Equine Tick-Borne Diseases: Diagnosis and Prevention

Horses in tick-endemic regions face exposure to multiple tick-borne pathogens that can cause fever, lethargy, limb edema, ataxia, and anemia. This article covers the major tick-borne diseases affecting horses, diagnostic methods, treatment approaches, and prevention strategies. The information is intended for horse owners and veterinarians managing horses in areas where tick exposure is a known risk.

At a Glance: Equine Tick-Borne Diseases

Major Tick-Borne Diseases in Horses

Lyme Disease (Equine Borreliosis)

Lyme disease in horses is caused by the spirochete bacterium Borrelia burgdorferi, transmitted primarily by Ixodes species ticks. The disease has been documented in horses across North America, Europe, and Asia. Clinical signs can include fever, shifting-leg lameness, joint swelling, uveitis, and neurologic signs such as ataxia or behavioral changes. The condition known as neuroborreliosis in horses involves direct infection of the nervous system and can present with more severe neurologic deficits. A current literature study examining methods of diagnosis and treatment, as well as preventive measures for Lyme disease in horses, provides relevant background on this condition (Lyme Disease in the Horse - A current literature study considering methods of diagnosis and treatment, as well as preventive measures, 2017). The Merck Veterinary Manual offers general information on Lyme disease in horses (Merck Veterinary Manual). The AAEP provides resources for horse owners on tick-borne disease prevention (AAEP).

Diagnosis relies on serologic testing (ELISA with Western blot confirmation) and PCR on blood, synovial fluid, or cerebrospinal fluid. Treatment typically involves prolonged antibiotic therapy, though specific protocols should be determined by a veterinarian based on the individual case. Horse owners should understand that serologic testing cannot distinguish active infection from past exposure, and vaccinated horses will test positive on some assays.

Equine Granulocytic Anaplasmosis

Equine granulocytic anaplasmosis (EGA) is caused by Anaplasma phagocytophilum, a bacterium that infects granulocytes. The disease is transmitted by Ixodes ticks and has been reported in North America, Europe, and other regions. Clinical signs include acute fever, lethargy, limb edema, petechial hemorrhages, ataxia, and reluctance to move. A systematic review and meta-analysis on clinico-pathological findings, diagnosis, and therapeutic management of EGA was published in Veterinary Sciences in 2024 (Equine Granulocytic Anaplasmosis: A Systematic Review and Meta-Analysis on Clinico-Pathological Findings, Diagnosis, and Therapeutic Management, 2024). Case reports have documented EGA in Croatia (A first case of equine granulocytic anaplasmosis in Croatia - A case report, 2017), Saskatchewan (Granulocytic anaplasmosis in a horse from Saskatchewan, 2012), and central West Brazil (Serologic evidence of equine granulocytic anaplasmosis in horses from central West Brazil, 2010). The Veterinary Clinics of North America Equine Practice published a review on equine granulocytic anaplasmosis in 2023 (Equine Granulocytic Anaplasmosis, 2023).

Diagnosis is confirmed by PCR on blood, identification of morulae in granulocytes on blood smear, or seroconversion. PCR for Venezuelan equine anaplasmosis has been described in the literature (Diagnosis of Venezuelan equine anaplasmosis by polymerase chain reaction, 2016). Treatment with oxytetracycline or doxycycline is commonly used, with rapid clinical improvement expected within 24 to 48 hours. Horse owners should monitor for fever and limb edema during tick season and contact a veterinarian promptly if these signs appear.

Equine Piroplasmosis

Equine piroplasmosis is caused by the protozoan parasites Babesia caballi and Theileria equi, transmitted by various tick species including Dermacentor, Rhipicephalus, and Hyalomma. The disease is endemic in tropical and subtropical regions and is reportable in many countries, including the United States. Clinical signs range from subclinical infection to acute fever, anemia, icterus, hemoglobinuria, and death. Chronic carriers can remain infected for years. The Veterinary Clinics of North America Equine Practice published a review on equine piroplasmosis in 2014 (Equine piroplasmosis, 2014). The World Organisation for Animal Health (WOAH) provides international standards for disease surveillance and control (Animal Health and Welfare, WOAH).

