Equine Influenza:Improving Biosecurity and Calculating Risk

Nicola Pusterla, DVM, PhD, Diplomate ACVIMDepartment of Medicine and Epidemiology

School of Veterinary MedicineUniversity of California-Davis

Davis, CA 95616

Part I. Biosecurity for the Prevention of Equine Influenza OutbreaksA recent, 2-year surveillance study conducted at the University of Cal-ifornia-Davis confirmed that equine influenza virus (EIV) continues to be a prevalent viral respiratory patho-gen of horses throughout all regions of the U.S.1 Diagnostic samples from a total of 761 clinical respiratory cases from 2008 to 2010 were evalu-

ated by PCR assay for four common equine upper respiratory disease (URD) pathogens: equine herpesvirus type 1 (EHV-1), EHV type 4 (EHV-4), EIV, and Streptococcus equi sub-species equi. Of these, 201 (26.4%) cases were positive for one of the four target pathogens. EHV-4 was most often diagnosed (82 cases), followed by EIV (60 cases), S. equi ss equi (49 cases), and EHV-1 (23 cases). The study was noteworthy for its large sample size and the broad

diagnostic sampling from 23 states. Thirty-nine percent (300/761) of study cases were vaccinated for EIV and 45% of cases had an unknown vac-cination history.

Compared with other surveillance data, the survey may understate the true prevalence of equine influenza (EI), particularly during the spring and winter and in at-risk populations. To illustrate, a 56.5% EIV morbidity rate was reported in a Canadian sur-

veillance study of acute and conva-lescent serum samples (n=115) from 2003 to 2005.2 A study by Colorado State University investigators identi-fied EIV infection in 28.5% (43/151) of horses with clinical URD.3

Collectively, these and other studies indicate that, despite widespread vaccination, EIV is commonly di-agnosed in clinical cases of URD in horses. Part I of this report discusses how practitioners can lower the risk of this prevalent and costly disease and mitigate its impact when it oc-curs. Part II describes a novel meth-od of risk calculation and an Equine Immunization Support Guarantee Program available to veterinarians who use Pfizer Animal Health’s EIV vaccines.

What has changed, what has notIncreased exposure riskIn contrast to the 1990s and ear-lier decades, today there are few boundaries to intermixing of the North American equine popula-tion. Horses now travel throughout North America and the world. Their destinations are increasingly large events, such as the 2012 Olympic equestrian competition where horses from numerous origins con-gregated and were subjected to the stress of transportation, adverse environmental conditions, and the demands of physical performance. Many of these horses are properly vaccinated, but some are not. In these man-made settings, equine URD exposure, infection, and clinical disease are all but inevitable and present a greater clinical challenge than preventing disease in closed horse populations. Severe outbreaks resulting from international move-ment of horses can occur when EIV-infected animals are introduced into susceptible populations previ-ously free of disease or not previously exposed to the infecting EIV strain.4

In the 2003 Newmarket outbreak in the U.K., for example, approximately 1,300 vaccinated and unvaccinated racehorses suffered severe EIV-asso-ciated respiratory signs lasting up to nine weeks.5,6

Disease characteristics conducive to infection and economic lossNone of the three most prevalent equine viral URD pathogens (EHV-1, EHV-4, and EIV) have a pathog-nomonic respiratory sign. Fever, depression, anorexia, higher preva-lence in young horses without active immunity, and a progression from se-rous to mucoid nasal discharge are characteristic of URD caused by all of these respiratory viruses. However, a feature of clinical EI is propulsive, dry coughing, usually to a greater extent than occurs with EHV-associ-ated URD.1,5 This mechanism enables dispersion of infectious respiratory aerosols among commingled horses. EIV has a short incubation period, 1 to 3 days. As a result, rapid and widespread infection of horses in a group setting may have occurred by the time a clinical outbreak is observed, usually 3 to 5 days after exposure, and before biosecurity measures can be implemented.

Pathogenesis of EIV can lead to a lengthy convalescence that de-lays return to activity. EIV infection results in denudation of the respira-tory epithelial cells and impairment of the mucociliary apparatus. This allows attachment of the EIV hem-agglutinin antigens to the surface of the epithelial cells. Cell necrosis and desquamation of the upper airway epithelium then occur, requiring up to 6 weeks or longer for restoration of normal cellular architecture. In fact, severe infections may render horses unfit for competition for as long 3 months.7 Presence of second-ary bacterial infection or premature return to activity can lead to sequel-ae such as pleuropneumonia that prolong recovery.

