsid precursor protein associated with serotype and viral pathogenesis, revealed extensive variation in amino acid sequence within each subtype: TAstV-2 (81.5%-100%), ANV (69.9%-100%), and CAstV (85.3%-97.9%). However, this region was more conserved in TAstV-1's (96.2%-100%). Furthermore, a novel astrovirus was detected in chicken samples and found to be <64% similar to ANV and <30.6% similar to CAstV. The results of this study underline the great genetic variability of avian astroviruses and indicate that there are most likely multiple serotypes of each avian astrovirus circulating in commercial poultry.

Detection and molecular characterization of enteric viruses in breeder turkeys

The present study was undertaken to detect and characterize enteric viruses (rotavirus, astrovirus, reovirus, and coronavirus) in breeder poults. Five turkey breeder flocks were selected. Faecal samples were collected from all flocks at 1 week of age and then every other week until the poults reached 9 weeks of age. The faecal samples were pooled in groups of five. Of the 193 pools ("samples") tested by reverse transcription-polymerase chain reaction, 47.2%, 30.6%, and 10.4% samples were positive for astrovirus, rotavirus, and reovirus, respectively. No coronavirus was detected in any of the samples. Overall, 118 (61.1%) samples were positive for one or more enteric viruses. Of the 118 samples, 70 (59.3%) were positive for a single virus and 48 (40.7%) for a combination of viruses. Phylogenetic analysis based on the polymerase gene showed that astroviruses clustered into two groups with sequence homology ranging from 85.6 to 100% at the nucleotide level. Based on NSP4 gene sequences, rotaviruses clustered in a group and had 96.3 to 99.9% sequence homology at the nucleotide level. The reoviruses, based on their S4 gene sequences, clustered in a single group with sequence homology of 96.9 to 100%. Differing amino acid sequences of all three viruses may affect the antigenicity and/or pathogenicity of these viruses and may merit further study. The presence of two or three different viruses in combination may affect the dynamics of turkey health and disease.

Epidemiological aspects of astrovirus and coronavirus in poults in the South Eastern Region of Brazil

A survey of Turkey Coronavirus (TCoV) and Astrovirus (TAstV-2) prevalence was carried out from February to December during 2006 year in semiarid region of Brazil, from a turkey producer area, localized in South Eastern of Brazil. To asses the risk factor related to clinical material, climatic condition and type of RT-PCR applied, cloacal swabs (CS), faeces, sera, bursa of Fabricius (BF), thymus (TH) and spleen (SP) and ileum-caeca region were collected from 30-day-old poults suffering of enteritis episode characterized as poult enteritis mortality syndrome (PEMS). The PEMS clinical features were characterized by watery to foamy faeces, light brown-yellow in colour and low mortality rate. Meteorological data (rainfall and relative humidity) observed during along the study presented monthly average temperature ranging from 39.3 and 31.2ºC, precipitation in rainy season from 40 to 270.3 mm/month, and no rain during dry season. Simplex RT-PCR gave odds ratio (OR) values suggesting that ileum-caeca region is at higher chance (OR=1.9; p=0.9741) to have both viral RNA than faeces (OR=1.5; p=0.7319). However, multiplex RT-PCR showed 3.98 (p=0.89982) more chance to give positive results in faeces than CS at dry season. The major risk factors seem to be low rate of humidity and high temperatures at winter, probably responsible for spread, easily, the TCoV and TAstv-2 among the flocks. The positive results of both virus suggested that they can play an important role in enteric disorders, associated to low humidity and high temperatures frequently found in tropical countries.

Pathogenesis of type 2 turkey astroviruses with variant capsid genes in 2-day-old specific pathogen free poults

The pathogenicity of three different type 2 turkey astroviruses (TAstV-2) was studied in specific pathogen free turkeys. These viruses differ based on sequence analysis of the capsid gene. Poults were inoculated at 2 days of age and examined during 14 days for clinical signs and virus shedding. All inoculated poults presented signs of enteric disease including diarrhoea and growth depression. Virus presence and shedding was detected by real-time reverse transcriptase-polymerase chain reaction from intestinal contents and cloacal swabs collected at 3, 7 and 14 days post-inoculation. Viraemia was also confirmed by this method. Common lesions observed at necropsy were dehydration; distended intestines filled with watery contents and undigested feed, and dilated caeca with foamy contents. Microscopic lesions present in the intestines consisted of mild crypt hyperplasia, villous atrophy and lymphocytic infiltration, and were most common in the jejunum. Presence of the viruses was demonstrated by immunohistochemistry and by in situ hybridization in both villi and crypt enterocytes in the jejunum and, less frequently, the duodenum, ileum and caeca. Mild lesions consisting mainly of lymphocytic infiltration were also observed in other organs including the pancreas, liver, spleen and kidneys. Mild to moderate bursal atrophy occurred in all TAstV-2-infected poults examined; however, no specific viral staining was observed in this organ or any other tissues examined apart from the intestines. In conclusion, TAstV-2 viruses with variant capsids produce a similar enteric disease in young turkeys and may also affect the immune system of the birds by causing bursal lymphoid depletion.