Diagnosis involves blood smear examination, PCR, and serology (competitive ELISA). Treatment with antiprotozoal drugs such as imidocarb dipropionate is used, but elimination of T. equi is difficult. Chemotherapy against babesiosis has been reviewed in the veterinary literature (Chemotherapy against babesiosis, 2006). Horse owners importing horses from endemic regions should be aware of testing requirements and quarantine protocols.

Equine Ehrlichiosis (Potomac Horse Fever)

Potomac horse fever is caused by Neorickettsia risticii, a bacterium transmitted by aquatic insects (caddisflies, mayflies) instead of ticks. The disease is most common in horses grazing near rivers and streams in North America. Clinical signs include acute fever, depression, anorexia, diarrhea, colic, and laminitis. Diagnosis is confirmed by PCR on blood or feces, or by serology. Treatment with oxytetracycline is effective if initiated early. The AAEP provides information on Potomac horse fever for horse owners (AAEP).

Other Tick-Borne Diseases of Concern

Tick-borne encephalitis has been documented in domestic animals, including horses, in certain regions of Europe and Asia. A study on tick-borne encephalitis in domestic animals was published in Acta Virologica in 2020 (Tick-borne encephalitis in domestic animals, 2020). Molecular surveillance of tick-borne diseases affecting horses in Poland used PCR to detect multiple pathogens (Molecular surveillance of tick-borne diseases affecting horses in Poland-Own observations, 2021). Molecular characterization of livestock-associated ticks and tick-borne bacteria in Xinjiang, northwestern China, published in Parasites and Vectors in 2025, provides additional context on the diversity of tick-borne pathogens affecting equids (Molecular characterization of livestock-associated ticks and tick-borne bacteria in Xinjiang, northwestern China, 2025). Horse owners in regions where these diseases are reported should discuss regional risks with their veterinarian.

Diagnostic Methods

Serology

Serologic testing detects antibodies against tick-borne pathogens. Common methods include ELISA, indirect fluorescent antibody (IFA) tests, and Western blot for confirmation. Serology is useful for population screening and for horses with clinical signs consistent with tick-borne disease. Limitations include the inability to distinguish active infection from past exposure, and the potential for cross-reactivity between related pathogens. For Lyme disease, serology should be interpreted in conjunction with clinical signs and vaccination history, as vaccinated horses will test positive on some assays.

Polymerase Chain Reaction (PCR)

PCR detects pathogen DNA in blood, synovial fluid, cerebrospinal fluid, or tissue samples. PCR is highly sensitive and specific for active infection. It is the preferred method for diagnosing equine granulocytic anaplasmosis and equine piroplasmosis during the acute phase. PCR can also be used to monitor treatment response. A study on molecular surveillance of tick-borne diseases affecting horses in Poland used PCR to detect multiple pathogens (Molecular surveillance of tick-borne diseases affecting horses in Poland-Own observations, 2021). PCR for Venezuelan equine anaplasmosis has been described in the literature (Diagnosis of Venezuelan equine anaplasmosis by polymerase chain reaction, 2016).

Blood Smear Examination

Blood smear examination allows direct visualization of pathogens within red blood cells (piroplasmosis) or granulocytes (anaplasmosis). This method is rapid and inexpensive but requires experienced personnel and is most sensitive during the acute phase when parasitemia is high. Morulae (intracytoplasmic inclusions) of Anaplasma phagocytophilum can be seen in neutrophils on Giemsa-stained blood smears.

Cerebrospinal Fluid Analysis

For horses with neurologic signs suggestive of neuroborreliosis, cerebrospinal fluid (CSF) analysis may be performed. CSF can be tested for antibodies against Borrelia burgdorferi and for the presence of the pathogen by PCR. CSF analysis also helps rule out other causes of neurologic disease.

Treatment Approaches

Antibiotic Therapy

Antibiotics are the mainstay of treatment for bacterial tick-borne diseases. Doxycycline and oxytetracycline are commonly used for anaplasmosis and ehrlichiosis. For Lyme disease, intravenous or oral doxycycline, or intravenous ceftriaxone, may be used. Treatment duration varies from 7 to 30 days depending on the disease and clinical response. Specific drug doses and withdrawal periods must be determined by a veterinarian based on the horse's weight, clinical condition, and applicable regulations.

