Equine picornaviruses: Well known but poorly understood
Introduction
Of the many members that comprise the family Picornaviridae, only two species are known to infect horses: equine rhinitis A virus (ERAV) and equine rhinitis B virus (ERBV). The equine picornaviruses were first isolated in the 1960s (Plummer, 1962) and 1970s (Hofer et al., 1972) and despite similarities that initially saw them grouped together in the genus Rhinovirus, they separate into two distinct taxonomical groups. Both ERAV and ERBV are associated with respiratory disease in horses; however, their importance as equine pathogens and the specific roles they have in respiratory disease is still unclear. The clinical manifestations of ERAV and ERBV infection remain ill-defined, with isolates recovered from clinically healthy animals, as well as those with signs of respiratory disease (Carman et al., 1997, Flammini and Allegri, 1970, Fukunaga et al., 1983, Hofer et al., 1972, Li et al., 1997, Mumford and Thomson, 1978, Plummer and Kerry, 1962, Studdert and Gleeson, 1978). Their frequent detection in combination with other viral and bacterial pathogens suggests they may have a contributing role in enhancing disease length and severity.
Equine respiratory disease is of particular importance in the performance horse industry, representing a major burden due to the additional veterinary bills, lost training time and decreased performance. The high prevalence of ERAV and ERBV in horse populations indicates that further studies are required to better understand the pathogenesis and epidemiology of these widespread viruses.
The bulk of research on equine picornaviruses was performed in the 1960s–1980s, with more recent work mostly limited to studies of seroprevalence or detection by molecular methods. In more recent years, genome sequencing has resulted in a significant reclassification of the equine picornaviruses and some reassessment of the ERBV serotype groupings. This review aims to collate current knowledge of the fundamental features of the equine picornaviruses, including current approaches for their detection that may provide an insight into the pathogenesis of these viruses.
Section snippets
Taxonomy
Historically, the equine picornaviruses were classified within the genus Rhinovirus, family Picornaviridae, due to their physical properties and their identification as respiratory pathogens (Ditchfield and McPherson, 1965, Fukunaga et al., 1981, Plummer, 1962, Steck et al., 1978b). Four distinct groups were identified based on serology and acid lability: equine rhinovirus-1, equine rhinovirus-2, equine rhinovirus-3 and acid-stable equine picornavirus. Following comparative genomic studies,
Genomic organisation and function
Picornaviruses have single-stranded, positive-sense RNA genomes between 8800 (in erboviruses) and 7209 (in human rhinoviruses) nucleotides long. The 3′ polyadenylated genome contains a single open reading frame, flanked at the 5′ and 3′ terminal ends by untranslated regions (UTR) (reviewed by Racaniello, 2007). The 5′ terminus is covalently bound to a small virally encoded protein (VPg) that acts as a protein primer for the initiation of RNA synthesis by the viral polymerase (Ferrer-Orta et
Structural proteins and virion assembly
The picornaviruses are non-enveloped particles of approximately 30 nm in diameter. The icosahedral capsid is assembled from three external (VP1, VP2 and VP3) and one internal (VP4) viral protein. The capsid proteins VP1, VP2 and VP3 share a similar protein topology consisting of eight-stranded, antiparallel β-barrels arranged as a wedge-like structure with loops connecting the strands (reviewed by Racaniello, 2007). Sixty copies of each capsid protein form the icosahedral virion where the
Genomic and antigenic variation among equine picornaviruses
Equine rhinitis A virus exists as a single serotype despite nucleotide variation in the capsid-coding region. An analysis of ten geographically distinct isolates revealed 79.6–96.6% nucleotide identity and 96.8–99.3% identity at the amino acid level (Varrasso et al., 2001). The majority of amino acid changes are located within a 95 amino acid region at the 3′ end of VP1 (Li et al., 1997). In cross-neutralisation experiments, using a panel of equine polyclonal serum, a small proportion of
Physical properties
Equine rhinitis A virus is resistant to ether and chloroform, and has a CsCl density of 1.45 g/ml (Newman et al., 1977). The VP1 protein dominates the surface profile of the capsid, with the carboxyl terminus and the βE to βF loop associating to form a prominent crown, extending to an outer radius of 159 Å, and marked depressions at the five-fold axis (Tuthill et al., 2009). The capsid is acid labile, dissociating into pentamers at pH levels between pH 6.5 and 5.5 (Tuthill et al., 2009).
Epidemiology
Equine
Physical properties
The ERBV genome is one of the largest in the Picornaviridae at approximately 8800 nucleotides in length. Complete particles have a buoyant density that can range from 1.41 to 1.46 g/ml and are resistant to ether and chloroform (Mumford and Thomson, 1978, Newman et al., 1977). Equine rhinitis B virus is inactivated at 50 °C in water but stabilised with the addition of MgCl2 (Mumford and Thomson, 1978). For ERBV1 and ERBV2, infectivity is lost below pH 5.4 (Newman et al., 1977, Steck et al., 1978a
Detection of equine picornavirus infection
Equine picornavirus infections are demonstrated by direct detection in clinical samples or by an increase in virus-specific antibodies between acute and convalescent serum samples. Equine picornaviruses are detected in nasopharyngeal swab samples either through virus isolation or by the detection of viral RNA using reverse transcription PCR (RT-PCR). Given the high levels of ERAV shed in the urine of infected animals, this sample, if available, should be tested in conjunction with respiratory
Conclusion
Equine respiratory disease remains a significant economic and welfare burden for both performance and pleasure horses worldwide, though there is great potential to reduce this burden with the identification of causative agents and the development of methods for their control. While respiratory viruses such as equine influenza virus, and equine herpesvirus-1 and -4 are known to contribute significantly to this disease burden, in many cases the causative agents of infectious upper respiratory
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Each author contributed equally to this work.