Monitoring Immunosuppressive Therapy in Veterinary Patients

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

• Immunosuppressive therapy necessitates structured monitoring to balance efficacy and dose-dependent toxicity, focusing on verifying adequate drug exposure, detecting subclinical toxicity, and providing objective criteria for dose adjustment.
• Therapeutic drug monitoring, particularly trough concentrations for cyclosporine, is crucial due to variable absorption and a narrow therapeutic index, with interpretation dependent on assay methodology and precise sample timing (e.g., 12 hours post-dose).
• Azathioprine toxicity is primarily hematologic (neutropenia, thrombocytopenia) and hepatic, requiring serial complete blood counts and liver enzyme monitoring, with cats exhibiting increased susceptibility to myelosuppression.
• Glucocorticoid adverse effects are monitored clinically and biochemically, focusing on iatrogenic hyperadrenocorticism, insulin resistance, and protein catabolism, rather than drug concentrations.
• Species differences, particularly in drug metabolism (e.g., slower in cats for azathioprine), necessitate tailored monitoring protocols and a lower threshold for vigilance in feline patients.
• Comprehensive monitoring integrates drug concentrations with clinical response, hematologic parameters (CBC), biochemistry panels, urinalysis, and physical examinations to guide dose adjustments and detect complications like opportunistic infections or nephrotoxicity.

Immunosuppressive therapy in dogs and cats requires structured monitoring to balance therapeutic efficacy against dose-dependent toxicity. This article outlines the scientific basis for monitoring protocols, the pharmacokinetic principles that govern drug level interpretation, and the practical parameters used to detect adverse effects before they become irreversible. It is written for veterinary students and practitioners who need a decision framework for cyclosporine, azathioprine, glucocorticoids, and other immunosuppressive agents used in small animal practice.

The clinical question this article answers is direct: once an immunosuppressive drug has been selected, how does the clinician know whether the dose is adequate, excessive, or failing? Monitoring serves three distinct purposes. First, it verifies that drug exposure reaches a range associated with efficacy. Second, it detects subclinical toxicity before clinical signs develop. Third, it provides objective criteria for dose adjustment when the patient's response is ambiguous. These purposes overlap, but they require different monitoring tools and different sampling schedules.

The evidence base for therapeutic drug monitoring in veterinary medicine draws heavily on human transplantation medicine, where the relationship between blood drug concentrations and clinical outcomes has been studied extensively. The principles established in that setting, including the value of trough concentration measurement and the correlation between exposure and toxicity, inform veterinary practice even where species-specific data are limited. Where veterinary evidence is lacking, this article identifies the gap explicitly instead of extrapolating silently.

At a Glance

Pharmacologic Basis for Monitoring

Immunosuppressive drugs share a common feature that makes monitoring necessary: their therapeutic index is narrow. The dose required to suppress a pathologic immune response approaches the dose that causes unacceptable toxicity. This is true for calcineurin inhibitors such as cyclosporine, antiproliferative agents such as azathioprine and mycophenolate, and the mammalian target of rapamycin (mTOR) inhibitors. Each drug class has a distinct mechanism of action, and each produces a distinct pattern of adverse effects that determines which parameters require surveillance.

Cyclosporine binds to cyclophilin and inhibits calcineurin, blocking the dephosphorylation of nuclear factor of activated T cells (NFAT) and thereby preventing interleukin-2 transcription. The drug's absorption is highly variable between patients and is influenced by food, bile flow, and gastrointestinal disease. This variability means that a fixed milligram per kilogram dose produces unpredictable blood concentrations. Therapeutic drug monitoring is therefore the only reliable method to confirm adequate exposure.

Azathioprine is a prodrug converted to 6-mercaptopurine, which incorporates into DNA and inhibits purine synthesis. Its onset of action is slow, often requiring four to six weeks for full effect. The principal toxicity is bone marrow suppression, particularly neutropenia, which can develop abruptly. Because the drug's metabolite profile varies with individual enzyme activity, particularly thiopurine methyltransferase, hematologic monitoring is mandatory even when the dose appears conservative.

Glucocorticoids act through intracellular receptors that modulate gene transcription across multiple inflammatory pathways. Their adverse effects are dose-dependent and cumulative, and they include iatrogenic hyperadrenocorticism, insulin resistance, and protein catabolism. Unlike cyclosporine or azathioprine, glucocorticoid effects are not typically monitored by blood concentration. Clinical parameters such as body weight, muscle mass, and glucose tolerance provide more useful information.

