Viral Respiratory Disease in Horses: Vaccines, Show Biosecurity and When to Worry

By Dr Duncan Houston

Horse shows, sales, races and clinics bring horses from different properties into the same airspace. They also bring shared stables, equipment, people and water sources, which creates an ideal opportunity for respiratory disease to spread.

The difficult part is that an infected horse may arrive looking completely healthy. Fever can develop later, signs may initially be subtle, and some horses can shed respiratory pathogens without obvious illness.

Vaccination is important, but it is only one layer of protection. The best defence combines appropriate vaccines, temperature monitoring, separation after travel, dedicated equipment and immediate isolation of any horse that develops fever, coughing or nasal discharge.

Quick Answer

The principal viral respiratory risks for travelling horses are equine influenza virus, EHV-1 and EHV-4. Vaccination reduces the risk and severity of respiratory disease and may reduce viral shedding, but vaccinated horses can still become infected.

High-risk show and performance horses commonly need equine influenza and EHV-1/EHV-4 boosters every six months, depending on the horse, vaccine label, event rules and veterinary advice. Routine boosters should be completed before travel, not on loading day. No licensed vaccine currently prevents neurological EHV, known as EHM. (AAEP)

When Is Viral Respiratory Disease an Emergency?

There are two kinds of urgency with infectious respiratory disease.

A horse with mild coughing, nasal discharge and fever may not be in immediate danger, but it is still a biosecurity emergency because the horse could infect others.

A horse with breathing difficulty, severe depression or neurological signs is a medical emergency.

Call a veterinarian urgently if your horse has:

• Laboured, noisy or visibly difficult breathing

• Flared nostrils and excessive abdominal effort at rest

• Blue, grey or very pale gums

• Collapse or inability to stand

• Severe weakness

• Persistent high fever

• Refusal to eat or drink

• Rapidly worsening depression

• Thick, foul or bloody nasal discharge

• Signs of pneumonia, including increased resting respiratory rate

• Neurological signs such as wobbliness, toe dragging or hindlimb weakness

• Loss of tail tone

• Urine dribbling or inability to urinate

• A dog-sitting posture

• Recumbency or inability to rise

• Abortion in a pregnant mare

Fever plus any neurological abnormality should be treated as an EHM emergency until proven otherwise. EHV-1 can cause respiratory disease, abortion, neonatal disease and EHM, while EHV-4 primarily causes respiratory disease but can occasionally cause abortion or neurological disease. (APHI Service)

Which Viruses Cause Respiratory Disease in Horses?

Equine Influenza Virus

Equine influenza is one of the most contagious respiratory infections of horses.

Typical signs include:

• Sudden fever

• A harsh, dry cough

• Nasal discharge

• Lethargy

• Reduced appetite

• Enlarged lymph nodes

• Reduced performance

The incubation period is often short, with signs potentially appearing within one to three days of exposure. Young horses and horses between approximately one and five years old are commonly affected more severely because they may have less established immunity and tend to mix frequently at training and competition facilities.

Vaccinated horses can still be infected, but vaccination usually reduces clinical severity and may shorten viral shedding. In unvaccinated horses, contagious shedding commonly lasts around seven to ten days. PCR testing can remain positive after the horse is no longer shedding infectious quantities of virus, so a positive test must be interpreted alongside clinical signs, timing and veterinary assessment.

EHV-1

EHV-1 can cause:

• Fever

• Nasal discharge

• Coughing

• Respiratory disease

• Abortion and stillbirth

• Neonatal disease

• Equine herpesvirus myeloencephalopathy

Many horses are exposed to EHV-1 early in life, and latent infection can persist. The virus can later become active again, although the factors controlling reactivation and transmission are complex and not completely understood.

Younger horses are more likely to develop respiratory disease, while older horses are generally at greater risk of EHM during an outbreak. (AAEP)

EHV-4

EHV-4 primarily causes respiratory disease, particularly in foals and young horses.

Signs are usually similar to other respiratory infections:

• Fever

• Nasal discharge

• Cough

• Depression

• Reduced appetite

• Enlarged lymph nodes

EHV-4 is less commonly associated with abortion or neurological disease than EHV-1, but these complications are still possible. (APHI Service)

Other Respiratory Pathogens

Influenza and EHV are not the only causes of coughing and nasal discharge.

