Equine Granulocytic Anaplasmosis

Equine granulocytic anaplasmosis is a tick-borne infectious disease caused by the bacterium Anaplasma phagocytophilum, which specifically targets and infects white blood cells called granulocytes. This condition was previously known as equine ehrlichiosis before the causative organism was reclassified, and it represents one of the most significant tick-transmitted diseases affecting horses in North America and Europe. The bacteria invade neutrophils and other granulocytes, compromising the horse's immune function and causing a range of systemic symptoms that can vary from mild to severe depending on the animal's age and immune status.

Equine granulocytic anaplasmosis occurs in regions where the primary tick vectors are present, particularly areas with populations of Ixodes species ticks including the black-legged tick and the western black-legged tick. The disease is most commonly reported in the northern and western United States, as well as parts of Europe where suitable tick habitats exist. Horses of all breeds and ages can be affected, though the severity of clinical signs tends to be more pronounced in older horses and those with compromised immune systems. Seasonal patterns follow tick activity, with most cases occurring during late fall, winter, and early spring when tick populations are active.

The impact of equine granulocytic anaplasmosis on horse health can be significant, particularly when diagnosis and treatment are delayed. Affected horses typically experience high fever, depression, limb edema, and reluctance to move, which can severely impact performance and daily function. The infection causes temporary immunosuppression by destroying white blood cells, potentially leaving horses vulnerable to secondary infections during the acute phase. Performance horses may require extended rest periods, and the disease can result in significant economic losses due to veterinary costs and lost training or competition time.

Fortunately, equine granulocytic anaplasmosis is highly treatable when diagnosed early, with most horses making complete recoveries following appropriate antibiotic therapy. The disease does not spread directly from horse to horse, as transmission requires a tick vector, making isolation unnecessary for affected animals. Early recognition of clinical signs and prompt veterinary intervention are crucial for optimal outcomes, as delayed treatment can lead to more severe symptoms and prolonged recovery periods. Most horses develop some degree of immunity following infection, though the duration of protection remains unclear and reinfection may occur in subsequent tick seasons.

The primary cause of equine granulocytic anaplasmosis is infection with the obligate intracellular bacterium Anaplasma phagocytophilum, an organism that cannot survive outside of host cells. This bacterium belongs to the family Anaplasmataceae and specifically targets granulocytes, particularly neutrophils, which are essential components of the horse's innate immune defense system. The organism was previously classified as Ehrlichia equi before genetic analysis led to its reclassification, which is why the disease was historically called equine ehrlichiosis. Understanding the bacterial cause is essential for appropriate treatment selection, as standard antibiotics may not effectively penetrate infected cells.

There is no known genetic or breed predisposition to equine granulocytic anaplasmosis, as the disease results entirely from environmental exposure to infected ticks rather than inherited factors. All horses are susceptible to infection when exposed to the bacterium through tick bites, regardless of their breeding or genetic background. However, individual immune responses vary, with some horses experiencing mild or subclinical infections while others develop severe clinical disease. Age appears to be the most significant host factor, with horses over eight years of age typically showing more severe clinical signs than younger animals.

Environmental and management factors play crucial roles in disease transmission and risk. Horses pastured in wooded areas, brushy terrain, or regions with high tick populations face significantly increased exposure risk. Geographic location is a primary determinant, with cases concentrated in tick-endemic areas of the northeastern, upper midwestern, and Pacific coastal regions of North America. Seasonal management practices, such as turnout schedules and pasture maintenance, can influence tick exposure levels. Properties adjacent to wildlife habitats may see higher tick populations, as deer and small mammals serve as reservoir hosts for the bacteria.

Several risk factors increase the likelihood of infection and disease severity. Horses with access to tick-infested pastures during peak tick activity periods face the highest exposure risk. Age remains a significant factor, with older horses generally experiencing more severe symptoms and longer recovery times. Horses that are immunocompromised due to other conditions, stress, or concurrent illness may be more susceptible to developing clinical disease following exposure. Previous infection may provide some protective immunity, though the duration and completeness of this protection are not fully understood.

