Lymphocytosis and lymphadenopathy in a dog arising from two distinct lymphoid neoplasms

A 10-year-old intact male Golden Retriever was presented to The Ohio State University Veterinary Medical Center for acute, non-painful facial swelling of the right mandibular region. On physical examination, the right mandibular swelling was found to represent marked lymphadenopathy of the submandibular lymph node. At this time, marked lymphadenopathy of the prescapular and popliteal lymph nodes was also appreciated. The CBC showed a moderate leukocytosis (38.4 × 10⁹ cells/L, reference interval [RI] 4.8-13.9 × 10⁹ cells/L) characterized by a moderate lymphocytosis (28.4 × 10⁹ cells/L, RI 1.0-4.6 × 10⁹ cells/L). Evaluation of peripheral blood and enlarged prescapular and popliteal lymph nodes revealed two morphologically different populations of homogeneous lymphocytes, with the lymphocyte population in the lymph nodes being distinct from that in the blood smear. Flow cytometry of peripheral blood revealed CD45-, CD5+, CD4-, CD8-, variably CD21+ neoplastic lymphocytes compatible with T-zone lymphocytes due to the absence of CD45 expression. Flow cytometry of the lymph node aspirate indicated a distinct population of CD21+ lymphocytes consistent with a B-cell phenotype along with a smaller proportion of the T-zone lymphocytes observed in the blood confirming the presence of two distinct populations of neoplastic lymphocytes, one involving T cells, and the other involving B cells.     

Differentiation of Chronic Lymphocytic Leukemia in the Horse: A Report of Two Cases

Chronic lymphocytic leukemia (CLL) was diagnosed in two horses: an 18-year-old Quarter Horse gelding that was examined because of edema of the prepuce and ventral abdomen; and a 20-year-old mixed breed gelding that was referred because of lymphocytosis, ventral edema, and weight loss. The first horse had enlarged peripheral lymph nodes and cool nonpainful pitting edema of the ventral abdomen and prepuce. The second horse had enlarged peripheral lymph nodes, cool nonpainful pitting edema of the ventral thorax and cranial ventral abdomen, and a 3/5 holosystolic heart murmur. The diagnosis of CLL was based on increased blood lymphocyte counts and infiltration of marrow and other tissues by lymphocytes. In horse 1, the lymphocytosis persisted for 2 months between initial examination and death. The results of flow cytometric analysis on blood lymphocytes using anti-lymphocyte antibodies suggested that horse 1 had T-cell CLL, and horse 2 had B-cell CLL. In addition, the second horse had a monoclonal gammopathy (IgG), with light-chain proteinuria.     

A case of T-cell chronic lymphocytic leukemia progressing to Richter syndrome with central nervous system involvement in a dog

An 8-year-old neutered Beagle dog was presented with polyuria and polydipsia. Routine clinicopathologic testing showed a significant lymphocytosis and proteinuria. Lymphocytes were of small to intermediate in size with a mature morphology. Infectious disease screening was negative. PCR for antigen receptor gene rearrangements showed a clonal T-cell receptor (TCR) rearrangement consistent with T-cell chronic lymphocytic leukemia (CLL). Bone marrow cytology showed <30% lymphocytes, while the proportion in splenic fine-needle aspirate cytology was considered increased. The dog was initially monitored but started on prednisolone and chlorambucil therapy 2 months later due to worsening clinical signs and progressive lymphocytosis. After an additional 2 weeks, the dog developed multifocal spinal pain and single-node lymphadenomegaly. Cytology of the lymph node showed a monomorphic population of large lymphoblasts consistent with lymphoma. Cytology of a cerebrospinal fluid sample also showed large lymphoblasts. PCR for antigen receptor gene rearrangement at both sites showed a clonal TCR rearrangement of the same molecular size as in the initial leukemic cells. The dog was diagnosed with a transformation of the CLL to Richter syndrome (RS) with involvement of the central nervous system (CNS). Therapy was started with L-asparaginase and an increased dose of prednisolone; however, the dog was euthanized due to progressive clinical signs. To our knowledge, this is the first report of canine RS with direct involvement of the CNS.     