Other infectious possibilities include:

• Equine rhinitis A virus

• Equine rhinitis B virus

• Equine viral arteritis

• Strangles caused by Streptococcus equi

• Bacterial pneumonia

• Pleuropneumonia

• Secondary bacterial infection following a viral illness

This is why respiratory disease cannot be diagnosed from the sound of the cough alone. A dry cough may suggest influenza, but temperature, exposure history, lymph nodes, nasal discharge, respiratory effort and laboratory testing all matter.

Has the Term “Rhino” Been Retired?

No.

“Rhino” and “rhinopneumonitis” are still commonly used to describe respiratory disease associated with EHV-1 and EHV-4. Current AAEP vaccination guidance and USDA information continue to use the term rhinopneumonitis.

The term can still be confusing because equine rhinitis viruses are separate viruses. For clarity:

• Rhino or rhinopneumonitis generally refers to EHV-1 and EHV-4 disease.

• Equine rhinitis virus refers to equine rhinitis A or B virus.

Calling the vaccine a “flu and EHV vaccine” is often clearer than saying “flu and rhino,” particularly when discussing test results or an outbreak.

Why Do Horse Shows and Sales Increase the Risk?

The event itself does not manufacture viruses. The risk comes from what happens around it.

At shows and sales, horses may:

• Travel long distances

• Mix with horses from many home properties

• Share stable airspace

• Stand nose-to-nose over doors

• Use communal wash areas

• Encounter shared hoses, buckets or equipment

• Be handled by multiple people

• Experience altered sleep, feeding and exercise routines

• Return home while incubating disease

Respiratory pathogens can spread directly through respiratory secretions and indirectly through hands, clothing, buckets, tack, grooming equipment, stall fittings and other contaminated surfaces.

One horse arriving home bright and apparently normal can therefore expose resident horses before anyone realises there is a problem. That is particularly concerning where young horses, pregnant mares or recently weaned foals are housed on the same property.

Can Healthy-Looking Horses Shed Respiratory Pathogens?

Yes, but the evidence needs to be described accurately.

A frequently quoted Colorado State University study examined nasal shedding of EHV-1 and EHV-4 in predominantly juvenile horses attending four equine events and sales. It did not show that influenza, EHV-1 and EHV-4 were all present at every event.

The study involved 451 horses. EHV-1 genetic material was detected in 3.8% of arrival samples and 1.8% of repeat samples taken several days later. The findings demonstrated that apparently healthy horses at events can test positive for EHV, but they should not be interpreted as proof that every positive horse was actively contagious or that all major respiratory viruses were circulating at every event. (Worms & Germs Blog)

A later study examined 639 nasal swabs from 484 clinically healthy sport horses after a multi-county EHM outbreak in California. Common respiratory pathogens were detected in 9.8% of swabs overall, including:

• EHV-1 in 3.8%

• EHV-4 in 1.4%

• Equine influenza in 0.6%

• Streptococcus equi in 2.8%

• Equine rhinitis B virus in 1.3%

Clusters of EHV-1, EHV-4, influenza and S. equi were found within individual barns. However, this was a post-outbreak population undergoing quarantine and testing, so the percentages should not be treated as normal prevalence figures for every horse show. (MDPI)

The practical conclusion is not that every healthy horse is secretly infectious. It is that appearance alone cannot guarantee safety.

How Worried Should You Be?

Low Risk

This includes a horse that:

• Has no fever

• Is eating and behaving normally

• Has no cough or nasal discharge

• Has not recently travelled

• Has no known infectious disease exposure

• Is appropriately vaccinated for its risk

What to do: maintain routine vaccination and normal health monitoring.

Moderate Risk

This includes a recently travelled horse that:

• Appears clinically normal

• Has been at a show, sale, clinic or training facility

• Has mixed with unfamiliar horses

• Has no known outbreak exposure

What to do: separate the horse from resident animals after returning home, monitor rectal temperature and watch closely for coughing, discharge, depression or reduced appetite.

