Clinical signs and humoral immune response in horses following equine herpesvirus type-1 infection and their susceptibility to equine herpesvirus type-4 challenge

Viremia and nasal shedding for the diagnosis of equine herpesvirus-1 infection in domesticated horses

BACKGROUND

Equine herpesvirus type 1 (EHV-1) infection is associated with upper respiratory disease, EHM, abortions, and neonatal death.

RESEARCH QUESTIONS

Are nasal secretions a more sensitive biological sample compared to blood for the detection of EHV-1 infection? How long is EHV-1 detectable after primary infection by PCR?

METHODS

MedLine and Web of Science searches identified original peer-reviewed reports evaluating nasal shedding and viremia using virus isolation methods or PCR published in English before October 9, 2023.

RESULTS

Sixty experimental and 20 observational studies met inclusion criteria. EHV-1 detection frequency by qPCR in nasal secretions and blood from naturally-infected horses with fever and respiratory signs were 15% and 9%, respectively; qPCR detection rates in nasal secretions and blood from horses with suspected EHM were 94% and 70%, respectively. In experimental studies the sensitivity of qPCR matched or exceeded that seen for virus isolation from either nasal secretions or blood. Detection of nasal shedding typically occurred within 2 days after EHV-1 inoculation with a detection period of 3 to 7 days. Viremia lasted 2 to 7 days and was usually detected ≥1 days after positive identification of EHV-1 in nasal secretions. Nasal shedding and viremia decreased over time and remained detectable in some horses for several weeks after inoculation.

CONCLUSIONS AND CLINICAL IMPORTANCE

Under experimental conditions, blood and nasal secretions have similar sensitivity for the detection of EHV-1 when horses are sampled on multiple consecutive days. In contrast, in observational studies detection of EHV-1 in nasal secretions was consistently more successful.

Respiratory Disease: Diagnostic Approaches in the Horse

Evaluation of the upper and lower respiratory tract of horses requires strategic selection of possible diagnostic tests based on location of suspected pathologic lesions and purpose of testing and must also include consideration of patient status. This article discusses the various diagnostic modalities that may be applied to the respiratory system of horses under field conditions, indications for use, and aspects of sample collection, handling, and laboratory processing that can impact test results and ultimately a successful diagnosis in cases of respiratory disease.

Equine herpesvirus type 4 UL56 and UL49.5 proteins downregulate cell surface major histocompatibility complex class I expression independently of each other

Major histocompatibility complex class I (MHC-I) molecules are critically important in the host defense against various pathogens through presentation of viral peptides to cytotoxic T lymphocytes (CTLs), a process resulting in the destruction of virus-infected cells. Herpesviruses interfere with CTL-mediated elimination of infected cells by various mechanisms, including inhibition of peptide transport and loading, perturbation of MHC-I trafficking, and rerouting and proteolysis of cell surface MHC-I. In this study, we show that equine herpesvirus type 4 (EHV-4) modulates MHC-I cell surface expression through two different mechanisms. First, EHV-4 can lead to a significant downregulation of MHC-I expression at the cell surface through the product of ORF1, a protein expressed with early kinetics from a gene that is homologous to herpes simplex virus 1 UL56. The EHV-4 UL56 protein reduces cell surface MHC-I as early as 4 h after infection. Second, EHV-4 can interfere with MHC-I antigen presentation, starting at 6 h after infection, by inhibition of the transporter associated with antigen processing (TAP) through its UL49.5 protein. Although pUL49.5 has no immediate effect on overall surface MHC-I levels in infected cells, it blocks the supply of antigenic peptides to the endoplasmic reticulum (ER) and transport of peptide-loaded MHC-I to the cell surface. Taken together, our results show that EHV-4 encodes at least two viral immune evasion proteins: pUL56 reduces MHC-I molecules on the cell surface at early times after infection, and pUL49.5 interferes with MHC-I antigen presentation by blocking peptide transport in the ER.