Analytic errors in Sysmex‐generated hematology results in blood from a dog with chronic lymphocytic leukemia

A blood sample from a 14‐year‐old dog was submitted to the veterinary diagnostic laboratory of the University of Milan for marked leukocytosis with atypical cells. A diagnosis of chronic T‐cell lymphocytic leukemia (CLL) was made based on blood smear evaluation and flow cytometric phenotyping. A CBC by Sysmex XT‐2000iV revealed a moderate normocytic normochromic anemia. Red blood cells counted by optic flow cytometry (RBC‐O) resulted in a higher value than using electrical impedance (RBC‐I). The relative reticulocyte count based on RNA content and size was 35.3%, while the manual reticulocyte count was < 1%. The WBC count of 1,562,680 cells/μL was accompanied by a flag. Manual counts for RBC and WBC using the Bürker chamber confirmed the Sysmex impedance results. Finally the manual PCV was lower than HCT by Sysmex. While Sysmex XT can differentiate between RBC and WBC by impedance, even in the face of extreme lymphocytosis due to CLL, RBC‐O can be affected by bias, resulting in falsely increased RBC and reticulocyte numbers. Overestimation of RBC‐O may be due to incorrect Sysmex classification of leukemic cells or their fragments as reticulocytes. This phenomenon is known as pseudoreticulocytosis and can lead to misinterpretation of regenerative anemia. On the other side PCV can be affected by bias in CLL due to the trapping of RBC in the buffy coat, resulting in a pink hue in the separation area. As HGB concentration is not affected by flow cytometric or other cell‐related artifacts it may represent the most reliable variable to assess the degree of anemia in cases of CLL.     

Disseminated histiocytic sarcoma with peripheral blood involvement in a Bernese Mountain dog

Abstract: A 6‐year‐old Bernese Mountain dog was presented with a history of lethargy and weight loss of 2 weeks duration. On physical examination the dog had pale mucous membranes and tachypnea. Ultrasound examination revealed hepatomegaly, splenomegaly, and mesenteric lymphadenomegaly. Results of a CBC included marked normocytic normochromic nonregenerative anemia, marked thrombocytopenia, and moderate leukocytosis with mild neutrophilia and a large population of unclassified round cells (6.2 × 10³/μL). The unclassified cells occasionally were bi‐ or multinucleated and had variably abundant pale basophilic cytoplasm that contained multiple irregular clear vacuoles and occasionally erythrocytes. Fine needle aspirate specimens of the mesenteric lymph nodes and spleen were composed of a population of round pleomorphic cells with the same features as the circulating cells. On flow cytometric analysis of peripheral blood, the unclassified cells expressed CD18, CD45, CD11c, CD1c, and CD14; immunocytochemical analysis of blood smears also indicated the cells were positive for CD1c, CD1a, and CD11c. The dog died a few hours after referral. The histologic interpretation of samples collected from spleen, liver, and lymph nodes was malignant neoplasia of histiocytic origin. Immunohistochemical staining yielded negative results for CD11d, a marker of red‐pulp macrophages, ruling out hemophagocytic histiocytic sarcoma. Based on clinical and pathologic findings, the final diagnosis was disseminated histiocytic sarcoma (DHS) with peripheral blood involvement. To our knowledge, DHS in a dog with evidence and immunophenotyping of neoplastic cells in peripheral blood has been reported only rarely.     