Pharmacokinetic Principles in Therapeutic Drug Monitoring

The relationship between drug dose, blood concentration, and clinical effect follows principles established in human transplantation pharmacology. For cyclosporine, trough concentration measured immediately before the next dose correlates with the area under the concentration-time curve (AUC), which represents total drug exposure. The same relationship has been described for sirolimus, where trough whole blood concentrations correlate with AUC and with the incidence of rejection in renal transplant recipients. This correlation is the foundation for using a single trough sample instead of serial sampling to guide therapy.

Assay methodology matters when interpreting drug concentrations. Cyclosporine can be measured by immunoassay, which detects both parent drug and metabolites, or by high-performance liquid chromatography with mass spectrometry, which detects only the parent compound. These methods produce different numerical results for the same blood sample. The clinician must know which assay the reference laboratory uses and must interpret the result against the therapeutic range established for that specific assay. Applying a chromatographic assay range to an immunoassay result will produce systematic misreading of drug exposure.

Timing of sample collection is equally critical. A trough sample must be drawn at the same interval after dosing each time, typically 12 hours for twice-daily cyclosporine. Samples drawn at random times after dosing produce concentrations that reflect absorption and distribution phases instead of steady-state exposure, and they cannot be compared to trough-based reference ranges. The clinical history accompanying the sample must state the dose, the route, the time of the last dose, and the time of sampling.

Species Differences in Drug Handling

Dogs and cats differ substantially in their handling of immunosuppressive drugs, and these differences affect monitoring strategy. Cyclosporine absorption is more erratic in cats than in dogs, and oral bioavailability is lower. This means that cats more frequently require dose adjustment based on measured concentrations instead of on clinical response alone. The target trough range for cats is generally lower than for dogs, although published ranges vary by indication and by assay.

Azathioprine metabolism differs between species in ways that are not fully characterized. Cats appear to be more sensitive to the myelosuppressive effects of azathioprine than dogs, and some authors recommend avoiding the drug in cats altogether. When it is used, more frequent hematologic monitoring is warranted. The evidence base for these species differences is largely extrapolated from clinical experience instead of from controlled pharmacokinetic studies, and the clinician should maintain a lower threshold for rechecking blood counts in cats.

The Role of Clinical Response in Monitoring

Drug concentrations provide objective data, but they do not replace assessment of the underlying disease. A cyclosporine trough concentration within the therapeutic range does not guarantee remission of immune-mediated hemolytic anemia, and a concentration below range does not automatically require dose escalation if the patient is responding. Clinical response remains the primary endpoint, and drug levels are interpreted in the context of disease activity.

This principle is particularly important when monitoring is used to justify dose reduction. In human renal transplantation, some patients achieve a state of clinical operational tolerance in which immunosuppression can be withdrawn without rejection. This state is exceptional and is identified retrospectively, not by any single monitoring test. In veterinary medicine, the goal is rarely complete drug withdrawal. The more common scenario is gradual tapering to the lowest dose that maintains remission, guided by disease activity scores and by the absence of drug toxicity.

Limitations of Current Monitoring Approaches

The therapeutic ranges used in veterinary medicine are largely adapted from human transplantation data. The evidence supporting these ranges in dogs and cats is limited, and the ranges vary between reference laboratories and between published sources. A trough concentration that falls outside a published range should prompt clinical evaluation, but it should not be treated as an absolute threshold separating efficacy from failure.

Pharmacodynamic monitoring, which measures the drug's effect on immune function instead of its concentration, remains largely experimental in veterinary medicine. Assays that measure T cell proliferation or cytokine production in response to drug exposure have been described in human medicine, but they are not widely available for clinical use in dogs and cats. Until such assays become practical, concentration monitoring combined with careful clinical and hematologic surveillance remains the standard approach.

Structured Monitoring Protocols for Individual Drugs

Cyclosporine

Cyclosporine monitoring in dogs and cats rests on trough whole blood concentrations measured by high-performance liquid chromatography or immunoassay. For atopic dermatitis in dogs, target trough concentrations generally fall between 300 and 600 ng/mL when the drug is given twice daily, though clinical response may be adequate at lower concentrations in some patients. The MSD Veterinary Manual provides species-specific reference ranges and sampling guidance that should be consulted before interpreting results.

