Equine Herpesvirus Respiratory Disease

Equine herpesvirus respiratory disease represents a significant and highly contagious viral infection affecting the respiratory tract of horses worldwide. Caused primarily by equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4), this disease presents with fever, nasal discharge, coughing, and general malaise. While EHV-4 typically causes respiratory disease alone, EHV-1 carries the additional risk of causing abortion in pregnant mares and the devastating neurological syndrome known as equine herpesvirus myeloencephalopathy (EHM). Understanding this viral disease is essential for all horse owners and facilities due to its highly contagious nature and potential for serious complications.

Equine herpesvirus infections occur globally and are endemic in horse populations worldwide, meaning the virus is continuously present and circulating. Studies suggest that up to 80 percent of horses have been exposed to equine herpesviruses by the time they reach adulthood, with many becoming latent carriers capable of reactivating and shedding virus during periods of stress. The disease most commonly affects young horses under five years of age during their first exposures, though adult horses remain susceptible to reinfection and can develop clinical disease. Outbreaks frequently occur at competitions, shows, breeding farms, and other locations where horses from multiple sources congregate.

The impact of equine herpesvirus respiratory disease on horse health ranges from mild, self-limiting illness to severe disease with long-lasting consequences. Uncomplicated respiratory infections typically resolve within two to three weeks with appropriate supportive care. However, EHV-1 strains capable of causing neurological disease pose life-threatening risks, with affected horses potentially developing paralysis, inability to urinate, and other debilitating symptoms. Economic impact on the equine industry is substantial, including direct costs of treatment, quarantine measures, event cancellations, and breeding losses from EHV-1-induced abortions.

While no cure exists for equine herpesvirus infection, supportive treatment enables most horses with respiratory disease to recover fully. Early detection and implementation of biosecurity measures are crucial for preventing spread during outbreaks. Vaccination plays an important role in reducing disease severity and viral shedding, though it does not completely prevent infection. Working with a licensed equine veterinarian to develop appropriate vaccination protocols and respond rapidly to suspected cases helps protect individual horses and the broader equine community from this persistent viral threat.

The primary cause of equine herpesvirus respiratory disease is infection with equine herpesvirus type 1 or type 4, both members of the Alphaherpesvirinae subfamily. EHV-4 is the most common cause of respiratory disease, typically producing uncomplicated upper and lower respiratory tract infection. EHV-1 causes similar respiratory signs but additionally can disseminate throughout the body, infecting the uterus of pregnant mares causing abortion, or the central nervous system causing neurological disease. Transmission occurs through direct contact with infected horses, exposure to respiratory secretions through aerosol spread over distances up to 35 feet, or contact with contaminated fomites including tack, grooming equipment, water buckets, and human hands and clothing.

Genetic factors do not predispose specific breeds to equine herpesvirus infection, as all equids are susceptible regardless of breed or genetic background. However, research has identified genetic variations within EHV-1 strains that influence their pathogenicity, particularly a mutation in the DNA polymerase gene that correlates with increased risk of neurological disease. This neuropathogenic strain variant has been associated with numerous EHM outbreaks. Host immune factors and individual variation in immune response affect disease severity, with some horses mounting more effective responses than others despite similar exposure.

Environmental and management factors significantly influence transmission dynamics and outbreak risk. Congregating horses from multiple sources creates ideal conditions for virus spread, making competitions, sales, breeding farms, and training facilities high-risk environments. Stress from transportation, competition, changes in routine, or concurrent illness can trigger reactivation of latent virus in carrier horses, initiating new chains of transmission. Seasonal patterns exist, with outbreaks more common during late fall through spring when horses are stabled more closely and respiratory viruses circulate more readily. Poor ventilation, overcrowding, and inadequate biosecurity protocols amplify outbreak potential.

Risk factors for equine herpesvirus infection include age, immune status, stress level, and exposure opportunities. Young horses experiencing their first exposure typically develop more pronounced clinical signs than adults with prior immunity. Immunocompromised horses or those stressed by illness, hard training, transportation, or other factors face increased susceptibility. Pregnant mares are at particular risk for EHV-1-induced abortion, especially during the last trimester. Horses traveling frequently or housed at facilities with high turnover have greater exposure opportunities. Unvaccinated horses lack even the partial protection vaccination provides, increasing disease severity upon infection.

