Equine influenza vaccination as reported by horse owners and factors influencing their decision to vaccinate or not

BACKGROUND

Equine influenza virus is a highly contagious respiratory pathogen that causes pyrexia, anorexia, lethargy and coughing in immunologically naïve horses. Vaccines against equine influenza are available and vaccination is mandatory for horses that participate in affiliated competitions, but this group forms a small proportion of the total horse population. The aims of this study were to: i) identify the equine influenza vaccination rate as reported in 2016 by horse owners in the United Kingdom (UK); ii) examine the demographics of owners and horses which were associated with significantly lower influenza vaccination rates and iii) explore factors that influence horse owners' decisions around influenza vaccine uptake.

RESULTS

Responses from 4837 UK horse owners who were responsible for 10,501 horses were analysed. An overall equine influenza vaccination rate of 80% (8385/10501) was reported. Several owner demographic characteristics were associated with significantly lower (p<0.05) reported equine influenza vaccination rates including: some geographical locations, increasing horse owner age, annual household income of less that £15,000 and owning more than one horse. Horse-related features which were associated with significantly lower reported equine influenza vaccination rates included age ranges of <4 years and > 20 years, use as a companion or breeding animal or leaving their home premises either never or at most once a year. The most common reasons cited for failing to vaccinate horses was no competition activity, lack of exposure to influenza and expense of vaccines. In contrast, the most common underlying reasons given by horse owners who vaccinated their horse were protection of the individual horse against disease, veterinary advice and to protect the national herd. Owners of vaccinated horses had less previous experience of an influenza outbreak or adverse reaction to vaccination compared with owners of unvaccinated horses.

CONCLUSIONS

This study documented a high rate of equine influenza vaccination as reported by owners in a substantial number of horses in the UK, but this does not reflect the level of protection. Sub-populations of horses which were less likely to be vaccinated and the factors that influence each owner's decision around vaccination of their horses against equine influenza were identified, but may alter following the 2019 European influenza outbreak. This information may nevertheless help veterinary surgeons identify "at-risk" patients and communicate more personalised advice to their horse-owning clients. It may also influence educational campaigns about equine influenza directed to horse owners, which aim to improve uptake of vaccination against this pathogen.

Polarisation of equine pregnancy outcome associated with a maternal MHC class I allele: Preliminary evidence

Identification of risk factors which are associated with severe clinical signs can assist in the management of disease outbreaks and indicate future research areas. Pregnancy loss during late gestation in the mare compromises welfare, reduces fecundity and has financial implications for horse owners. This retrospective study focussed on the identification of risk factors associated with pregnancy loss among 46 Thoroughbred mares on a single British stud farm, with some but not all losses involving equid herpesvirus-1 (EHV-1) infection. In a sub-group of 30 mares, association between pregnancy loss and the presence of five common Thoroughbred horse haplotypes of the equine Major Histocompatibility Complex (MHC) was assessed. This involved development of sequence specific, reverse transcriptase polymerase chain reactions and in several mares, measurement of cytotoxic T lymphocyte activity. Of the 46 mares, 10 suffered late gestation pregnancy loss or neonatal foal death, five of which were EHV-1 positive. Maternal factors including age, parity, number of EHV-1 specific vaccinations and the number of days between final vaccination and foaling or abortion were not significantly associated with pregnancy loss. In contrast, a statistically significant association between the presence of the MHC class I B2 allele and pregnancy loss was identified, regardless of the fetus/foal's EHV-1 status (p=0.002). In conclusion, this study demonstrated a significantly positive association between pregnancy loss in Thoroughbred mares and a specific MHC class I allele in the mother. This association requires independent validation and further investigation of the mechanism by which the mare's genetic background contributes to pregnancy outcome.

Report of the Second Havemeyer EHV-1 Workshop, Steamboat Springs, Colorado, USA, September 2008

This report summarises the findings of the Second Havemeyer EHV-1 Workshop, which was held in Steamboat Springs, Colorado, USA in September 2008. A total of 38 delegates, consisting of veterinary clinicians and scientists from academia and industry participated in a series of sessions that focused on equine herpesvirus myeloencephalopathy (EHM). Each session consisted of a review, followed by short presentations on current research topics. The sessions included EHM epidemiology, in vivo and in vitro models for studying EHM, EHV-1 virulence determinants, real-time PCR diagnostics, antiviral medications and new vaccination technologies. The report summarises the key advances identified during and since the meeting. Citations are restricted to selected reviews and papers published since the workshop.

