Equine Influenza: What the Research Says
Evidence from 30 peer-reviewed studies
1 Systematic Review
5 RCT
6 Cohort Study
6 Case Report
12 Expert Opinion
What Professionals Should Know
- •Practitioners in high-risk regions (Asia and Americas) should implement enhanced biosecurity and vaccination protocols, particularly in areas with high horse density
- •EI surveillance should intensify during cold quarters in identified high-risk zones, as seasonal precipitation patterns influence disease occurrence
- •Geographic range and timing of EI risk is predicted to shift eastward in coming years; management strategies should be adapted regionally based on these forecasts
- •Consider initiating multivalent vaccination protocols at 3 months (90 days) rather than waiting until 6 months in high-risk foals or situations where earlier immunity is needed; immune responses are comparable to conventional timing
- •Maternal antibodies do not prevent immune activation in young foals—do not assume early vaccination will be ineffective
- •Ensure booster vaccination at 11 months regardless of initial vaccination age, as this is critical for establishing robust memory immune responses and immunoglobulin G production
- •Longer vaccination intervals permitted by racing authorities create periods of increased susceptibility to equine influenza, requiring strategic timing around exposure risk periods
- •Poor responders to initial vaccination are common in yearlings, suggesting this age group may need additional monitoring or alternative vaccination strategies
- •Vaccination schedules can be extended without compromising immune response to booster doses, allowing flexibility in management planning
- •Current equine influenza vaccines can provide meaningful cross-protection against emerging strains even when not specifically updated for new variants—vaccinated horses had minimal clinical signs and substantially reduced viral shedding
- •The Matrix-C adjuvant platform appears to support cross-protective immunity; vaccinated horses showed reduced fever and clinical disease compared to unvaccinated controls after challenge
- •Regular vaccination remains worthwhile for reducing disease severity and transmission risk even when vaccine strains don't perfectly match circulating field strains
- •This combination vaccine effectively prevents clinical disease and reduces viral shedding when horses are challenged with American-lineage equine influenza, supporting its use in vaccination programmes
- •Protective immunity is established within 4-6 weeks of initial vaccination and booster doses at 6 and 32 weeks maintain protective titres for at least 26 weeks, informing practical revaccination schedules
- •The vaccine's ability to reduce virus excretion is valuable for limiting disease transmission in populations, particularly important in competition and boarding settings
- •A single intranasal dose of this cold-adapted vaccine provides meaningful protection against clinical influenza for at least 6 months, substantially reducing disease severity even when horses face live-virus challenge
- •Vaccinated horses shed significantly less virus over shorter periods, reducing transmission risk within premises and supporting herd-level disease control
- •This vaccine offers an effective tool to reduce the frequency, severity and duration of influenza outbreaks in North American equine operations without adverse vaccination effects
- •Implement mandatory vaccination and post-arrival quarantine protocols for all new horse arrivals, particularly during October-December when outbreak risk is highest
- •Strengthen biosecurity measures during horse transport and at sales facilities, especially for non-breeding stock imported from Ireland
- •Monitor Irish horse imports as a key surveillance indicator for predicting UK equine influenza outbreaks and timing of control interventions
- •Reverse genetics-derived EI vaccines provide equivalent protection to traditional commercial vaccines, offering a platform for rapid vaccine strain updates when new equine influenza variants emerge
- •Both primary and booster vaccination schedules with RG-derived bivalent vaccines achieve expected immune responses in Thoroughbreds, supporting their use in routine vaccination programs
- •RG vaccine technology enables flexibility in multivalent vaccine development, potentially improving response to evolving equine influenza strains in field populations
- •While shorter vaccination intervals (1 month) achieve protective immunity faster in foals, a 3-month interval between first and second doses produces stronger peak antibody levels and more reliable protection during the gap before the third dose
- •Practitioners should consider the 2-3 month interval range for primary foal vaccination to balance early protection with antibody durability and reduced window of vulnerability
- •All tested intervals achieved adequate protection, so choice can be individualized based on management schedule and risk assessment, but avoid the shortest (1-month) interval if foals face high disease exposure risk