The pathophysiology of equine herpesvirus infection begins with viral replication in the respiratory epithelium following inhalation of infectious particles. Initial replication damages respiratory tract lining, producing inflammation, mucus accumulation, and the characteristic clinical signs of respiratory disease. EHV-1 additionally enters white blood cells (leukocytes), enabling systemic spread through the bloodstream during a period of viremia. Infected leukocytes can transport virus to the uterus, causing placental and fetal infection resulting in abortion, or to blood vessel endothelial cells in the central nervous system, causing vasculitis and subsequent neurological damage. Following acute infection, the virus establishes latency in lymphoid tissues and neurons, remaining dormant but capable of reactivation and renewed shedding throughout the horse's lifetime.

Early warning signs of equine herpesvirus respiratory disease often develop within two to ten days of exposure, a period known as the incubation period. Initial symptoms may be subtle, with affected horses appearing slightly dull or off-feed before more obvious signs develop. Mild lethargy and reduced appetite frequently precede the characteristic fever spike. Owners may notice affected horses are quieter than usual or show decreased interest in their surroundings. Because horses instinctively hide signs of illness, these early changes may be overlooked unless temperature monitoring is being performed, emphasizing the value of routine temperature checks during outbreak situations.

Common symptoms of established respiratory herpesvirus infection include fever, nasal discharge, coughing, and submandibular lymph node enlargement. Fever typically reaches 102 to 106 degrees Fahrenheit and may occur in a biphasic pattern with two distinct spikes. Nasal discharge begins as clear and serous before progressing to thicker, mucopurulent material as secondary bacterial involvement develops. Coughing ranges from occasional to frequent and may be dry or productive depending on disease stage. Enlarged lymph nodes beneath the jaw are readily palpable and may be mildly painful. Conjunctivitis with watery eye discharge and mild limb swelling occasionally accompany respiratory signs.

Behavioral changes during equine herpesvirus infection reflect the systemic nature of the illness. Affected horses typically demonstrate reduced appetite, sometimes refusing grain while still picking at hay, or becoming completely anorexic in severe cases. Depression and lethargy are common, with horses standing quietly rather than displaying normal curiosity and social behaviors. Exercise intolerance develops as respiratory compromise and systemic inflammation reduce stamina. Some horses become irritable when handled, particularly around the head and throat where lymph node swelling causes discomfort. Isolation-seeking behavior may occur as horses feel unwell.

Physical signs on examination include elevated temperature, increased respiratory rate and effort, abnormal lung sounds, and visible nasal discharge. Auscultation may reveal increased bronchial sounds, wheezes, or crackles depending on the extent of lower respiratory involvement. Heart rate is often elevated corresponding to fever and respiratory compromise. Mucous membranes may appear congested or mildly hyperemic. Dehydration can develop in horses with prolonged fever and reduced water intake. Mild limb edema, particularly affecting the hind legs, occasionally presents in some cases. Weight loss may occur with prolonged illness or poor appetite.

Symptom progression in uncomplicated respiratory disease typically follows a pattern of worsening during the first three to five days followed by gradual improvement over one to three weeks. Fever usually resolves within five to seven days with appropriate supportive care. Nasal discharge may persist longer, particularly if secondary bacterial infection develops. Coughing can continue for several weeks after other signs resolve as damaged respiratory epithelium heals. In cases that progress to neurological involvement from EHV-1, additional signs develop including hind limb weakness, ataxia, urinary incontinence, and potentially recumbency, typically appearing seven to twelve days after initial respiratory signs.

Emergency symptoms requiring immediate veterinary care include any signs suggestive of neurological involvement, as EHM can progress rapidly from mild ataxia to complete paralysis within hours. Signs warranting urgent attention include stumbling, hind limb weakness, difficulty rising, inability to posture normally to urinate or defecate, and recumbency. High fevers exceeding 105 degrees Fahrenheit that do not respond to treatment require prompt evaluation. Signs of severe respiratory distress, rapid deterioration, or development of complications such as pneumonia necessitate immediate veterinary assessment. Pregnant mares showing signs of EHV infection should be monitored closely for abortion, which typically occurs two to four weeks after infection but can happen at any time.

Physical examination by a licensed equine veterinarian provides initial assessment of horses suspected of equine herpesvirus infection. Vital parameters including temperature, heart rate, and respiratory rate are evaluated, with fever being a hallmark finding. Thorough auscultation of the respiratory tract assesses airway sounds and identifies any abnormalities suggesting lower respiratory involvement. Palpation of submandibular and retropharyngeal lymph nodes evaluates enlargement and pain. Neurological examination assesses gait, coordination, tail and anal tone, and bladder function to identify any signs suggesting progression to EHM. The history of potential exposure, recent travel, or contact with horses from other facilities informs the index of suspicion.

