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Applied and Environmental Microbiology, March 2004, p. 1448-1454, Vol. 70, No. 3
0099-2240/04/$08.00+0 DOI: 10.1128/AEM.70.3.1448-1454.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Microbiology, University of Barcelona, 08028 Barcelona,1 Department of Animal Health and Anatomy, Centre de Recerca en Sanitat Animal, Autonomous University of Barcelona, 08193 Bellaterra, Spain2
Received 20 June 2003/ Accepted 5 December 2003
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In spite of the existence of reported cases of pneumoenteritis or encephalitis (19), PAVs do not normally produce clinically severe pathologies. Several serotypes of BAVs have been isolated from calves suffering from keratoconjunctivitis, acute febrile disease, or pneumoenteritis (19), and some, such as BAV10 and BAV subgroup II, have even been associated with acute and fatal episodes in cattle (28). Moreover, in vitro inoculations have resulted in viral recovery from swabs of nasal and conjunctival mucosa and from feces of all infected calves during the course of clinical observation, indicating that a large and continuous viral excretion follows transient viremia (19). Extended periods of excretion in feces have also been described in cases of HAd infection, including those involving strains producing respiratory diseases. The study of PAV, OAV, and BAV will thus provide new information about the molecular epidemiology of these viruses and the diversity and prevalence of adenoviruses infecting farm animals.
In a previous study, a methodology for the detection of HAd in environmental samples was developed and applied to the analysis of viral contamination in sewage, river water, seawater, and shellfish (25). HAds have been shown to be more prevalent and stable than enteroviruses in the environment and in shellfish in widely divergent geographical areas (13, 21, 24), and the nested PCR assay developed for their specific detection has been proposed as a molecular index for the presence of fecal contamination of human origin in the environment, water, and food (24). The sensitivity of this nested PCR test was previously established to be as high as one viral particle, and the same procedure has been applied to the detection of animal adenoviruses. In the present study, two sets of oligonucleotides were designed, one for the specific detection of BAV and OAV and the other for the specific detection of PAV, to evaluate the level of excretion of adenoviruses by farm animals and to devise a test to trace the specific source of viral contamination in the environment.
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Sewage samples.
Twelve urban-sewage samples were collected monthly from the Barcelona sewage network between August 2000 and July 2001. The samples were collected at the entry of a depuration plant receiving 670,000 m3/day from the urban area of Barcelona, with a population of approximately 1,800,000 inhabitants, without a significant animal wastewater content. All samples were gathered in sterile 500-ml polyethylene containers and kept at 4°C for less than 8 h prior to analysis of viral-particle concentration in phosphate-buffered saline (PBS; pH 7.4). Samples were subsequently stored at -80°C.
Concentration of viral particles from fecal samples.
Pools of fecal samples from three, four, or five animals were prepared. One gram from each pool was eluted in 3.5 ml of 0.25 N glycine buffer (pH 9.5), kept in ice for 30 min, and then centrifuged (9,200 x g for 15 min) to separate any suspended solids. Finally, the supernatant was concentrated by ultracentrifugation (110,000 x g for 1 h at 4°C) and viral particles were resuspended in 100 µl of PBS and stored at -80°C.
Concentration of viral particles from sewage samples.
Recovery of viral particles was performed as described previously (25). Briefly, 40 ml of sewage was ultracentrifuged (110,000 x g for 1 h at 4°C) to pellet all the viral particles together with any suspended material. The pellet was eluted with 4 ml of 0.25 N glycine buffer (pH 9.5), and suspended solids were separated by centrifugation at 12,000 x g for 15 min. Viruses were finally concentrated by ultracentrifugation (110,000 x g for 1 h at 4°C), resuspended in 100 µl of PBS, and stored at -80°C.
Nucleic acid extraction.
The nucleic acids from the viral concentrates were extracted using a method selected in previous studies for its high sensitivity and efficiency in PCR inhibition. The procedure is based on the use of guanidinium isothiocyanate to denature viral capsid proteins and silica particles to bind viral nucleic acids (6). Finally, nucleic acids were eluted from the support in TE buffer (10 mM Tris-0.1 mM EDTA [pH 7.4]) and stored at -20°C.
Design of specific oligonucleotides.