Studies have shown that exercise has an immunomodulatory effect in horses. A decline in lymphoprolifera-tive response, phagocytosis, and oxi-dative burst activity in immune cells has been demonstrated within hours after horses were subjected to pro-tracted or high-intensity training.8-10 Challenge studies have shown that even moderately exercised horses are more susceptible to EIV and had clinical EI of increased severity com-pared to non-exercised horses.8,11

The implications for equine practitio-ners are that EIV often has a dispro-portionate effect in performance horses, both in terms of disease se-verity and loss of use. In addition, the importance of adequate rest during the recovery period should not be underestimated. A standard recom-mendation for recovery from clinical EI is one week of complete rest for every day of elevated tempera-ture, followed by a gradual return to activity.12 More conservative man-agement consists of an additional week of rest for every day that fever persists beyond 3 days, an extended recovery period designed to avoid risk of serious complications.10

Unlike EHV, EIV does not cause latent infection of asymptomatic carrier horses. However, in partially immune horses EIV infection can exist during subclinical disease, generally for up to 10 days but as long as 21 days.10,13 Asymptomatic and convalescent shedders maintain a temporary reservoir of virus in the equine popu-lation, with the potential to insidiously infect susceptible horses.

The critical role of client educationTo implement a truly effective EI biosecurity program, the client must be an active participant. This begins by making sure the horse owner is aware of the early clinical signs of EI, particularly the characteristic cough-ing that occurs. Subtleties such as coughing frequency and duration, type of nasal discharge, and occur-rence of fever should be noted. All horse owners should have an equine digital rectal thermometer and know that the normal equine body tem-perature in a resting horse is 98-101° F and that a temperature of 101.5-102° F indicates pyrexia. Owners should monitor horses for respiratory signs every day, and be especially vigilant after horses are exercised, when they return from off-site events, and when new horses enter the premises.

Veterinarians should counsel their clients on the basic aspects of infec-tious disease control – what we call biosecurity – if an EI outbreak occurs. Recognition of clinical disease is the

necessary first step in implement-ing biosecurity measures to limit EIV transmission. Interestingly, studies have shown that horses with rela-tively high concentrations of EIV serum antibody titers have a lower risk of EHV disease during EHV out-breaks. Similarly, horses with a high EHV serologic response had a lower risk of EIV during equine influenza outbreaks. This suggests that risk of exposure to one respiratory patho-gen corresponds to risk of exposure to other equine respiratory patho-gens, emphasizing the importance of non-specific, biosecurity control measures.

Appropriate biosecurity response to an EI outbreakOutbreak alertThe single most common and impor-tant mistake that occurs during an EI outbreak is lack of communication among personnel at the local site. It is critical that all individuals involved in the care of horses be alerted in the early stages of an outbreak so that biosecurity lapses do not occur. Properly executed, an “outbreak alert” is akin to a fire drill where ev-eryone is immediately informed and knows exactly what to do.Ideally, individual assignments and procedures for responding to an EI outbreak should be written down in a protocol or standard operating procedure. If instructions are in writ-ing, compliance tends to be good. The protocol should be limited to half a dozen points at most, in which case compliance will probably be close to 100%. The likelihood of com-pliance with a multi-page protocol, or even having anyone read it, is low.

The basic components of an on-site biosecurity program, conducted by the horse owner, for equine viral respiratory disease are:

• Daily monitoring of all horses for clinical EI to enable early intervention if an outbreak occurs• Recognition of clinical signs, especially fever, depression, nasal discharge, and coughing

In case of a suspected outbreak:

• Informing the regular or attending veterinarian• Informing all on-site personnel involved in care of the horses• Isolation of any sick horse from contact with other animals

A biosecurity protocol for EI should identify which individual is respon-sible for each of these actions and specify how they are to be performed. The involvement of a veterinarian after an initial, on-site biosecurity response by the horse owner will facilitate further mitigation measures. These interventions in-clude obtaining diagnostic samples, serologic testing, and vaccination. A nasal swab is the preferred EIV diagnostic sample for suspect horses with nasal secretions. The collection of nasal secretions should ideally be left to a veterinarian using appropri-ate protective equipment including latex gloves and coveralls. The swab should be placed in a red-top tube and sent to a diagnostic laboratory for overnight testing. A PCR assay is fast, highly accurate, and afford-able, and is the diagnostic test of choice. EIV diagnostic services can be obtained from the veterinary di-agnostic laboratories at the Universi-ty of California-Davis or at the Gluck Equine Research Center, University of Kentucky.

