Turkey astrovirus

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Higher order taxa

Virus; ssRNA positive-strand viruses, no DNA stage; Astroviridae; Avastrovirus

Species

NCBI: Taxonomy

Genus species

Description and significance

Astrovirus originated from ‘astron’, a Greek word for star because of the five-pointed or six-pointed side projection which can be detected by negative stained electron microscopy (EM). They were first described by Madeley & Cosgrove in 1975 as the causal for gastroenteritis in infants ⁴. Astrovirus disease is known as the causal for gastroenteritis ², it is usually mild but more serious cases have been discovered in poultry. Astrovirus is most commonly found in turkeys and can lead to mortality. In 1980, turkey astrovirus was first described and was linked with turkey poults in the UK that were suffering from diarrhea and increased mortality. They were discovered in the U.S. the same year ⁴. Although replication is only routinely detected in the intestines, experimentally infected poults show thymus and bursal atrophy and the virus can be isolated in other tissues.

Genome structure

Genomically, astrovirus have positive-sense, single stranded, RNA genome which are about 6.8 to 7.9 kb in length. Their genomes have three open reading frames (ORFs) which are organized from 5’ to 3'. ORF1a encodes a serine protease, ORF1b encodes the RNA dependent polymerase and ORF 2 encodes the structural proteins. The space between the ORF1b (open reading frame) stop codon and the ORF2 is 18 nucleotides ¹. There is also a translation machinery for ORF1b, although its sequence do not give a clear picture to the overall translation strategy. The frameshift structure allows for the translation of ORF1a and ORF1b to occur in order for polyprotein to cleave into functional subunits.

Virion Structure

Turkey Astrovirus are non-eneveloped, spherical, icosahedral capsid of 28 to 30nm. On the surface, they appear rough, spikes protruding from the 12 verticals. As previously stated, the capsid is not enveloped but round with polyhedral symmetry ⁴.

Ecology

Turkey Astrovirus are found in turkey poults which are mainly on farms. This disease then have the ability to transfer to human after being consumed. Complete sanitation of all materials and restricted access to facilities with the affected poultry is required to limit the outbreak to the farm affected. The only known method of preventing and controlling infections with any of the astrovirus is strict containment. Strict biosecurity prophylactically is the most practical prevention method that can be used ⁴.

Pathology

After the turkey has been infected and then consumed by human, it typically cause watery diarrhea which lasts for about 2-4 days. It can also cause although less commonly, vomiting, headache, fever, abdominal pains, and anorexia in children under 2, elderly and immunocompromised people ¹. Diarrhea is the third leading infectious cause of death worldwide.

Research done by Koci et.al. shows that turkey astrovirus causes growth depression, decreased thymus size and enteric infection in infected turkeys ⁵. This was done by removal of a novel strain from turkey with the disease poult enteritis mortality syndrome. The result shows that although there was severe diarrhea, histopathology changes in the intestine were not deadly and there was no inflammation. This result may be because of increased activation of the “potent immunosuppressive cytokine" transforming growth factor beta during astrovirus infection.

Turkey virus can be isolated in embryonated eggs but there are no tools to detect the presence of antibodies against the virus. RT-PCR primers specific for virus has been the only accurate way to detect new astrovirus isolates that are similar to the ones available in the GenBank ⁴. The only known method of preventing and controlling infections with any of the astrovirus is strict containment. Also, the lab experiment shows that 0.3% formaldehyde, 1.5% Virkon S, 0.1% β-propriolactone, and 90% methanol are the only effective products that can be used for the inactivation.

Current Research

There have been some evidence that there are distinct subtypes of turkey astrovirus. Strain et.al., performed experiment in differentiating the types of turkey astrovirus. Predicted capsid amino acid sequences were first used to assessed the relationship of the novel clinical TAstV isolates within the Astroviridae ⁶. The TAstV-1 capsid sequence was found in a clade containing avian nephritis virus, with distance between it and the TAstV-1. Sequence analysis of TastV-1 and TastV-2 diagnostic amplicons was used to demonstrate that the levels of variation among TastV-1 and TastV-2 are very diverse. The phylogenetic analysis of the full-length capsid genes of both types of Tast-V shows that both may have originated from separate introductions into the turkey species and that there are at least two lineages which are distinct subtypes instead of serotypes. There are potential serotypes to appear within each subtypes.

