Humoral response and antiviral cytokine expression following vaccination of thoroughbred weanlings--a blinded comparison of commercially available vaccines

Evaluation of immunogenicity and protective efficacy of bacteriophage conjugated haemagglutinin based subunit vaccine against equine influenza virus in a murine model

Equine influenza (EI) is a highly contagious acute respiratory disease of equines caused by the H3N8 subtype of Influenza A virus i.e. equine influenza virus (EIV). Vaccination is an important and effective tool for the control of EI in equines. Most of the commercial influenza vaccines are produced in embryonated hen's eggs which has several inherent disadvantages. Hence, subunit vaccine based on recombinant haemagglutinin (HA) antigen, being the most important envelope glycoprotein has been extensively exploited for generating protective immune responses, against influenza A and B viruses. We hypothesized that novel vaccine formulation using baculovirus expressed recombinant HA1 (rHA1) protein coupled with bacteriophage will generate strong protective immune response against EIV. In the present study, the recombinant HA1 protein was produced in insect cells using recombinant baculovirus having cloned HA gene of EIV (Florida clade 2 sublineage) and the purified rHA1 was chemically coupled with bacteriophage using a crosslinker to produce rHA1-phage vaccine candidate. The protective efficacy of vaccine preparations of rHA1-phage conjugate and only rHA1 proteins were evaluated in mouse model through assessing serology, cytokine profiling, clinical signs, gross and histopathological changes, immunohistochemistry, and virus quantification. Immunization of vaccine preparations have stimulated moderate antibody response (ELISA titres-5760 ± 640 and 11,520 ± 1280 for rHA1 and rHA1-phage, respectively at 42 dpi) and elicited strong interferon (IFN)-γ expression levels after three immunizations of vaccine candidates. The immunized BALB/c mice were protected against challenge with wild EIV and resulted in reduced clinical signs and body weight loss, reduced pathological changes, decreased EIV antigen distribution, and restricted EIV replication in lungs and nasopharynx. In conclusion, the immune responses with moderate antibody titer and significantly higher cytokine responses generated by the rHA1-phage vaccine preparation without any adjuvant could be a novel vaccine candidate for quick vaccine preparation through further trials of vaccine in the natural host.

An epidemiological overview of the equine influenza epidemic in Great Britain during 2019

BACKGROUND

During 2019, an epidemic of equine influenza (EI) occurred in Europe.

OBJECTIVES

To describe the epidemiology of the 2019 EI epidemic within Great Britain (GB).

STUDY DESIGN

Retrospective descriptive study of laboratory confirmed EI cases.

METHODS

Epidemiological data were obtained from veterinary surgeons referring samples for EI virus testing. Where available, data on confirmed cases and their wider resident population on EI-infected premises were collated and described. On a national level, spatial and temporal representations, consisting of choropleth maps and epidemic curves, described the spread of EI. EI-infected premises-level factors associated with the first of two epidemic phases were investigated using ordinary logistic regression analysis.

RESULTS

There were 412 confirmed cases and 234 EI-infected premises, with the first of two epidemic phases occurring between January and April, followed by a second phase through to August. The median age of confirmed cases was 5 years and Sports horses (24%) and Cobs (16%) made up the highest proportions by general horse type and breed. Among confirmed cases 72% were unvaccinated and 18% were vaccinated against EI. New horses arriving within 2 weeks of a confirmed case were reported by 42% of EI-infected premises. Investigation of EI-infected premises biosecurity measures indicated that 23% quarantined new arrivals, 37% had isolation facilities and 57% of resident horses were vaccinated. EI-infected premises were more likely in the first than second epidemic phase to be classified as professional, have a vaccinated confirmed case and EI confirmed in a newly arrived animal.

MAIN LIMITATIONS

Data were collected at a single time point for each EI-infected premises with no follow ups performed.

CONCLUSIONS

During 2019, EI-infected premises generally had low levels of population vaccine coverage and implemented limited preventive biosecurity measures, particularly linked to horse movements. Without substantial improvements in infectious disease prevention and control, the GB equine population remains at risk of future EI epidemics.

