Equine rhinopneumonitis (infection with equid herpesvirus-1 and -4)

DAMIANI, ARMANDO MARIO; PETER TIMONEY; DEBRA ELTON; ANN CULLINANE

Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2017

OIE

Año: 2018; p. 1 - 13

Equine rhinopneumonitis (ER) is a collective term for any one of several contagious, clinical disease entities of equids that may occur as a result of infection by either of two closely related herpesviruses, equid herpesvirus-1 and -4 (EHV-1 and EHV-4). Infection with EHV-1 is listed by the OIE. EHV-1 and EHV-4 are endemic in most domestic equine populations worldwide.Primary infection by either EHV-1 or EHV-4 is characterised by upper respiratory tract disease of varying severity that is related to the age and immunological status of the infected animal. EHV-1 also causes the more serious complications of abortion, perinatal foal death, or paralytic neurological disease (equine herpesvirus myeloencephalopathy). EHV-4 has been associated with sporadic cases of abortion, but not the large outbreaks associated with EHV-1. Like other herpesviruses, EHV-1 and 4 induce long-lasting latent infections and can be reactivated following stress or pregnancy. Most horses are likely to be re-infected multiple times during their lifetime, often mildly or subclinically. Detection of viral DNA or anti-EHV antibodies should therefore be interpreted with care.Identification of the agent: The standard method of identification of EHV-1 and EHV-4 from appropriate clinical or necropsy material is by polymerase chain reaction (PCR), followed by laboratory isolation of the virus in cell culture. Positive identification of viral isolates as EHV-1 or EHV-4 can be achieved by type-specific PCR. Viruses can be isolated in equine cell culture from nasal or nasopharyngeal swab extracts taken from horses during the febrile stage of respiratory tract infection, from the placenta and liver, lung, spleen, or thymus of aborted fetuses and early foal deaths, and from the leukocyte fraction of the blood of animals with acute EHV-1 infection. Unlike EHV-4, EHV-1 will also grow in various non-equine cell types such as the RK-13 cell line and this property can be used to distinguish between the two viruses.A rapid presumptive diagnosis of abortion induced by EHV-1 or (infrequently) EHV-4 can be achieved by direct immunofluorescent detection of viral antigen in cryostat sections of placenta and tissues from aborted fetuses, using a conjugated polyclonal antiserum.Post-mortem demonstration of histopathological lesions of EHV-1 in placenta and tissues from aborted fetuses, cases of perinatal foal death or in the central nervous system of neurologically affected animals complements the laboratory diagnosis.Serological tests: Most horses possess some level of antibody to EHV-1/4, the demonstration of specific antibody in the serum collected from a single blood sample is therefore not confirmation of a positive diagnosis of recent infection. Paired, acute and convalescent sera from animals suspected of being infected with EHV-1 or EHV-4 should be tested for a four-fold or greater rise in virus-specific antibody titre by either virus neutralisation (VN) or complement fixation (CF) tests. Neither of these assays is type-specific but both have proven useful for diagnostic purposes especially since the CF antibody response to recent infection is relatively short-lived. Limited use has also been made of a type-specific enzyme-linked immunosorbent assay (Crabb et al., 1995; Hartley et al., 2005).Requirements for vaccines: Both live attenuated and inactivated viral vaccines are available for use in assisting in the control of EHV-1/4. Vaccination is helpful in reducing the severity of respiratory infection in young horses and the incidence of abortion in mares, however current vaccines are not licenced to protect against neurological disease. Vaccination should not be considered a substitute for sound management practices known to reduce the risk of infection. Revaccination at frequent intervals is recommended in the case of each of the products, as the duration of vaccine-induced immunity is relatively short.Standards for production and licensing of both attenuated and inactivated EHV-1/4 vaccines are established by appropriate veterinary regulatory agencies in the countries of vaccine manufacture and use. A single set of internationally recognised standards for EHV vaccines is not available. In each case, however, vaccine production is based on the system of a detailed outline of production employing a well characterised cell line and a master seed lot of vaccine virus that has been validated with respect to virus identity, safety, virological purity, immunogenicity and the absence of extraneous microbial agents.