Pasteurellosis

Pasteurellosis is a bacterial infection caused by organisms of the genus Pasteurella, most commonly Pasteurella caballi and Pasteurella multocida in horses, that typically affects the respiratory tract and can progress to severe pneumonia or disseminated disease. These bacteria are considered normal inhabitants of the upper respiratory tract in many horses, becoming pathogenic when the animal's natural defenses are compromised by stress, concurrent viral infection, transport, or other predisposing factors. The condition forms an important component of what is commonly called shipping fever complex, a respiratory disease syndrome frequently observed in horses following transportation.

Pasteurellosis occurs worldwide wherever horses are kept, with incidence increasing in situations involving stress, crowding, and transport. The disease affects horses of all ages, breeds, and disciplines, though young horses, recently transported animals, and those with compromised immune function face particularly elevated risk. Outbreaks may occur in training facilities, sales venues, and competition grounds where horses from multiple sources commingle and stress levels are high. The opportunistic nature of Pasteurella organisms means that even horses carrying these bacteria asymptomatically can develop clinical disease when conditions favor bacterial multiplication.

The impact of pasteurellosis on equine health ranges from mild upper respiratory symptoms that resolve spontaneously to life-threatening pneumonia requiring intensive care. Respiratory compromise affects athletic performance significantly, as even mild cases may require weeks of rest before return to training. Severe cases with pulmonary abscessation or pleuritis may have long-term consequences for respiratory function and athletic careers. Economic impacts include treatment costs, lost training time, and potential permanent reduction in athletic capacity for severely affected horses.

Treatability of pasteurellosis is generally favorable when infection is recognized early and appropriate antibiotic therapy initiated promptly. Mild cases may resolve with oral antibiotics and rest, while severe pneumonia or pleuropneumonia requires intensive treatment including intravenous antibiotics, anti-inflammatory medications, and supportive care. Early detection through awareness of risk factors and recognition of initial symptoms allows intervention before infection progresses to difficult-to-treat severe disease. Veterinary evaluation at the first signs of respiratory illness in at-risk horses improves outcomes significantly.

The primary causes of equine pasteurellosis are infection with Pasteurella caballi, Pasteurella multocida, and occasionally other Pasteurella species, with these bacteria frequently existing as commensal organisms in the equine upper respiratory tract before becoming pathogenic. The transition from harmless colonization to active infection typically requires predisposing factors that compromise the respiratory tract's normal defense mechanisms. Viral respiratory infections, particularly equine influenza and equine herpesvirus, damage the respiratory epithelium and impair mucociliary clearance, creating conditions favorable for secondary bacterial proliferation and invasion.

No genetic or breed predisposition has been established for pasteurellosis, as susceptibility relates primarily to exposure and immune status rather than hereditary factors. All breeds develop pasteurellosis at similar rates under comparable conditions of stress and exposure. However, horses with congenital or acquired immune deficiencies may be more susceptible to severe disease. Young horses lacking mature immune function and geriatric horses with age-related immune decline may be at somewhat increased risk of progression to severe disease.

Environmental and management factors significantly influence pasteurellosis risk through their effects on stress levels and pathogen exposure. Transportation represents one of the most important risk factors, with the combination of physical stress, temperature fluctuations, exhaust fume exposure, head elevation preventing normal airway drainage, and exposure to unfamiliar pathogens creating optimal conditions for bacterial multiplication. Overcrowding, poor ventilation, and mingling of horses from multiple sources at sales, shows, and training facilities facilitate transmission and increase exposure to virulent strains.

Risk factors for pasteurellosis include recent transport particularly over long distances, concurrent respiratory viral infection, inadequate ventilation in housing, temperature extremes, dust exposure, and immunosuppression from any cause. Horses new to a facility face elevated risk as they encounter unfamiliar pathogen strains while simultaneously experiencing stress from environmental change. Intensive training schedules that push horses beyond their conditioning level create stress that may impair immune function. Failure to allow adequate rest following transport or viral illness allows bacterial infections to establish before defenses recover.

The pathophysiology of pasteurellosis involves bacterial colonization of damaged or compromised respiratory epithelium, followed by multiplication and invasion of deeper tissues. Pasteurella organisms produce several virulence factors including capsular polysaccharides that resist phagocytosis, toxins that damage tissue and impair immune responses, and enzymes that facilitate tissue invasion. The inflammatory response to infection produces the clinical signs of respiratory disease while simultaneously causing tissue damage. Severe cases may develop bacteremia with spread to distant sites including joints, meninges, and other tissues, producing disseminated disease.