Antiprotozoal Therapy

Equine piroplasmosis is treated with antiprotozoal drugs. Imidocarb dipropionate is the most commonly used agent for Babesia caballi and Theileria equi infections. Treatment may require multiple doses, and elimination of T. equi is often incomplete, resulting in persistent carrier status. Chemotherapy against babesiosis has been reviewed in the veterinary literature (Chemotherapy against babesiosis, 2006). Treatment should only be administered under veterinary supervision due to potential adverse effects and regulatory restrictions.

Supportive Care

Supportive care is essential for horses with severe clinical signs. This may include intravenous fluids for dehydrated or anorexic horses, anti-inflammatory drugs for fever and joint pain, and nursing care for recumbent or ataxic horses. Horses with Potomac horse fever may require treatment for laminitis, which is a common complication.

Prevention Strategies

Tick Control

Tick control is the most effective prevention strategy for tick-borne diseases. This includes environmental management (pasture rotation, brush removal, keeping grass short) and application of acaricides to horses. Products containing permethrin, cypermethrin, or fipronil are commonly used. The AAEP provides guidelines on tick control for horses (AAEP). Regular tick checks and prompt removal of attached ticks reduce the risk of pathogen transmission, as most tick-borne pathogens require 24 to 48 hours of attachment before transmission occurs.

Vaccination

Vaccines are available for some tick-borne diseases in horses. A vaccine for Potomac horse fever is available in North America and is recommended for horses in endemic areas. No commercially available vaccine exists for Lyme disease, anaplasmosis, or piroplasmosis in horses. Research on vaccine development continues.

Quarantine and Testing

Horses imported from or traveling to endemic areas should be tested for tick-borne diseases before introduction to a naive population. Equine piroplasmosis is reportable in many countries, and import requirements often include negative serologic tests. The World Organisation for Animal Health (WOAH) provides international standards for testing and movement of horses (Animal Health and Welfare, WOAH).

Practical Implementation Steps for Horse Owners

Step 1: Assess Tick Exposure Risk

Evaluate your horse's environment for tick habitat. Ticks are most active in spring and fall, but can be active year-round in warmer climates. Pastures with tall grass, brush, and wooded edges are high-risk areas. Horses that travel to shows, trails, or boarding facilities may encounter ticks from other regions.

Step 2: Implement Tick Control Measures

Apply veterinary-approved acaricides according to label directions. Rotate pastures and keep grass mowed. Remove brush and leaf litter from fence lines and paddocks. Consider using tick tubes or other environmental control methods. Conduct daily tick checks, especially in spring and fall.

Step 3: Monitor for Clinical Signs

Be alert for fever, lethargy, lameness, limb edema, ataxia, or changes in appetite or behavior. Take and record your horse's temperature daily during tick season. Normal equine temperature is 99 to 101 degrees Fahrenheit (37.2 to 38.3 degrees Celsius). Any temperature above 102 degrees Fahrenheit (38.9 degrees Celsius) warrants veterinary evaluation.

Step 4: Record Observations

Maintain a health log for each horse. Record daily temperature, appetite, manure consistency, and any abnormal findings. Note tick exposure events, travel history, and vaccination status. This information is valuable for your veterinarian when diagnosing tick-borne disease.

Step 5: Consult Your Veterinarian

If you suspect tick-borne disease, contact your veterinarian immediately. Early diagnosis and treatment improve outcomes. Provide your veterinarian with a complete history, including tick exposure, travel, and clinical signs. Follow your veterinarian's recommendations for diagnostic testing and treatment.

Records and Measurements

Health Log Template

Diagnostic Test Results Record

Treatment Record

Common Failure Patterns

Delayed Diagnosis

Failure to recognize early clinical signs of tick-borne disease can lead to delayed diagnosis and more severe illness. Horse owners may attribute fever and lethargy to other causes, such as viral respiratory infection or colic. Regular temperature monitoring and prompt veterinary evaluation of febrile horses are essential.

Incomplete Treatment

Incomplete antibiotic or antiprotozoal treatment can result in persistent infection or relapse. Owners must follow the veterinarian's treatment protocol exactly, including completing the full course of medication even if the horse appears clinically normal. For equine piroplasmosis, carrier status may persist despite treatment.

Inadequate Tick Control

Relying solely on one tick control method is often insufficient. Integrated pest management combining environmental control, acaricide application, and daily tick checks provides the best protection. Failure to treat all horses on the premises can maintain a reservoir of ticks and pathogens.