Sampling timing is critical. A true trough sample is collected immediately before the next dose, after a minimum of 5 to 7 days of consistent dosing to reach steady state. Food significantly alters cyclosporine absorption in dogs, so samples should be interpreted with knowledge of whether the drug was given with food. Cats absorb cyclosporine more erratically, and individual variation is substantial enough that a single subtherapeutic trough should prompt a repeat measurement before dose adjustment.

Adverse effect monitoring for cyclosporine includes gastrointestinal signs, gingival hyperplasia, and papillomatosis. Routine biochemistry and complete blood count every 3 to 6 months during chronic therapy detect less common effects such as elevated liver enzymes or mild anemia. In cats, monitoring for opportunistic infections, particularly toxoplasmosis in seropositive individuals, is warranted during prolonged therapy.

Azathioprine

Azathioprine is monitored primarily through hematologic toxicity surveillance instead of drug concentrations. The active metabolite, 6-thioguanine nucleotide, can be measured in humans but is not routinely available or validated for veterinary use. The MSD Veterinary Manual lists azathioprine among drugs requiring periodic blood count monitoring in dogs and cats.

A complete blood count should be performed 7 to 14 days after initiating therapy, then every 2 to 4 weeks for the first 3 months, and every 1 to 3 months thereafter. Myelosuppression, particularly neutropenia and thrombocytopenia, is the principal dose-limiting toxicity. Cats are especially susceptible to severe bone marrow suppression, and some clinicians avoid azathioprine in this species entirely or use it with heightened vigilance.

Liver enzyme monitoring is recommended because azathioprine can cause hepatotoxicity, particularly cholestasis. Biochemistry panels at each hematology check detect early changes. Pancreatitis is a reported adverse effect in dogs and should be considered if vomiting, anorexia, or abdominal pain develops during therapy.

Glucocorticoids

Monitoring glucocorticoid therapy relies on clinical response and adverse effect surveillance instead of drug concentrations. Iatrogenic hyperadrenocorticism is the primary concern during chronic therapy. Physical examination findings such as muscle wasting, alopecia, and abdominal distension should be documented at each recheck.

Biochemistry monitoring includes alkaline phosphatase elevation in dogs, which is expected and not itself an indication to reduce dose. Fasting glucose, cholesterol, and triglycerides may increase. Urinalysis is recommended periodically because glucocorticoid therapy predisposes to urinary tract infection, particularly in dogs. The MSD Veterinary Manual provides guidance on tapering protocols and monitoring intervals for chronic glucocorticoid use.

Mycophenolate and Leflunomide

Mycophenolate mofetil is monitored through complete blood count and gastrointestinal tolerance. Diarrhea is the most common adverse effect and often limits dose escalation. Hematologic monitoring follows a schedule similar to azathioprine, with initial checks at 7 to 14 days and monthly intervals thereafter.

Leflunomide is monitored by measuring the active metabolite teriflunomide in whole blood, with target concentrations typically between 20 and 40 μg/mL in dogs. Sampling is performed after 5 to 7 days of therapy. Gastrointestinal signs and anemia are the principal adverse effects. The MSD Veterinary Manual includes leflunomide among drugs where therapeutic monitoring is available and clinically useful in veterinary patients.

Monitoring Schedule Framework

The schedule above assumes stable disease and no dose changes. Any dose adjustment resets the timeline, with repeat hematology and drug concentration checks 7 to 14 days after the change. Patients receiving combination immunosuppressive therapy require more frequent monitoring than those on a single agent.

Decision Points That Change Monitoring Intensity

Patient status alters the correct monitoring approach. Geriatric patients, those with pre-existing renal or hepatic disease, and patients with marginal bone marrow reserve require more frequent hematologic checks. Cats generally require closer monitoring than dogs for azathioprine and cyclosporine due to species differences in metabolism and toxicity risk.

Available equipment changes the monitoring plan. Practices with in-house analyzers can perform weekly complete blood counts economically. Practices without in-house laboratory capacity may need to extend intervals or use point-of-care devices with acknowledged limitations. Therapeutic drug monitoring for cyclosporine requires an external laboratory in most settings, and sample handling, including EDTA anticoagulation and prompt shipment, affects result reliability.