High Risk

This includes a horse with:

• Fever

• Repeated coughing

• Nasal discharge

• Known contact with an infected horse

• Attendance at an event where respiratory disease has been reported

• Several affected horses in the same barn

• Recent travel followed by reduced appetite or depression

• Incomplete vaccination

• Exposure to young horses, foals or pregnant mares

What to do: isolate immediately, stop unnecessary horse movement and contact your veterinarian the same day. Inform the stable manager or event organiser where relevant.

Critical

This includes:

• Fever with neurological abnormalities

• Breathing difficulty at rest

• Collapse

• Inability to rise

• Severe weakness

• Urinary dysfunction

• Rapid deterioration

• Abortion during a suspected EHV event

What to do: seek emergency veterinary assistance. Keep the horse isolated and avoid moving it unless instructed by the veterinarian.

Do Vaccines Prevent Viral Respiratory Disease?

Vaccines reduce risk. They do not create an invisible force field around the horse.

Equine influenza vaccines can reduce:

• The likelihood of clinical disease

• Fever severity

• Cough severity

• Duration of illness

• Viral shedding

EHV vaccines are available to reduce respiratory disease and, with appropriately labelled products, abortion risk. They may also reduce nasal shedding and therefore help limit transmission. However, infection and clinical disease can still occur despite vaccination. (AAEP)

Vaccination works best as part of a layered system:

1. Appropriate vaccine selection

2. Correct primary course

3. Risk-based boosters

4. Biosecurity during travel

5. Temperature monitoring

6. Isolation after returning home

7. Immediate separation of sick horses

A vaccination certificate is useful. It is not permission to share a bucket with every horse at the showground.

How Often Should Show Horses Be Vaccinated?

Vaccination should be based on the horse’s age, previous vaccine history, pregnancy status, travel frequency, event rules and product label.

A general adult-horse framework is:

Horse or situation	Equine influenza	EHV-1 and EHV-4
Low exposure risk	Usually annually	Usually annually
Ongoing travel or exposure	Commonly every 6 months	Consider every 6 months
Performance or show horse at high risk	Every 6 months is commonly advised	Every 6 months is commonly advised
Horse under 5 years old	Consider 6-month intervals	Consider 6-month intervals
Horse on a breeding farm or around pregnant mares	Risk-based	Consider 6-month intervals
Pregnant mare	Booster commonly given 4 to 6 weeks before foaling	Abortion-labelled EHV-1 vaccine commonly given at months 5, 7 and 9, plus respiratory vaccination as advised

Current AAEP guidance recommends six-month equine influenza revaccination for horses with ongoing exposure. It also advises considering six-month EHV-1/EHV-4 intervals for horses under five, horses on breeding farms or in contact with pregnant mares, and high-risk performance or show horses. (AAEP)

Competition rules may be stricter than general veterinary recommendations. For example, current USEF rules require documentation of equine influenza and EHV vaccination within the six months before entering competition stables. Other organisations and countries have their own rules. (U.S. Equestrian)

The final schedule should follow:

• The specific vaccine label

• Your veterinarian’s risk assessment

• Competition or facility requirements

• The horse’s previous vaccination history

An unvaccinated horse may need a two-dose or three-dose primary course rather than a single injection. One rushed vaccine before a show does not replace a completed primary series.

When Should Vaccination Be Given Before Travel?

Do not leave routine vaccination until loading day.

AAEP influenza guidance states that, during an outbreak, a booster in a previously vaccinated horse is unlikely to be useful unless it can be administered at least ten days before exposure. As a practical planning rule, routine show-season boosters are often scheduled at least ten to fourteen days before travel, while still following the product label and the treating veterinarian’s advice.

Giving vaccines shortly before travel can also complicate monitoring because some horses develop temporary lethargy, soreness or a mild temperature increase following vaccination.

For a major competition season, review the schedule several weeks in advance. Vaccination planning done beside the loaded trailer is not really planning. It is optimistic paperwork.

Does EHV-1 Have to Mutate to Cause Neurological Disease?

No.

EHM should not be described simply as EHV-1 “mutating” inside an individual horse.