New assays to measure equine influenza virus-specific Type 1 immunity in horses

Equine influenza virus (EIV) is a leading cause of respiratory disease in horses. Equine influenza infection induces a long-term immunity to re-infection. Recent strategies of vaccination aim to mimic this immunity by stimulating both antibody and cellular immune responses. Cell-mediated immunity (CMI) to influenza is well defined in man, but little has been done to characterise the responses in the horse. Additionally, the development of reliable assays for the measurement of equine CMI has lagged behind serological methods and vaccine development. In this study, two methods of measuring EIV-specific T lymphocyte responses have been developed. An EIV 'bulk' cytotoxic T lymphocytes (CTL) assay using equine dermal fibroblasts as target cells has been adapted from a method used in the 1980s. This method was also complemented with a new EIV-specific IFNgamma synthesis assay. When compared with the measurement of EIV-specific IFNgamma synthesis previously described, this method required the amplification of EIV-specific lymphocytes by culture and was sensitive enough to detect stimulation of EIV-specific T lymphocytes induced by experimental infection with EIV or vaccination with recombinant canarypox viruses coding for EIV-HA molecules. This study provides the tools to characterise the stimulation of CMI by the new generation of vaccines against equine influenza.

Equine herpesvirus-1-specific interferon gamma (IFNgamma) synthesis by peripheral blood mononuclear cells in thoroughbred horses

REASONS FOR PERFORMING STUDY

An assay has been developed that measures EHV-1 specific interferon gamma synthesis (IFNgamma), a cytokine produced following the activation of memory T lymphocytes and therefore a measure of cell mediated immunity. The method requires validation in the field.

OBJECTIVES

To measure the frequency of EHV-1 specific, IFNgamma synthesising peripheral blood mononuclear cells (PBMC) in a population of Thoroughbred horses, and examine its relationship with age, gender, premises and history of vaccination or field infection with EHV-1.

METHODS

Lymphocytes from 200 Thoroughbred horses were stimulated with EHV-1 in vitro, and IFNgamma detected using a monoclonal antibody and indirect immunofluorescence. Percent positive cells were enumerated by flow cytometric analysis and the results described and compared statistically between groups.

RESULTS

The frequency of IFNgamma+ PBMC was significantly higher in animals age >5 years compared with 2-4 years, in females vs. males, on stud farms vs. training yards and following vaccination of 2-year-olds with inactivated virus compared with nonvaccinates. Age strongly confounded all these associations and care must therefore be taken interpreting these results. Mares exposed to a field infection with EHV-1 also had higher frequencies of IFNgamma+ PBMC than other vaccinated horses.

CONCLUSIONS

The frequency of EHV-1 specific, IFNgama+ PBMC among the sample Thoroughbred population was diverse but lowest in young, unvaccinated horses-in-training.

POTENTIAL RELEVANCE

The frequency of EHV-1 specific lymphocytes synthesising IFNgamma in this population may be associated with its susceptibility to infection with this virus. This easy technique may be applied to monitor the antigenicity of vaccines and their effectiveness at stimulating cellular immunity.

Antibody and IFN-γ responses induced by a recombinant canarypox vaccine and challenge infection with equine influenza virus