Pelger-Huët anomaly in an Arabian horse

Abstract Abstract: A 9‐year‐old Arabian mare was evaluated for a 7‐day history of malaise. Results of a CBC included a leukocyte concentration within the reference interval (8.4 × 10³/μL, reference interval 6.0–14.0 × 10³/μL) with an apparent degenerative left shift (segmented neutrophils 1.2 × 10³/μL, reference interval 2.5–7.5 × 10³/μL; hyposegmented neutrophils 1.8 × 10³/μL, reference interval 0.0–0.2 × 10³/μL). Serum clinical chemistry results included increased aspartate transaminase, alkaline phosphatase, and gamma‐glutamyltransferase activities. A presumptive diagnosis of hepatitis or cholangiohepatitis was made. The horse was treated with antimicrobials and the malaise quickly resolved. However, in a recheck CBC on day 13, the apparent degenerative left shift remained. Further evaluation of the blood smear revealed many hyposegmented granulocytes with coarse mature chromatin and normal cytoplasmic features. On the basis of the microscopic examination, the horse was diagnosed with Pelger‐Huët anomaly. The patient's offspring was subsequently also diagnosed with Pelger‐Huët anomaly on the basis of blood film examination. Neutrophil, eosinophil, and basophil mean nuclear scores in both affected horses (mare, range 1.5–2.6; offspring, range 1.6–3.2) were lower than those in 2 unrelated Arabian horses (range, 2.8–5.0) and 5 non‐Arabian control horses (range, 2.8–5.0). Results of immunophenotyping and phagocytosis/oxidative burst assays via flow cytometry showed no difference in the expression of myeloid–specific or adhesion molecules or in neutrophil function between affected and control horses. This is the second known report of equine Pelger‐Huët anomaly, both of which affected Arabian horses.     

Cutaneous T‐cell lymphoma – Sézary syndrome in a Boxer

A 7‐year‐old male Boxer with a 3.5‐year history of atopy and food hypersensitivity was presented with multiple poorly circumscribed nodules and maculae of the skin and tongue, and jaundiced mucosal membranes. Cytologic and histopathologic examination of the skin lesions revealed cutaneous epitheliotropic lymphoma. Cells were CD3⁺ and CD8⁺ in flow cytometry. The CBC showed a moderate leukocytosis with 16% atypical lymphocytes with irregularly cleaved nuclei. Flow cytometric phenotyping of peripheral blood showed an elevated proportion of the CD8⁺ T‐lymphocyte subpopulation, indicating a malignant population of T‐cell origin, and the electropherogram of the PCR antigen receptor rearrangement produced a monoclonal peak for TCRγ. Liver enzyme activities were markedly increased and abdominal ultrasound examination showed increased echogenicity of the liver and enlarged abdominal lymph nodes. Fine‐needle aspirates of the liver confirmed infiltration with lymphocytes exhibiting the same morphology as the cells detected in skin and peripheral blood. Treatment was induced with L‐asparaginase, lomustine, and prednisone. Partial clinical remission of the skin and tongue lesions was achieved within 10 days, and hematologic abnormalities resolved. Despite further treatment with L‐asparaginase and lomustine, the dog relapsed within one month and was euthanized. Presence of malignant lymphocytes in skin, peripheral blood, and liver indicate a rare variant of leukemic cutaneous T‐cell lymphoma, equivalent of Sézary syndrome in a dog. This case report describes the use of flow cytometry as a complementary tool for lymphocyte characterization of skin lesions for the first time.     

Pot-bellied Vietnamese pig with visceral mast cell tumors and mastocytemia

An approximately 12-year-old female Vietnamese Pot-Bellied Pig was presented to the Mississippi State College of Veterinary Medicine Food Animal Service for anorexia of 2 days duration. On physical examination, the patient appeared depressed and lethargic with significantly pale mucus membranes, open mouth breathing, and nostril flaring. On abdominal palpation, the abdomen was tense and uncomfortable. A complete blood count (CBC) and chemistry profile were performed. The CBC revealed significant anemia and mild leukocytosis characterized by mild neutrophilia with a left shift. Mast cells were rarely observed. Hematocrit = 8.1% (RI 22-50), RBC = 1.25 × 10⁶/μL (RI 3.6-7.8), WBC = 19.85 × 10³/μL (RI 5.2-17.9), Neutrophils = 15.08 × 10³/μL (RI 0-11.4), and Bands = 0.993 × 10³/μL (RI 0-0.019). The chemistry profile was unremarkable with a mildly elevated BUN and slightly decreased total protein and albumin (BUN = 39 mg/dL [RI 4.2-15.1], total protein = 6.2 g/dL [RI 6.6-8.9], and albumin = 2.5 g/dL [RI 3.6-5.0]). An abdominal ultrasound revealed numerous hypoechoic nodules diffusely scattered throughout the hepatic parenchyma. An FNA of one of the hepatic nodules was performed. A mild suppurative component and numerous variably granulated mast cells were observed. A presumptive cytologic diagnosis of mast cell tumor was made. Histopathology was performed, confirming the cytologic interpretation.     