- •Sex of the horse influences serological response to equine influenza vaccination, with females mounting better antibody responses—consider this when interpreting vaccination efficacy
- •Using different vaccine products over time may provide better immune protection than repeated doses of the same product, but excessive vaccination doses do not guarantee higher protection
- •Vaccination strategy should account for individual factors (age, sex, previous products used) rather than applying one-size-fits-all protocols to all racehorses in training
- •In outbreak situations, horses can be rapidly protected with a 14-day vaccination interval rather than waiting for standard schedules, enabling faster herd immunity development
- •The ALVAC-EIV vaccine on this accelerated schedule reliably produces protective antibody levels, supporting its use when quick disease control is needed
- •Booster vaccination at 105 days maintains immunity, providing practitioners with clear timing for follow-up vaccinations in accelerated programs
- •Consider timing of vaccination programs in foals with maternal antibodies present, as repeated early vaccination may be counterproductive
- •Monovalent influenza vaccines appear more immunologically effective in young horses than multivalent formulations
- •Adding an extra booster vaccination improves protective antibody levels in young racehorses, which may justify the additional cost and handling
- •Monitor vaccination status of your horses against equine influenza, as new viral clades may escape existing vaccine-induced immunity
- •Report suspected equine influenza cases (fever, respiratory signs) to your veterinarian promptly for PCR testing, as surveillance is incomplete in many regions
- •Coordinate with your veterinarian on vaccine strain recommendations, which may need updating as new EIV clades emerge in your geographic region
- •Ensure your horses are vaccinated against equine influenza — 72% of cases occurred in unvaccinated animals, making vaccination the single most important preventive measure
- •Implement strict quarantine protocols for new arrivals (at least 2 weeks isolation) and maintain isolation facilities, as new horse movements were implicated in 42% of infected premises
- •Work with your veterinarian to establish biosecurity measures appropriate to your operation size and type, as professional premises and those with better protocols had fewer secondary cases
- •Reverse genetics technology enables rapid updates of equine influenza vaccine strains, allowing faster response to emerging viral variants
- •RG-derived vaccines show adequate immune responses in both primary vaccination series and booster protocols in horses
- •Multiple vaccine doses (3-dose primary series) are recommended for optimal antibody development, particularly in younger horses
- •Annual booster vaccination alone is insufficient protection against equine influenza—consider more frequent booster protocols, especially for young horses in racing yards
- •Implement strict biosecurity measures when introducing new horses or after horses return from events, and avoid mixing racing and non-racing populations with different vaccination histories
- •Synchronize vaccination schedules across yards sharing horses to reduce high-risk periods for virus transmission and monitor for vaccine failure even in compliant horses
- •Vaccination alone is insufficient protection against equine influenza in your practice—regular vaccination combined with quarantine and movement restrictions for respiratory disease are essential
- •When influenza-like disease occurs in vaccinated horses, consider vaccine strain mismatch as a cause and report cases to identify circulating variants that may require updated vaccines
- •Stay informed about OIE vaccine strain recommendations and coordinate with your veterinarian to ensure your facility uses current vaccine formulations, as antigenic drift occurs continuously
- •Recognize that EI can spread despite vaccination; implement strict biosecurity measures including isolation, booster vaccinations every 6 months, and maintaining separate air spaces between stables
- •Move quickly to diagnose suspected EI cases using RT-PCR and isolate affected horses within 5 days of clinical signs to prevent spread through personnel, equipment, and fomites
- •Screen and vaccinate all new horse arrivals before introduction to premises, and maintain detailed vaccination records to track booster timing for each individual horse
- •This universal assay enables standardised influenza A detection across equine, avian, and swine populations using a single test, improving disease surveillance and outbreak response.
- •The high sensitivity (LOD ~10 templates) and broad host coverage make this tool practical for on-farm or clinic screening before confirmatory testing, supporting rapid risk assessment.