Detection of Turkey astrovirus in young poults affected with poult enteritis complex in Brazil

Reverse transcription polymerase chain reaction (RT-PCR) of turkey astrovirus (TAstV) capsid and polymerase genes was applied to the bursa of Fabricius (BF), thymus (TH), spleen (SP) and cloacal swabs (CS) of young poults with "Poult enteritis complex" (PEC). The histological lesions included atrophy, lymphoid depletion, cellular infiltration and necrosis of the BF, TH and SP, respectively. The RT-PCR reactions were positive for the polymerase gene of TAstV-2 in all 100 CSs, 7 out of 10 of BFs and 10 out of 20 THs and SPs, respectively. Five out of 10 THs and SPs samples, considered to be negative by RT-PCR, were positive when specific primers designed for the TAstV-2 capsid gene were applied. This is the first description of turkey astrovirus infection presenting PEC in Latin America.

Genomic analysis of closely related astroviruses

To understand astrovirus biology, it is essential to understand factors associated with its evolution. The current study reports the genomic sequences of nine novel turkey astrovirus (TAstV) type 2-like clinical isolates. This represents, to our knowledge, the largest genomic-length data set available for any one astrovirus type. The comparison of these TAstV sequences suggests that the TAstV species contains multiple subtypes and that recombination events have occurred across the astrovirus genome. In addition, the analysis of the capsid gene demonstrated evidence for both site-specific positive selection and purifying selection.

Co-circulation of distinct genetic lineages of astroviruses in turkeys and guinea fowl

Viruses belonging to the genus Astrovirus have been increasingly associated with enteritis in mammalian and avian species, including turkeys. More recently, astroviruses have also been detected in diseased guinea fowl. In turkeys, two genetically distinct types of astrovirus have been reported, namely turkey astrovirus 1 and 2 (TAstV1 and TAstV2). The prevalence and the pathogenesis of astrovirus infections in this species is currently unknown, with the exception of data generated in the USA. In the present report, we have demonstrated the co-circulation of distinct genetic lineages of astroviruses infecting turkeys and guinea fowl. The predominant lineage infecting turkeys is genetically related to the American TAstV2, but genetic variability within this lineage was demonstrated. Other isolates appeared to be either TastV1-related or unrelated to any other known isolate. Astroviruses infecting guinea fowl were more closely related to TAstV2, and interspecies transmissions between turkeys and guinea fowl is suggested as a possible mechanism by which these viruses may have evolved, based on the genetic data available. This investigation provides genetic and epidemiological information contributing to a better understanding of enteric viral infections in turkeys and guinea fowl.

Detection of antibodies against serotypes 1 and 2 infectious bursal disease virus by commercial ELISA kits

Two distinct serotypes of infectious bursal disease virus (IBDV) are recognized in chicken and turkey flocks in the United States. Serologic testing of chicken flocks for serotype 1 viruses is routinely performed to monitor disease status and vaccination. Earlier studies indicated that enzyme-linked immunosorbent assay (ELISA) test detects antibodies to both serotypes of the virus, while the virus neutralization (VN) test is serotype specific. It is useful to evaluate currently available commercial ELISA kits for their ability to differentiate between antibodies elicited by the two serotypes. Three trials were performed in which chickens were orally inoculated with either a high or a low dose of serotype 1 STC or serotype 2 OH strains of IBDV. Sera collected at 0, 7, 14, and 21 days from these chickens and antisera procured from naturally infected broiler (n=20) and layer (n=30) flocks were tested with five different commercial ELISA kits and by VN. All ELISA kits detected different levels of antibodies elicited against serotype 1 of the virus and moderate and high levels of antibodies against serotype 2 virus. A correlation existed between the ELISA and the VN titers of experimentally infected chickens. All serum samples tested from the commercial layer flocks and 65% of the broiler flocks had antibodies against the OH strain. However, no correlation between the VN titers and ELISA titers was observed for the commercial broilers and layers sera by the majority of the kits. The results indicated that currently available commercial ELISA kits detect antibodies elicited by the two serotypes of IBDV. Hence, the prevalence of serotype 2 antibodies in the flocks should be considered while determining antibody profiles of the flocks against serotype 1 viruses.