Early warning signs of pasteurellosis include subtle changes in attitude, mild decrease in appetite, and slight elevation in body temperature that may precede obvious respiratory symptoms. Horses may appear less enthusiastic about work or show decreased performance before developing visible illness. Because horses naturally tend to hide signs of illness as prey animals, these early indicators require attentive observation to detect. Monitoring temperature in horses following transport or viral illness exposure helps identify fever before other symptoms become apparent.

Common symptoms of pasteurellosis primarily involve the respiratory tract and include nasal discharge that progresses from clear and serous to thick, purulent, and often yellow or green-tinged as infection establishes. Coughing ranges from occasional throat-clearing to frequent, productive coughs depending on infection severity and location. Fever is typically present, often ranging from 102 to 105 degrees Fahrenheit, and may fluctuate during the disease course. Affected horses commonly show depression and reduced appetite as systemic illness develops.

Behavioral changes associated with pasteurellosis reflect both respiratory compromise and systemic illness. Horses become reluctant to exercise and may stop eating or drinking normally as they feel unwell. Depression manifests as decreased interaction with handlers and herd mates, with affected horses often standing quietly with lowered heads. Some horses show reluctance to lower their heads to eat from ground level, as this position increases pressure in congested airways. Performance horses may refuse jumps or show dramatic reductions in speed and stamina.

Physical signs detectable on examination include increased respiratory rate and effort, particularly notable at rest when normal horses breathe quietly and regularly. Auscultation of the lungs may reveal abnormal sounds including crackles, wheezes, or areas of decreased sound over consolidated regions. Lymph nodes in the head and neck may become enlarged and palpable. In severe cases, nostril flare with each breath, abdominal press during expiration, and extension of the head and neck indicate significant respiratory compromise. Pleuritis produces pain on chest compression and reluctance to move.

Symptom progression in untreated or severe pasteurellosis follows a concerning pattern of worsening respiratory compromise. Initial mild symptoms progress to obvious respiratory distress within days if infection is not controlled. Pleuropneumonia develops when infection extends to involve the pleural space surrounding the lungs, producing additional fluid accumulation and pain. Lung abscesses may form, creating pockets of infection that are difficult to resolve. Weight loss becomes apparent in protracted cases as appetite suppression and increased metabolic demands deplete body reserves.

Emergency symptoms requiring immediate veterinary intervention include severe respiratory distress with marked increased effort, labored breathing at rest, high fever exceeding 104 degrees Fahrenheit, refusal to eat or drink, extreme depression, or any signs of systemic shock. Horses that develop pleuritis often show reluctance to move, short choppy strides to minimize chest wall motion, and obvious discomfort. Rapid progression of symptoms, particularly in recently transported horses, warrants urgent evaluation as early treatment dramatically affects outcomes in serious respiratory infections.

Physical examination provides essential diagnostic information for suspected pasteurellosis, beginning with assessment of vital signs including temperature, heart rate, and respiratory rate. Elevated temperature above 101.5 degrees Fahrenheit supports infection, while increased heart and respiratory rates reflect systemic illness and respiratory compromise. Careful auscultation of both lung fields identifies abnormal respiratory sounds, areas of consolidation, and potential pleural involvement. Examination of nasal discharge, submandibular lymph nodes, and overall demeanor completes the initial assessment and helps characterize disease severity.

Diagnostic tests for pasteurellosis include complete blood count, which typically reveals elevated white blood cell count with neutrophilia in acute infection or neutropenia in overwhelming sepsis. Fibrinogen levels are often markedly elevated, indicating significant inflammation. Bacterial culture and sensitivity testing of transtracheal wash or bronchoalveolar lavage samples identify the specific causative organisms and guide antibiotic selection. Polymerase chain reaction testing may detect Pasteurella organisms when culture is negative due to prior antibiotic therapy or fastidious growth requirements.

Advanced diagnostics help characterize the extent and severity of respiratory involvement. Thoracic radiography reveals pulmonary infiltrates, consolidation patterns, and pleural fluid accumulation characteristic of different disease stages. Thoracic ultrasound provides excellent detail of pleural surfaces and fluid accumulation, allowing guided drainage of pleural effusions when present. Endoscopy of the upper airways may identify concurrent conditions or sources of discharge. In cases with systemic involvement, additional diagnostics evaluate affected organ systems.