The disease mechanism involves a complex interaction between the bacterium and host immune cells. When an infected tick feeds on a horse, Anaplasma phagocytophilum organisms are transmitted through tick saliva into the horse's bloodstream. The bacteria then seek out and invade neutrophils and other granulocytes, where they replicate within membrane-bound vacuoles called morulae. This intracellular replication protects the organisms from the host's immune defenses while simultaneously destroying the infected cells. The resulting decrease in functional white blood cells compromises the horse's immune response and triggers systemic inflammatory reactions that produce the characteristic clinical signs.

Early warning signs of equine granulocytic anaplasmosis can be subtle, particularly since horses are prey animals that instinctively mask signs of illness. The initial symptoms often include mild depression, decreased appetite, and a slight reduction in energy levels that may go unnoticed in the early stages. Owners and handlers should be vigilant for any changes in behavior or attitude, especially during tick season in endemic areas. Early detection is crucial for successful treatment, as clinical signs typically become more pronounced within the first few days of infection. A rectal temperature taken at the first sign of behavioral changes may reveal the beginning of fever before other symptoms become apparent.

The most common and characteristic symptoms of equine granulocytic anaplasmosis include high fever, often ranging from 102 to 106 degrees Fahrenheit, which represents one of the earliest and most consistent clinical findings. Affected horses typically display significant depression and lethargy, showing little interest in their surroundings or herd mates. Partial anorexia or complete loss of appetite is frequently observed, with horses refusing grain and sometimes even hay. Limb edema, particularly affecting the lower legs, develops in many cases and may be accompanied by reluctance to move or a stiff, stilted gait. Icterus, or yellowing of the mucous membranes, may be visible in more severely affected horses.

Behavioral changes in horses with equine granulocytic anaplasmosis can be quite pronounced and distressing to observe. Affected horses often stand quietly in corners of stalls or pastures, showing none of their usual alertness or curiosity. Depression may be severe enough that horses are reluctant to lift their heads or respond to stimuli. Some horses display signs of abdominal discomfort that may be mistaken for colic, including looking at their sides and mild pawing. Social horses may become withdrawn and avoid contact with herd mates, while normally independent horses may seek companionship. Changes in normal routines, such as not coming to the gate at feeding time, should prompt immediate investigation.

Physical signs of the disease extend beyond the commonly observed fever and edema. Petechial hemorrhages, appearing as small red spots on the gums, inner surface of the eyelids, or vulvar mucosa, may develop due to thrombocytopenia. Heart rate may be elevated, and some horses develop cardiac arrhythmias during the acute phase. Respiratory rate can increase, and mild nasal discharge may be present in some cases. Muscle stiffness and reluctance to move may be pronounced, with some horses appearing lame or unwilling to walk. Dehydration can develop secondary to decreased water intake and fever.

Symptom progression in equine granulocytic anaplasmosis typically follows a predictable pattern without treatment. Clinical signs usually appear seven to fourteen days after the tick bite that transmitted the infection. Fever develops first, followed within one to two days by depression, appetite loss, and the onset of limb edema. Symptoms typically peak around day three to five of clinical illness, with maximum fever and most pronounced depression. Without treatment, signs may persist for one to two weeks before gradual spontaneous improvement, though recovery without antibiotics is prolonged and complications are more likely.

Emergency symptoms requiring immediate veterinary attention include fever exceeding 105 degrees Fahrenheit, severe depression or recumbency, marked respiratory distress, significant cardiac arrhythmias, or signs of ataxia and neurological impairment. Pregnant mares showing signs of anaplasmosis require urgent care due to potential effects on the developing fetus. Any horse showing signs of severe limb swelling that impairs movement or circulation should receive immediate evaluation. Horses that become recumbent or unable to rise face serious complications and require emergency intervention. Secondary infections may develop due to immunosuppression, presenting as respiratory disease, wound infections, or other opportunistic conditions that require additional treatment.

Physical examination provides important initial information for diagnosing equine granulocytic anaplasmosis, though findings are often nonspecific. Veterinarians typically observe elevated body temperature, often exceeding 103 degrees Fahrenheit, along with depression and reluctance to move. Examination of mucous membranes may reveal pallor, icterus, or petechial hemorrhages depending on disease severity. Limb palpation often identifies ventral edema, particularly in the distal limbs, which may be warm and painful. Cardiac auscultation may detect tachycardia or arrhythmias, while thoracic auscultation helps rule out respiratory involvement. A thorough history including geographic location, recent tick exposure, and time of year helps establish clinical suspicion.