In horses, equine influenza virus (EIV) is a leading cause of respiratory disease. Conventional inactivated vaccines induce a short-lived immune response. By comparison, natural infection confers a long-term immunity to re-infection. An aim of new equine influenza vaccines is to more closely mimic natural infection in order to achieve a better quality of immunity. A new live recombinant vaccine derived from the canarypox virus vector and expressing haemagglutinin genes of EIV (subtype H3N8) has been developed. Stimulation of the immune system was studied after immunisation with this canarypox-based vaccine and challenge infection by exposure to a nebulised aerosol of EIV. The humoral immune response was evaluated by measuring serum antibody levels using the single radial haemolysis (SRH) assay. The cellular immune response was assessed by the measurement of interferon gamma (IFN-gamma) synthesis in peripheral blood mononuclear cells (PBMC). Clinical signs of the disease (temperature, coughing, nasal discharge, dyspnoea, depression and anorexia) and virus excretion were monitored after challenge infection. Clinical signs and virus shedding were significantly reduced in vaccinates compared with unvaccinated controls. EIV-specific immunity was stimulated by vaccination with a recombinant vaccine as serological responses were detected after immunisation. This study also provided the first evidence for increased IFN-gamma protein synthesis in vaccinated ponies following challenge infection with EIV compared with control ponies.

Vaccination against equine influenza: Quid novi?

Equine influenza virus is a leading cause of respiratory disease in the horse. Equine influenza vaccines containing inactivated virus were first developed in the 1960s. Despite their intensive use, equine influenza outbreaks still continue to occur and therefore new strategies of vaccination are necessary to improve vaccine efficacy. Numerous methods of vaccination have been evaluated and commercialised in the horse, the most recent being the cold-adapted influenza virus and poxvirus-based vaccines. As a large animal model, the horse is also a useful species in which to evaluate the potential of new generations of influenza vaccine such as live-attenuated influenza virus engineered by reverse genetics. This report details the equine immune responses conferring protection against influenza. It then undertakes a selective review of different strategies of vaccination against equine influenza that have been developed over the last two decades and discusses factors that may influence the efficacy of vaccination. Finally it outlines progress in the development of a novel vaccination strategy against equine influenza using reverse genetics.

Characterisation of CTL and IFN-γ synthesis in ponies following vaccination with a NYVAC-based construct coding for EHV-1 immediate early gene, followed by challenge infection

Equine herpesvirus-1 (EHV-1) is a ubiquitous pathogen of horses, which continues to cause respiratory and neurological disease and abortion, despite the widespread use of vaccines. Cell mediated immunity (CMI) is thought to play a major role in protection against infection with EHV-1. The aim of this study was to characterise the virus-specific CMI response in ponies vaccinated with vP1014, a vaccinia-based construct (NYVAC) coding for the immediate early gene (gene 64) of EHV-1. This gene product is a CTL target protein for an equine MHC class I allele expressed on the A3 haplotype. EHV-primed yearling ponies expressing this haplotype were vaccinated once (n = 1), three (n = 1), or four times (n = 2), and one pony was kept as an unvaccinated control. Cytotoxic T lymphocyte (CTL) activity and interferon gamma (IFN-gamma) synthesis were measured before and after vaccination and challenge infection with EHV-1. Multiple immunisations with vP1014 resulted in increased CTL activity and IFN-gamma synthesis specific for EHV-1 compared with unvaccinated or singly vaccinated ponies. The phenotype of EHV-1 specific T-cells synthesising IFN-gamma was also modified by immunisation. In the unvaccinated pony, the predominant population synthesising IFN-gamma after EHV-1 stimulation was CD8alpha+. In contrast, multiply vaccinated ponies demonstrated an increased proportion of CD8alpha- T-cells synthesising IFN-gamma. The results demonstrated that vaccination with a NYVAC-based construct coding for gene 64 stimulated CMI. This immune response alone did not protect against challenge infection. However, the study does illustrate that vaccinia-based vaccines can stimulate CMI in the horse and may therefore contribute to protection against disease caused by EHV-1.

Use of polarised equine endothelial cell cultures and an in vitro thrombosis model for potential characterisation of EHV-1 strain variation