Hematologic abnormalities and flow cytometric immunophenotyping results in dogs with hematopoietic neoplasia: 210 cases (2002–2006)

Background: Growing interest in veterinary oncohematology has facilitated the recent development and advancement of new techniques, such as flow cytometry, for immunophenotyping hematopoietic neoplasia in animals. Objective: The aim of this retrospective study was to characterize hematologic abnormalities and flow cytometric immunophenotyping (FCI) results in cases of hematopoietic neoplasia in dogs. Methods: Signalment, CBC data, and FCI results were obtained for 210 dogs with blood samples submitted to our laboratory. Immunophenotyping was carried out using an Epics XL‐MCL flow cytometer and a panel of 10 antibodies (CD45, CD3, CD4, CD8, CD79, CD21, CD14, CD34, CD41/61, CD61). The prevalence and severity of hematologic abnormalities was determined for the different types of hematopoietic neoplasms. Results: Based on cell morphology and phenotype, cases were classified as: acute lymphoblastic leukemia (ALL, n=51), acute myeloid leukemia (AML, n=33), chronic lymphocytic leukemia (CLL, n=61), and leukemic high‐grade lymphoma (L‐HGL, n=65). Most cases of ALL (47/51) and L‐HGL (41/65) had a B‐cell phenotype, while most cases of CLL (54/61) had a T‐cell phenotype, with a high prevalence of the large granular lymphocyte subtype (49/61). Anemia was found in 85% of all cases and was significantly more severe in ALL and AML compared with CLL and L‐HGL. Neutropenia was seen in 64–78% of acute leukemias (AML and ALL) in contrast to no cases of CLL and 11% of L‐HGL. Thrombocytopenia was seen in 88–90% of acute leukemias in contrast to 15% of CLL and 40% of L‐HGL. Thrombocytopenia was more prevalent (71% vs 22%) and significantly more severe in T‐cell vs B‐cell L‐HGL. Conclusion: A standard CBC is useful in suggesting the type of hemoproliferative disorder and may also help to predict the phenotype, especially in cases of L‐HGL.     

Treatment of a subchondral cystic-like lesion in the distal scapula with a translesional bone screw in a horse

A 2-year-old French Warmblood stallion was presented for lameness investigation. The diagnostic procedures identified the reason for lameness as being a subchondral cystic-like lesion (SCL) in the glenoid cavity of the scapula. The horse was surgically treated with a translesional 4.5 mm cortical bone screw inserted through the SCL under radiographic guidance. Sixty days after surgery, the horse was sound and the SCL showed significant radiographic signs of healing. Radiographs obtained 180 days after surgery showed further radiographic healing, with the SCL presenting a radiodensity similar to the surrounding bone. At 4 years post-operative follow-up, the horse is sound and competing successfully as a 6-year-old in Concours de Saut International (CSI) competitions.     

Pseudolymphoma in a cat on phenobarbital treatment

A two‐year‐old female spayed domestic shorthair cat was presented with apathy, inappetence and generalised lymphadenomegaly. Anamnestic data included a generalised seizure disorder and phenobarbital treatment started one month before presentation. Routine blood analysis revealed only mild abnormalities and FeLV and FIV tests were negative. Both popliteal lymph nodes were aspirated and cytology was consistent with reactive lymph node hyperplasia. PCR for antigen receptor rearrangement testing diagnosed a polyclonal cell population. In the absence of another cause, lymphadenomegaly was attributed to an adverse drug reaction and phenobarbital was discontinued. The cat's condition improved and lymph node size normalised over the next 10 days. The retrospective diagnosis was phenobarbital‐induced pseudolymphoma.     

