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Equine picornavirus infection

Equine picornavirus infection

Equine picornavirus infection

Previous authors: R J GERAGHTY AND J A MUMFORD

Current authors:
C A HARTLEY - Senior Lecturer in Veterinary Virology, BSc (Hons), PhD, Faculty of Veterinary and Agricultural Science, The University of Melbourne, Building 400, Corner of Flemington Road and Park Drive, Parkville, Victoria, 3010, Australia
J R GILKERSON - Professor of Veterinary Microbiology, BVSc, BSc (Vet), PhD, Centre of Equine Infectious Diseases, The University of Melbourne, Building 400, Corner of Flemington Road and Park Drive, Parkville, Victoria, 3010, Australia

GENERAL INTRODUCTION: PICORNAVIRIDAE

Introduction and aetiology

The equine rhinitis viruses cause upper respiratory tract infections in horses and have the potential for long term shedding in urine and faeces. While these viruses have long been detected in horses with respiratory disease, their role, especially the erboviruses, as the main driver of widespread respiratory disease remains to be unequivocally defined. Four types of equine rhinitis viruses have been identified within 2 genera of the family Picornaviridae.  With a publication history more than 50 years long, the names and classifications of these viruses have evolved along with the transformations in virus taxonomy that have occurred over this time. Equine rhinitis A virus (ERAV, formerly equine rhinovirus-1) contains a single serotype and is classified within the Aphthovirus genus together with foot-and-mouth disease virus (FMDV) and bovine rhinitis A and B viruses.  The newly named species erbovirus A is the only member of the Erbovirus genus and contains the 3 known serotypes of this species, known as equine rhinitis B virus (ERBV)-1 (formerly equine rhinovirus-2), ERBV2 (formerly equine rhinovirus-3) and ERBV3 (formerly acid stable equine picornavirus).44

Equine picornaviruses have single stranded positive sense RNA genomes from ~7600 nucleotides for ERAV27 to ~8800 bp for ERBV43 that are enclosed in small (~30 nm) non enveloped capsids.  The icosahedral capsid contains 60 copies of each of the capsid proteins (VP1, VP2, VP3 and VP4) where the sequence and three-dimensional structure of these capsid proteins determines the antigenic sites, serotype, and the physical properties of the capsid.  These proteins also mediate cell binding and entry and are important molecular determinants of virulence.44 Picornaviruses are resistant to ether and chloroform, but equine picornaviruses have variable acid stability; whereas most are labile below pH 5.5, ERBV3 only loses infectivity below pH 3.3.21

Most of the available studies that describe these viruses focus on ERAV and ERBV1, and the information presented in this chapter will reflect this focus, and provide information about ERBV2 and ERBV3 where available.

Epidemiology and pathogenesis

In all of regions of the world where studies have been performed, ERAV and ERBV have either been detected directly by virus isolation or RT-PCR or by antibody detection in seroprevalence surveys.2, 7, 9, 11, 16, 19, 21, 25, 31, 33, 35, 37, 39, 40, 42, 43 These studies suggest that these viruses occur endemically worldwide, or at least that their distribution aligns with the presence of significant horse populations, although their existence in the isolated Icelandic horse population has not been documented.

Infection by ERAV is widespread, where seroprevalence studies show that close to 100 per cent of older horses are seropositive.19, 31 Primary infection with ERAV is most likely to occur at a time after maternal antibody is waning (3- 9 months) prior to increased opportunity for contact with other horses, such as entry into training stables.2, 19 Infection occurs via the upper respiratory tract, where infectious virus can be detected in the nasopharynx and in oral secretions before the systemic phase of infection characterized by transient viraemia and virus dissemination to distant sites.18, 39  Viraemia ceases concomitant with the development of neutralizing antibody, at approximately Day 7 post- infection in experimental infection studies.  After the development of neutralizing antibodies, ERAV genome can still be detected in the nasopharynx by RT-PCR at least at 35 days post- infection.30 More strikingly, experimental infection studies show that the highest load of ERAV is shed in the urine of infected animals. Prolonged urinary shedding of ERAV is a feature of this virus in the natural host, with the viral load shed from this site significantly higher than that shed from the respiratory tract, both by volume and titre. Long-term urinary shedding can be detected for up to 143 days in both experimental and field studies.30, 31 This finding suggests that while ERAV may be transmitted to new hosts via respiratory aerosols, direct or in direct contact with urine-contaminated fomites or aerosols created by urine splash onto hard floor surfaces are also likely an important mechanism of spread.