Managing risk factorsMinimizing risk factors for equine respiratory disease will lower the odds of EIV exposure and a clinical outbreak. Well known risk factors include high stocking density, nose-to-nose contact among horses of variable origin or unknown history, poor ventilation that allows buildup of aerosolized pathogens, environ-mental stressors, conditions that compromise lower-airway defense mechanisms, and inadequate im-mune status.

Biosecurity in the strict sense consid-ers every horse to be a potential source of EIV infection. Even clini-cally normal horses may be as-ymptomatically infected with EIV, shedding virus to susceptible horses. The likelihood of exposure to EIV

declines in proportion to the extent that individual horses can be kept apart from other horses of unknown disease or immune status. If possible, newly arrived horses on the premises should be quarantined for several days in order to monitor their infec-tious disease status. Indirect transfer of EIV from horse to horse via human vectors or contaminated surfaces and fomites is a major factor in dis-ease transmission. How often have we seen horses in a barn with their heads sticking out of their stalls and someone walking down the aisle petting their muzzles one at a time? To cite another common example, grooms will often use the same cloth to wipe the face and muzzle of several horses. Barrier precautions, consisting of disposable gloves and gowns, rubber boots, and disinfec-tant footbaths are important ways of helping minimize indirect transmission risk. Personnel with contaminated hands represent the most frequent route of indirect EIV transmission.10 Frequent hand washing or use of sanitizing gels before and after han-dling horses is an excellent way of circumventing this route of exposure. Cleaning and sanitation practices directed at all surfaces, equipment, grooming supplies, tack, and ve-hicles that horses come in contact with will help break the cycle of indirect EIV transmission.

Vaccines and vaccinationPreventive vaccination has a promi-nent role in any EI control strategy. Immunologically naïve horses are at highest risk of infection. American Association of Equine Practitioners vaccination guidelines consider EIV vaccination to be risk-based, mean-ing that it is discretionary depend-ing on likelihood of exposure, and that the consequences of disease should be weighed against the cost of vaccination and potential for adverse vaccine-associated events. The low-mortality (<1% in uncompli-cated cases), self-limiting nature of EI and low risk of infection in closed herds explains why AAEP guidelines do not specify EIV vaccination to be a core immunization. However, the endemic nature of EI justifies routine vaccination of all horses that come

in contact with other horses. Repeat-ed outbreaks of EIV in sporadically vaccinated horses has led to the introduction of mandatory vaccina-tion of competition horses in several European countries, including the U.K. and Ireland. Due to apparent herd immunity, epizootic risk appears to decline in groups where the EIV vaccination rate is ≥75%.14

Various commercial EIV vaccine formulations are available. These include several parenteral killed-virus (KV) vaccines, an intranasal modi-fied live-virus (MLV) vaccine, and a parenteral live canarypox-vectored vaccine expressing recombinant EIV antigens. Although reported EIV vac-cine efficacy runs the gamut from very effective to limited efficacy, the following conclusions and recom-mendations can be made:

• Protection following EIV vaccination is variable and often short lived.14,15

• The vaccine that is used should contain an EIV strain closely related to endemic strains.• EIV vaccines ameliorate or provide protection from clinical disease but may not prevent transient, subclinical infection.16

• Vaccination should be employed in conjunction with other biosecurity precautions.• For competition or traveling horses or those with an ongoing exposure risk, vaccination at least twice a year is advisable; booster doses at even shorter intervals (3 months or less) are sometimes given to horses at high risk of exposure.17,18

The A/equi-2 (H3N8) subtype is the dominant EIV subtype worldwide. Due to antigenic drift, numerous H3N8 variants have emanated from the original A/equi-2/Miami/63 pro-totype (Miami/63) isolated in 1963 in South Florida.19 About 1987, the H3N8 subtype evolved into two geneti-cally distinct lineages, a European branch and a more dominant Amer-ican branch.20 The American lineage subsequently evolved into Kentucky and Florida sublineages, with the lat-ter being the most prevalent.21,22