Diagnostic tests confirm equine herpesvirus infection and distinguish it from other respiratory diseases. Polymerase chain reaction (PCR) testing of nasal swabs or nasopharyngeal secretions detects viral DNA with high sensitivity and specificity, providing rapid confirmation. PCR can also be performed on blood samples (whole blood or buffy coat) to detect viremia, which is particularly important for assessing EHM risk. Viral isolation from respiratory secretions remains the gold standard but takes longer than PCR. Paired serology comparing acute and convalescent blood samples for rising antibody titers confirms recent infection but is retrospective rather than useful for acute diagnosis. Complete blood count may reveal lymphopenia and neutropenia during early infection.

Advanced diagnostics become important when neurological involvement is suspected or for outbreak investigation. Cerebrospinal fluid analysis in horses with neurological signs may reveal increased protein concentration and changes in cell populations. Specialized PCR testing can identify the neuropathogenic variant of EHV-1 associated with higher EHM risk. Respiratory endoscopy evaluates the extent of airway involvement and allows sample collection from lower airways. Thoracic radiographs or ultrasound assess for secondary pneumonia development. During outbreaks, testing of in-contact horses even without clinical signs helps identify subclinical shedders and inform quarantine decisions.

Differential diagnosis for equine herpesvirus respiratory disease includes other infectious respiratory conditions producing similar clinical signs. Equine influenza causes fever, coughing, and nasal discharge but typically produces more severe coughing and rarely causes neurological disease. Streptococcus equi infection (strangles) produces similar lymph node enlargement and nasal discharge but abscessation and rupture of lymph nodes is characteristic. Equine viral arteritis can cause fever and respiratory signs along with limb edema and requires laboratory differentiation. Bacterial pneumonia presents with fever and respiratory signs but often with more severe systemic illness and abnormal lung sounds. Accurate diagnosis through laboratory testing is essential for appropriate management and biosecurity implementation.

Emergency and immediate treatment of equine herpesvirus respiratory disease focuses on supportive care, reducing fever, and preventing complications. Anti-inflammatory medications including non-steroidal anti-inflammatories (NSAIDs) such as flunixin meglumine or phenylbutazone reduce fever, alleviate inflammation, and improve patient comfort. Adequate hydration must be maintained, with intravenous fluid therapy considered for horses with severe illness or inadequate voluntary intake. Immediate isolation of affected horses prevents spread to susceptible herdmates. The environment should be optimized with good ventilation, appropriate bedding, and easy access to fresh water and palatable feed. Rest is essential, with all exercise discontinued until complete recovery.

Medical management of uncomplicated respiratory herpesvirus infection centers on supportive care allowing the horse's immune system to clear the infection. NSAIDs provide symptomatic relief and reduce fever, though prolonged use requires monitoring for gastrointestinal or renal side effects. Antiviral medications including valacyclovir and acyclovir have been used with variable success, particularly in horses at high risk for neurological disease or during outbreaks with known neuropathogenic strain involvement. The efficacy of antivirals in preventing EHM progression remains somewhat controversial, but early treatment may reduce viral replication and systemic spread. Antimicrobial therapy is reserved for horses with evidence of secondary bacterial infection rather than routine use.

Surgical options are not applicable to equine herpesvirus respiratory disease, as treatment is medical rather than surgical. However, horses that develop EHM-associated urinary retention may require bladder catheterization to prevent bladder rupture and manage urinary complications. Severely affected recumbent horses require intensive nursing care including maintenance of appropriate body position, prevention of pressure sores, and sometimes sling support to assist standing. These supportive interventions, while not surgical, require significant medical management and monitoring.

Supportive care encompasses the critical elements of nursing management that promote recovery. Isolation in a well-ventilated area away from other horses prevents transmission while maintaining appropriate environmental conditions. Palatable, easily consumed feeds encourage continued eating during illness. Fresh water should be readily accessible, with monitoring of intake to ensure adequate hydration. Soft bedding reduces physical stress and prevents injury in weak horses. Monitoring of temperature, appetite, attitude, and respiratory status allows early detection of complications or deterioration. Gradual reintroduction to normal routine follows resolution of fever and clinical signs.