Molecular Characterization of the Capsid Gene of Two Serotypes of Turkey Astroviruses

Astrovirus infections mainly cause acute gastroenteritis in children and young animals. Human astroviruses are well characterized antigenically and genetically. However, information on turkey astroviruses is limited. We isolated two astroviruses (TAstV1987 and TAstV2001) from turkeys and classified them as two different serotypes using a virus neutralization test. To elucidate the differences between these two isolates at the molecular level, further genetic characterization and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis were carried out. The sequences of the complete capsid protein gene of these two isolates were obtained by cloning and sequencing. The percentage nucleotide and predicted amino acid identities for these two sequences along with those of 16 other capsid protein gene sequences from human and animal astroviruses retrieved from GenBank were calculated using MegAlign. The results showed that TAstV1987 and TAstV2001 had 73.3% nucleotide and 82.8% amino acid identities, respectively. An unrooted Neighbor-joining phylogenetic tree of these sequences was generated using MEGA 3 software with 1000 bootstrap replicates. The results of evolutionary analysis showed that TAstV1987 was closely related genetically to another virus, designated TAstV-2, whereas TAstV2001 was not as close to TAstV-2 as TAstV1987. The analysis of the capsid proteins of the two viruses by SDS-PAGE revealed that they had different band patterns, indicating that their capsid proteins consisted of different viral proteins. The findings in this study revealed the molecular differences in the capsid protein gene of TAstV1987 and TAstV2001, which may provide the molecular basis of the antigenic differences between these two serotypes of turkey astroviruses.

Development of antigen-capture enzyme-linked immunosorbent assay and RT-PCR for detection of turkey astroviruses

Turkey astrovirus (TAstV) is an important agent of poult enteritis. The diagnosis of astroviruses has been dependent mainly on electron microscopy (EM) or immune EM (IEM). To develop other simple, rapid, and reliable diagnostic assays, two antigen-capture enzyme-linked immunosorbent assays (AC-ELISAs), polyclonal AC-ELISA and monoclonal AC-ELISA, were developed in this study. Monoplex and multiplex reverse transcription-polymerase chain reactions (RT-PCRs) were also developed using nondegenerate primer sets specific to the capsid region and degenerate primer pairs specific to the polymerase area of two TAstV. EM was included for comparison. Fecal or intestinal contents samples from naturally and experimentally infected poults with enteritis were examined using the developed assays. The polyclonal AC-ELISA had higher sensitivity and wider detection spectrum than the monoclonal AC-ELISA with group-specific monoclonal antibody (MAb), whereas the monoclonal AC-ELISA had very high specificity but lower sensitivity, which was estimated at 0.06 microg of viral proteins. Small round viruses (SRV) that could be astroviruses or other small viruses were detected in 34.4% of the samples examined by EM. The monoplex RT-PCR results amplified with primers SRV-1-3 and SRV-1-5 revealed that the positive rate of astroviruses was 45.3%, which was 10.9% higher than that of EM even if other SRVs were not excluded. Multiplex RT-PCR with SRV-1-3 and SRV-1-5 and AFCP-F1 and AFCP-R1 and the monoplex RT-PCR with degenerate primers verified that the positive rate of astroviruses was 59.4%, which was 25% higher than that of EM. Both RT-PCRs showed good specificity and wider detection spectrum compared with earlier published data.

Interference Between Mild and Pathogenic Strains of Infectious Bursal Disease Virus in Chickens

Infectious bursal disease virus is a contagious, immunosuppressive disease of young chickens that is controlled by vaccination. Cross-protection occurs between different strains of the virus as a result of shared neutralizing epitopes. However, interactions between two antigenically similar strains (a mild and a pathogenic) coinfecting the same host have not been investigated. Groups of specific-pathogen-free chickens were inoculated with a mild strain followed by a pathogenic strain at 0, 16, 24, or 48 hr postinoculation (PI) with a mild strain. Virus persistence and the predominant strain of the virus were determined by reverse transcriptase-polymerase chain reaction and restriction fragment length polymorphism analysis, respectively, in bursas at 2, 4, 8, 14, and 21 days PI with the pathogenic strain. Severity of infection was assessed by the bursa/body weight ratios and histopathologic lesion scores. The mild virus interfered with replication of the pathogenic virus. The greatest interference was observed when the pathogenic strain was inoculated 24 hr PI with the mild strain. The interference phenomenon observed might be due to competition for host receptor sites or production of cytokine(s). This interference phenomenon could have practical implications for vaccine usage and protection.