Different EHV-1 genetic variants have been studied, including D752, N752 and H752 variants. Although some variants were historically associated more often with particular disease patterns, the association is not absolute. The current ACVIM consensus states that no specific EHV-1 variant can reliably predict whether neurological disease will occur in the field, and genotype should not determine routine outbreak management. (AAEP)

What matters clinically is that any EHV-1 infection should be taken seriously when fever or neurological abnormalities develop.

Can Vaccination Prevent EHM?

No licensed vaccine is currently labelled to prevent EHM.

EHV vaccination may reduce respiratory disease, nasal shedding and possibly the chance of widespread transmission, but it does not guarantee that a horse will avoid viraemia or neurological disease.

AAEP also states that strict biosecurity, isolation, quarantine and monitoring are currently more effective for controlling an EHV outbreak than any existing vaccination protocol.

This does not mean EHV vaccination is pointless. It means vaccination cannot carry the entire biosecurity plan on its back.

Biosecurity Before Travelling to an Event

Before leaving:

1. Check vaccination records early.
   Make sure the primary course and boosters meet both veterinary and event requirements.

2. Establish a normal temperature baseline.
   Knowing what is normal for your horse makes subtle fever easier to identify.

3. Do not travel a sick horse.
   Coughing, unexplained nasal discharge, fever, depression or reduced appetite should be assessed before travel.

4. Ask about recent exposure.
   Do not travel if the horse or stable has had recent contact with an infectious respiratory case unless cleared by the veterinarian and relevant authorities.

5. Pack dedicated equipment.
   Bring your own water buckets, feed tubs, grooming kit, thermometer, lead ropes, towels and mucking tools where practical.

6. Clean the trailer.
   Remove organic material before disinfecting. Disinfectant works poorly through manure, mucus and dirty bedding.

7. Make an isolation plan before returning.
   Deciding where the horse will go after the show is much easier before the barn suddenly has a coughing horse.

Biosecurity at the Show

Take Temperatures Twice Daily

AAEP biosecurity guidance recommends taking and recording equine temperatures twice daily at events. Temperatures should be checked after the horse has cooled down, not immediately after exercise.

A resting temperature above approximately 101.5°F or 38.6°C should be reported according to event and veterinary protocols. Individual baseline and environmental conditions still matter. (AAEP)

Use Your Own Buckets

Do not allow horses to drink from communal troughs or shared buckets.

Do not place the end of a shared hose inside your bucket. A hose that has been submerged in several buckets is essentially a travelling saliva collection service. AAEP specifically recommends preventing hoses from touching or entering water buckets. (AAEP)

Prevent Nose-to-Nose Contact

Avoid allowing horses to:

• Touch noses with unfamiliar horses

• Reach into neighbouring stalls

• Share feed or hay

• Lick common rails

• Congregate unnecessarily in narrow wash areas

Do Not Share Equipment

Avoid sharing:

• Tack

• Grooming tools

• Towels

• Feed tubs

• Water buckets

• Lead ropes

• Bits

• Twitch handles

• Thermometers

Use Hand and Clothing Hygiene

Wash or sanitise hands between horses, particularly after handling nasal discharge.

Where disease is suspected, use dedicated clothing, footwear and equipment for the isolated horse.

Report Signs Immediately

Do not hide a cough because the horse has a class in an hour.

A repeated cough, fever, nasal discharge or sudden depression should be reported to the event veterinarian or manager. Continuing to move the horse around the venue risks exposing other horses and making the eventual disruption much larger.

What Should You Do After Returning Home?

Post-travel separation is one of the most effective ways to protect the resident herd.

AAEP influenza guidance recommends isolating horses returning from shows, exhibitions or trail rides for at least 14 days. USDA APHIS takes a more conservative EHV-focused approach and recommends isolating new and returning horses for at least 21 days.