Fortunately, EIV evolves on a much

less frequent basis than human influenza virus, which requires vac-cine reformulation on an annual basis. No EIV antigenic shift to a new subtype has occurred since the H3N8 emerged in 1963 as a subtype distinct from H3N7. Because EIV is relatively stable, commercial EIV vaccines have remained clinically relevant against endemic EIV strains. Evidence suggests that the adjuvant used in KV-EIV vaccines may be at least as important as the immunizing antigen in determining vaccine ef-ficacy.5,14 Long-term clinical experi-ence is a good guide to choosing an EIV vaccine. A practitioner’s best option is probably to rely on a vaccine that he or she has used successfully on a regular basis and has confidence in. In the face of an active outbreak involving EIV-naïve horses, a MLV-EIV vaccine will confer a partial immune response within 7 days.23

It is important to note that vaccina-tion is only one tool for controlling equine viral respiratory disease. For the most complete protection against EI, horses should be vac-cinated in combination with good biosecurity protocols. Reports in-dicate that even well vaccinated horses can be infected, develop clinical disease, and shed EIV.6,24-26 Thus, vaccination will only be partial-ly effective unless it is administered in the context of a biosecurity program consisting of non-specific infection control measures.

Part II. Calculating Equine Influenza RiskPhysical and environmental risk fac-tors for EI include age of the horse, likelihood of horse-to-horse contact, and vaccination status. Age <2 years has been associated with an increased risk of EI.7 For example, one study evaluated EI risk at a Canadian thoroughbred race track that experienced outbreaks for 3 consecutive years.24 The risk of dis-ease increased incrementally as the horse’s age decreased. Horses ≤2 years of age were 5 to 8 times more likely to develop URD than horses ≥5 years of age. Although older horses can develop EI, particularly if they

are exposed to an EIV strain they have not previously encountered, their apparently lower risk of clini-cal disease is probably due to some level of immunity from prior exposure or vaccination.

Increased EI risk is also correlated with horse-to-horse contact, which is often a function of how the animal is used.10 For example, a South Ameri-can study of an EIV outbreak (n=213 horses) found that show-jumping horses had a 2.4-fold greater risk of EI compared to horses engaged in a less rigorous activity such as pa-rade or draft use.27 The investigators attributed the higher disease risk in the show-jumping horses to intermin-gling with horses from other facilities. Exercise-induced immunosuppres-sion has been verified in horses, and may also have had a role in the higher disease incidence in the show-jumping horses.10,28 In this study, younger horses (≤3 years of age) had an attack rate 1.8 times greater than horses 3 to 10 years of age. In a Canadian study of EI epizootics in racetrack horses (n=1,163), exercise ponies had a 6.7-fold greater risk of disease compared to racehorses. The exercise ponies had more fre-quent and variable contact with other horses, including those from other facilities.24

Naturally acquired immunity, vac-cination status, and by extension, serologic status, are strong predictive factors for EI. The Canadian race-track study mentioned above found that EIV-seropositive horses were 10 to 40 times less likely to develop clini-cal EI during disease outbreaks com-pared to seronegative horses.10,24 This longitudinal study estimated that timely EIV vaccination prior to exposure could prevent 25% of EI cases, underscoring the advisability of timely vaccination prior to antici-pated exposure.10 Statistical models for calculating EI morbidity found that the probability of EIV infection was greatest in non-vaccinated horses (93.2%) compared to horses that received annual vaccination (43.3%) or semiannual vaccination (26.5%).16 Past experience suggests that blanket EIV vaccination in the face of an outbreak can be benefi-

cial in suppressing clinical disease at racetracks or in other settings where sizeable numbers of horses are pres-ent.29,30

Calculating riskAAEP guidelines recommend an-nual EIV vaccination for horses at low risk of exposure, and semiannual vaccination for horses with ongo-ing exposure risk. However, due to widely varying exposure patterns and variation in commercial vac-cine formulations, there is no uni-versal standard for EIV vaccination. As previously noted, more frequent revaccination intervals may be appropriate for performance or traveling horses, those undergoing strenuous activity, or horses hav-ing frequent encounters with other horses with an unknown history. Pfizer Animal Health has developed an Equine Risk Calculator (ERC) that estimates the risk of EIV infection in an individual horse and the costs associated with clinical disease (Figure 1), based on vaccination frequency. The ERC first calculates the probability of clinical disease (expressed as a percentage) and disease severity (mild, moderate, or severe) based on vaccination status of the horse and the overall herd. In addition, the ERC calculates the economic impact of clinical dis-ease based on the average cost of treatment (for diagnostics, steroids, antimicrobials, mucolytics), disease severity, and the number of days the horse misses training or physical activity. As shown in Figure 1, the number of days of loss-of-use of the horse, determined by disease sever-ity, can be the dominant factor in calculating the economic impact of EI. Data suggests days of loss-of-use to be 7.7 and 14.4 days for mild and severe EI cases, respectively.31 However, in cases where secondary bacterial involvement or coinfection with other viral respiratory agents occurs, convalescence from EI can be protracted. Clinical data from EI epizootics in Australia determined that severe cases can require 50 to 100 days to full recovery.32