An inventory of adjuvants used for vaccination in horses: the past, the present and the future

Vaccination is one of the most widely used strategies to protect horses against pathogens. However, available equine vaccines often have limitations, as they do not always provide effective, long-term protection and booster injections are often required. In addition, research efforts are needed to develop effective vaccines against emerging equine pathogens. In this review, we provide an inventory of approved adjuvants for equine vaccines worldwide, and discuss their composition and mode of action when available. A wide range of adjuvants are used in marketed vaccines for horses, the main families being aluminium salts, emulsions, polymers, saponins and ISCOMs. We also present veterinary adjuvants that are already used for vaccination in other species and are currently evaluated in horses to improve equine vaccination and to meet the expected level of protection against pathogens in the equine industry. Finally, we discuss new adjuvants such as liposomes, polylactic acid polymers, inulin, poly-ε-caprolactone nanoparticles and co-polymers that are in development. Our objective is to help professionals in the horse industry understand the composition of marketed equine vaccines in a context of mistrust towards vaccines. Besides, this review provides researchers with a list of adjuvants, either approved or at least evaluated in horses, that could be used either alone or in combination to develop new vaccines.

Nasal transmission of equine parvovirus hepatitis

BACKGROUND

Equine parvovirus hepatitis (EqPV-H) is highly prevalent and causes subclinical to fatal hepatitis, which can occur in outbreaks. Whereas iatrogenic transmission is well documented, the mode of horizontal transmission is not known. The virus is shed in nasal, oral and fecal secretions, and PO transmission has been reported in a single horse.

HYPOTHESIS/OBJECTIVE

Investigate the efficiency of PO and nasal transmission of EqPV-H in a larger cohort.

METHODS

Prospective experimental transmission study. Eleven EqPV-H-negative horses were inoculated with 5 × 10⁶ genome equivalents EqPV-H. Serum PCR and serology for EqPV-H were performed weekly and monthly, respectively. Horses first were inoculated PO, and then intranasally 8 weeks later.

RESULTS

No horse became viremic or seroconverted within 8 weeks after PO inoculation. After intranasal inoculation, 5 horses became viremic within 6 to 12 weeks and seroconverted within 10 to 19 weeks. After a period without monitoring from 12 to 19 weeks postinoculation, another 5 horses were found to be viremic at 19 to 22 weeks. The second set of 5 horses could have been infected by horizontal transmission from the first 5 because of cohousing.

CONCLUSIONS AND CLINICAL IMPORTANCE

We demonstrated that EqPV-H can be transmitted nasally. The prolonged eclipse phase before detectable viremia indicates biosecurity measures to control spread could be impractical.

A Review on Equine Influenza from a Human Influenza Perspective

Influenza A viruses (IAVs) have a main natural reservoir in wild birds. IAVs are highly contagious, continually evolve, and have a wide host range that includes various mammalian species including horses, pigs, and humans. Furthering our understanding of host-pathogen interactions and cross-species transmissions is therefore essential. This review focuses on what is known regarding equine influenza virus (EIV) virology, pathogenesis, immune responses, clinical aspects, epidemiology (including factors contributing to local, national, and international transmission), surveillance, and preventive measures such as vaccines. We compare EIV and human influenza viruses and discuss parallels that can be drawn between them. We highlight differences in evolutionary rates between EIV and human IAVs, their impact on antigenic drift, and vaccine strain updates. We also describe the approaches used for the control of equine influenza (EI), which originated from those used in the human field, including surveillance networks and virological analysis methods. Finally, as vaccination in both species remains the cornerstone of disease mitigation, vaccine technologies and vaccination strategies against influenza in horses and humans are compared and discussed.

Assessment of Humoral and Long-Term Cell-Mediated Immune Responses to Recombinant Canarypox-Vectored Equine Influenza Virus Vaccination in Horses Using Conventional and Accelerated Regimens Respectively