The American Veterinary Medical Association practice resources address laboratory quality assurance and interpretation of diagnostic tests, which supports decisions about in-house versus referral laboratory monitoring.

Documenting Monitoring Findings

Each monitoring encounter should record the drug, dose, route, and interval since the last dose. For drugs with therapeutic monitoring, the sampling time relative to the last dose must be documented. Laboratory results should be recorded with reference intervals and trended over time instead of interpreted in isolation.

A standardized monitoring log, whether paper or electronic, reduces the risk of missed checks. The log should include columns for date, body weight, drug dose, laboratory values, drug concentration, adverse effects observed, and the clinician's assessment of whether therapy continues unchanged, requires dose adjustment, or should be discontinued. This documentation supports clinical decisions and provides a record of the rationale for dose changes when therapy is adjusted.

Adverse effects should be graded for severity and documented with a clear action plan. Mild transient gastrointestinal signs may warrant continued therapy with symptomatic support, while neutropenia below established thresholds requires dose reduction or drug withdrawal. The Davis-Thompson Foundation pathology resources offer case material that illustrates the histopathologic changes associated with drug toxicity and opportunistic infection, which supports interpretation of biopsy findings when they arise.

Infection Surveillance During Immunosuppression

Opportunistic infection is a recognized complication of immunosuppressive therapy. Clinical signs are the primary trigger for investigation, but routine surveillance is warranted in specific circumstances. Cats receiving cyclosporine should be assessed for Toxoplasma reactivation if they are known seropositive. Dogs receiving multiple immunosuppressive drugs may develop fungal infections, particularly in endemic regions.

The World Organization for Animal Health terrestrial animal health standards address surveillance principles for infectious diseases that are relevant when immunosuppressed patients present with febrile illness of unknown origin. While these standards target population-level surveillance, the diagnostic approach to individual patients should include consideration of regionally endemic pathogens that may emerge during immunosuppression.

Routine vaccination during immunosuppressive therapy is generally avoided, particularly with modified live vaccines. The decision to vaccinate an immunosuppressed patient requires weighing disease risk against the potential for vaccine-associated illness or inadequate response.

Recognized Complications and Early Detection

The principal failure modes in immunosuppressive therapy are infection, myelosuppression, nephrotoxicity, hepatotoxicity, and gastrointestinal injury. Each has a characteriztic temporal profile that determines the appropriate surveillance interval.

Opportunistic infection is the most common serious complication. Detection depends on a low threshold for investigation instead of scheduled screening alone. Patients receiving combination therapy, particularly regimens that pair calcineurin inhibitors with antiproliferative agents, carry the highest risk. Serial physical examination, body temperature measurement, and hematology are the minimum monitoring standard. Thoracic radiography is indicated when fever, cough, or tachypnoea develops, because fungal and protozoal pneumonias may present with minimal auscultatory change.

Myelosuppression is the dominant dose-limiting toxicity for azathioprine, mycophenolate, and leflunomide. The nadir for azathioprine-induced neutropenia typically occurs 7 to 14 days after a dose change, whereas mycophenolate affects leukocyte counts more rapidly. Serial complete blood counts detect this before clinical signs appear. Thrombocytopenia and anemia may develop independently of neutropenia, so all three cell lines require assessment at each visit.

Nephrotoxicity complicates cyclosporine and the mammalian target of rapamycin inhibitors. Cyclosporine reduces glomerular filtration through afferent arteriolar vasoconstriction, a hemodynamic effect that is partially reversible with dose reduction. Serial serum creatinine and symmetric dimethylarginine measurement in cats detect declining function before azotaemia becomes marked. Blood pressure measurement is mandatory, because hypertension may precede biochemical change.

Hepatotoxicity occurs with azathioprine and, less commonly, with cyclosporine. Alanine aminotransferase and alkaline phosphatase should be measured at each monitoring visit. A rising trend, even within the reference interval, warrants dose review.

Gastrointestinal signs, including vomiting, diarrhea, and inappetence, are common with mycophenolate and leflunomide. These are often concentration-independent and may respond to dose splitting or temporary withdrawal. Weight measurement at each visit provides an objective index of tolerance.

Common Errors and Corrective Action

The most frequent error is interpreting a single drug concentration without reference to the sampling time. A trough sample drawn after the dose has been administered produces a falsely elevated result and may prompt an unnecessary dose reduction. Confirm the dosing schedule and the interval between the last dose and blood collection before acting on any value.