Misinterpretation of Serology

Serologic testing cannot distinguish active infection from past exposure. A positive serologic test in a horse without clinical signs does not necessarily indicate disease. Conversely, a negative serologic test early in infection does not rule out tick-borne disease. PCR testing is more reliable for diagnosing active infection.

Limitations and Safety Context

Diagnostic Limitations

No single diagnostic test is 100% sensitive or specific for any tick-borne disease. Serology may be negative in early infection (window period) and positive in vaccinated horses. PCR may be negative if the pathogen is not present in the sample collected. Blood smear examination is insensitive when parasitemia is low. A combination of tests and clinical evaluation is often necessary for accurate diagnosis.

Treatment Limitations

Treatment outcomes vary depending on the disease, stage of infection, and individual horse factors. Some horses may not respond fully to treatment, particularly those with chronic or neurologic disease. Equine piroplasmosis carriers may remain infected despite treatment, posing a risk to other horses and complicating international movement.

Regulatory Context

Equine piroplasmosis is a reportable disease in many countries, including the United States, Canada, and Australia. Suspected or confirmed cases must be reported to state or federal animal health authorities. Movement restrictions and quarantine may be imposed. The World Organisation for Animal Health (WOAH) provides international standards for disease reporting and control (Animal Health and Welfare, WOAH).

Professional Escalation Criteria

Contact your veterinarian immediately if your horse shows any of the following signs:

• Fever (temperature above 102 degrees Fahrenheit or 38.9 degrees Celsius)
• Lethargy, depression, or reluctance to move
• Limb edema or swelling
• Lameness or joint swelling
• Ataxia, incoordination, or neurologic signs
• Diarrhea or colic
• Anemia or icterus (yellow mucous membranes)
• Petechiae (small red spots on mucous membranes)
• Hemoglobinuria (red or dark urine)

If your veterinarian is unavailable, contact an equine veterinary emergency service or a veterinary teaching hospital.

Practical Decision Framework for Selecting Diagnostic Tests and Interpreting Results in Suspected Equine Tick-Borne Disease

Selecting the appropriate diagnostic test for a horse with suspected tick-borne disease requires a systematic approach that considers clinical presentation, disease prevalence in the region, timing relative to exposure, and the specific diagnostic limitations of each test method. Horse owners and veterinarians often face uncertainty when interpreting test results, particularly when serology is positive but clinical signs are ambiguous, or when initial testing is negative despite strong clinical suspicion. This section provides a structured decision framework to guide test selection, result interpretation, and subsequent management decisions based on published evidence and clinical experience.

Diagnostic Decision Algorithm Based on Clinical Presentation

The first decision point in the diagnostic pathway is the horse's clinical presentation. Different tick-borne diseases produce characteristic clinical syndromes that can guide initial test selection. For horses presenting with acute fever, lethargy, and limb edema without gastrointestinal signs, equine granulocytic anaplasmosis (EGA) should be a primary consideration. The Veterinary Clinics of North America Equine Practice review on equine granulocytic anaplasmosis from 2023 provides detailed clinical descriptions that support this presentation pattern (Equine Granulocytic Anaplasmosis, 2023). In these cases, whole blood PCR for Anaplasma phagocytophilum is the preferred initial test because it detects active infection with high sensitivity during the acute febrile phase. Blood smear examination for morulae in neutrophils can provide rapid preliminary results, but the systematic review and meta-analysis on EGA published in Veterinary Sciences in 2024 notes that blood smear sensitivity varies with parasitemia levels and examiner experience (Equine Granulocytic Anaplasmosis: A Systematic Review and Meta-Analysis on Clinico-Pathological Findings, Diagnosis, and Therapeutic Management, 2024).

For horses presenting with fever, shifting-leg lameness, joint swelling, or neurologic signs, Lyme disease (borreliosis) should be considered. The current literature study on Lyme disease in horses from 2017 emphasizes that clinical diagnosis is complicated by the nonspecific nature of many signs (Lyme Disease in the Horse - A current literature study considering methods of diagnosis and treatment, as well as preventive measures, 2017). In these cases, serologic testing with ELISA followed by Western blot confirmation is the standard approach, but interpretation requires careful consideration of vaccination history. PCR on synovial fluid or cerebrospinal fluid may be more useful for confirming active infection in horses with localized clinical signs.