- •Implementation of this assay supports One Health biosecurity protocols by allowing consistent detection standards across multiple species, facilitating earlier identification of zoonotic risks and cross-species transmission events.
- •Current UK vaccination practices may not provide optimal immunity despite compliance with competition rules—review your protocols against manufacturer datasheets to ensure proper spacing between vaccine doses
- •Most adverse vaccination reactions are transient (stiffness, swelling, lethargy, fever), but underreporting to authorities limits safety monitoring—consider reporting significant cases to improve surveillance data
- •Address owner vaccine hesitancy by discussing the balance between immunity requirements, competitive rules, and individual horse risk rather than adopting blanket vaccination schedules
- •Vaccination contingency planning for EI should be based on economic modelling that weighs outbreak costs against prevention costs
- •Non-endemic countries should evaluate control strategies proactively rather than reactively to optimize resource allocation
- •Decision-makers need integrated epidemiologic and economic data to justify EI management investments to stakeholders
- •Maintain current vaccination protocols for equine influenza as a critical disease prevention strategy in your practice
- •Stay informed about evolving equine influenza strains and vaccination recommendations from authoritative sources like the Animal Health Trust
- •Educate horse owners about the infectious nature of equine influenza and the importance of vaccination compliance
- •Stay informed about seasonal disease trends and surveillance data relevant to equine practice
- •Equine influenza remains a monitored infectious disease requiring awareness for clinical diagnosis
- •Surveillance reports provide epidemiological context for regional disease occurrence affecting practice decisions
- •Monitor for equine influenza signs during summer when horses are mixed together more frequently due to increased movement and competition activity
- •Maintain awareness of diagnostic approaches for equine influenza to enable prompt identification and management of cases
- •Implement biosecurity measures during periods of increased horse population mixing to reduce transmission risk
- •Ensure vaccination protocols include both Fc1 and Fc2 sublineage viruses as recommended by OIE to maintain optimal protection against current circulating strains
- •Monitor for potential vaccine efficacy gaps in regions using older vaccine formulations that lack recommended viral sublineages
- •Be aware that minor genetic changes in influenza viruses can reduce vaccine effectiveness, necessitating periodic vaccine updates
- •Monitor for clinical signs of equine influenza and EHV-1 neurological disease in your practice; outbreaks were active in this period
- •Maintain awareness of disease prevalence across regions to inform biosecurity and vaccination protocols
- •Refer to quarterly surveillance reports from Defra, Animal Health Trust, and BEVA for up-to-date disease occurrence data
- •Mathematical models can help predict equine influenza spread patterns and identify optimal vaccination and biosecurity strategies for your facility
- •Evidence from modelling studies supports targeted control measures that may reduce disease incidence and economic losses from outbreaks
- •Understanding model predictions about transmission can guide herd management decisions and inform when to implement enhanced preventive protocols
- •Horse managers who underestimate their vulnerability to disease outbreaks are less likely to implement biosecurity measures—education must address perceived risk, not just factual risk
- •Biosecurity communication strategies should incorporate protection motivation theory to increase perceived vulnerability and strengthen compliance with disease control protocols
- •Post-outbreak conditions provide a critical window to assess and influence long-term biosecurity attitudes and practices among horse owners and managers
- •Stay updated on which equine influenza strains are in circulation in your region through surveillance data to ensure vaccination protocols remain effective
- •Be aware that vaccinated horses can still shed virus subclinically and spread disease internationally; implement biosecurity measures accordingly
- •Work with your veterinarian to monitor ESP vaccine strain recommendations annually and adjust vaccination strategies based on current epidemiological data
- •Flu Avert IN is a safe, stable live attenuated vaccine suitable for field use with minimal risk of reversion or unintended spread to unvaccinated horses