Equine herpesvirus-1 (EHV-1) is responsible for respiratory disease and abortion in pregnant mares. Some high virulence isolates of EHV-1 also cause neurological disease. The pathogenesis of both abortion and neurological disease relates in part, to thrombus formation occurring in the pregnant uterus and central nervous system. The differences in disease outcome may relate to differing abilities of high and low virulence EHV-1 isolates to cause cell-associated viraemia, infect endothelial cells and cause thrombosis at sites distant from the respiratory tract. This study attempted to identify in vitro assays, which could be used to characterise the interaction between these isolates, equine endothelial cells and clotting factors. No significant difference was found between the growth kinetics of high and low virulence isolates of EHV-1 in polarised endothelial cells. For both isolates, virus was released preferentially from the apical surface of the polarised cells. The functional effects of viral infection on endothelial cells, with reference to virally-induced thrombosis were then investigated. Endothelial cells were grown on microcarrier beads, infected with EHV-1 and assayed for procoagulant activity. No significant difference in clotting time was observed between mock and EHV-1 infected endothelial cells in microcarrier cultures. Thus the degree of thrombosis may reflect a more complex interaction between endothelial cells, circulating leucocytes and other factors in the microenvironment.

Equine interferon gamma synthesis in lymphocytes after in vivo infection and in vitro stimulation with EHV-1

Equine cytotoxic T lymphocyte (CTL) responses to equine herpesvirus-1 (EHV-1) are well characterised but little is known about the cytokine response after infection or vaccination. EHV-1 is common in horses and infects lymphocytes in vivo. This virus was used as a model to measure the synthesis of interferon gamma (IFN-gamma) by equine peripheral blood mononuclear cells (PBMC) after in vivo infection and/or in vitro stimulation with EHV-1. Both flow cytometry and ELISPOT assays were used to quantify equine IFN-gamma using a mouse anti-bovine IFN-gamma monoclonal antibody (clone CC302; shown to cross-react with recombinant equine IFN-gamma) and a rabbit anti-canine IFN-gamma polyclonal antibody. The percentage of PBMC synthesising IFN-gamma after in vitro stimulation with EHV-1 increased with age. In yearlings infected experimentally with EHV-1, PBMC showed two peaks of IFN-gamma synthesis, 11 and 56 days after infection. The IFN-gamma synthesis was principally associated with CD8(+) cells. The patterns of IFN-gamma synthesis detected by intracellular IFN-gamma staining or ELISPOT were compared with CTL data and shown to be similar. These methods were also applied successfully to frozen samples of PBMC. Measurement of equine IFN-gamma using these simple techniques can now be applied to future studies on protective cellular immune responses following virus infection and/or vaccination of horses.

Pre-infection frequencies of equine herpesvirus-1 specific, cytotoxic T lymphocytes correlate with protection against abortion following experimental infection of pregnant mares

In general, vaccines containing inactivated equine herpesvirus-1 (EHV-1) fail to prevent abortion in pregnant mares following infection with a virulent strain of EHV-1. We have tested the hypothesis that resistance to EHV-1-induced abortion in pregnant mares is associated with high frequencies of EHV-1 specific, major histocompatibility complex (MHC) class I-restricted, cytotoxic T lymphocytes (CTL) in the circulation. To test this theory, three groups of pregnant mares were assembled with varying backgrounds of infection or vaccination in an attempt to mimic the immune status of the general population. Group 1 mares (n=9) were untreated controls selected at random. Group 2 mares (n=5) were vaccinated three times intramuscularly with inactivated EHV-1. Group 3 mares (n=3) had been infected with EHV-1 on four previous occasions. The frequency of CTL in blood leucocytes was measured by limiting dilution analysis at three time points; at the beginning of pregnancy (approximately 28 weeks before infection) in the Group 2 and Group 3 mares (4-7 weeks of gestation) (Group 1 was unavailable for sampling) and then 2 weeks before (30-40 weeks of gestation) and 3 weeks after experimental infection in all the mares. Serum samples were collected to monitor complement fixing (CF) antibody titres. Mares in all three groups were infected experimentally with EHV-1 strain Ab4/8 by the intranasal route after which they were monitored clinically to determine the outcome of pregnancy and samples were collected to determine the duration of nasopharyngeal shedding and cell-associated viraemia. The untreated control mares showed low pre-infection CTL. After experimental infection, they all seroconverted, aborted and demonstrated expected clinical and virological signs. Some vaccinated mares (3/5) had elevated titres of CF antibody prior to their first vaccination. All the vaccinated mares seroconverted after vaccination and exhibited higher CTL frequencies than controls before infection. Four of the five foaled normally. The multiply infected mares had low CF antibody titres prior to infection and showed neither seroconversion nor clinical or virological signs after infection. All multiply infected mares exhibited high frequencies of CTL before infection and they all foaled normally. The CTL frequencies observed differed significantly from the expected frequencies in the control and multiply infected groups at 2 weeks pre-infection (P=0.034) and between the foaling and aborting mares at 2 weeks pre-infection (P=0.005) and 3 weeks post-infection (P=0.015). The results show a positive correlation between the number of virus-specific CTL in the peripheral blood of pregnant mares and their protection against abortion induced by EHV-1 infection. Therefore, as indicated by this study, rational approaches to the development of new vaccines for EHV-1 should stimulate cytotoxic immune responses and develop virus-specific CTL as pre-requisites for protection against abortion.