Horses presenting with fever, anemia, icterus, and hemoglobinuria, particularly those with recent travel history to tropical or subtropical regions, should be evaluated for equine piroplasmosis. The Veterinary Clinics of North America Equine Practice review on equine piroplasmosis from 2014 describes the classic clinical presentation and emphasizes that acute cases may have detectable parasitemia on blood smear (Equine piroplasmosis, 2014). PCR is the most sensitive method for detecting Babesia caballi and Theileria equi DNA, while competitive ELISA serology is used for screening and regulatory testing.

For horses with fever, diarrhea, and laminitis, particularly those grazing near waterways in North America, Potomac horse fever (equine ehrlichiosis) should be suspected. PCR on blood or feces is the preferred diagnostic method during the acute phase, as seroconversion may not occur until later in the disease course.

Timing Considerations for Test Selection

The timing of sample collection relative to disease onset significantly affects test accuracy. During the first three to seven days of clinical signs, PCR and blood smear examination are most likely to detect active infection. Serologic tests may be negative during this window period because antibody production has not yet reached detectable levels. The molecular surveillance study of tick-borne diseases affecting horses in Poland used PCR to detect multiple pathogens and demonstrated that PCR positivity is highest during acute clinical disease (Molecular surveillance of tick-borne diseases affecting horses in Poland-Own observations, 2021).

After the first week of illness, serologic testing becomes more reliable as antibody titers rise. However, a single positive serologic result cannot distinguish active infection from past exposure. For horses with clinical signs lasting more than two weeks, paired serology (acute and convalescent samples taken two to four weeks apart) can demonstrate a rising antibody titer consistent with active infection. The Merck Veterinary Manual provides general guidance on interpreting serologic results in horses (Merck Veterinary Manual).

For horses with chronic or intermittent clinical signs, PCR may be negative if the pathogen has been cleared from the bloodstream but persists in tissues. In these cases, serology combined with clinical assessment and response to treatment may be the most practical diagnostic approach. The AAEP provides resources for horse owners on understanding diagnostic test limitations (AAEP).

Decision Matrix for Test Selection

The following decision matrix can guide test selection based on clinical presentation and disease duration:

Interpreting Discordant Test Results

Discordant test results occur when different diagnostic methods yield conflicting findings. For example, a horse may have positive serology for Borrelia burgdorferi but negative PCR on blood. This pattern is common and can indicate past exposure instead of active infection, particularly in endemic areas where seroprevalence is high. The Lyme disease literature study from 2017 notes that seropositivity in clinically normal horses is well documented and does not necessarily indicate disease (Lyme Disease in the Horse - A current literature study considering methods of diagnosis and treatment, as well as preventive measures, 2017).

Conversely, a horse with negative serology but positive PCR indicates early active infection before antibody production has occurred. This pattern is most common in the first week of clinical signs and supports a diagnosis of acute tick-borne disease. The systematic review on EGA from 2024 emphasizes that PCR is the most sensitive method for detecting acute Anaplasma phagocytophilum infection (Equine Granulocytic Anaplasmosis: A Systematic Review and Meta-Analysis on Clinico-Pathological Findings, Diagnosis, and Therapeutic Management, 2024).

When blood smear examination is positive but PCR is negative, this may indicate that the pathogen DNA has degraded during sample handling or that the PCR assay targets a different genetic region. The diagnosis of Venezuelan equine anaplasmosis by polymerase chain reaction described in the literature highlights the importance of using validated PCR assays for specific pathogens (Diagnosis of Venezuelan equine anaplasmosis by polymerase chain reaction, 2016).

When serology is positive for multiple pathogens, cross-reactivity should be considered. Some tick-borne bacteria share antigenic epitopes that can cause false-positive results on serologic assays. In these cases, PCR testing for each suspected pathogen can help clarify the diagnosis.