Diagnostic blood tests are essential for confirming equine granulocytic anaplasmosis and assessing disease severity. Complete blood count analysis typically reveals thrombocytopenia, with platelet counts sometimes dropping below 100,000 per microliter, representing one of the most consistent laboratory findings. Leukopenia, particularly affecting neutrophils and lymphocytes, is commonly observed during the acute phase. Mild to moderate anemia may develop in prolonged cases. Serum chemistry panels often show elevated bilirubin levels consistent with jaundice, along with mild elevations in liver enzymes. The hallmark diagnostic finding is the presence of morulae, which are inclusion bodies containing clusters of bacteria visible within neutrophils on blood smear examination, though these may only be present in a small percentage of circulating cells.

Advanced diagnostic testing provides definitive confirmation of Anaplasma phagocytophilum infection. Polymerase chain reaction testing on whole blood samples offers the most sensitive and specific diagnosis, capable of detecting bacterial DNA even when morulae are not visible on blood smears. PCR testing is particularly valuable in early infection before antibody responses develop and in cases where blood smear examination is inconclusive. Serologic testing using indirect fluorescent antibody or ELISA methods can detect antibodies to the organism, though these may not appear until seven to ten days after infection onset. Paired serology samples collected two to three weeks apart showing a fourfold rise in antibody titers provides strong diagnostic evidence in retrospective cases.

Differential diagnosis for equine granulocytic anaplasmosis includes several conditions that can produce similar clinical signs. Equine infectious anemia presents with fever, depression, and edema, though it typically causes more severe anemia and has a different disease course. Equine viral arteritis may produce similar signs including fever, limb edema, and depression. Purpura hemorrhagica causes dramatic limb swelling but typically follows respiratory infection and involves different immunologic mechanisms. Other tick-borne diseases, including Lyme disease and piroplasmosis, should be considered in endemic areas. Immune-mediated thrombocytopenia may produce hemorrhagic signs similar to severe anaplasmosis. Careful clinical evaluation combined with appropriate laboratory testing allows accurate differentiation among these conditions.

Emergency and immediate treatment for equine granulocytic anaplasmosis focuses on initiating appropriate antibiotic therapy as quickly as possible once the diagnosis is suspected or confirmed. Oxytetracycline administered intravenously at a dose of 7 mg/kg once daily represents the gold standard treatment and typically produces rapid clinical improvement. Horses often show reduced fever within 12 to 24 hours of the first antibiotic dose, with progressive improvement in attitude and appetite over the following days. Initial treatment should be administered by a veterinarian who can ensure proper dosing based on accurate body weight estimation and monitor for potential adverse reactions. Horses with severe symptoms may require additional supportive care during the first 24 to 48 hours of treatment.

Medical management of equine granulocytic anaplasmosis centers on completing a full course of antibiotic therapy to eliminate the infection. Intravenous oxytetracycline is typically continued for five to seven days, though the duration may be extended in severe cases or if clinical response is slower than expected. Some practitioners transition to oral doxycycline at 10 mg/kg twice daily after initial improvement with intravenous therapy, which can simplify treatment logistics while maintaining therapeutic effect. Tetracycline antibiotics are the only class with proven efficacy against Anaplasma phagocytophilum, as the intracellular nature of the organism limits effectiveness of many other antibiotics. Treatment should continue for at least 48 hours beyond resolution of clinical signs to ensure complete bacterial elimination.

Surgical intervention is not applicable for equine granulocytic anaplasmosis, as this infectious disease responds entirely to appropriate medical therapy. There are no anatomical abnormalities or structural problems associated with the condition that would require surgical correction. The focus of management remains entirely on eliminating the bacterial infection through antibiotic therapy and supporting the horse through the recovery period. However, horses that develop complications such as severe thrombocytopenia with significant hemorrhage may require more intensive medical intervention including blood transfusions in rare circumstances.

Supportive care plays an important role in managing horses with equine granulocytic anaplasmosis, particularly in moderate to severe cases. Nonsteroidal anti-inflammatory drugs such as flunixin meglumine or phenylbutazone may be administered to reduce fever, alleviate pain, and improve comfort during the acute phase. Intravenous fluid therapy benefits dehydrated horses and those with significantly reduced water intake due to depression. Nutritional support, including offering highly palatable feeds and ensuring fresh water availability, encourages horses to maintain intake during recovery. Stall rest in a comfortable, well-bedded environment reduces stress and energy expenditure. Cold hosing or standing wraps may help manage limb edema and improve comfort.