During Australia's first and only outbreak of equine influenza (EI), which was restricted to two northeastern states, horses were strategically vaccinated with a recombinant canarypox-vectored vaccine (rCP-EIV; ProteqFlu™, Merial P/L). The vaccine encoded for haemagglutinin (HA) belonging to two equine influenza viruses (EIVs), including an American and Eurasian lineage subtype that predated the EIV responsible for the outbreak (A/equine/Sydney/07). Racehorses in Victoria (a southern state that remained free of EI) were vaccinated prophylactically. Although the vaccine encoded for (HA) belonged to two EIVs of distinct strains of the field virus, clinical protection was reported in vaccinated horses. Our aim is to assess the extent of humoral immunity in one group of vaccinated horses and interferon-gamma ((EIV)-IFN-γ)) production in the peripheral blood mononuclear cells (PBMCs) of a second population of vaccinated horses. Twelve racehorses at work were monitored for haemagglutination inhibition antibodies to three antigenically distinct equine influenza viruses (EIVs) The EIV antigens included two H3N8 subtypes: A/equine/Sydney/07) A/equine/Newmarket/95 (a European lineage strain) and an H7N7 subtype (A/equine/Prague1956). Cell-mediated immune responses of: seven racehorses following an accelerated vaccination schedule, two horses vaccinated using a conventional regimen, and six unvaccinated horses were evaluated by determining (EIV)-IFN-γ levels. Antibody responses following vaccination with ProteqFlu™ were cross-reactive in nature, with responses to both H3N8 EIV strains. Although (EIV)IFN-γ was clearly detected following the in vitro re-stimulation of PBMC, there was no significant difference between the different groups of horses. Results of this study support reports of clinical protection of Australian horses following vaccination with Proteq-Flu™ with objective evidence of humoral cross-reactivity to the outbreak viral strain A/equine/Sydney/07.

Evaluation of concurrent vaccinations with recombinant canarypox equine influenza virus and inactivated equine herpesvirus vaccines

Despite vaccination, equine influenza virus (EIV) and equine herpesvirus (EHV) infections still cause highly contagious respiratory diseases in horses. Recently, concurrent vaccination with EIV and EHV was suggested as a new approach; however, there have been no reports of concurrent vaccination with recombinant canarypox EIV and inactivated EHV vaccines. In this study, we aimed to compare the EIV-specific immune responses induced by concurrent administrations of a recombinant canarypox EIV vaccine and an inactivated bivalent EHV vaccine with those induced by a single recombinant canarypox EIV vaccine in experimental horse and mouse models. Serum and peripheral blood mononuclear cells (PBMCs) were collected from immunized animals after vaccination. EIV-specific serum antibody levels, serum hemagglutinin inhibition (HI) titers, and interferon-gamma (IFN-γ) levels were measured by enzyme-linked immunosorbent assay, HI assay, and quantitative polymerase chain reaction, respectively. Concurrent EIV and EHV vaccine administration significantly increased IFN-γ production, without compromising humoral responses. Our data demonstrate that concurrent vaccination with EIV and EHV vaccines can enhance EIV-specific cellular responses in horses.

Novel Vaccine Technologies in Veterinary Medicine: A Herald to Human Medicine Vaccines

The success of inactivated and live-attenuated vaccines has enhanced livestock productivity, promoted food security, and attenuated the morbidity and mortality of several human, animal, and zoonotic diseases. However, these traditional vaccine technologies are not without fault. The efficacy of inactivated vaccines can be suboptimal with particular pathogens and safety concerns arise with live-attenuated vaccines. Additionally, the rate of emerging infectious diseases continues to increase and with that the need to quickly deploy new vaccines. Unfortunately, first generation vaccines are not conducive to such urgencies. Within the last three decades, veterinary medicine has spearheaded the advancement in novel vaccine development to circumvent several of the flaws associated with classical vaccines. These third generation vaccines, including DNA, RNA and recombinant viral-vector vaccines, induce both humoral and cellular immune response, are economically manufactured, safe to use, and can be utilized to differentiate infected from vaccinated animals. The present article offers a review of commercially available novel vaccine technologies currently utilized in companion animal, food animal, and wildlife disease control.

Immunogenicity of Calvenza-03 EIV/EHV® Vaccine in Horses: Comparative In Vivo Study

Equine influenza (EI) is a highly contagious acute respiratory disease of equines that is caused mainly by the H3N8 subtype of influenza A virus. Vaccinating horses against EI is the most effective strategy to prevent the infection. The current study aimed to compare the kinetics of EI-specific humoral- and cell-mediated immunity (CMI) in horses receiving either identical or mixed vaccinations. Two groups of horses were previously (six months prior) vaccinated with either Calvenza 03 EIV EHV^® (G1) or Fluvac Innovator^® (G2) vaccine. Subsequently, both groups received a booster single dose of Calvenza 03 EIV EHV^®. Immune responses were assessed after 10 weeks using single radial hemolysis (SRH), virus neutralization (VN), and EliSpot assays. Our results revealed that Calvenza-03 EIV/EHV^®-immunized horses had significantly higher protective EI-specific SRH antibodies and VN antibodies. Booster immunization with Calvenza-03 EIV/EHV^® vaccine significantly stimulated cell-mediated immune response as evidenced by significant increase in interferon-γ-secreting peripheral blood mononuclear cells. In conclusion, Calvenza-03 EIV/EHV^® vaccine can be safely and effectively used for booster immunization to elicit optimal long persisting humoral and CMI responses even if the horses were previously immunized with a heterogeneous vaccine.