Differential diagnosis for pasteurellosis includes other bacterial pneumonias caused by Streptococcus equi subspecies zooepidemicus, Rhodococcus equi in young horses, and various other opportunistic organisms. Viral respiratory infections including equine influenza and equine herpesvirus may present similarly and often precede bacterial infection. Strangles can produce some overlapping symptoms but typically has characteristic lymph node involvement. Non-infectious causes of coughing and nasal discharge including inflammatory airway disease, recurrent airway obstruction, and exercise-induced pulmonary hemorrhage should also be considered. Clinical context, particularly recent transport or viral exposure, supports pasteurellosis diagnosis.

Emergency and immediate treatment of pasteurellosis focuses on stabilizing respiratory function and initiating antimicrobial therapy. Horses with severe respiratory distress may require oxygen supplementation to maintain adequate blood oxygen levels. Intravenous fluid therapy corrects dehydration from fever and reduced water intake while supporting circulation. Anti-inflammatory medications including flunixin meglumine reduce fever, control inflammation, and provide comfort. Hospitalization is recommended for severe cases to allow intensive monitoring and aggressive intervention.

Medical management of pasteurellosis centers on appropriate antibiotic therapy guided by culture and sensitivity results when available. Empirical antibiotic selection typically begins with broad-spectrum agents effective against common respiratory pathogens, including penicillin combined with gentamicin, or trimethoprim-sulfonamide combinations. Once sensitivity results are available, therapy may be adjusted to optimize efficacy and minimize resistance development. Treatment duration varies from one to several weeks depending on severity, with complicated cases requiring extended courses to ensure complete resolution.

Surgical intervention may be necessary for horses developing significant pleural effusion or lung abscessation. Thoracic drainage through chest tubes allows removal of infected fluid accumulation that impairs lung expansion and harbors bacteria. Repeated lavage of the pleural space with sterile saline may help remove fibrin and debris that accumulate in pleuropneumonia. In rare cases of localized lung abscessation, surgical approaches to drain or remove affected lung tissue may be considered when medical management fails.

Supportive care during pasteurellosis treatment includes rest in a well-ventilated, low-dust environment that minimizes airway irritation. Nutritional support maintains body condition during illness, with palatable feeds encouraging intake in horses with reduced appetite. Mucolytics and expectorants may help mobilize and clear respiratory secretions. Environmental humidification can soothe irritated airways. Nursing care including regular temperature monitoring, assessment of appetite and attitude, and observation of respiratory effort tracks treatment response.

Rehabilitation and return to work following pasteurellosis requires adequate time for complete resolution of infection and healing of damaged respiratory tissues. Horses should remain afebrile for at least one week before gradually reintroducing exercise. Return to training should be gradual, with careful monitoring for exercise intolerance, coughing, or recurrence of symptoms that would indicate incomplete recovery. Repeat veterinary examination before returning to full work may include thoracic auscultation, ultrasound, or endoscopy to confirm resolution.

Treatment decision factors include severity of respiratory compromise, presence of complications such as pleuritis or abscessation, response to initial therapy, and practical considerations including facilities for intensive care. Mild cases managed promptly have excellent prognosis and may be treated on an outpatient basis. Severe or complicated cases require intensive care and carry more guarded prognosis. Early aggressive treatment generally produces better outcomes and shorter recovery times than delayed or inadequate initial therapy. Financial considerations should account for potential extended treatment needs in complicated cases.

Recovery timeline for pasteurellosis varies considerably based on initial severity and presence of complications. Uncomplicated upper respiratory infections may resolve within one to two weeks of appropriate treatment, with horses returning to light work soon after completing antibiotic courses. Pneumonia without pleural involvement typically requires three to six weeks of rest and treatment before returning to exercise. Pleuropneumonia or cases complicated by lung abscessation may require months of treatment and rehabilitation, with some horses requiring six months or longer before returning to athletic use.