Rehabilitation and return to work following equine granulocytic anaplasmosis should be gradual to allow complete recovery. Most horses can resume light work within one to two weeks of completing antibiotic therapy, assuming clinical signs have fully resolved and appetite has returned to normal. A progressive exercise program starting with hand walking, progressing to turnout, and then gradually increasing ridden work helps rebuild fitness without overstressing the recovering horse. Monitoring for any return of symptoms during the rehabilitation period is essential, as incomplete treatment could result in relapse. Performance horses may require four to six weeks before returning to full competition level, depending on fitness loss during the illness period.

Treatment decisions should consider several important factors specific to each case. The horse's age and overall health status influence both prognosis and treatment intensity, with older or debilitated horses potentially requiring more aggressive supportive care. Geographic location and likelihood of reexposure may influence discussions about long-term tick prevention strategies. Competition horses require attention to drug withdrawal times, as tetracycline antibiotics have regulated withdrawal periods in many equestrian disciplines. Cost considerations, including the expense of intravenous versus oral therapy, should be discussed with owners to ensure compliance with the full treatment course. Early treatment generally results in faster recovery and fewer complications, emphasizing the importance of prompt veterinary attention when the disease is suspected.

Recovery timeline for equine granulocytic anaplasmosis is generally favorable when appropriate treatment is initiated promptly. Most horses show significant clinical improvement within 24 to 48 hours of starting intravenous oxytetracycline therapy, with fever reduction typically being the first observable response. Complete resolution of clinical signs, including return of normal appetite, resolution of limb edema, and restoration of normal energy levels, usually occurs within five to seven days of treatment initiation. Younger horses and those with mild to moderate disease often recover more quickly than older horses or those with severe symptoms. The total recovery period from diagnosis to return to full work typically ranges from two to four weeks for uncomplicated cases.

Post-treatment care and monitoring are essential components of successful recovery from equine granulocytic anaplasmosis. Daily temperature monitoring should continue for at least one week following completion of antibiotic therapy to ensure fever does not return. Observation of appetite, attitude, and manure production helps confirm continued recovery. Complete blood count analysis one to two weeks after treatment completion can verify normalization of platelet counts and white blood cell numbers. Any return of clinical signs should prompt immediate veterinary reevaluation and potential retreatment. Horses should remain in a low-stress environment during the recovery period, with gradual reintroduction to normal activities.

Prognosis factors for equine granulocytic anaplasmosis are generally positive, with several variables influencing individual outcomes. Horses that receive prompt treatment within the first few days of clinical signs typically make complete recoveries without long-term complications. Age significantly impacts both disease severity and recovery, with horses over eight years potentially experiencing more severe illness and slower recovery. Concurrent health conditions or immunosuppression may complicate recovery and extend the timeline. Compliance with the full antibiotic course is essential, as incomplete treatment may result in relapse or treatment failure. Horses in good body condition and overall health prior to infection generally have better outcomes.

Long-term soundness outlook for horses recovering from equine granulocytic anaplasmosis is excellent in the vast majority of cases. The disease does not cause permanent damage to joints, muscles, or other structures that would impair future athletic function. Horses typically return to their previous level of performance without lasting effects once recovery is complete. Some degree of immunity develops following infection, which may provide protection against future exposure, though reinfection can occur particularly in subsequent tick seasons. Rare complications such as secondary infections acquired during the immunosuppressed phase may extend recovery but generally do not affect long-term soundness. Owners can expect affected horses to have normal working lives following successful treatment of this condition.

Management practices form the foundation of preventing equine granulocytic anaplasmosis by reducing tick exposure. Pasture management including regular mowing, brush clearing, and removal of leaf litter eliminates tick habitat and reduces populations. Avoiding turnout in heavily wooded areas or brushy terrain during peak tick activity seasons significantly decreases exposure risk. Creating buffer zones between wooded areas and pastures using gravel or wood chips can reduce tick migration into grazing areas. Regular inspection of horses for attached ticks, particularly after turnout in high-risk areas, allows prompt removal before disease transmission occurs. Guinea fowl and chickens can help reduce tick populations when maintained in pasture areas.