Rehabilitation and return to work proceed gradually following complete resolution of clinical signs. Horses should remain resting until fever has been absent for at least 24 to 48 hours and appetite and attitude have normalized. Return to exercise begins with light activity such as hand walking, progressing slowly to normal work over two to four weeks depending on illness severity. Pushing horses back into work too quickly risks relapse and prolonged recovery. Continued monitoring during the return-to-work period ensures any setbacks are identified promptly. Horses that developed secondary complications may require longer rehabilitation periods.

Treatment decision factors include disease severity, presence of neurological signs, pregnancy status, and outbreak context. Uncomplicated respiratory cases in otherwise healthy horses often recover with basic supportive care. Horses showing any neurological signs require more intensive management including antiviral therapy consideration. Pregnant mares warrant close monitoring for abortion and may benefit from antiviral treatment. During outbreaks, treatment decisions must balance individual horse care with broader biosecurity needs and resource limitations. Financial considerations influence treatment intensity, though basic supportive care requirements are generally affordable. Close communication with the attending veterinarian guides appropriate treatment choices for each situation.

Recovery timeline for uncomplicated equine herpesvirus respiratory disease typically spans two to four weeks from illness onset to full return to normal activity. Fever usually resolves within five to seven days with appropriate supportive care. Nasal discharge and coughing may persist for one to three weeks beyond fever resolution as respiratory epithelium regenerates. Appetite and attitude commonly normalize within a few days of fever breaking. Return to full work capacity may take three to four weeks as fitness rebuilds following the illness-related deconditioning. Horses that develop secondary bacterial pneumonia or other complications require longer recovery periods extending to six weeks or more.

Post-treatment care and monitoring continue after acute illness resolves to ensure complete recovery and detect any complications. Temperature monitoring should continue for several days after fever resolves to confirm sustained improvement. Respiratory rate and effort should return to normal, with any persistent abnormalities prompting reassessment. Gradual reintroduction of normal feeding and turnout proceeds as the horse demonstrates improved strength and stamina. Monitoring for signs of secondary complications including pneumonia or neurological involvement continues throughout the recovery period. Horses recovering from EHV-1 infection warrant particular vigilance for late-developing neurological signs.

Prognosis factors for equine herpesvirus respiratory disease recovery include the viral strain involved, promptness of treatment initiation, development of complications, and individual immune response. Uncomplicated EHV-4 respiratory infections carry an excellent prognosis, with nearly all horses achieving full recovery. EHV-1 infections carry somewhat more guarded prognoses due to potential for neurological complications, though uncomplicated respiratory cases still recover well. Horses that develop secondary bacterial pneumonia face longer recovery periods and slightly increased risk of chronic respiratory issues. Development of EHM dramatically worsens prognosis, with outcomes ranging from full recovery to persistent neurological deficits to death depending on severity.

Long-term outlook following equine herpesvirus respiratory infection is generally excellent for horses with uncomplicated disease. Most horses return to their previous level of work and performance without lasting effects. However, the virus establishes latency following infection, meaning recovered horses become lifelong carriers capable of reactivating and shedding virus during future stress episodes. This latent carrier state has no clinical significance for the individual horse's health but has important implications for biosecurity and outbreak prevention. Horses that experienced EHM may have variable long-term outcomes, with some recovering completely and others retaining permanent neurological deficits affecting gait or bladder function. Repeated respiratory infections may occur throughout life as immunity is not complete or permanent.

Management practices form the foundation of equine herpesvirus prevention, with biosecurity protocols being paramount. New horses entering a facility should be quarantined for a minimum of 21 days with daily temperature monitoring before joining the resident population. Horses returning from competitions, breeding farms, or veterinary hospitals should undergo similar isolation. Minimizing commingling of horses from different sources reduces transmission opportunities. At events, avoiding nose-to-nose contact between unfamiliar horses and not sharing water buckets, tack, or equipment decreases risk. Handlers should wash hands between horses and consider footwear disinfection when moving between barn areas. During outbreaks, strict quarantine with dedicated personnel and equipment for affected horses contains spread.

Nutritional factors do not directly prevent equine herpesvirus infection but maintaining optimal nutrition supports robust immune function. Well-nourished horses with adequate vitamin and mineral status mount more effective immune responses when exposed to pathogens. Avoiding nutritional stress that might compromise immunity reduces disease severity if infection occurs. Proper nutrition also supports recovery, enabling horses to heal respiratory tract damage and regain condition following illness. Adequate hydration maintains healthy respiratory secretions and mucosal defenses.