Record System for Diagnostic Decision Making

Maintaining a structured record of diagnostic decisions and results improves clinical management and facilitates communication with veterinarians. The following record template can be used for each suspected tick-borne disease case:

Case Record: Suspected Tick-Borne Disease

Diagnostic Test Selection Record

Test Results and Interpretation

Troubleshooting Common Diagnostic Challenges

Challenge 1: Seropositive horse with no clinical signs

This is a common finding in endemic areas. The horse may have been exposed to the pathogen but successfully cleared the infection, or may be a chronic carrier without clinical disease. For Borrelia burgdorferi, seropositivity in clinically normal horses is well documented. The Merck Veterinary Manual advises that serologic testing should only be performed when clinical signs are present, as positive results in healthy horses can lead to unnecessary treatment (Merck Veterinary Manual). For Theileria equi, seropositive horses without clinical signs may be chronic carriers that can transmit the pathogen to other horses through tick vectors or blood contamination.

Action: Do not treat seropositive horses without clinical signs. Repeat serology in 30 to 60 days to check for rising titers. If the horse remains seropositive and clinically normal, document the finding and monitor for future clinical signs. For piroplasmosis, regulatory testing requirements may apply.

Challenge 2: Clinically affected horse with negative initial testing

This situation can occur when testing is performed too early (before PCR or serology becomes positive), when the wrong test is selected, or when the pathogen is not present in the sample collected. The molecular surveillance study from Poland demonstrated that PCR sensitivity depends on sample type and timing (Molecular surveillance of tick-borne diseases affecting horses in Poland-Own observations, 2021).

Action: Repeat testing 48 to 72 hours later if clinical signs persist. Consider testing a different sample type (e.g., synovial fluid instead of blood for Lyme disease). Add serology if only PCR was performed initially. Consider testing for alternative tick-borne pathogens that may produce similar clinical signs.

Challenge 3: Positive PCR but negative serology

This pattern indicates very recent infection, typically within the first week of clinical signs. The horse has not yet produced detectable antibodies. This is most common with acute anaplasmosis and piroplasmosis.

Action: Start treatment based on PCR results. Repeat serology in two to four weeks to document seroconversion, which confirms the diagnosis and provides a baseline for future reference.

Challenge 4: Positive serology but negative PCR in a clinically affected horse

This pattern can indicate past exposure with current clinical signs caused by a different condition, or active infection where the pathogen has been cleared from the bloodstream but persists in tissues. For Lyme disease, Borrelia burgdorferi may be sequestered in joints or nervous tissue and not detectable in blood.

Action: Consider PCR on affected tissues (synovial fluid, cerebrospinal fluid). Evaluate for other causes of clinical signs. If Lyme disease is strongly suspected based on clinical presentation and response to treatment, a treatment trial may be warranted under veterinary supervision.

Common Failure Patterns in Diagnostic Decision Making

Failure Pattern 1: Testing without clinical indication

Some horse owners request tick-borne disease testing as part of routine health screening or pre-purchase examinations. This practice can lead to discovery of seropositive horses that are clinically normal, causing unnecessary concern and potential treatment. The AAEP advises that testing should be reserved for horses with clinical signs consistent with tick-borne disease (AAEP).

Prevention: Only test horses with clinical signs suggestive of tick-borne disease. Discuss the rationale for testing with your veterinarian before submitting samples.

Failure Pattern 2: Relying on a single test method

Each diagnostic method has limitations. Serology cannot distinguish active from past infection. PCR may be negative if the pathogen is not in the sample. Blood smear is insensitive when parasitemia is low. Relying on a single test can lead to misdiagnosis.

Prevention: Use a combination of test methods appropriate for the clinical presentation and disease duration. Confirm positive results with a second method when possible.

Failure Pattern 3: Interpreting serology without vaccination history

Vaccination for Potomac horse fever produces antibodies that are detected by serologic assays. Similarly, some Lyme disease vaccines used in other species can cross-react with equine serologic tests. Interpreting serology without knowing vaccination history can lead to false attribution of clinical signs to tick-borne disease.

Prevention: Always document vaccination history before interpreting serologic results. If vaccination status is unknown, consider PCR testing to confirm active infection.

Failure Pattern 4: Delaying treatment while awaiting test results

Some tick-borne diseases, particularly equine granulocytic anaplasmosis and Potomac horse fever, respond best to early treatment. Delaying treatment for several days while awaiting PCR or serology results can worsen outcomes.