Detection of equine arteritis virus (EAV)-specific cytotoxic CD8+ T lymphocyte precursors from EAV-infected ponies

Equine arteritis virus (EAV) causes a systemic infection in equids with variable outcome, ranging from subclinical infections to severe disease, and also has the capacity to induce abortion in pregnant mares and persistent infections in stallions. The serum virus-neutralizing antibody response that invariably develops in the infected animal lasts for many months or years and is believed to play an important role in virus clearance. However, very little is known about cellular immunity against EAV because of a lack of methods for evaluating these immune responses. In the present study, we describe methods for detecting cytotoxic T lymphocyte (CTL) precursors in the peripheral blood of EAV-convalescent ponies using a 51Cr release cytolysis assay. Primary equine dermal cells, used as CTL targets, were shown to express MHC I but not MHC II and to retain 51Cr efficiently and support EAV replication. Peripheral blood mononuclear cells (PBMC) collected from EAV-convalescent ponies that had been incubated with or without live EAV were used as effectors. EAV-induced PBMC cultures showed evidence of expansion and activation of lymphoblasts, with an increase in the CD8+/CD4+ ratio in comparison with mock-induced PBMC. The cytotoxicity induced by EAV-stimulated PBMC was virus specific, showed genetic restriction, was mediated by CD8+ T lymphocytes and could be detected for periods of 4 months to more than 1 year post-infection. These findings and methods will hopefully contribute to an understanding of virus–host interactions in horses, in particular the mechanisms of virus clearance occurring during EAV infection.

The effect of aging on T cell responses in the horse

Horses greater than 20 years of age exhibit alterations in their immune responses similar to those observed in other aged individuals. The purpose of this study was to characterize immunosenescence in a population of aged ponies. The peripheral blood mononuclear cells (PBMC) from aged ponies exhibited a decreased proliferative response to various mitogens that was not overcome by the addition of interleukin 2 (IL-2) to the cultures. No difference in overall expression of the IL-2 receptor was seen between young and aged ponies, though CD8(+) cells from aged ponies exhibited increased levels of IL-2 receptor expression. The kinetics of the response to both mitogen and IL-2 did not appear to be affected in the aged PBMCs. These results indicate that the age-related decrease in the proliferative response to mitogens is not due to a failure to produce or respond to IL-2 but probably involves some other process.

Clinical and virological evaluation of the efficacy of an inactivated EHV1 and EHV4 whole virus vaccine (Duvaxyn EHV1,4). Vaccination/challenge experiments in foals and pregnant mares

Pregnant mares and young foals were vaccinated with Duvaxyn EHV1,4, an inactivated and adjuvanted vaccine containing both the EHV-1 and 4 antigens. SN and CF antibody titres were induced two weeks after first vaccination. Antibody levels were boosted after second vaccination, however they never reached the levels induced after virus challenge. Young foals were challenged with virulent EHV-1 and EHV-4 field viruses. Pregnant mares were challenged with the highly abortigenic EHV-1 strain Ab4. Vaccinated animals showed a clear reduction in clinical signs and virus excretion compared to unvaccinated control animals. Log transformed antibody levels could be correlated to duration of virus excretion. The incidence of EHV-1 induced abortions was drastically reduced in vaccinated mares. Therefore, although vaccinated animals are not fully protected against disease, Duvaxyn EHV1,4 clearly reduces clinical symptoms, the duration of virus shedding and the quantity of virus shed. It can be concluded that vaccination of foals and pregnant mares with Duvaxyn EHV1,4 significantly reduces the risk of abortions and outbreaks of respiratory disease caused by circulating field viruses.