Exercise and conditioning considerations relate to stress management rather than direct prevention. Horses subjected to excessive training stress, overwork, or inadequate rest may experience immune suppression increasing susceptibility to infection. Balanced training programs that build fitness without excessive stress support immune function. Transportation, a known stressor, should be minimized when possible or managed with attention to minimizing stress through appropriate vehicle conditions, driver behavior, and rest stops. Allowing adequate recovery time following competition or other stressful events before returning to intensive training reduces vulnerability to opportunistic infections.

Environmental factors influencing equine herpesvirus transmission include ventilation, population density, and sanitation. Well-ventilated barns reduce airborne viral concentrations and limit transmission distances. Avoiding overcrowding provides physical separation between horses. Regular cleaning and disinfection of shared equipment, stalls, and common areas reduces environmental viral load. Equine herpesviruses are susceptible to many common disinfectants including quaternary ammonium compounds, chlorhexidine, and dilute bleach solutions. Sunlight and drying also inactivate the virus relatively quickly, making outdoor environments lower risk than enclosed spaces. Facilities should have isolation areas available for immediate use if respiratory disease occurs.

Vaccination protocols play an important role in equine herpesvirus prevention, though available vaccines have limitations. Vaccines are available against both EHV-1 and EHV-4 and have been shown to reduce severity of respiratory disease and decrease viral shedding, thereby reducing transmission risk. However, no available vaccine completely prevents infection or reliably prevents EHM. Core vaccination recommendations include vaccination every six months for horses at elevated risk due to frequent travel, competition, or contact with transient populations. Pregnant mares should be vaccinated at months 5, 7, and 9 of gestation to reduce abortion risk. Vaccination should be completed two to four weeks before anticipated exposure for optimal protection. Veterinary guidance helps determine appropriate protocols for individual horses and facility risk profiles.

Daily management adjustments during equine herpesvirus infection prioritize rest, supportive care, and infection control. Affected horses should be isolated in a well-ventilated area with dedicated equipment to prevent transmission to herdmates. Temperature should be monitored twice daily, with any fever spikes prompting anti-inflammatory treatment. Fresh water and palatable feed should be readily accessible, with intake monitored to ensure adequate nutrition and hydration. Stall conditions should be optimized with appropriate bedding and protection from weather extremes. Handlers should follow biosecurity protocols including hand washing, wearing dedicated clothing or coveralls, and using foot baths when entering and leaving the isolation area. The horse should be disturbed as little as necessary for monitoring and care.

Housing and turnout considerations during illness and recovery balance infection control needs with horse welfare. Complete stall rest is appropriate during the febrile phase of illness. Once fever resolves and the horse shows improvement, limited turnout in a small paddock away from other horses provides mental stimulation and gentle exercise while maintaining biosecurity. Full return to group turnout should await completion of viral shedding, typically about three weeks post-infection, though individual variation occurs. The isolation facility should offer good ventilation, appropriate shelter, and safe fencing. Bedding should be managed to maintain cleanliness and comfort while disposing of soiled material in a manner that does not spread contamination.

Exercise modifications progress gradually through recovery phases. No forced exercise is appropriate during active illness when fever is present. Hand walking may begin once fever has resolved for 48 hours and the horse shows interest in moving. Light turnout follows as strength and attitude improve. Return to ridden work begins at a walk, progressing through gaits as stamina demonstrates tolerance. Full return to previous work intensity may take three to four weeks following illness resolution. Monitoring respiratory rate and effort during exercise identifies any limitations requiring continued work restriction. Pushing recovery too quickly risks setbacks and prolonged overall recovery.

Monitoring and ongoing care extend beyond acute illness resolution. Temperature monitoring should continue for several days after normalization to confirm stability. Appetite, attitude, and respiratory status provide daily indicators of recovery progress. Any deterioration or development of new signs such as neurological abnormalities requires prompt veterinary reassessment. Respiratory tract healing continues for weeks after clinical sign resolution, and horses should be monitored for signs of secondary bacterial infection such as persistent or worsening nasal discharge or cough. Record keeping documents illness course and recovery, providing useful information for future health management.