An Evaluation of Three Different Primary Equine Influenza Vaccination Intervals in Foals

In order to evaluate the effect of three different primary vaccination intervals on EI vaccine response, 21 unvaccinated thoroughbred foals were randomly divided into three groups of 7 and vaccinated with three different intervals of primary immunization (i.e., with 1, 2 or 3 months intervals between V1 and V2, respectively). The antibody response was measured for up to 1 year after the third immunization V3 (administered 6 months after V2) by single radial hemolysis (SRH) assay. All weanlings had seroconverted and exceeded the clinical protection threshold 2 weeks after V2 and 1 month after V3 until the end of the study. Significant differences were measured at the peak of immunity after V2 and for the duration of the immunity gap between V2 and V3. The group with one month primary vaccination interval had a lower immunity peak after V2 (158.05 ± 6.63 mm²) and a wider immunity gap between V2 and V3 (18 weeks) when compared with other groups (i.e., 174.72 ± 6.86 mm² and 16 weeks for a two months interval, 221.45 ± 14.48 mm² and 12 weeks for a 3-month interval). The advantage observed in the group with 1 month primary vaccination interval, which induces an earlier protective immunity, is counterbalance with a lower peak of immunity and a wider immunity gap after V2, when compared with foals vaccinated with 2- and 3-month intervals.

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.

Evaluation of Current Equine Influenza Vaccination Protocols Prior to Shipment, Guided by OIE Standards

To facilitate the temporary importation of horses for competition and racing purposes, with a minimum risk of transmitting equine influenza, the World Organisation for Animal Health (Office International des Epizooties, or OIE), formally engaged in a public-private partnership with the Federation Equestre Internationale (FEI) and the International Federation for Horseracing Authorities (IFHA) to establish, within the context of existing OIE standards, a science-based rationale to identify the ideal time period for equine influenza vaccination prior to shipment. Field trials using vaccines based on different technologies were carried out on three continents. The antibody response post-booster vaccination at intervals aligned with the different rules/recommendations of the OIE, FEI, and IFHA, was monitored by single radial haemolysis. It was determined that 14 days was the optimum period necessary to allow horses adequate time to respond to booster vaccination and for horses that have previously received four or more doses of vaccine and are older than four years, it is adequate to allow vaccination within 180 days of shipment. In contrast, the results indicate that there is a potential benefit to younger (four years old or younger) horses in requiring booster vaccination within 90 days of shipment, consistent with the current OIE standard.

Impact of Mixed Equine Influenza Vaccination on Correlate of Protection in Horses

To evaluate the humoral immune response to mixed Equine Influenza vaccination, a common practice in the field, an experimental study was carried out on 42 unvaccinated thoroughbred weanling foals divided into six groups of seven. Three groups were vaccinated using a non-mixed protocol (Equilis^® Prequenza-Te, Proteqflu-Te^® or Calvenza-03^®) and three other groups were vaccinated using a mix of the three vaccines mentioned previously. Each weanling underwent a primary EI vaccination schedule composed of two primary immunisations (V1 and V2) four weeks apart followed by a third boost immunisation (V3) six months later. Antibody responses were monitored until one-year post-V3 by single radial haemolysis (SRH). The results showed similar antibody responses for all groups using mixed EI vaccination and the group exclusively vaccinated with Equilis^® Prequenza-TE, which were significantly higher than the other two groups vaccinated with Proteqflu-TE^® and Calvenza-03^®. All weanlings (100%) failed to seroconvert after V1 and 21% (9/42) still had low or no SRH antibody titres two weeks post-V2. All weanlings had seroconverted and exceeded the clinical protection threshold one month after V3. The poor response to vaccination was primarily observed in groups exclusively vaccinated with Proteqflu-Te^® and Calvenza-03^®. A large window of susceptibility (3–4.5-month duration) usually called immunity gap was observed after V2 and prior to V3 for all groups. The SRH antibody level was maintained above the clinical protection threshold for three months post-V3 for the groups exclusively vaccinated with Proteqflu-Te^® and Calvenza-03^®, and six months to one year for groups using mixed EI vaccination or exclusively vaccinated with Equilis^® Prequenza-Te. This study demonstrates for the first time that the mix of EI vaccines during the primary vaccination schedule has no detrimental impact on the correlate of protection against EIV infection.