Using the ERC, the horse owner or veterinarian can determine EI risk depending on the horse’s vaccina-

tion status (unvaccinated, vaccinat-ed annually, or vaccinated semian-nually) and that of the herd. The ERC also estimates the cost of clinical disease, with or without an Equine Immunization Support Guarantee (see box). With this information, the horse owner in conjunction with the veterinarian can then decide on a vaccination strategy, including which vaccine is to be used, timing of vaccination, number of doses to give, and the revaccination interval.

FLUVAC INNOVATOR (Pfizer Animal Health) is a KV-EIV vaccine contain-ing the Kentucky/97 subtype of EIV in combination with METASTIM, a pro-prietary oil emulsion adjuvant with immunostimulating properties. Ken-tucky/97 is included in the dominant EIV American lineage-Florida sublin-eage. As a vaccine antigen, it has been demonstrated to be effective against clinically relevant EIV strains

prevalent in North America, includ-ing the Ohio/03 (Pfizer Study Report No. B671-08-004R) and Kentucky/07 strains (Data on file, April 19, 2012).Vaccination is a key element of a comprehensive biosecurity strat-egy for equine infectious respira-tory disease. Pfizer Animal Health’s Equine Risk Calculator can be used to devise a rational EIV vaccina-tion program suitable for the horse’s individual degree of risk. Vaccina-tion with FLUVAC INNOVATOR line of vaccines, backed by the Equine ISG program, provides horse owners and veterinarians with a high degree of assurance that their biosecurity program includes reliable protection against EIV infection.

AcknowledgementThe authors acknowledge the con-tribution of Mark Dana of Scientific Communications Services, LLC in the writing and editing of this report.

Pfizer Animal Health has introduced an Equine Immunization

Support Guarantee (ISG) program to encourage horse owners to

maintain a veterinarian-client-patient relationship, and to make

EIV vaccination an integral part of their infectious disease bios-

ecurity program. The Equine ISG program reimburses the horse

owner up to $5,000 for reasonable diagnostic and treatment

costs for any horse properly vaccinated with Pfizer’s FLUVAC

INNOVATOR® line of KV-EIV vaccines, develops EIV infection con-

firmed by diagnostic evaluation. Vaccination must be performed

by a licensed veterinarian with whom the horse owner has an es-

tablished client-patient relationship. No enrollment in the Equine

ISG program is necessary, and there is no cost to veterinarians or

horse owners to participate.

In a case of suspected EI in a vaccinated horse, a participat-

ing veterinarian should contact Pfizer Animal Health’s Veterinary

Medical Information and Product Support (VMIPS) group at the

company’s web site or by phoning 800-366-5288. Depending on

the horse’s symptoms, VMIPS may recommend diagnostic tests

for other equine respiratory pathogens as well as EIV. This helps

alert practitioners of various respiratory diseases that may be

endemic in the local area, allowing them to take appropriate

measures. All diagnostic testing is conducted at no cost to the

veterinarian or horse owner. If the diagnostic test shows a positive

EIV result, treatment costs up to the $5,000 limit will be covered

by the Equine ISG. Other Pfizer Animal Health products eligible

for the Equine ISG include vaccines for West Nile virus, tetanus,

eastern and western equine encephalomyelitis, and Venezuelan

equine encephalomyelitis.