Serum concentration of copper, zinc, iron, and cobalt and the copper/zinc ratio in horses with equine herpesvirus-1

The serum concentrations of copper, zinc, iron, and cobalt and copper/zinc ratio were investigated in horses infected with equine herpesvirus-1 (EHV-1). Nine horses were naturally infected with the virus and nine healthy horses served as controls. The concentrations of copper, zinc, iron, and cobalt were determined spectrophotometrically in the blood serum of all horses. The results were (expressed in micrograms per deciliters) copper 2.80 +/- 0.34 vs 1.12 +/- 0.44, zinc 3.05 +/- 0.18 vs 0.83 +/- 0.06, iron 2.76 +/- 0.17 vs 3.71 +/- 0.69, cobalt 0.19 +/- 0.37 vs 0.22 +/- 0.45, and copper/zinc ratio 0.72 +/- 0.38 vs 1.41 +/- 0.36 for control vs infected group, respectively. In conclusion, copper and zinc concentrations of the infected group were lower than the control group (p < 0.001), whereas iron concentration and the copper/zinc ratio of the infected group were higher than the control group (p < 0.05 and p < 0.001). The cobalt concentration was not found to be statistically different between two groups. It might be emphasized that copper/zinc ratio was significantly affected by the EHV-1 infection, so it could be taken into consideration during the course of infection.

Comparison of antibody detection assays for the diagnosis of equine herpesvirus 1 and 4 infections in horses

OBJECTIVE

To compare methods of detecting equine herpesvirus type 1 (EHV1)- and EHV4-specific antibodies in horse sera.

SAMPLE POPULATION

33 acute and convalescent serum samples from experimentally or naturally infected horses after confirmed EHV1 or EHV4 infection.

PROCEDURE

For each sample, serum antibody titers against EHV1 and EHV4 were determined by use of virus neutralization (VN) and complement fixation (CF) assays. The ELISA absorbance values for each serum sample were determined against the EHV1 and EHV4 recombinant ELISA antigens. Values obtained for acute and convalescent sera in each assay were compared.

RESULTS

Following experimental infection of foals, EHV1 or EHV4 antibodies that were specific for the inoculating virus were detected only by use of the ELISA. Results of VN and CF assays indicated that the foals seroconverted to EHV1 and EHV4 following infection with EHV4 only. After EHV1-induced abortion, myeloencephalitis, or respiratory tract disease, the VN and CF assay results revealed seroconversion to EHV1 and EHV4, whereas results of the ELISA revealed seroconversion to EHV1 only. Similarly, after confirmed EHV4-induced respiratory tract disease, increases in EHV4-specific antibodies were detected only by use of the ELISA with no indication of an increase in EHV1 antibodies. The CF and, to a lesser degree, VN assays revealed that seroconversion to EHV1 and EHV4 occurred between the time of obtaining acute and convalescent serum samples.

CONCLUSIONS AND CLINICAL RELEVANCE

The EHV1/EHV4 type-specific antibody ELISA clearly identifies horses that have been infected with EHV1 or EHV4 by use of acute and convalescent sera. Results of VN and CF assays indicate that cross-reactive antibodies greatly limit their use.

Evaluation of alveolar macrophage function after experimental infection with equine herpesvirus-1 in horses

The role of the pulmonary alveolar macrophages (PAM) in the lung defense mechanism was evaluated in horses infected with equine hespesvirus-1 (EHV-1). Five adult horses were exposed to 10(6.6) TCID50 EHV-1 by intranasal instillation. Cytology of bronchoalveolar lavage (BAL) was performed using cytocentrifugation of samples and slides stained by Rosenfeld. Cell concentration was adjusted to 2<FONT FACE=Symbol>´</FONT>10(6) cells/ml, for the measurement of macrophage activity - spreading, phagocytosis of zymosan particles and release of hydrogen peroxide (H2O2). All animals were positive in virus isolation on the second, third and fifth days post-inoculation (DPI). Seroconversion was observed on the 14th DPI. Lymphocytosis was observed by BAL cytology on the 16th DPI. Measurement of macrophage activity demonstrated a marked increase in the spreading rate, on the 23rd and 30th DPI. Phagocytosis was decreased on the second DPI, and returned to levels similar to those observed before inoculation on the 23rd DPI. The amount of H2O2 released by PAM declined on day 2, but, by day 16, they returned to values similar to those observed before inoculation. The decline in PAM activity in the acute phase of disease is indirect evidence that these cells have an important role in lung defense mechanisms against this agent.

Seroprevalence of equine herpesvirus 1 in thoroughbred foals before and after weaning

OBJECTIVE

To investigate the seroprevalence of equine herpesvirus 1 in foals around weaning and after weaning on two large Thoroughbred farms using a type-specific enzyme-linked immunosorbent assay to determine exposure to infection.

DESIGN

A longitudinal population study in groups of Thoroughbred weanling foals.