Post-treatment care and monitoring emphasize ensuring complete resolution before resuming normal activities. Temperature monitoring should continue twice daily until consistently normal for at least one week following antibiotic completion. Appetite and attitude should return fully to normal before exercise resumes. Periodic veterinary rechecks may include repeat thoracic ultrasound to confirm resolution of any pleural abnormalities. Return to exercise should be gradual, with immediate cessation and reevaluation if coughing, nasal discharge, or exercise intolerance recurs.

Prognosis factors influencing recovery include timeliness of treatment initiation, severity of lung involvement at diagnosis, development of complications, and completeness of treatment course. Horses treated early in disease course before significant lung consolidation develops have excellent prognosis for full recovery. Those developing pleuropneumonia have more guarded prognosis, with some studies reporting survival rates around seventy to eighty percent for treated cases. Lung abscessation carries the most guarded prognosis, though many horses still recover with aggressive, prolonged treatment.

Long-term soundness outlook for horses recovering from pasteurellosis depends primarily on the degree of permanent lung damage sustained. Uncomplicated cases typically recover completely with no lasting effects on respiratory function or athletic capacity. Horses recovering from severe pneumonia or pleuropneumonia may have reduced exercise tolerance due to pulmonary scarring or adhesion formation. Some horses develop recurrent airway obstruction following severe respiratory infection. Performance expectations should be adjusted based on respiratory function assessment following recovery, with some horses returning to their previous level of work while others may be limited to lower-intensity activities.

Management practices to prevent pasteurellosis focus on minimizing stress and optimizing conditions that maintain respiratory tract health. Quarantine of new arrivals for two to three weeks allows observation for respiratory illness before introducing horses to established groups. Minimizing transport stress through adequate rest stops, proper ventilation during travel, and allowing recovery time following arrival reduces shipping fever complex risk. Avoiding overcrowding and maintaining appropriate group sizes reduces pathogen transmission pressure.

Nutritional prevention supports immune function through adequate diet providing all essential nutrients. Vitamin C supplementation may support immune function during stress periods including transport and competition. Ensuring adequate water intake maintains respiratory tract hydration and mucociliary function. Avoiding dusty feeds and providing soaked hay reduces airway irritation that may predispose to infection. Body condition maintenance prevents the immunosuppressive effects of both under- and over-nutrition.

Exercise and conditioning programs should build fitness gradually without inducing overtraining stress that impairs immunity. Adequate warm-up and cool-down periods protect respiratory tissues from exercise-induced stress. Avoiding work during extreme environmental conditions including very cold, hot, or dusty conditions reduces respiratory tract irritation. Rest following viral respiratory infection should continue until the horse is fully recovered, as premature return to exercise predisposes to secondary bacterial infection.

Environmental factors significantly influence respiratory health and pasteurellosis risk. Excellent ventilation in stabling prevents accumulation of ammonia, dust, and airborne pathogens while avoiding direct drafts on horses. Low-dust bedding options reduce airway irritation. Arena and paddock management should control dust through watering or appropriate footing choices. Separation of horses by age and health status reduces transmission from carriers to susceptible individuals.

Vaccination and deworming protocols indirectly reduce pasteurellosis risk by preventing conditions that predispose to bacterial infection. Vaccination against equine influenza and equine herpesvirus reduces viral respiratory infections that damage airways and allow secondary bacterial invasion. Maintaining current vaccination status is particularly important before transport or events involving exposure to horses from multiple sources. Regular deworming prevents parasitic damage to tissues and the immunosuppressive effects of heavy parasite burdens. No specific vaccine against equine pasteurellosis is widely available, making prevention of predisposing conditions the primary strategy.

Daily management adjustments for horses recovering from pasteurellosis or those at high risk focus on respiratory health optimization and stress reduction. Providing housing with excellent ventilation ensures fresh air supply without drafts, reducing airborne pathogen concentration and ammonia exposure. Feeding practices should minimize dust through hay soaking or steaming and using low-dust concentrates. Water availability should encourage adequate hydration that maintains respiratory tract moisture. Daily observation for any signs of respiratory distress or illness allows early intervention.

Housing and turnout considerations for horses with pasteurellosis history or current illness prioritize air quality and stress minimization. Indoor housing during recovery should feature low-dust bedding, excellent ventilation, and separation from obviously ill horses. Turnout during recovery should be in clean, low-dust paddocks with shelter from inclement weather. Avoiding turnout with aggressive herd mates reduces stress during the recovery period. Gradual reintroduction to normal housing and turnout routines follows clinical recovery.