The Equine Immunization

Support Guarantee Program

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from 2006 to 2007. Vet Microbiol. 2009;138:41-52.22. Lai AC, Rogers KM, Glaser A, Tudor L, Chambers T. Alternate circulation of recent equine-2 influenza viruses (H3N8) from two distinct lineages in the United States. Virus Res. 2004;100:159-164.23. American Association of Equine Practitioners Vaccination Guidelines, Equine Influenza. Available at: http://www.aaep.org/equine_influenza.htm. Ac-cessed: July 30, 2012.24. Morley PS, Townsend HG, Bogdan JR, et al. Risk factors for disease associated with influenza virus infections during three epidemics in horses. J Am Vet Med Assoc. 2000;216:545-50.25. Paillot R, Prowse L, Donald C, et al. Efficacy of a whole inactivated EI vaccine against a recent EIV outbreak isolate and comparative detection of virus shedding. Vet Immunol Immunopathol. 2010;136:272-283.26. van Maanen C, van Essen GJ, Minke J, et al. Di-agnostic methods applied to analysis of an outbreak of equine influenza in a riding school in which vac-cine failure occurred. Vet Microbiol. 2003;93:291-306.27. Manley L, Caceres P. Retrospective cohort study of an equine influenza outbreak in the Chilean Army in the metropolitan region of Santiago, Chile, during 2006. International Symposia on Veterinary Epide-miology and Economics (ISVEE) proceedings: Pro-

ceedings of the 12th Symposium of the International Society for Veterinary Epidemiology and Economics, Durban, South Africa, Equine, Disease distribution & determinants. 2009;64.28. Lunn DP, Hussey S, Sebing R, et al. Safety, effi-cacy, and immunogenicity of a modified-live equine influenza virus vaccine in ponies after induction of exercise-induced immunosuppression. J Am Vet Med Assoc. 2001;218:900-906.29. Barquero N, Daly JM, Newton JR. Risk factors for influenza infection in vaccinated racehorses: lessons from an outbreak in Newmarket, UK in 2003. Vac-cine. 2007;25:7520-7529.30. Garner MG, Cowled B, East IJ, et al. Evaluating the effectiveness of the response to equine influenza in the Australian outbreak and the potential role of early vaccination. Aust Vet J. 2011;89 Suppl 1:143-145.31. Gildea S, Arkins S, Cullinane A. Management and environmental factors involved in equine influ-enza outbreaks in Ireland 2007-2010. Equine Vet J. 2011;43:608-617.32. Marshall D. Equine influenza. University of Dela-ware Cooperative Extension, 2007. Available at: http://ag.udel.edu/anfs/faculty/documents/Equin-eInfluenza8-20-07.pdf. Accessed: July 13, 2012.

Figure 1: Pfizer Equine Influenza (EI) Risk CalculatorFigure 1: Pfizer Equine Influenza (EI) Risk Calculator

A spreadsheet from the Pfizer Equine Influenza (EI) Risk Calculator shows the likelihood of clinical disease depending on vaccination status (upper right), cost of treatment and number of days off training depending on disease severity (lower left), and the financial impact of loss-of-use of the horse. Costs are compared with and without Pfizer’s Equine Immunization Support Guarantee (lower right). In this case, the risk of an unvaccinated horse developing clinical disease following an EIV outbreak is 93% vs. 42% for a horse vaccinated annually, and 27% for a horse vaccinated twice annually. The Equine Immunization Support Guarantee for horses vaccinated with a Pfizer Animal Health EIV vaccine covers diagnostic and treatment costs up to $5,000 per horse for a laboratory-confirmed case of equine influenza. The financial impact of a case of clinical equine influenza in a properly vaccinated horse is less than a fourth of that for an unvaccinated horse. The model is based on current market prices and data from research trials. User-defined costs can be substituted for the default cost variables by the veterinarian or horse owner.

10

A spreadsheet from the Pfizer Equine Influenza (EI) Risk Calculator shows the likeli-hood of clinical disease depending on vaccination status (upper right), cost of treat-ment and number of days off training depending on disease severity (lower left), and the financial impact of loss-of-use of the horse. Costs are compared with and without Pfizer’s Equine Immunization Support Guarantee (lower right). In this case, the risk of an unvaccinated horse developing clinical disease following an EIV outbreak is 93% vs. 42% for a horse vaccinated annually, and 27% for a horse vaccinated twice annually. The Equine Immunization Support Guarantee for horses vaccinated with a Pfizer Animal Health EIV vaccine covers diagnostic and treatment costs up to $5,000 per horse for a laboratory-confirmed case of equine influenza. The financial impact of a case of clinical equine influenza in a properly vaccinated horse is less than a fourth of that for an unvaccinated horse. The model is based on current mar-ket prices and data from research trials. User-defined costs can be substituted for the default cost variables by the veterinarian or horse owner.

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