Ultrasonographic findings post laryngoplasty in the horse

Laryngoplasty is commonly used to treat laryngeal hemiplegia in Thoroughbred racehorses. Evaluation of the success of the laryngoplasty is traditionally determined using endoscopy. Laryngeal ultrasonography and normal ultrasonographic appearance have been reported in the standing horse, but post‐laryngoplasty and ventriculectomy ultrasonographic evaluation has limited literature coverage. A prospective case series of 10 Thoroughbred racehorses with left laryngeal hemiplegia was examined ultrasonographically and endoscopically prior to 3‐10 days, 30‐50 days, and 6‐12 months after laryngoplasty and ventriculectomy. Anatomical structures and Plica vocalis movements were described and measurements and gradings analyzed by repeated means analysis of variance (P < .05). Postsurgical ultrasonographic visualization of Ventriculus laryngis entrances was possible. The distance between Plica vocalis in exhalation was significantly larger than that during inhalation (P < .05). Pre‐ and postsurgical caudal Basihyoideum and rostral Cartilago thyroidea depth was significantly different in some instances (P < .05). No significant differences in the Muscularis cricoarytenoideus lateralis measurements were found. Complications in the extra‐luminal structures were found in seven horses including soft tissue swelling, seroma, and hematoma. A luminal Plica vocalis abscess and Plica vocalis granuloma were also detected ultrasonographically. Ultrasonography can be used to evaluate the post‐laryngoplasty horse for assessing the success of the procedure, monitoring healing, and detecting complications.     

CONTRAST‐ENHANCED HARMONIC ULTRASONOGRAPHY OF MEDIAL ILIAC LYMPH NODES IN HEALTHY DOGS

Herein, we describe the normal contrast‐enhanced harmonic, color, and power Doppler ultrasonographic characteristics of the medial iliac lymph nodes in healthy dogs. Contrast‐enhanced harmonic ultrasonography of the medial iliac lymph nodes was performed on 14 healthy dogs after intravenous administration of the lipoprotein‐bound inert gas‐filled microbubble contrast media Definity^®. Time–pixel intensity curves were generated for 1‐min postinjection. Quantification of these curves was performed using Philips QLab software. Non‐contrast‐enhanced power and color Doppler examinations were performed in each node to assess vascular patterns subjectively. Normal lymph nodes exhibited a mean contrast wash‐in phase beginning at 6.3 s from the time of injection with mean peak pixel intensity at 12.1 s. Angioarchitecture was best visualized with contrast‐enhanced harmonic ultrasound compared with power and color Doppler. Normal lymph nodes in dogs have a central artery with a centrifugal and uniform branching pattern. Contrast‐enhanced harmonic ultrasonography is a noninvasive examination that demonstrates improved visibility of the intranodal architecture of healthy medial iliac lymph nodes in dogs compared with conventional, non‐contrast‐enhanced Doppler methods that may have future clinical applications.     

B‐cell lymphoma with Mott cell differentiation in a cat

A 12‐year‐old, male castrated Domestic Shorthair cat was presented to Animal Medical Center of Gifu Univeristy with anorexia and vomiting. Physical examination revealed an enlarged left tonsil and right mandibular lymph node (approximately 2–3× the normal size), and a submucosal mass on the right side of the epiglottis (1.5 × 2.0 cm). On computed tomography images, an enlarged left tonsil, and enlarged right mandibular, right pharyngeal, and left and right cervical lymph nodes were observed. Cytologic examination of smears of tonsil and lymph nodes revealed numerous medium‐ to large‐sized neoplastic lymphoid cells, approximately half of which contained one or several light‐blue homogenous globoid cytoplasmic inclusions (5–10 μm), which stained magenta with periodic acid–Schiff (PAS) stain. Histopathologic examination of the left tonsil revealed diffuse proliferation of medium‐ to large‐sized neoplastic lymphoid cells effacing the original lymphoid architecture. Half of the cells contained one or several eosinophilic globoid cytoplasmic inclusions, which stained magenta with PAS and showed positive immunohistochemical reactions for immunoglobulin M (IgM) and λ light chain. Neoplastic lymphoid cells were also CD20⁺, Pax5⁺, and MUM1⁺, and CD3^?. Thus, the neoplastic lymphoid cells expressed a B‐cell immunophenotype, and the globoid cytoplasmic inclusions represented an aberrant IgM λ light chain accumulation, similar to Russell bodies. B‐cell lymphoma with Mott cell differentiation was diagnosed based on cytologic, histopathologic, and immunohistochemical features. This is the first report of B‐cell lymphoma with Mott cell differentiation in a cat.     