Specific oligonucleotides for the detection of PAV and of BAV and OAV were designed based on alignments of all available sequences of the hexon gene present in the GenBank and EMBL (European Molecular Biology Laboratory) databases. Detailed sequences of the primer sets are presented in Table 1. Furthermore, two sets of oligonucleotides were designed to amplify a region of the protease gene of PAV and of the new genus Atadenovirus.
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TABLE 1. Oligonucleotide primers used for PCR amplification and sequencing of HAd, PAV, and BAV
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Quality control of amplification method.
Standard precautions were applied in order to reduce the probability of sample contamination. Every sample was analyzed twice in independent experiments, and negative controls were added after every two samples. Positive amplifications were confirmed by sequencing of the amplified DNA.
Sequencing and analysis of amplicons.
Amplicons obtained after nested PCR were purified using a QIAquick purification kit (Qiagen, Inc.) according to the manufacturer's instructions. Sequencing reactions were prepared in both orientations with an ABI PRISM Big Dye terminator cycle sequencing ready reaction kit with AmpliTaq DNA polymerase FS (Applied Biosystems) according to the manufacturer's instructions. Sequencing was performed using an ABI PRISM 377 analyzer (Applied Biosystems) by Serveis Científico-Tècnics of the University of Barcelona, and the sequences were compared with those in the GenBank database by using the BLAST program of the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov/BLAST/). Sequences were aligned using the ClustalX 1.8 program, and phylogenetic analysis was performed using the neighbor-joining program of the PHYLIP software package for comparison with representative sequences characterized previously.
Nucleotide sequence accession numbers.
The sequences reported in this paper have been deposited in the GenBank database under accession numbers AY288805 to AY288813 (porcine hexon sequences), AY288814 to AY288818 (bovine hexon sequences), and AY288819 to AY288821 (bovine protease sequences).
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The number of detectable viral particles in swine feces was estimated to be 101 to 103 genome equivalents/g of pooled fecal sample by limiting decimal dilution analysis using the PAV hexon primers. No significant relationships were established between positive cases and the ages of animals or their commercial aim, even though excretion of PAV was detected in sows and especially in fattening pigs.
Cattle fecal samples.
In the two farms studied, three of the four bovine fecal samples were positive for the presence of BAVs. Both farms were located in the same area and were controlled by the same sanitary staff. The amplicons detected were 410 bp long and presented differences in their nucleotide sequences. The six identified strains were named BovA to BovF.
The number of viral particles was estimated to be 101 to 104 genomic equivalents/g of pooled fecal sample by limit dilution analysis using the BAV hexon oligonucleotide set. Although more samples should be tested in order to establish reliable relationships, in this study half of the young-animal samples and all of the old-animal samples were positive for BAV.
Sewage samples.
Twelve sewage samples from the Barcelona sewage network were collected between August 2000 and July 2001. All samples were analyzed for PAVs, BAVs, and HAds by using the specific primers designed in the hexon region. As expected, none of the 12 samples were positive for PAV or BAV, whereas 11 samples were positive for HAd.
Sequence analysis of adenovirus excreted by swine.
The nucleotide sequence alignment of detected strains (Por1 to Por9) showed that all of the strains were closely related to the IAF strain of PAV type 3 (accession no. AJ237815) (Fig. 1). Sequences shared 93 to 98% similarity. This classification was confirmed by amplification of a 356-bp fragment of the protease gene, which showed 98% similarity to the PAV type 3 IAF strain.
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FIG. 1. Sequence alignment of the hexon regions of nine PAV-positive samples in reference to PAV3 strain IAF. Dots indicate sequence identities, and dashes indicate lack of nucleotide. M = A + C, Y = C + T, and W = A + T.
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FIG. 2. Sequence alignment of the hexon regions of six BAV-positive samples. Dots indicate sequence identities, and dashes indicate lack of nucleotide. M = A + C, Y = C + T, and W = A + T.
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FIG. 4. Phylogenetic reconstruction of BAV strains analyzed in this study. Reference strains of BAV2, BAV3, BAV4, and BAV7 (accession no. AF252854, AF030154, NC_002685, and AF238232, respectively), EDS (accession no. Y09598), FAV1 (fowl adenovirus type 1; accession no. U46933), HAd2, HAd12, HAd17, and HAd40 (accession no. J01917, X73487, AF108105, and L19433, respectively) are represented. The neighbor-joining tree was constructed using 437 nucleotides of a region of the protease gene of BAV genomes.
