Protective effect of intranasal vaccination of horses with a modified live equine herpesvirus type 1 vaccine

Abstract

Mucosal immunity in the nasal cavity is considered to play an essential role in protecting horses from disease caused by equine herpesvirus type 1 (EHV-1), and immunoglobulin A (IgA) antibody in the nasal secretions is a key component of this mucosal immunity. Although intranasal administration of live or killed EHV-1 vaccines has been attempted with the aim of protecting horses from viral challenge, induction of virus-specific IgA after these vaccinations was not observed, and only slight mitigation of clinical signs was observed after viral challenge. Here, we measured immunological responses and performed a challenge experiment in horses intranasally immunized with a modified live EHV-1 vaccine (Nisseiken, Tokyo, Japan) that was originally licensed for intramuscular injection. Horses (n ¼ 3) were intranasally inoculated with the vaccine (1.3 106 plaque-forming units [pfu]/10 mL/head) twice with a 4-week interval. Control horses (n ¼ 2) were inoculated in the same way with dilution medium. Four weeks after the second dose had been given, the horses were challenged by intranasal inoculation of EHV-1 (strain 10-I-224, 4.0 106 pfu/10 mL/head). Sequential nasal wash samples were concentrated by ultrafiltration and subjected to ELISA to measure EHV-1-specific IgA. Viral nasal shedding and viremia were monitored by virus isolation and real-time PCR. The results of real-time PCR were considered negative if there were � 10 gene copies in the analyzed sample. EHV-1-specific IgA levels in the nasal wash increased in all of the vaccinated horses after two intranasal doses of vaccine, whereas those in the control horses remained stable. Serum virus-neutralizing titers also increased in the vaccinated horses. After viral challenge, pyrexia over 39.0 � C was observed at 1 day post-inoculation (dpi) in both of the control horses. In contrast, none of the vaccinated horses was pyretic (over 39.0 � C) throughout the experiment, although one horse had a temperature of 38.7 � C at 7 dpi. Viral nasal shedding in the control horses was detected at 1 to 6 dpi, with a highest mean gene copy number of 103.8. In contrast, viral nasal shedding in the vaccinated horses was detected only at 1 to 2 dpi, with a highest mean gene copy number of 102.3. Viremia was detected in all horses at 5 to 11 dpi, with individual durations of 2 to 6 days. Intranasal immunization of horses with the live EHV-1 vaccine resulted in the production of EHV-1-specific IgA in the nasal mucosa and virus-neutralizing antibody in the whole body. Virus replication in the nasal mucosa after viral challenge was lower in vaccinated horses than in control horses; virus-specific IgA induced by the intranasal vaccination might have been responsible for this. The absence of pyrexia over 39.0 � C in the vaccinated horses indicated that the immunity induced by intranasal vaccination was effective in protecting horses from EHV-1-induced pyrexia.