Nutritional strategies do not directly prevent equine granulocytic anaplasmosis, as the disease results from infectious exposure rather than nutritional deficiency. However, maintaining horses in optimal body condition and nutritional status supports robust immune function that may help minimize disease severity if exposure occurs. Adequate protein, vitamin, and mineral intake ensures the immune system can mount effective responses to pathogens. Antioxidant supplementation may support overall immune health, though no specific supplements have been proven to prevent anaplasmosis. A well-balanced diet appropriate for the horse's age and workload provides the foundation for general disease resistance.

Exercise and conditioning considerations relate primarily to managing tick exposure during training and turnout activities. Trail riding through wooded areas or brushy terrain during tick season increases exposure risk and should be minimized when possible. Post-exercise tick checks should become routine, particularly examining areas where ticks commonly attach including the mane, tail base, chest, and between the hind legs. Keeping horses in well-maintained paddocks or arenas during peak tick activity periods reduces exposure while maintaining fitness. Conditioning programs that strengthen overall health may help horses recover more quickly if infection does occur.

Environmental factors significantly impact disease prevention strategies. Geographic awareness of tick-endemic regions allows horse owners to implement appropriate prevention measures based on local risk levels. Seasonal timing is crucial, with most transmission occurring during late fall, winter, and early spring when Ixodes ticks are most active, contrary to the warm-weather activity of many other tick species. Property assessment to identify high-risk areas such as stone walls, brush piles, and woodland edges guides targeted tick control efforts. Topical tick repellents and permethrin-based products applied to horses can reduce tick attachment when turnout in high-risk areas is unavoidable. Consultation with local agricultural extension services may provide region-specific recommendations.

No vaccine currently exists to prevent equine granulocytic anaplasmosis, making environmental and management-based prevention strategies essential. Routine deworming programs do not affect anaplasmosis risk, as the causative organism is a bacterium transmitted by ticks rather than an intestinal parasite. However, maintaining overall health through appropriate vaccination against other equine diseases ensures horses are not dealing with concurrent infections that might complicate anaplasmosis if it occurs. Annual veterinary examinations provide opportunities to discuss tick-borne disease risks and update prevention strategies based on local conditions. Owners in endemic areas should maintain awareness of the disease and understand early clinical signs to ensure prompt treatment if prevention measures fail.

Daily management adjustments for horses recovering from or previously affected by equine granulocytic anaplasmosis focus on supporting continued health and preventing reinfection. Morning and evening health checks should include temperature monitoring during recovery and observation of attitude, appetite, and manure production. Fresh water must be readily available at all times, as proper hydration supports immune function and overall health. Feeding schedules should provide high-quality forage and appropriately balanced concentrate to maintain optimal body condition. Stress reduction through consistent routines and comfortable housing helps support immune function during recovery. Daily grooming sessions provide opportunities to check for ticks and assess overall health status.

Housing and turnout considerations should account for tick exposure risk, particularly in endemic regions. Stabling during peak tick activity periods, particularly during late fall through early spring, significantly reduces exposure risk. If turnout is necessary, well-maintained pastures away from wooded areas and brush are preferred over natural or wooded paddocks. Run-in sheds and shelter areas should be kept clear of leaf litter and debris that could harbor ticks. Bedding should be changed regularly to maintain cleanliness and reduce stress. Night turnout may be preferable in some situations, though tick activity patterns vary by species and location.

Exercise modifications during and after recovery from equine granulocytic anaplasmosis should follow veterinary guidance based on clinical response. During acute illness, complete rest is essential, with the horse confined to a stall or small paddock. As fever resolves and appetite returns, hand walking can begin, typically within a few days of treatment completion. Turnout in a small paddock allows natural movement while limiting exertion during early recovery. Gradual return to work over two to four weeks prevents setbacks and ensures complete recovery. Horses should be monitored for any return of symptoms during exercise progression, with immediate reduction in activity if concerns arise.

Monitoring and ongoing care for horses in endemic areas requires vigilance even after recovery from infection. Regular tick checks should become part of daily management routines, with careful examination of common attachment sites. Owners should maintain awareness of early disease signs and monitor temperatures if behavioral changes are noted. Follow-up blood work as recommended by the veterinarian confirms normalization of hematologic parameters. Documentation of previous infection may be valuable for future veterinary reference, as some degree of immunity may influence clinical presentation with subsequent exposures. Communication with farm managers and barn staff ensures all caretakers understand monitoring requirements.