A practical approach is:

• At least 14 days after routine show travel where no specific EHV concern exists

• 21 days for new arrivals, known EHV exposure, high-risk travel or properties containing pregnant mares and young horses

• Longer when directed by the attending veterinarian, event organiser or animal health authority

During separation:

• Check temperature at least daily

• Use twice-daily monitoring where exposure risk is higher

• Use separate buckets and equipment

• Avoid nose-to-nose contact

• Handle resident horses first

• Monitor appetite, manure production and behaviour

• Watch for cough, discharge or swollen lymph nodes

• Record every abnormality

A horse that remains healthy throughout the monitoring period can then rejoin the resident group according to the farm’s veterinary plan.

What Should You Do if a Horse Develops Fever or Coughing?

Step 1: Isolate Immediately

Do not wait for test results before separating the horse.

Use a stall or paddock away from other horses, ideally with separate airspace and no shared fence line.

Step 2: Stop Horse Movement

Pause unnecessary movement on and off the property until the situation has been assessed.

This does not always mean the entire property will require formal quarantine, but uncontrolled movement should stop while the risk is clarified.

Step 3: Contact Your Veterinarian

Provide:

• Current rectal temperature

• Previous temperature records

• Vaccination history

• Travel dates and locations

• Onset and frequency of coughing

• Description of nasal discharge

• Appetite and demeanour

• List of exposed horses

• Any neurological abnormalities

Step 4: Monitor All Exposed Horses

Take rectal temperatures at least twice daily.

Fever may appear before coughing or nasal discharge, particularly with EHV.

Step 5: Establish Clean and Dirty Zones

Use separate:

• Buckets

• Feed equipment

• Clothing

• Footwear

• Thermometers

• Grooming tools

• Mucking equipment

Handle healthy horses first and isolated horses last.

Step 6: Notify the Relevant People

Depending on the situation, this may include:

• Stable owner

• Event organiser

• Transporter

• Owners of exposed horses

• Referring veterinary hospital

• State or national animal health authorities

Reporting requirements vary by disease and location.

How Do Vets Diagnose Viral Respiratory Disease?

Diagnosis may involve:

• Physical examination

• Rectal temperature monitoring

• Respiratory rate and effort assessment

• Lung and tracheal auscultation

• Nasopharyngeal or nasal swabs

• PCR testing

• Whole blood testing

• Blood count and inflammatory markers

• Ultrasound or radiographs if pneumonia is suspected

• Endoscopy or airway sampling in selected cases

For suspected influenza, diagnostic swabs are most useful early, ideally within approximately 24 to 48 hours after signs begin.

For suspected EHV-1, current consensus recommendations support testing both nasal secretions and uncoagulated whole blood with real-time quantitative PCR. Testing only one sample type can miss part of the infection pattern. (AAEP)

Testing may also include Streptococcus equi, EHV-4, equine rhinitis viruses and other pathogens depending on the horse’s signs and outbreak history.

What Else Can Look Like Viral Respiratory Disease?

Not every cough after a show is influenza or EHV.

Important alternatives include:

• Strangles

• Equine viral arteritis

• Bacterial pneumonia

• Pleuropneumonia or shipping fever

• Guttural pouch infection

• Inflammatory airway disease

• Equine asthma

• Dust or mould exposure

• Aspiration

• Exercise-induced pulmonary haemorrhage

• Tracheal irritation

• Cardiac disease

• Lungworm in relevant populations

• Foreign material in the airway

A single cough after entering a dusty arena is different from repeated coughing accompanied by fever and nasal discharge.

What matters most is the pattern:

• Is there fever?

• Is the horse bright or depressed?

• Are several horses affected?

• Did signs begin after travel?

• Is the discharge clear, thick or foul?

• Is breathing normal at rest?

• Are neurological abnormalities present?

That is the difference between reasonable monitoring and hoping the problem disappears while it tours the entire barn.

How Long Does Recovery Take?

Many horses with uncomplicated viral respiratory disease recover well with rest and supportive care.

However, clinical recovery and return to athletic work are not the same thing.

With influenza:

• Fever may settle before the cough

• Coughing can persist for several weeks

• Airway inflammation may outlast the obvious illness

• Returning to work too early can prolong recovery

• Secondary bacterial pneumonia can complicate the course

Exercise plans should be guided by the horse’s temperature, cough, respiratory examination and intended workload. A horse that has stopped coughing in the stable may still not be ready for speed work.