- •This vaccine provides cross-protection against diverse equine influenza strains currently circulating globally, reducing the need for frequent vaccine updates
- •Low spontaneous transmissibility makes this vaccine practical for use in mixed vaccinated/unvaccinated populations without concerns about accidental exposure of non-target animals
Key Research Findings
517 equine influenza occurrences from 2005-2022 identified 14 significant spatiotemporal clusters globally
Maxent predictive model achieved high accuracy (AUC = 0.920 ± 0.008) for identifying suitable EI occurrence areas
Annual average ultraviolet radiation, horse density, and precipitation of coldest quarter are the three most important environmental variables for EI occurrence
High-risk regions include Asia (India, Mongolia, China) and Americas (Brazil, Uruguay, USA, Mexico) with predicted eastward expansion and range changes under different climate scenarios
Foals vaccinated at 90 days showed comparable cellular immune responses (CD4+, CD8+, interleukin-4, interferon-γ, granzyme B) to those vaccinated at 180 days across six major antigens
Both groups demonstrated antigen-specific immune activation 30 days after initial vaccination and sustained responses at age 344 days
Both treatment and control groups showed significant increases in antigen-specific immunoglobulin G following booster vaccination at 11 months, indicating memory immune responses were established
Early vaccination at 3 months elicited comparable immune activation despite presence of maternal antibodies, providing evidence for earlier immunisation in high-risk situations
Longer vaccination intervals increase immunity gaps between doses but do not inhibit antibody response to second and third vaccinations
Response to second and third doses was similar regardless of vaccination regime used
Yearlings had the greatest number of poor responders to initial vaccination compared to other age groups
2- and 3-year-old horses showed superior vaccination responses compared to weanlings and yearlings
Vaccinated horses showed significantly lower clinical scores (P=0.0345) and total clinical scores (P=0.0180) compared to controls following challenge
Vaccination significantly reduced virus excretion extent (P=0.0006), duration (P<0.0001), and total viral load (P=0.0006)
A non-updated Matrix-C-based equine influenza subunit vaccine demonstrated cross-protective capacity against a heterologous Florida clade 2 challenge strain
Evidence Base
Spatiotemporal pattern and suitable areas analysis of equine influenza in global scale (2005-2022).
Ding Jiafeng, Wang Yu, Liang Jinjiao et al. (2024) — Frontiers in veterinary science
Systematic Review
Characterisation of immune responses in healthy foals when a multivalent vaccine protocol was initiated at age 90 or 180 days.
Davis E G, Bello N M, Bryan A J et al. (2015) — Equine veterinary journal
RCT
Comparison of primary vaccination regimes for equine influenza: working towards an evidence-based regime.
Cullinane A, Gildea S, Weldon E (2014) — Equine veterinary journal
RCT
Efficacy of a non-updated, Matrix-C-based equine influenza subunit-tetanus vaccine following Florida sublineage clade 2 challenge.
Pouwels H G W, Van de Zande S M A, Horspool L J I et al. (2014) — The Veterinary record
RCT
Efficacy and duration of immunity of a combined equine influenza and equine herpesvirus vaccine against challenge with an American-like equine influenza virus (A/equi-2/Kentucky/95).
Heldens J G M, Pouwels H G W, van Loon A A W M (2004) — Veterinary journal (London, England : 1997)
RCT
Efficacy of a cold-adapted, intranasal, equine influenza vaccine: challenge trials.
Townsend H G, Penner S J, Watts T C et al. (2001) — Equine veterinary journal
RCT
What happened after the epidemic? Equine influenza surveillance sheds light on sources and seasonal risk in the United Kingdom.
Whitlock Fleur, Grewar John, Newton Richard (2026) — Equine veterinary journal
Cohort Study
Antibody Responses to a Reverse Genetics-Derived Bivalent Inactivated Equine Influenza Vaccine in Thoroughbred Horses.
Ohta Minoru, Bannai Hiroshi, Kambayashi Yoshinori et al. (2022) — Journal of equine veterinary science
Cohort Study
An Evaluation of Three Different Primary Equine Influenza Vaccination Intervals in Foals.
Dilai Mohamed, Fassi Fihri Ouafaa, El Harrak Mehdi et al. (2021) — Journal of equine veterinary science
Cohort Study
The impact of different equine influenza vaccine products and other factors on equine influenza antibody levels in Thoroughbred racehorses.