A second error is monitoring drug concentration while ignoring clinical and clinicopathological parameters. A therapeutic cyclosporine trough does not exclude infection, hypertension, or renal injury. Conversely, a subtherapeutic concentration in a patient with controlled disease does not automatically mandate dose escalation. The clinical picture governs the decision.

A third error is failing to recheck hematology after a dose adjustment. Any change in azathioprine or mycophenolate dose requires a follow-up complete blood count within 7 to 14 days. Delaying this check until the next scheduled visit risks allowing severe neutropenia to develop undetected.

A fourth error is abrupt discontinuation of glucocorticoids after prolonged therapy. Adrenocortical suppression requires gradual tapering. This principle applies regardless of the reason for withdrawal.

Limitations of the Evidence and Divergent Expert Opinion

The evidence base for therapeutic drug monitoring in veterinary immunosuppression is extrapolated largely from human transplantation. The relationship between trough concentrations and clinical outcomes is better established for some drugs than for others. Sirolimus monitoring, for example, has a proposed relationship between trough concentrations, area under the curve, and rejection incidence, but the strength of this association varies by indication and concomitant therapy. Expert opinion differs on whether routine sirolimus monitoring is mandatory or reserved for patients with suspected toxicity or non-adherence.

There is no consensus on the optimal target trough for cyclosporine in canine atopic dermatitis versus immune-mediated disease. Dermatologic protocols often use lower targets than those recommended for transplantation, and some clinicians monitor clinical response alone. Similarly, the value of routine monitoring for azathioprine is contested because its active metabolite, 6-thioguanine nucleotide, is not measured by standard assays, and the parent drug concentration correlates poorly with effect.

Species differences compound the uncertainty. Cats metabolise azathioprine slowly and are more prone to severe myelosuppression, yet published monitoring protocols for cats remain less detailed than those for dogs. The evidence for leflunomide monitoring is limited to small case series, and target concentrations are not firmly established.

Referral, Consultation, and Reporting

Referral to a specialist is warranted when disease fails to respond despite therapeutic drug concentrations, when adverse effects recur at successively lower doses, or when the diagnosis itself is uncertain. A veterinary clinical pathologist should be consulted for interpretation of complex hematologic changes, particularly when bone marrow suppression is suspected and cytology or biopsy is contemplated.

A veterinary pharmacologist or clinical pharmacologist can assist with dose adjustments in patients with concurrent hepatic or renal disease, where clearance is unpredictable. Therapeutic drug monitoring laboratories should be contacted before sampling to confirm the required sample type, volume, and handling. Whole blood is required for cyclosporine and sirolimus assays, whereas serum is used for mycophenolate.

Regulatory reporting obligations vary by jurisdiction. Suspected adverse drug reactions should be reported to the relevant national pharmacovigilance program, and the AVMA practice resources provide guidance on reporting pathways in the United States. Where zoonotic infection is suspected in an immunosuppressed patient, public health authorities may require notification. International movement of animals receiving immunosuppressive therapy may be restricted, and the World Organization for Animal Health terrestrial animal health standards address disease surveillance and trade-related health certification.

Frequently Asked Questions

How Should I Prioritize Monitoring When Therapeutic Drug Assays Are Unavailable or Cost-Prohibitive?

When drug concentration measurement is not feasible, monitoring shifts to structured clinical and laboratory surveillance. Schedule serial complete blood counts, serum biochemistry panels, and urinalyses at intervals appropriate to the drug's known toxicity profile. For cyclosporine, measure trough levels at least once early in therapy if any assay is accessible, then rely on clinical response and adverse effect tracking. For azathioprine, weekly complete blood counts during the first month remain the standard safeguard against myelosuppression. Document the monitoring limitation explicitly in the medical record and inform the owner that dose adjustments will rely on indirect markers. The MSD Veterinary Manual provides species-specific guidance on expected adverse effects that should direct this laboratory surveillance.

What Is the Minimum Viable Monitoring Protocol for a Dog on Long-Term Cyclosporine When Cost Is a Constraint?