Concurrent vaccination against equine influenza and equine herpesvirus - a practical approach

Background

There is a lack of information concerning concurrent administration of vaccines against equine influenza virus (EIV) and equine herpesvirus 1 and 4 (EHV‐1/4).

Objectives

The primary objective of this study was to determine the impact of the concurrent use of EIV and EHV‐1/4 vaccines in Thoroughbred racehorses on their humoral immune response to EIV.

Methods

This study was carried out on a population of 30 horses using an inactivated whole‐virus EIV vaccine and an inactivated EHV‐1/4 vaccine. Horses were randomly allocated to vaccination group A or B. Horses in group A were vaccinated against EIV and EHV‐1/4 2 weeks apart. Horses in group B were vaccinated against EIV and EHV‐1/4 on the same day. Whole‐blood samples were collected on the day of vaccination and 2 weeks and 6 weeks post‐vaccination. Antibody levels against EIV and EHV‐1/4 were measured using the single radial haemolysis and serum neutralisation test, respectively.

Results

The pattern of EIV antibody response post‐vaccination was similar for both groups. Highest EIV antibody levels were recorded 2 weeks post‐vaccination, and a significant decrease in antibody level was observed 4 weeks later. Horses in group B demonstrated a significantly higher EIV antibody response post‐vaccination. Overall, there was no significant difference in EHV‐1/4 antibody response between the two groups post‐vaccination.

Conclusion

In this study, concurrent vaccination against EIV and EHV‐1/4 increased the response to EIV and did not compromise the humoral immune response to EHV‐1/4.

Simultaneous detection and quantification of six equine cytokines in plasma using a fluorescent microsphere immunoassay (FMIA)

Cytokines are cell signalling proteins that mediate a number of different physiological responses. They are also biomarkers for inflammatory conditions and potential diagnostic references for diseases. Until recently, simultaneous quantification of cytokine profiles had not been possible. Now however, fluorescent microsphere immunoassays (FMIA) are able to measure multiple cytokines in a single sample. The following pro-inflammatory and anti-inflammatory cytokines were quantified in equine plasma and serum samples: interleukin (IL)-2, IL-4, IL-6, IL-10, interferon (IFN)-γ, tumor necrosis factor (TNF)-α.

•

The objective of this study was to quantify six equine cytokines simultaneously using the BioPlex^® 200 system in equine EDTA-plasma and serum.

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It demonstrates an increased number of detectable cytokines over published studies.

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This technology has the advantage of reduced sample volume and assay time compared to traditional sandwich ELISAs.

Interaction between anthelmintic treatment and vaccine responses in ponies naturally infected with cyathostomins

Anthelmintics and vaccines are commonly given concurrently in routine equine management, but it is unknown to what extent an interaction between the two exists. Cyathostomins can modulate the local immune response by stimulating a type 2 helper T cell (Th2) response. In addition, anti-inflammatory effects of ivermectin have been found in rodent models. It is unknown whether these anti-inflammatory effects affect the acute phase response elicited by commonly used vaccines. This study evaluated how the acute phase inflammatory response, leukocyte expression of pro-inflammatory cytokines, and vaccine-specific titers induced by simultaneous injection of three vaccines (West Nile Virus, Equine Herpes Rhinopneumonitis, and Keyhole Limpet Hemocyanin) were modulated by concurrent administration of ivermectin or pyrantel pamoate in ponies naturally infected with cyathostomins. Mixed-breed yearling ponies were blocked by gender and fecal strongyle egg count, then randomly assigned to three treatment groups: ivermectin (n=8), pyrantel pamoate (n=8), and control (n=7). All ponies received vaccinations intramuscularly on days 0 and 29, and anthelmintics were administered on the same days. Whole blood, serum and plasma samples were collected one, three and 14 days after each vaccination. Samples were analyzed for concentrations of acute phase reactants (haptoglobin, serum amyloid A, fibrinogen and iron), mRNA expression levels of cytokines (interleukin (IL)-1β, IL-4, IL-10, tumor necrosis factor (TNF)-α and interferon (IFN)-γ) in leukocytes, and vaccine-specific antibody titers. A marked acute-phase response was noted following both vaccinations. In contrast, the pattern of change in cytokine expression was less pronounced and more variable. Statistical differences were observed between groups for haptoglobin, fibrinogen, IL-1β, IL-4, and IL-10, but differences were generally small and none of the vaccine titers were different between the groups. Taken together, the study found some signs of modulation of immunologic or inflammatory responses to the administered vaccines, when anthelmintics were administered concurrently, but these are unlikely to have practical implications for vaccination routines.