Ryan M, Gildea S, Walsh C et al. (2015) — Equine veterinary journal
Cohort Study
Accelerated vaccination schedule provides protective levels of antibody and complete herd immunity to equine influenza.
El-Hage C M, Savage C J, Minke J M et al. (2013) — Equine veterinary journal
Cohort Study
Field studies on equine influenza vaccination regimes in thoroughbred foals and yearlings.
Cullinane A, Weld J, Osborne M et al. (2001) — Veterinary journal (London, England : 1997)
Cohort Study
First Reported Circulation of Equine Influenza H3N8 Florida Clade 1 Virus in Horses in Italy.
Ricci Ida, Tofani Silvia, Lelli Davide et al. (2024) — Animals : an open access journal from MDPI
Case Report
An epidemiological overview of the equine influenza epidemic in Great Britain during 2019.
Whitlock Fleur, Grewar John, Newton Richard (2023) — Equine veterinary journal
Case Report
Growth properties and immunogenicity of a virus generated by reverse genetics for an inactivated equine influenza vaccine.
Ohta Minoru, Bannai Hiroshi, Kambayashi Yoshinori et al. (2022) — Equine veterinary journal
Case Report
Annual booster vaccination and the risk of equine influenza to Thoroughbred racehorses.
Gildea Sarah, Lyons Pamela, Lyons Rachel et al. (2020) — Equine veterinary journal
Case Report
Multifocal outbreak of equine influenza in vaccinated horses in Argentina in 2018: Epidemiological aspects and molecular characterisation of the involved virus strains.
Olguin-Perglione C, Vissani M A, Alamos F et al. (2020) — Equine veterinary journal
Case Report
Management and environmental factors involved in equine influenza outbreaks in Ireland 2007-2010.
Gildea S, Arkins S, Cullinane A (2011) — Equine veterinary journal
Case Report
A universal RT-qPCR assay for "One Health" detection of influenza A viruses.
Nagy Alexander, Černíková Lenka, Kunteová Kateřina et al. (2021) — PloS one
Expert Opinion
Equine influenza vaccination in the UK: Current practices may leave horses with suboptimal immunity.
Wilson Amie, Pinchbeck Gina, Dean Rachel et al. (2021) — Equine veterinary journal
Expert Opinion
Show 10 more references
An economic analysis of a contingency model utilising vaccination for the control of equine influenza in a non-endemic country.
Rosanowski Sarah M, Carpenter Tim E, Adamson David et al. (2019) — PloS one
Expert Opinion
Equine influenza: evolution of a highly infectious virus.
(2018) — The Veterinary record
Expert Opinion
Equine disease surveillance: quarterly summary.
(2017) — The Veterinary record
Expert Opinion
Diagnosis of equine influenza.
Rash Adam (2017) — The Veterinary record
Expert Opinion
The potential impact of a single amino-acid substitution on the efficacy of equine influenza vaccines.
Yamanaka T, Cullinane A, Gildea S et al. (2015) — Equine veterinary journal
Expert Opinion
Equine disease surveillance: quarterly summary.
(2014) — The Veterinary record
Expert Opinion
What can mathematical models bring to the control of equine influenza?
Daly J M, Newton J R, Wood J L N et al. (2013) — Equine veterinary journal
Expert Opinion
Perceptions of vulnerability to a future outbreak: a study of horse managers affected by the first Australian equine influenza outbreak.
Schemann Kathrin, Firestone Simon M, Taylor Melanie R et al. (2013) — BMC veterinary research
Expert Opinion
The molecular epidemiology of equine influenza in Ireland from 2007-2010 and its international significance.
Gildea S, Quinlivan M, Arkins S et al. (2012) — Equine veterinary journal
Expert Opinion
A new modified live equine influenza virus vaccine: phenotypic stability, restricted spread and efficacy against heterologous virus challenge.
Chambers T M, Holland R E, Tudor L R et al. (2001) — Equine veterinary journal
Expert Opinion