Equid herpesvirus-induced immunosuppression is associated with lymphoid cells and not soluble circulating factors

A paresis isolate of equid herpesvirus 1 (EHV1, Ab4/8) and a plaque-purified virus derived from it (EHV1, Ab4/13), induced long-term suppression of both mitogenic and antigen-specific lymphocyte proliferations in adult outbred ponies. Peripheral blood mononuclear cells (PBMC) taken from a pony after EHV1 infection suppressed the in vitro function of normal cells but serum did not. This showed that the observed immune suppression was associated with circulating PBMC and/or their products rather than circulating soluble factors such as antigen or immune complexes. The results suggested that productive infection of lymphocytes by EHV1 was unlikely to result in the observed in vitro effects. Moreover, prostaglandin release from monocytes was not likely to have caused the observed suppression, because lymphocyte responsiveness was not restored in the presence of indomethacin.

Determination of equid herpesvirus 1-specific, CD8+, cytotoxic T lymphocyte precursor frequencies in ponies

The frequency of antigen-specific, genetically restricted cytotoxic T lymphocyte precursors (CTLp) was measured in peripheral blood mononuclear cells (PBMC) of ponies before and after infection with equid herpesvirus 1 (EHV1). Split-well limiting dilution analysis (LDA) was developed to measure CTLp frequency using EHV1-infected 51Cr-labelled lymphoblasts as targets. Extensive characterisation showed that recombinant human interleukin-2, autologous antigen presenting cells and equine serum containing virus neutralising antibody were necessary for maturation of CTLp into effector CTL in vitro. CTLs were not induced when the equine serum (containing VN antibody) was replaced with either foetal calf serum or foetal equine serum (without VN antibody), or seronegative equine serum. CTLp frequency decreased significantly when CD8+ lymphocytes were depleted from the induction cultures. There was good inter- and intra-assay reproducibility using both fresh and recovered cryopreserved PBMC. Both EHV1 and EHV4 could be used to induce effector CTL which lysed EHV1-infected target cells. CTLp frequencies were measured in 2 groups of ponies: Group 1 consisted of two ponies (approx. 9 years old), which had multiple previous experimental infections with EHV1; Group 2 comprised five young (1-2 years) and two older (7 years) ponies which had presumed natural exposure to EHV1/EHV4 but no previous experimental infections. The results showed that CTLp frequencies were higher in the ponies of Group 1 compared with the others. Moreover, ponies with the higher CTLp frequencies were better protected against re-challenge infection with EHV1, showing reduced or absent clinical and virological signs. Consequently, measurement of EHV1-specific CTLp frequency is a potential in vitro correlate of immunity which may be useful for screening new vaccines in horses before embarking upon challenge protection studies to confirm efficacy.

Report of the Second Equine Leucocyte Antigen Workshop, Squaw valley, California, July 1995

The final assignment of antibody clusters for leucocyte antigens and immunoglobulins, as described in detail in Sections 3 and 4, is summarized in Table 4. Together with other mAbs developed outside of ELAW II (Table 9) this pool of reagents represent a powerful array of tools for the study of equine immunity. The Second Equine Leucocyte Antigen Workshop made considerable advances in pursuing the objectives of establishing the specificities of mAbs and achieving consensus on the nomenclature for equine leucocyte and immunoglobulin molecules. Of equal importance, several productive collaborations were fostered among the participating laboratories and observers. Overall, enormous advances have been made in the past decade since mAbs specific for equine leucocyte antigens and immunoglobulins were first reported. There remains enormous scope and need for further studies of equine leucocyte antigens and immunoglobulins, both for the purposes of comparative immunology and for the good of the horse. In the future novel techniques will be required to develop reagents for specific target antigens such as the orthologues of the CD25 or CD45 isoforms. In studies of equine immunoglobulins the functional role of the IgG isotypes must be better established, reagents for IgE must be developed, and cloning of the immunoglobulin heavy chain genes will be essential if the complexities of the IgG sub-isotypes are to be elucidated. The tasks still facing the currently small group of equine immunologists throughout the world remain formidable, and will only be tackled successfully in a spirit of collaboration.