Quality of life and use considerations following equine herpesvirus infection are generally favorable for horses with uncomplicated recovery. Most horses return to their previous level of work without lasting effects. However, awareness of latent carrier status informs biosecurity decisions, as stress may trigger viral reactivation and shedding. Horses with history of EHV infection should have vaccination status maintained and be monitored during stressful events for signs of recurrence. Horses that experienced complications such as pneumonia or neurological involvement may require modified expectations depending on the extent of permanent damage. Overall, most horses resume normal quality of life and athletic function following equine herpesvirus respiratory disease.

High-risk breeds for equine herpesvirus infection are not defined by breed-specific genetic susceptibility, as all horses are susceptible regardless of breed. Exposure opportunity rather than inherent breed characteristics determines infection risk. However, certain breed populations face elevated risk due to typical management practices and use patterns. Thoroughbreds and racing breeds experience high rates of exposure due to frequent transportation, dense population housing at racetracks, and mixing of horses from multiple sources. Warmbloods and sport horses competing regularly face similar exposure patterns at competitions and training facilities. Breeding stock, particularly mares at large breeding operations where horses from multiple farms commingle, face increased risk, with EHV-1 abortion posing particular concern.

Use and discipline considerations significantly influence equine herpesvirus exposure risk. Horses in disciplines involving frequent travel and competition, including racing, show jumping, dressage, and eventing, encounter repeated exposure opportunities. Sale horses passing through auction and consignment facilities contact numerous transient populations. Horses at large training centers with continuous turnover of individuals face ongoing exposure. Conversely, horses maintained at private facilities with limited outside contact experience reduced risk. Breeding operations face unique concerns due to EHV-1 abortion potential, necessitating strict biosecurity protocols and vaccination programs for pregnant mares. Understanding exposure risks associated with different uses helps guide appropriate preventive measures.

Genetic testing and breeding recommendations do not apply to equine herpesvirus infection in the traditional sense, as no genetic test identifies susceptibility. Research has focused on viral genetics rather than host genetics, identifying strain variations affecting pathogenicity. From a breeding management perspective, EHV-1 abortion poses significant concern, and breeding operations should implement comprehensive vaccination protocols for pregnant mares. Mares should receive rhinopneumonitis vaccination at months 5, 7, and 9 of gestation. Strict biosecurity preventing exposure of pregnant mares to outside horses is critical. Breeding farm design should separate pregnant mare populations from horses with higher exposure risk. Stallion management should recognize that breeding shed activities can introduce infection to mare populations.

Commonly co-occurring conditions with equine herpesvirus respiratory disease include secondary bacterial infections and complications of systemic illness. Bacterial pneumonia may develop when viral damage to respiratory defenses allows opportunistic bacterial invasion, presenting with persistent fever, worsening respiratory signs, and purulent nasal discharge. Pharyngitis and guttural pouch inflammation can complicate upper respiratory involvement. Dehydration and weight loss occur in horses with prolonged illness and reduced intake. In EHV-1 infections, equine herpesvirus myeloencephalopathy (EHM) represents the most serious co-occurring syndrome, developing in some horses as viral-induced vasculitis damages the central nervous system. Abortion may occur in pregnant mares infected with EHV-1.

Conditions with similar symptoms requiring differentiation from equine herpesvirus infection include other infectious respiratory diseases of horses. Equine influenza produces fever, coughing, and nasal discharge with typically more severe coughing and rapid spread through populations. Streptococcus equi subspecies equi infection (strangles) causes fever, nasal discharge, and lymph node enlargement but progresses to abscess formation. Streptococcus equi subspecies zooepidemicus can cause respiratory infection with similar initial presentation. Equine viral arteritis shares some features including respiratory signs and limb edema but produces more prominent vascular signs. Rhodococcus equi causes pneumonia primarily in foals. Laboratory testing distinguishes these conditions, enabling appropriate management.

Potential complications of equine herpesvirus infection extend beyond uncomplicated respiratory disease. Equine herpesvirus myeloencephalopathy (EHM) represents the most serious complication, causing neurological signs ranging from mild ataxia to complete paralysis with urinary and fecal incontinence. EHM-affected horses may become recumbent and unable to rise, creating welfare concerns and sometimes necessitating euthanasia. Abortion storms affecting multiple pregnant mares can devastate breeding operations. Secondary bacterial pneumonia prolongs illness and may cause permanent lung damage. Prolonged viral shedding from some recovered horses creates ongoing biosecurity challenges. Chronic respiratory issues including persistent cough or exercise intolerance may follow severe infections. Understanding these potential complications emphasizes the importance of prevention and early intervention.