A minimum protocol includes a complete blood count and serum biochemistry panel at baseline, again at four to six weeks, then every three to four months. Measure trough whole blood cyclosporine concentration once steady state is reached, ideally two weeks after initiation or dose change. If the owner cannot afford repeated assays, prioritize one trough measurement to confirm the drug is within the reported therapeutic range, then monitor clinical response and gastrointestinal signs. Track body weight at each visit because dose adjustments follow weight changes. Advise owners that skipping blood work increases the risk of unrecognized infection or renal injury. The Davis-Thompson Foundation pathology resources can help interpret biopsy findings if opportunistic infection is suspected.

How Does Monitoring Differ for a Cat Receiving Immunosuppressive Therapy Compared With a Dog?

Cats metabolize several immunosuppressive drugs more slowly than dogs, so steady state is reached later and dose adjustments should be smaller and less frequent. Cyclosporine absorption in cats is more variable, particularly with food, so trough sampling should be standardized to a consistent feeding schedule. Feline patients require more frequent monitoring of renal parameters because azathioprine and cyclosporine can both affect kidney function. Cats are also more susceptible to certain opportunistic infections, including toxoplasmosis and fungal disease, so serologic or antigen testing may be warranted before and during therapy. The MSD Veterinary Manual notes species-specific contraindications and adverse effect profiles that should guide the monitoring interval.

What Should I Do When a Trough Level Is Above the Therapeutic Range but the Patient Is Clinically Well?

Do not reduce the dose based on a single elevated trough value without confirming the sampling time and assay method. Verify that the sample was drawn immediately before the next dose and that the laboratory uses a validated assay for the species and drug. If the elevation is confirmed, consider whether the patient is experiencing subclinical toxicity such as mild thrombocytopenia or hyperlipidemia. A modest dose reduction with repeat trough measurement in seven to fourteen days is reasonable. If the patient remains clinically stable and no laboratory abnormalities appear, some clinicians continue at the current dose with closer monitoring. The relationship between trough concentrations and toxicity is better established than the relationship between trough concentrations and efficacy, as noted in reviews of sirolimus pharmacokinetic monitoring.

How Should I Document Monitoring Decisions to Support Continuity of Care?

Record the indication for therapy, the drug and formulation, the baseline laboratory values, and the monitoring schedule in the initial entry. For each recheck, document the clinical signs relevant to efficacy, all laboratory results, the current dose in milligrams per kilogram, and the rationale for any dose change. Note the timing of blood sampling relative to drug administration. If a therapeutic drug assay was performed, record the laboratory, assay method, and result. Include a plan for the next monitoring interval and any owner communication about expected adverse effects. The AVMA practice resources offer guidance on medical record standards that support defensible documentation.

How Do I Explain the Importance of Monitoring to an Owner Who Wants to Skip Blood Work?

Frame monitoring as the mechanism that keeps the drug safe, not as an optional extra. Explain that immunosuppressive drugs have a narrow margin between effective and harmful blood concentrations, and that individual animals vary widely in how they absorb and clear these drugs. Use concrete examples: a dog that appears healthy may still develop silent bone marrow suppression or a urinary tract infection. State that skipping blood work means dose adjustments are made without information, which increases the risk of either treatment failure or toxicity. Offer a reduced-cost monitoring plan if finances are the concern. The WOAH terrestrial animal health standards emphasize that responsible therapeutic use includes appropriate surveillance, a principle that applies to companion animal practice as well.

Related Clinical & Scientific Guides

• Hypersensitivity Reactions: Types and Mechanisms
• Therapeutic Decision-Making for Respiratory Infections in Cattle
• Monitoring Fluid Therapy in Critically Ill Veterinary Patients

References and Further Reading

• Harnessing cellular therapeutics for type 1 diabetes mellitus: progress, challenges, and the road ahead.. 2025.
• Monitoring of Human Uterus Transplantation With Cervical Biopsies: A Provisional Scoring System for Rejection.. 2017.
• Sirolimus: the evidence for clinical pharmacokinetic monitoring.. 2005.
• Clinical operational tolerance after renal transplantation: current status and future challenges.. 2010.
• Sirolimus, a new, potent immunosuppressive agent.. 1997.
• Mycobacterium bovis infections in San Diego: a clinicoepidemiologic study of 73 patients and a historical review of a forgotten pathogen.. 1993.
• Davis-Thompson Foundation Veterinary Pathology Resources. Davis-Thompson Foundation.
• MSD Veterinary Manual, Professional Edition. MSD Veterinary Manual.
• American Veterinary Medical Association Practice Resources. American Veterinary Medical Association.

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This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.