Exercise modifications during and after pasteurellosis are essential to allow complete healing without setbacks. Complete rest is indicated during acute illness until fever resolves and appetite returns. Hand walking may begin once acute signs resolve, gradually progressing to light turnout. Mounted exercise should not resume until the horse has been afebrile for at least one week and shows no respiratory symptoms at rest. Return to full work should be gradual over several weeks, with any return of symptoms prompting immediate rest and veterinary reevaluation.

Monitoring and ongoing care for horses recovering from pasteurellosis includes twice-daily temperature checks, observation of respiratory rate and effort at rest, monitoring of appetite and water intake, and assessment of nasal discharge character if present. Recording these parameters helps identify trends indicating improvement or concern. Completing the full prescribed antibiotic course is essential even if the horse appears recovered, as premature discontinuation risks relapse or development of resistant organisms.

Quality of life and use considerations following pasteurellosis depend on recovery completeness and any residual respiratory compromise. Horses recovering fully can return to their previous discipline and level of work without restriction. Those with residual effects may perform best in activities with lower respiratory demands. Avoiding situations that previously triggered illness, such as long-distance transport without adequate rest periods, reduces recurrence risk. Open communication with trainers and riders about any residual limitations helps set appropriate expectations and training goals.

Pasteurellosis does not demonstrate breed predisposition, as this opportunistic infection relates to stress exposure and immune status rather than genetic factors. All horse breeds develop pasteurellosis under appropriate conditions of stress, pathogen exposure, and compromised respiratory defenses. Thoroughbreds, Warmbloods, Quarter Horses, Arabians, draft breeds, ponies, and mixed breeds are all susceptible when exposed following transport, viral infection, or other predisposing situations. No breed-specific prevention measures beyond general good management are indicated.

Use and discipline considerations reveal that horses in activities involving frequent transport face increased pasteurellosis risk regardless of breed. Show horses traveling regularly to competitions encounter repeated stress and pathogen exposure. Racehorses shipped between tracks experience similar risk factors. Sales horses transported to auctions face particularly high risk due to the combination of transport stress and exposure to horses from many sources. Event horses and endurance horses traveling to competitions should receive appropriate rest and monitoring following transport.

Genetic testing and breeding recommendations are not applicable to pasteurellosis as no hereditary component influences susceptibility. Breeding decisions need not account for pasteurellosis history from a genetic standpoint. However, breeding stock that will be transported for breeding purposes should receive appropriate pre-transport care and post-transport monitoring to reduce infection risk. Pregnant mares are particularly important to protect, as high fever from pasteurellosis can cause pregnancy complications. Foals from mares with recent pasteurellosis should receive adequate colostrum intake to ensure passive immunity transfer.

Commonly co-occurring conditions with pasteurellosis include viral respiratory infections that frequently precede and predispose to bacterial invasion. Equine influenza and equine herpesvirus type 1 and 4 damage respiratory epithelium, impairing normal defenses and allowing Pasteurella multiplication. Streptococcal infections may occur simultaneously with pasteurellosis, as both organisms exploit similar predisposing conditions. Shipping fever complex typically involves multiple pathogens including Pasteurella, Streptococcus, and various viruses acting in combination.

Conditions with similar symptoms that must be differentiated from pasteurellosis include other causes of bacterial pneumonia. Streptococcus equi subspecies zooepidemicus pneumonia produces essentially identical clinical signs and may coexist with pasteurellosis. Rhodococcus equi infection in young horses causes similar respiratory symptoms but has characteristic features in foals under six months of age. Strangles produces nasal discharge and fever but characteristically involves lymph node abscessation. Non-infectious conditions including inflammatory airway disease, recurrent airway obstruction, and exercise-induced pulmonary hemorrhage may cause respiratory symptoms requiring differentiation.

Potential complications of pasteurellosis include progression to pleuropneumonia when infection extends to involve the pleural space, producing fluid accumulation, adhesion formation, and more severe illness. Lung abscessation occurs when pockets of infection become walled off, creating difficult-to-treat focal infections. Laminitis may develop secondary to systemic toxemia in severe cases. Thrombophlebitis can complicate intravenous catheter sites used for treatment. Long-term complications may include reduced athletic capacity from pulmonary scarring, chronic airway disease, and exercise intolerance requiring modification of use expectations.