The seroprevalence of the erboviruses is also very high. Up to 100 per cent of older horses have antibody to the ERBVs in most of the populations studied in the USA, European countries, New Zealand and Australia.2, 7, 9, 11, 12, 19, 23, 31, 37, 38, 41, 43 Primary infection occurs at four to six months of age, and there is some evidence to suggest that recrudescence or reinfection may continue to occur over time.3, 20, 33, 38 A study of sequential samples from 50 weanlings between four and 13 months of age over a nine month period has shown highest seroprevalence to ERBV3 (~60 per cent) compared to ERBV1 (~38 per cent) and lowest to ERBV2 (~13 per cent).20 While ERBV1 and ERBV2 have been isolated in locations across the world, ERBV3 isolations have been reported in Australia, Japan and the United Kingdom.16, 21, 33 Multiple examples of dual infection with two ERBV serotypes have been identified through virus isolation from clinical samples,21 and this is consistent with serosurveys that also suggest that co-infection is common.3, 20, 33, 38

Infection occurs primarily in the upper respiratory tract after transmission through direct and indirect contact with respiratory secretions and aerosols, consistent with the isolation of these viruses from respiratory sites.33, 36 While there are no reports of viraemia or urinary shedding, there is a report of the detection of ERBV2 and ERBV3 by RT-PCR and sequencing in four faecal samples from horses in Dubai.42

Clinical signs and pathology

Equine rhinitis A viruses have been isolated from horses with acute febrile respiratory disease28, 39, 43 and can cause similar clinical signs after experimental infection.34 In addition, subclinical infections have been reported in both experimental17 and natural infection studies.18, 31 These two clinical outcomes have been observed experimentally, with the most severe cases reported in studies from the late 1960s .34 A more recent study has reported clinical signs after ERAV infection in horses treated with dexamethasone for three days prior to infection.8

Equine rhinitis A virus has been detected in horses that have no clinical disease as well as those with signs of acute respiratory disease. When detected in natural or experimental infections, clinical signs are most commonly reported to be fever (up to 41.4°C) lasting up to three days during the viraemic phase, with anorexia (four to five days), copious serous nasal discharge that later becomes mucopurulent, pharyngitis, bronchitis, coughing and swollen lymph nodes.5, 10, 15, 18, 28, 39, 43

Like ERAV, both natural and experimental infections with the ERBVs are often subclinical.33, 36 All three serotypes of ERBV have been isolated from horses with clinical signs of acute respiratory disease (fever, serous nasal discharge, anorexia, coughing, lymphadenitis), as well as oedema of the legs.31, 36, 43 There have been few experimental infection studies with ERBVs. In one study, infection of gnotobiotic and conventional foals showed an antibody response in the absence of clinical signs or re-isolation of the virus, while a second study reported ERBV1 infection of already seropositive horses with minimal clinical signs.33, 36

As with many equine viral respiratory infections, recovery from ERAV and ERBV may be complicated by bacterial infections.38

No information is available on the pathogenicity of ERBV3 infections. While their clinical significance remains obscure, their acid stable phenotype is consistent with picornaviruses that can establish enteric infections and this potential remains to be more clearly resolved.

Diagnosis and differential diagnosis

Detection of equine picornavirus infections can occur by direct agent detection or by the demonstration of a rise in virus-specific antibody between acute and convalescent sera.

Samples for agent detection are similar for ERAV and ERBV, since both these viruses have been detected in nasopharyngeal and oral secretions.  Given the high levels and long term shedding of ERAV in urine, this sample should be included for testing when available. Faecal samples have yielded ERBV2 and ERBV3.42

Molecular tools for the detection of these viruses are conventional RT-PCR or quantitative RT-PCR assays.  With RNA genomes, amplification by these methods must target well conserved genomic regions such as 3Dpol and the untranslated regions of the 5’ and 3’ genomic termini, but some assays have been described with targets within the more variable VP1 region of ERAV.1, 13, 32, 35 The capsid encoding region of the ERBV genomes appears to be highly variable within and amongst the three serotypes, while the P2 and P3 non-structural protein regions are well conserved across the serotypes. Therefore, at the current time, there are no RT-PCR based assays that are able to distinguish between the ERBV serotypes. In future this may more likely be resolved by direct sequencing of clinical samples.

Virus isolation of both ERAV and the ERBVs is supported by rabbit kidney (RK13), African green monkey kidney (Vero), equine foetal lung and kidney and equine dermis cells.28, 31, 36, 39 Primary isolation from clinical specimens can be complicated by some non-cytopathic viruses28 which, combined with the high seroprevalence of these viruses, suggests that the true prevalence of equine picornavirus infection may not yet be well reported.

Both virus neutralization and complement fixation tests have been used to measure antibody to equine picornaviruses,4, 19, 33 in which ERAV in particular induces extremely high levels of virus neutralizing antibody, and therefore this provides a highly sensitive test.  While most studies have used virus neutralization assays for antibody detection, more recently ELISAs have been developed, but these are not in widespread use.  ELISAs for ERAV have been described26, 29 but appear less sensitive than virus neutralization assays, given the high sensitivity of these assays for ERAV and the highly conformational nature of the immunodominant epitope of ERAV that appears difficult to replicate in these assays.  ELISAs for ERBV124 were first developed, prior to the development of a serotyping ELISA for each of the three ERBV serotypes.20, 22  Given the high seroprevalence of these viruses, a single serum sample is not helpful in confirming a recent infection and paired acute and convalescent sera 10 to 14 days apart are necessary to demonstrate evidence of a recent infection.

Infection with these viruses should be borne in mind in the differential diagnosis of acute upper respiratory tract disease in horses (see Equine influenza, and Equid herpesvirus 1 and 4 infections).

Control

A killed virus vaccine has been released on a conditional license in the USA for ERAV,44 but there is no vaccine available for ERBVs.

References

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