Prevention: If clinical signs are strongly suggestive of a treatable tick-borne disease, discuss with your veterinarian the option of starting treatment empirically while awaiting test results. The systematic review on EGA from 2024 notes that rapid clinical response to tetracycline antibiotics can be both therapeutic and diagnostic (Equine Granulocytic Anaplasmosis: A Systematic Review and Meta-Analysis on Clinico-Pathological Findings, Diagnosis, and Therapeutic Management, 2024).

Professional Escalation Criteria for Diagnostic Uncertainty

Contact your veterinarian or seek referral to a veterinary teaching hospital or internal medicine specialist if any of the following situations arise:

• Discordant test results that cannot be resolved with additional testing
• Clinically affected horse with persistently negative test results despite repeated testing
• Seropositive horse with progressive neurologic signs
• Suspected piroplasmosis in a region where the disease is not endemic (requires regulatory notification)
• Horse with tick-borne disease that does not respond to appropriate treatment within 48 to 72 hours
• Multiple horses on the same premises developing clinical signs of tick-borne disease
• Horse with tick-borne disease and concurrent immunosuppression or other complicating conditions

The World Organisation for Animal Health provides international standards for reporting and managing reportable tick-borne diseases such as equine piroplasmosis (Animal Health and Welfare, WOAH). Veterinarians should be familiar with local reporting requirements.

Practical Implementation Steps for Horse Owners

Step 1: Recognize Clinical Patterns

Learn to recognize the common clinical presentations of tick-borne diseases in horses. Fever, lethargy, and limb edema suggest anaplasmosis. Fever with lameness and joint swelling suggests Lyme disease. Fever with anemia and icterus suggests piroplasmosis. Fever with diarrhea suggests Potomac horse fever. Record all clinical signs in your health log.

Step 2: Document Disease Duration

Note when clinical signs first appeared. This information helps your veterinarian select the most appropriate diagnostic tests. Acute signs (less than 7 days) are best evaluated with PCR and blood smear. Chronic signs (more than 14 days) may require serology and tissue-specific testing.

Step 3: Gather History

Collect information on tick exposure, travel history, vaccination status, and previous tick-borne disease diagnoses. This context is essential for interpreting test results. The AAEP recommends maintaining accurate health records for all horses (AAEP).

Step 4: Consult Your Veterinarian

Discuss the clinical presentation and history with your veterinarian before testing. Your veterinarian can recommend the most appropriate tests based on regional disease prevalence, current season, and your horse's specific risk factors. The Merck Veterinary Manual provides general guidance on diagnostic approaches for equine tick-borne diseases (Merck Veterinary Manual).

Step 5: Follow Up on Results

After testing, discuss results with your veterinarian and develop a treatment or monitoring plan. Record all test results in your health log for future reference. If treatment is prescribed, complete the full course and monitor for clinical response.

Frequently Asked Questions

What are the most common tick-borne diseases in horses?

The most common tick-borne diseases affecting horses include Lyme disease (borreliosis) caused by Borrelia burgdorferi, equine granulocytic anaplasmosis caused by Anaplasma phagocytophilum, equine piroplasmosis caused by Babesia caballi and Theileria equi, and equine ehrlichiosis (Potomac horse fever) caused by Neorickettsia risticii. The geographic distribution of these diseases varies, with Lyme disease and anaplasmosis more common in temperate regions and piroplasmosis more common in tropical and subtropical areas.

How is Lyme disease diagnosed in horses?

Lyme disease in horses is diagnosed through a combination of clinical signs, history of tick exposure, and laboratory testing. Serologic testing (ELISA with Western blot confirmation) detects antibodies against Borrelia burgdorferi. PCR testing on blood, synovial fluid, or cerebrospinal fluid can detect the pathogen's DNA. For horses with neurologic signs, cerebrospinal fluid analysis may be performed. Diagnosis should be made by a veterinarian, as serologic results must be interpreted in the context of vaccination history and clinical findings.

What is equine granulocytic anaplasmosis?

Equine granulocytic anaplasmosis (EGA) is a tick-borne disease caused by the bacterium Anaplasma phagocytophilum. It is transmitted by Ixodes ticks and causes acute fever, lethargy, limb edema, petechiae, and ataxia. Diagnosis is confirmed by PCR on blood or identification of morulae in granulocytes on blood smear. Treatment with oxytetracycline or doxycycline typically results in rapid clinical improvement. EGA has been reported in North America, Europe, and other regions.