EHV respiratory disease is often mild in otherwise healthy horses, but EHM, abortion and neonatal disease carry very different prognoses. Any neurological change requires urgent reassessment.

Common Mistakes Horse Owners Make

Vaccinating on Loading Day

A vaccine needs time to generate a useful immune response. Last-minute vaccination is not a substitute for advance planning.

Assuming Vaccinated Means Non-Infectious

Vaccinated horses can still become infected and shed pathogens, even when their clinical signs are mild.

Taking Temperatures Only When the Horse Looks Sick

Fever may appear before coughing, discharge or obvious depression. Temperature monitoring is most valuable before the problem becomes obvious.

Taking the Temperature Immediately After Exercise

Exercise can temporarily increase temperature. Allow the horse to cool down before interpreting the result.

Sharing Hoses and Buckets

A clean-looking bucket is not necessarily a pathogen-free bucket, particularly when a communal hose has been placed inside it.

Returning Straight to the Resident Herd

A horse may be incubating disease despite looking normal on arrival home.

Continuing to Compete With a Cough

A cough can be caused by dust or airway irritation, but repeated coughing, fever or discharge must be treated as potentially infectious until assessed.

Waiting for Neurological Signs Before Isolating an EHV Case

By the time EHM becomes apparent, respiratory shedding and horse-to-horse exposure may already have occurred. Isolation should begin at the fever or respiratory stage.

Can Viral Respiratory Outbreaks Be Prevented Completely?

No system eliminates every risk.

Horses travel, viruses circulate, vaccine protection is imperfect and some infected horses show few signs.

The goal is to reduce the chance that one exposure becomes a property-wide outbreak.

The most effective approach combines:

• Risk-based vaccination

• Advance vaccine timing

• Accurate records

• Twice-daily temperatures at events

• Dedicated buckets and grooming tools

• No communal water

• Reduced nose-to-nose contact

• Immediate isolation of abnormal horses

• Separation after returning home

• Rapid veterinary testing

• Clear communication between owners, organisers and veterinarians

The individual steps are not glamorous. There is no dramatic slow-motion montage of someone labelling buckets.

They are still what works.

FAQs

Can a vaccinated horse still catch equine influenza?

Yes. Vaccination does not prevent every infection, but it generally reduces the likelihood and severity of clinical disease and may shorten viral shedding.

How long before a show should my horse be vaccinated?

Routine boosters should generally be completed at least ten to fourteen days before likely exposure, subject to the product label and your veterinarian’s advice. An unvaccinated horse may require a complete primary series over several weeks.

How long should a horse be isolated after returning from a show?

AAEP influenza guidance recommends at least 14 days. USDA APHIS recommends 21 days for new or returning horses as an EHV precaution. Use the longer period where EHV exposure, young horses or pregnant mares are a concern.

Does an EHV vaccine prevent neurological herpes?

No. There is currently no licensed vaccine labelled to prevent EHM. EHV vaccination may reduce respiratory disease and nasal shedding, but biosecurity remains essential.

Should one cough stop a horse from competing?

A single cough can result from dust or airway irritation. Repeated coughing, fever, nasal discharge, depression or known disease exposure should prompt withdrawal, isolation and veterinary assessment.

Final Thoughts

Viral respiratory disease is an unavoidable risk wherever horses travel and mix, but outbreaks are not simply bad luck.

The most important protections are layered:

• Vaccinate according to the horse’s actual risk

• Complete boosters before travel

• Record temperatures twice daily at events

• Never share buckets or equipment

• Separate horses after returning home

• Isolate fever, coughing or nasal discharge immediately

• Treat fever with neurological signs as an emergency

A horse can look healthy while incubating or shedding a respiratory pathogen. That does not mean every travelling horse is dangerous. It means good management cannot rely on appearance alone.

Vaccination reduces the risk. Monitoring detects the problem. Biosecurity stops one sick horse from becoming everyone’s sick horse.

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Planning a competition season or concerned about coughing, fever or nasal discharge after travel? ASK A VET™ can help you organise a vaccination, temperature-monitoring and post-travel plan, while your local veterinarian provides examination, testing and treatment when hands-on care is needed.