A coronavirus was isolated from feces of a diarrheic foal and serially propagated in human rectal adenocarcinoma (HRT-18) cells. Antigenic and genomic characterizations of the virus (isolate NC99) were based on serological comparison with other avian and mammalian coronaviruses and sequence analysis of the nucleocapsid (N) protein gene. Indirect fluorescent-antibody assay procedures and virus neutralization assays demonstrated a close antigenic relationship with bovine coronavirus (BCV) and porcine hemagglutinating encephalomyelitis virus (mammalian group 2 coronaviruses). Using previously described BCV primers, the N protein gene of isolate NC99 was amplified by a reverse transcriptase PCR (RT-PCR) procedure. The RT-PCR product was cloned into pUC19 and sequenced; the complete N protein of NC99 (446 amino acids) was then compared with published N protein sequences of other avian and mammalian coronaviruses. A high degree of identity (89.0 to 90.1%) was observed between the N protein sequence of NC99 and published sequences of BCV (Mebus and F15 strains) and human coronavirus (strain OC43); only limited identity (<25%) was observed with group 1 and group 3 coronaviruses. Based on these findings, the virus has been tentatively identified as equine coronavirus (ECV). ECV NC99 was determined to have close antigenic and/or genetic relationships with mammalian group 2 coronaviruses, thus identifying it as a member of this coronavirus antigenic group.

Modified-live (ML) infectious laryngotracheitis (ILT) vaccine viruses, both tissue-culture-origin (TCO) and chicken-embryo-origin (CEO), were passaged 20 times in specific-pathogen-free chickens. After serial bird-to-bird passage, increased virulence was observed for CEO virus but not TCO virus. Increased mortality and increased severity and duration of respiratory disease were observed in chickens inoculated with chicken-passaged CEO viruses; only mild respiratory disease (no mortality) occurred in chickens inoculated with chicken-passaged TCO viruses. These findings suggest that ML ILT vaccine viruses may increase in virulence after bird-to-bird passage.

Several different viruses have been identified as causes of gastrointestinal tract infections in poultry. These include rotaviruses, coronaviruses, enteroviruses, adenoviruses, astroviruses, and reoviruses. In addition, a number of other viruses of unknown importance have been associated with gastrointestinal diseases in poultry based on electron microscopic examination of feces and intestinal contents. Viral infections of the gastrointestinal tract of poultry are known to negatively impact poultry production, and they likely contribute to the development of other, extragastrointestinal diseases. Our current understanding of the viruses that cause gastrointestinal tract infections in poultry is reviewed, with emphasis given to those of greatest importance.

Turkey coronavirus (TCoV) is the cause of an acute highly contagious enteric disease of turkeys. In recent years, TCoV has been increasingly recognized in North America as an important pathogen of young turkeys, resulting in economic loss due to impaired growth and poor feed conversion. While the epidemiology and pathogenesis of TCoV have been extensively studied, TCoV remains one of the least characterized of the known coronaviruses.Avian and mammalian coronaviruses have been subdivided into distinct antigenic/genotypic groups; however, classification of TCoV has been controversial. Previous studies indicated that TCoV was closely related to bovine coronavirus and other group 2 mammalian coronaviruses, but more recent antigenic and genome sequence analyses contradict these findings and, instead, provide evidence that TCoV is closely related to avian infectious bronchitis virus (IBV). Additionally, experimental studies have indicated that the host range of TCoV, once thought to be restricted to turkeys, includes chickens. These studies have raised additional questions regarding the classification of TCoV; particularly, whether IBV and TCoV are taxonomically distinct viruses, or whether TCoV is merely a variant of IBV.Sequence analyses of TCoV have given credence to the idea that TCoV is a variant of IBV, as these studies have shown that TCoV and IBV are very closely related. However, these studies have been limited to only three TCoV strains and relatively small portions of the TCoV genome. TCoV is readily distinguished from IBV based on antigenic and biological differences, and these differences suggest that TCoV should be considered a distinct virus species.Additional studies will be needed to better define the relationship between TCoV and IBV, and to resolve this taxonomic question. Based on our current understanding, it seems prudent to consider TCoV and IBV as distinct virus species that share a close phylogenetic relationship and together comprise group 3 of the coronavirus major antigenic groups.

The complete genome sequence of the first equine coronavirus (ECoV) isolate, NC99 strain was accomplished by directly sequencing 11 overlapping fragments which were RT-PCR amplified from viral RNA. The ECoV genome is 30,992 nucleotides in length, excluding the polyA tail. Analysis of the sequence identified 11 open reading frames which encode two replicase polyproteins, five structural proteins (hemagglutinin esterase, spike, envelope, membrane, and nucleocapsid) and four accessory proteins (NS2, p4.7, p12.7, and I). The two replicase polyproteins are predicted to be proteolytically processed by three virus-encoded proteases into 16 non-structural proteins (nsp1-16). The ECoV nsp3 protein had considerable amino acid deletions and insertions compared to the nsp3 proteins of bovine coronavirus, human coronavirus OC43, and porcine hemagglutinating encephalomyelitis virus, three group 2 coronaviruses phylogenetically most closely related to ECoV. The structure of subgenomic mRNAs was analyzed by Northern blot analysis and sequencing of the leader-body junction in each sg mRNA.