Quality of life and use considerations for horses affected by equine granulocytic anaplasmosis are generally excellent following recovery. The disease does not cause permanent limitations or ongoing health problems in successfully treated cases. Horses can return to all previous activities, including high-level competition, without restrictions once recovery is complete. Breeding horses can continue reproductive activities without concern for vertical transmission, as the disease is not spread directly between horses. Living in endemic areas does require ongoing tick prevention attention, but this should not significantly impact quality of life or intended use. With appropriate management and prompt treatment if reinfection occurs, horses in tick-endemic regions can continue to lead full, active lives.

Equine granulocytic anaplasmosis does not demonstrate any breed predisposition, as susceptibility is determined entirely by exposure to infected ticks rather than genetic factors. All breeds of horses, ponies, donkeys, and mules can develop the disease if bitten by ticks carrying Anaplasma phagocytophilum. Thoroughbreds, Quarter Horses, Warmbloods, draft breeds, and pony breeds all show similar susceptibility when exposed to the pathogen. The geographic distribution of cases reflects tick habitat rather than breed populations, with horses of various breeds affected wherever the tick vectors are present. Individual variation in disease severity appears related to age and immune status rather than breed characteristics.

Use and discipline considerations relate primarily to management practices and geographic exposure rather than breed-specific vulnerabilities. Trail horses and those used for hunting or cross-country activities face increased tick exposure due to their work in natural environments. Horses maintained on properties adjacent to wooded areas or wildlife corridors experience higher exposure regardless of their discipline. Competition horses may face additional considerations regarding drug withdrawal times for tetracycline antibiotics used in treatment. Working horses in endemic regions may benefit from topical tick prevention products applied before activities in high-risk areas. Pleasure horses with regular turnout in natural settings require the same prevention attention as performance animals.

Genetic testing is not applicable for equine granulocytic anaplasmosis, as there are no hereditary factors influencing susceptibility. Breeding recommendations do not need to consider anaplasmosis history, as the condition is not transmitted from mare to foal and does not affect reproductive function. Horses that have recovered from the disease can be bred without concern for passing increased susceptibility to offspring. The only breeding-related consideration involves geographic location of breeding operations, with farms in endemic areas needing to implement appropriate tick prevention measures for all horses regardless of breeding status. Mare and foal pairs in endemic regions should receive the same tick prevention attention as other horses on the property.

Several conditions commonly co-occur with or may complicate equine granulocytic anaplasmosis due to shared risk factors or disease effects. Secondary bacterial infections may develop during the immunosuppressed phase when white blood cell counts are low, potentially affecting the respiratory system, skin, or other organs. Lyme disease, caused by Borrelia burgdorferi and transmitted by the same Ixodes tick species, may occur as a co-infection in endemic regions. Stress-related conditions such as gastric ulcers may develop in horses experiencing prolonged fever and reduced appetite. Horses with severe limb edema may develop secondary skin problems if swelling persists or if supportive wraps are improperly applied.

Conditions with similar symptoms that must be differentiated from equine granulocytic anaplasmosis include several infectious and non-infectious diseases. Equine infectious anemia presents with similar signs of fever, depression, and edema but has a different disease course and is caused by a retrovirus rather than bacteria. Equine viral arteritis produces fever, limb edema, and depression through viral infection of blood vessel walls. Piroplasmosis, another tick-borne disease caused by protozoan parasites, can produce fever and anemia with somewhat similar clinical presentation. Purpura hemorrhagica causes dramatic limb swelling but typically follows strangles or other streptococcal infections. Immune-mediated hemolytic anemia may produce similar signs of depression, fever, and icterus.

Potential complications of equine granulocytic anaplasmosis primarily relate to the temporary immunosuppression caused by the disease and delays in treatment. Secondary infections may establish during the period of low white blood cell counts, requiring additional antibiotic therapy beyond treatment for the primary condition. Severe thrombocytopenia can occasionally lead to hemorrhagic complications, though clinically significant bleeding is uncommon. Prolonged fever and reduced intake may result in weight loss and deconditioning that extend the recovery period. Rare cases of myocarditis or cardiac arrhythmias may develop, though permanent cardiac damage is unlikely with appropriate treatment. Pregnant mares may experience complications including abortion, though this appears uncommon when treatment is initiated promptly.