A computer analysis was done by Jonassesen et.al. that focused on the ORF1a gene product sequences and RNA sequences around the 5’end of the subgenomic RNA. In addition, reverse transcription, cDNA amplification, and DNA sequencing were performed ³. The ORF1 translational frameshift signal and overall genomic organization of TastV-1 corresponded to the ones published for other astrovirus. The analysis of the virus using TMHMM 2.0 tool which is used for predicting transmembrane helices in proteins showed five and six transmembrane helices, in the virus. As a result, analysis of the astrovirus that were available showed difference in the postulated transmembrane region and also two protein regions with the potential for forming coiled coils. Also, turkey astrovirus 2 make a single subgenomic mRNA that share boundary with the 3’ end of the genome.

As previously stated, RT-PCR is one of the ways to detect the Turkey astrovirus. Tang et.al performed ssRNA experiment with internal control (IC) and multiplex RT-PCR to detect the astrovirus ⁷. It is a specific and sensitive assay which detects minute quantities of nucleic acid molecules of target pathogens. It would be affected by many factors which in turn lowers the sensitivity by causing false results because of its complexity. This study designed easy way to the generation of a ssRNA IC template reagent for use with multiplex RT-PCR developed in this lab to detect turkey astrovirus. As for result, the positive clones were confirmed by EcoR1 digestion and electrophoresis and sequence analysis of recombinant plasmids. Detection limit of the internal control template was determined by RT-PCR reaction of 10x serial ssRNA dilutions with primer sets of SRV and AFCP. Using ssRNA IC as a template, the RT-PCR products with primer sets SRV and AFCP were 576 and 575 bp in length while the positive RT-PCR products detected in the fecal samples by co-amplification with the same primer sets were 473 or 464. The result of the multiplex RT-PCR with the IC as co-amplification component revealed a test inhibition rate of 12.5%. This result shows that majority of inhibitors are small molecules.

References

¹Finkbeiner, S.R., Kirkwood C.D., and Wang, D. 2008. Complete genome sequence of a highly divergent astrovirus isolated from a child with acute diarrhea. Virology Journal, v. 5, p. 117.

²Jonassen, C. M., Jonassen T. O., Saif Y.M., Snodgrass, D. R., Ushijima, H., Shimizu, M., and Grinde, B. 2001. Comparison of capsid sequences from human and animal astroviruses. Journal of General Virology, v. 82, p. 1061-1067.

³Jonassen, C.M., Jonassen, T.O., Sveen, T.M., and Grinde, B. 2003. Complete genomic sequences of astroviruses from sheep and turkey: comparison with related viruses. Virus Research, v. 91, p. 195-201.

⁴Koci, M.D. and Schultz-Cherry, S. 2002. Avian Astrovirus. Avian Pathology, v. 31, p. 213-227.

⁵Koci, M.D., Moser, L.A., Kelley, L.A., Larsen, D., Brown, C.C., and Schultz-Cherry, S. 2003. Astrovirus Induces Diarrhea in the Absence of Inflammation and Cell death. Journal of Virology, v. 77 no. 21, p. 11798-11808.

⁶Strain, E., Kelley, L., Schultz-Cherry, S., Muse S.V., and Koci M.D. 2008. Genomic Analysis of Closely Related Astroviruses. Journal of Virology, v. 82 no. 10, p. 5099-5103.

⁷Tang, Y., Wang, Q., and Saif Y.M. 2005. Development of a ssRNA internal control template reagent for a multiplex RT-PCR to detect turkey astroviruses. Journal of Virological Methods, v. 126, p. 81-86.

Edited by Adedoyin Odewale, student of Emily Lilly at University of Massachusetts Dartmouth.