What is equine piroplasmosis and why is it reportable?

Equine piroplasmosis is a tick-borne disease caused by the protozoan parasites Babesia caballi and Theileria equi. It is reportable in many countries because it can cause significant disease, infected horses can become chronic carriers, and it can be transmitted through blood transfusion or contaminated equipment. The disease is endemic in tropical and subtropical regions. Import requirements often include negative serologic tests. The World Organisation for Animal Health (WOAH) provides international standards for testing and control (Animal Health and Welfare, WOAH).

Can horses get neuroborreliosis?

Yes, neuroborreliosis in horses is a manifestation of Lyme disease where the spirochete Borrelia burgdorferi infects the nervous system. Clinical signs can include ataxia, behavioral changes, head tilt, and other neurologic deficits. Diagnosis is challenging and may require cerebrospinal fluid analysis. Treatment with intravenous antibiotics is typically required. Neuroborreliosis is considered a severe form of Lyme disease and warrants immediate veterinary attention.

How can I prevent tick-borne diseases in my horse?

Prevention of tick-borne diseases involves integrated tick control measures. Apply veterinary-approved acaricides to horses according to label directions. Manage the environment by keeping grass short, removing brush and leaf litter, and rotating pastures. Conduct daily tick checks and remove attached ticks promptly. For horses in endemic areas, consider vaccination for Potomac horse fever. Quarantine and test new horses before introducing them to your herd. Consult your veterinarian for a prevention plan tailored to your region and horse's risk factors.

What should I do if I find a tick on my horse?

Remove the tick promptly using fine-tipped tweezers. Grasp the tick as close to the skin as possible and pull upward with steady, even pressure. Do not twist or jerk, as this may cause the mouthparts to break off. Clean the bite area with soap and water or an antiseptic. Monitor the horse for signs of illness over the following weeks. Record the date and location of the tick bite in your health log. If the horse develops fever, lameness, or other signs, contact your veterinarian.

When should I call a veterinarian for a suspected tick-borne disease?

Call a veterinarian immediately if your horse shows fever (temperature above 102 degrees Fahrenheit or 38.9 degrees Celsius), lethargy, limb edema, lameness, ataxia, diarrhea, colic, anemia, icterus, petechiae, or hemoglobinuria. Early diagnosis and treatment improve outcomes for tick-borne diseases. If your regular veterinarian is unavailable, contact an equine emergency service or veterinary teaching hospital. Provide a complete history including tick exposure, travel, and clinical signs.

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• Diagnostic Medical Sonography Programs

References and Further Reading

• aaep.org
• www.merckvetmanual.com
• www.acvim.org
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• Molecular surveillance of tick-borne diseases affecting horses in Poland-Own observations.. Veterinary medicine and science, 2021.
• Equine piroplasmosis.. The Veterinary clinics of North America. Equine practice, 2014.
• Tick-borne encephalitis in domestic animals.. Acta virologica, 2020.
• Equine Granulocytic Anaplasmosis.. The Veterinary clinics of North America. Equine practice, 2023.
• Chemotherapy against babesiosis.. Veterinary parasitology, 2006.
• Molecular characterization of livestock-associated ticks and tick-borne bacteria in Xinjiang, northwestern China.. Parasites & vectors, 2025.
• LYME DISEASE IN HORSES. 2009.
• Lyme Disease in the Horse - A current literature study considering methods of diagnosis and treatment, as well as preventive measures. 2017.
• A first case of equine granulocytic anaplasmosis in Croatia - A case report. Veterinarski Arhiv, 2017.
• Equine Granulocytic Anaplasmosis: A Systematic Review and Meta-Analysis on Clinico-Pathological Findings, Diagnosis, and Therapeutic Management. Veterinary Sciences, 2024.
• Diagnosis of Venezuelan equine anaplasmosis by polymerase chain reaction. Revista Cientifica De La Facultad De Ciencias Veterinarias De La Universidad Del Zulia, 2016.
• Equine granulocytic anaplasmosis. Journal of Equine Veterinary Science, 2013.
• Serologic evidence of equine granulocytic anaplasmosis in horses from central West Brazil. Revista Brasileira De Parasitologia Veterinaria, 2010.
• Granulocytic anaplasmosis in a horse from Saskatchewan. Canadian Veterinary Journal, 2012.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.