STUDY POPULATION

Two hundred weanling Thoroughbred foals from a population of about 380 foals were enrolled on two adjacent stud farms in the Hunter Valley of New South Wales. Foals on both farms were weaned from February to May 1995 into randomly selected groups of 10 to 15 foals. Farms were selected because of their willingness to cooperate in the survey and because their detailed records of foals and their movements. They were representative of well-managed large Thoroughbred stud farms in New South Wales. Both studs had upper respiratory tract disease among weanling foals around weaning each year although the seroprevalence of viral respiratory disease on either farm was not known before the study.

PROCEDURE

Serum was collected from foals within each group at fortnightly intervals from 9th February until 1st June 1995, and at a single follow-up period in August 1995. Each sample was tested in triplicate using an antibody-detection ELISA which is type-specific for EHV-1 and EHV-4 antibodies.

RESULTS AND CONCLUSIONS

There was serological evidence of EHV-1 infection both before and after weaning. The prevalence of EHV-1 antibody in the sample population increased during the study and individual cases of EHV-1 infection were identified. The increase was caused both by the seroconversion of foals within the groups and by the recruitment into the study of foals with pre-existing EHV-1 antibody. Evidence of EHV-1 infection in Thoroughbred foals after weaning has not been reported previously in Australia and this has implications for vaccination regimens.

Role of T-cells, virus neutralising antibodies and complement-mediated antibody lysis in the immune response against equine herpesvirus type-1 (EHV-1) infection of C3H (H-2k) and BALB/c (H-2d) mice

The suitability of C3H (H-2k) and BALB/c (H-2d) mice for use as small animal models to study immunity to EHV-1 was assessed. An in vitro T cell response mediated by both CD4+ and CD8+ T cells was detected both during the acute phase of infection and after challenge with a second dose of EHV-1 at two months in lymphocyte populations taken from the spleens of both types of mouse. The responses were apparent until at least 61 days after the primary inoculation. After challenge, T cells from mice previously infected with EHV-1 responded by as early as day 3 after infection and higher levels of T cell proliferation were reached than in mice undergoing a primary infection. Immunological cross-reactivity with the closely related virus, EHV-4 was detected and some activity against herpes simplex virus type-1 (HSV-1) was observed during the acute phase of infection. T cell responses were detected in the draining cervical lymph nodes but not in the inguinal lymph nodes of the mice and these were the primary sites of T cell activation. Complement-dependent virus neutralising antibodies were present by day 8 after infection. These antibodies were also able to lyse EHV-1 infected target cells in vitro. Complement-independent virus neutralising antibodies were found before challenge only in C3H mice. The clinical signs and duration of virus shedding were reduced after challenge. The time course of the appearance of the different immune effector mechanisms is discussed in relation to the clearance of virus from the infected mice. The results suggest that C3H mice provide a better model in which to study potential vaccine candidates against EHV-1 infections of the horse than BALB/c mice.

Modulation of the serological response of specific pathogen-free (EHV-free) foals to EHV-1 by previous infection with EHV-4 or a TK-deletion mutant of EHV-1

EHV-1 was inoculated into specific pathogen-free (SPF) foals in order to study uncomplicated primary responses. Infection resulted in a strong serological response recognizing EHV-1-specific antigens; this contrasts with a previous publication where a weak response was recorded in SPF animals. Antibodies to EHV-1 were readily detected by four techniques (virus neutralization, complement fixation, Western blots and immune precipitation), yet there was comparatively little cross-reaction to EHV-4 target antigen. Re-inoculation with the same virus strain stimulated antibodies to EHV-1 but no additional antigens were recognized and antibodies cross-reacting with EHV-4 antigens were not enhanced. Having characterized the uncomplicated primary response to EHV-1 in SPF foals, further animals were exposed to either EHV-4 or a thymidine kinase-deficient mutant of EHV-1 prior to challenge with w/t EHV-1 to investigate how these infections might modulate the immune responses to EHV-1 or 4. Primary inoculation with EHV-4 or with a thymidine kinase-deficient mutant of EHV-1 produced productive infections as evidenced by virus shedding and pyrexia. In both these cases, however, in contrast to that with w/t EHV-1, the serological response was very weak. Re-infection of foals primed with either EHV-4 or TK-deficient EHV-1 with w/t EHV-1 resulted in a strong response to EHV-1 antigens detected by all four methods. In addition, in the foals given a primary inoculation with EHV-4, superinfection with EHV-1 resulted in a strong cross-reactive response to EHV-4 target antigens. The relevance of these observations to the interpretation of previously reported serological responses to EHVs in SPF and naturally reared animals is discussed.