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FIG. 3. Specificity assays performed with designed sets of oligonucleotides. Each adenovirus strain detected was tested with human (H), porcine (P), and bovine (B) oligonucleotides. The molecular sizes expected of the amplicons were 344 bp for swine, 410 bp for cattle, and 123 bp for human fragments. Lane M, molecular size marker ( X174 cleaved with HaeIII).
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The BAV isolates detected showed a much greater variability, including sequences related to the two BAV subgroups described to date. The BAV2-like strain is a member of BAV subgroup I. This type has been divided into two subspecies (A and B) based on hemagglutination properties and restriction enzyme cleavage patterns (3). A noticeable variation in this serotype could thus be detected. Further studies are being performed to characterize this isolate. The BAV4-like and BAV7-like strains belong to BAV subgroup II and therefore to the Atadenovirus genus. Experimental infections associate serotype 4 with weak bronchopneumonia. Serotype 7 was first described in blood samples from a cow that suffered an acute feverish episode (17), suggesting that a temporary viremia is a pathogenic process common to BAV infections. Thus, the divergence was high, indicating that several serotypes may coexist in a reduced number of individuals. Diverse adenoviral distribution has already been discussed for BAVs in relation to the proposal that they evolved exclusively with their hosts (classical Mastadenovirus members) or may have extended their spectrum of hosts to related species, such as sheep (1, 4), deer (20), and even moose (27).
The phylogenetic study based on protease sequences clearly generated two clusters (Fig. 4). One cluster contains classical HAd, PAV, and BAV subgroup I, whereas the other includes atypical OAV and avian adenovirus as well as BAV subgroup II. Hexon analysis had already provided evidence to support this arrangement (data not shown). The protease gene has been widely used in phylogenetic analysis, leading to the conclusion that BAV subgroup II, together with EDS virus and OAV287, belong to a new genus named Atadenovirus because of the viruses' high content of adenines and thymines (5, 29). All of the bovine and porcine fragments studied shared the same active residues, His51-Glu71-Cys122. This catalytic triad has been described in every adenovirus protease with the exception of HAd2 and HAd5, in which aspartic acid (Asp[D]) is found instead of glutamic acid (Glu[E]) (11). The Cys103 residue, determined to be a binding site of the cellular activator pVI peptide (22), was also conserved. Nevertheless, the amino acid Pro137, believed to be necessary for intracellular trafficking, was found in porcine strains but not in any of the bovine strains analyzed. This residue has been detected in all Mastadenovirus strains but not in Aviadenovirus or Atadenovirus (15).
Animal feces may contaminate the environment and water via runoff, drainage, or direct deposition or by contamination through slaughterhouse sewage. Although the analysis of a larger number of samples is necessary, this study reports a high PAV and BAV prevalence in animal feces, as has been described for HAd in humans. This finding as well as adenoviral features such as their DNA genome, uniform geographical and seasonal distribution (24), and stability under a wide range of environmental conditions (12, 18, 23) lead us to regard this viral family as a suitable indicator of viral contamination. The methodology reported in this work is reliable and specific. The sets of oligonucleotides used in the developed tests were designed for the hexon gene after consideration of all of the sequences available in the nucleotide databases, including those of Atadenovirus and Mastadenovirus. The hexon protein is composed of a variable triangular surface and two highly ß-barrel regions called P1 and P2. Our amplified fragment is located within a P1-P2 shared region. Experimentally, neither the BAV nor the PAV primer sets used in the nested PCR assays produced nonspecific amplifications in the fecal samples and urban-sewage samples tested. Furthermore, the set of primers described previously for the detection of HAds produced negative results with any of the samples containing the detected animal adenovirus strains. In a previous study, the specificity of HAd oligonucleotides had already been evaluated using nine slaughterhouse sewage samples containing only fecal contamination of animal origin, such as cattle, swine, and sheep (24). Similarly, the primers designed for detection of animal adenoviruses produced negative results when the HAd strains Ad2, Ad5, Ad12, and Ad41 were tested at concentrations of 107 viral particles/ml. These results suggest that further analysis of animal adenoviruses and evaluation of the potential application of these tests to identify PAVs and BAVs in addition to HAds may lead to a useful tool for tracing the source of fecal contamination in water, food, and the environment.
We thank Serveis Científico-Tècnics of the University of Barcelona for sequencing of PCR products. We also thank Rosa Bufías for her excellent technical assistance.
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