A Systematic Review of Recent Advances in Equine Influenza Vaccination

Equine influenza (EI) is a major respiratory disease of horses, which is still causing substantial outbreaks worldwide despite several decades of surveillance and prevention. Alongside quarantine procedures, vaccination is widely used to prevent or limit spread of the disease. The panel of EI vaccines commercially available is probably one of the most varied, including whole inactivated virus vaccines, Immuno-Stimulating Complex adjuvanted vaccines (ISCOM and ISCOM-Matrix), a live attenuated equine influenza virus (EIV) vaccine and a recombinant poxvirus-vectored vaccine. Several other strategies of vaccination are also evaluated. This systematic review reports the advances of EI vaccines during the last few years as well as some of the mechanisms behind the inefficient or sub-optimal response of horses to vaccination.

Epidemiological and virological investigations of equine influenza outbreaks in Ireland (2010-2012)

BACKGROUND

Outbreaks of equine influenza (EI) in endemic populations cause disruption and economic loss.

OBJECTIVES

To identify (i) factors involved in the spread of EI (ii) virus strains responsible for outbreaks (iii) single radial haemolysis (SRH) antibody levels correlating with protection against current virus strains (iv) evidence of vaccination breakdown.

METHODS

RT-PCR, virus isolation and SRH were carried out on nasopharyngeal swabs and blood samples collected from horses, ponies and donkeys on affected premises. Data relating to 629 samples from 135 equidae were analysed.

RESULTS AND CONCLUSIONS

Outbreaks were sporadic, self limiting and associated with the movement of horses. Vaccination status and age influenced clinical signs of disease while housing and fomites contributed to virus spread. Subclinical infection as defined as a horse which tested positive by one or more of the following; RT-PCR, virus isolation and seroconversion in the absence of clinical signs, was identified in 9% of animals. Of the horses with up to date vaccination records 32% developed clinical signs. Vaccine breakdown occurred among horses vaccinated with all four commercially available vaccines. Analysis of HA1 sequence data generated for 26 viruses indicated that they all belonged to clade 2 of the Florida sublineage. Higher SRH antibody levels were required for both clinical and virological protection than reported in studies where vaccine strains were antigenically and genetically similar to those circulating in the field. The results of this study therefore support the OIE recommendations that vaccines be updated to include representatives of both clades of the Florida sublineage.

Duration of the protective immune response after prime and booster vaccination of yearlings with a live modified cold-adapted viral vaccine against equine influenza

We previously created a live vaccine against equine influenza based the new reassortant cold-adapted (Ca) strain A/HK/Otar/6:2/2010. The live vaccine contains surface proteins (HA, NA) from the wild-type virus A/equine/Otar/764/2007 (Н3N8; American Lineage Florida Clade 2), and internal proteins (PB2, PB1, PA, NP, M, NS) from the attenuated Ca donor virus A/Hong Kong/1/68/162/35CA (H3N2). To determine the safety and duration of the protective immune responses, 90 yearlings were intranasally vaccinated in single mode, double mode at an interval of 42 days (10(7.0) EID50/animal for both vaccinations), or with PBS (control group). Ten animals from each group were challenged with the homologous wild-type virus A/equine/Otar/764/07 (Н3N8) at 1, 2, 3, 4, 5, 6, 9 and 12 months after vaccination. Similarly, 10 animals from each group were challenged with the heterologous wild-type virus A/equine/Sydney/2888-8/07 (Н3N8; American Lineage Florida Clade 1) 12 months after vaccination. The vaccine was completely safe, and single intranasal vaccination of yearlings was capable of inducing statistically significant (from P=0.03 to P<0.0001) clinical and virological protection against the homologous virus; however, only double mode vaccination generated significant (from P=0.02 to P<0.0001) protection against the heterologous virus at 12 months (observation period). Interestingly, this vaccine enables the differentiation of infected and vaccinated animals. On this basis of this study, we recommend double intranasal administration of this vaccine at an interval of 42 days in veterinary practice.