Residence and recruitment of leucocytes to the equine lung after EHV-1 infection

This study characterised bronchoalveolar leucocytes collected by lavage from five susceptible and two immune ponies before and after nebulised aerosol infection with equid herpesvirus-1 (EHV-1). Leucocyte counts and lymphocyte phenotypic analyses were performed using either differential staining or an indirect immunofluorescence assay with monoclonal antibodies specific for equine (Eq) CD4, CD5, CD8 and B lymphocytes. After EHV-1 infection, significant changes developed: a transient neutrophilia occurred on Day 2, coincident with a reduction in macrophage numbers and an EqCD5+, EqCD4+ and EqCD8+ lymphopaenia. On Day 21, a significant rise in EqCD8+ lymphocytes with an associated fall in the EqCD4+:EqCD4+ ratio and fluctuations in B lymphocyte phenotypes were observed. This study shows that following EHV-1 infection, the bronchoalveolar compartment is subject to dynamic leucocyte migration which may have an important role in immunopathogenesis and recovery from disease.

Distribution of equid herpesvirus-1 (EHV-1) in the respiratory tract of ponies: implications for vaccination strategies

Twelve adult ponies and 2 conventional foals were exposed to 10(6.6) TCID50 of Equid herpesvirus-1 (EHV-1), strain Ab4 and samples of respiratory tract tissues were recovered. Infectious virus in tissue homogenates was detected using susceptible cell monolayers and expression of viral antigens was monitored using indirect immunoperoxidase histochemistry of paraffin sections. The results illustrated the rapid dissemination of EHV-1 throughout the respiratory tract, with early replication in the lungs one day after exposure. Endothelial cell infection was prominent in all areas of the nasopharynx by Day 4 emphasising the role of endotheliotropism and viraemia in dissemination of this virus to sites of secondary replication. Clinical disease in the adult ponies was mild.

Distribution of equid herpesvirus-1 (EHV-1) in respiratory tract associated lymphoid tissue: implications for cellular immunity

Twelve adult ponies and 2 conventional foals were exposed intranasally to EHV-1, strain Ab4 (TCID50 10(-6.6) and samples of respiratory tract associated lymphoid tissues were recovered between 12 h and 13 days after infection. Infectious virus was detected in tissue homogenates using susceptible cell monolayers and expression of viral antigens was monitored using indirect immunoperoxidase histochemistry on paraffin sections. The results showed both infectious EHV-1 and viral antigens in respiratory tract associated lymph nodes 12 h after exposure. Infected leucocytes were identified morphologically as lymphocytes, monocytes, macrophages and plasma cells. The rapid intracellular localisation of EHV-1 in lymph nodes implies that cell mediated immunity is an important aspect of the equine response to this virus.

Expression cloning of an equine T-lymphocyte glycoprotein CD2 cDNA. Structure-based analysis of conserved sequence elements

An equine CD2 cDNA has been isolated by monoclonal antibody screening of a T-lymphocyte cDNA library. The cDNA contained an open reading frame of 1041 bp encoding a translated product of 347 amino acids. Northern blotting analysis revealed a single mRNA species expressed in spleen, thymus and activated peripheral lymphocytes. The predicted amino acid sequence has 50-65% identity with the human, rat and mouse CD2 sequences with greatest similarity shared with the human homologue. Evolutionarily conserved structural and functional domains in CD2 were identified by comparing the sequences of the equine, human, mouse and rat CD2 homologues in the context of the recently derived crystal structure of rat soluble CD2 [Jones, E. Y., Davis, S. J., Williams, A. F., Harlos, K. & Stuart, D. I. (1992) Nature 360, 232-239]. The key conserved features of the extracellular region included core residues necessary to preserve the structural integrity of the molecule, residues in the linker region likely to maintain the unique domain organization of CD2, an array of highly charged residues in the putative ligand-binding face of the molecule and glycosylation-signal distributions that render the putative ligand-binding GFCC'C" face of domain 1 relatively unhindered by glycosylation.