B‐cell lymphoma with plasmacytoid differentiation,atypical cytoplasmic inclusions,and secondary leukemia in a dog

A 7‐year‐old male castrated Jack Russell Terrier was presented to the oncology service at the University of California–Davis Veterinary Medical Teaching Hospital for evaluation of suspected lymphoma. The dog had several enlarged lymph nodes and moderate lymphocytosis. Aspirates of an enlarged inguinal lymph node contained a bimorphic population of large immature lymphocytes and smaller cells with plasmacytoid features. Both cell types often contained a single large cytoplasmic inclusion that varied from clear to pale pink to sky blue. Cytologic changes were interpreted as most consistent with lymphoid neoplasia. Based on the predominantly mature cell morphology and some morphologic heterogeneity, the peripheral lymphocytosis was interpreted as most likely reactive in nature. However, the immunophenotype of the cells (CD20+, CD21+, CD79a+, MUM‐1+, and MHCII+) and clonality assays showed that tissue and blood lymphocytes were neoplastic B cells with clonal identity despite their different morphologic appearances. The cytoplasmic inclusions were positive with periodic acid‐Schiff and were immunoreactive for IgM and IgG. By transmission electron microscopy, inclusions consisted of aberrant rough endoplasmic reticulum; a few small Russell bodies were also noted. A final diagnosis of high‐grade B‐cell lymphoma with plasmacytoid differentiation, atypical cytoplasmic inclusions, and secondary leukemia was made. Chemotherapy was initiated, but the dog was euthanized due to severe and uncontrolled seizures 9 months after the initial diagnosis. This case extends the morphologic repertoire of canine plasmacytoid neoplasms and emphasizes their continuum with multicentric lymphoma. This case also demonstrates the need for advanced diagnostic techniques in establishing blood involvement in lymphoma in some instances.     

Controversies in therapy of infections caused by Rhodococcus equi in foals

Pneumonia caused by Rhodococcus equi is one of the most important causes of disease and death in foals. R. equi can also be cultured from a large variety of extrapulmonary sites of infection. In the absence of an effective vaccine, ultrasonographic screening for early detection of pulmonary lesions has become routine practice at many farms endemic for pneumonia caused by R. equi. Consequently, the most frequently recognised form of R. equi infection at such farms is a subclinical form in which foals develop sonographic evidence of peripheral pulmonary consolidation or abscessation without necessarily manifesting clinical signs. Evidence exists that not all foals with ultrasonographic lesions will progress to develop clinical signs, and treating a large proportion of foals based on subclinical ultrasonographic findings has been linked to emergence of macrolide‐ and rifampin‐resistant R. equi at a horse farm. Selectively treating only those foals with larger lesion scores and monitoring foals with daily physical inspections and weekly thoracic ultrasonography offers an approach that could decrease antimicrobial drug use without significantly increasing mortality. Current evidence continues to support the combination of rifampin with a macrolide (azithromycin, clarithromycin or erythromycin) for treating clinical infections caused by R. equi despite recently described pharmacological interactions between these drugs. When infection with a macrolide‐resistant isolate is confirmed, limited effective alternatives exist.     

Comparisons between elastographic stiffness scores for benign versus malignant lymph nodes in dogs and cats

Lymph node evaluation is an important component of oncology staging in dogs and cats, however diagnosis of malignancy currently requires cytology or histopathology. Elastography is an ultrasound technique that allows estimation of tissue stiffness. In people, it has been shown to increase the sensitivity of sonography in detection of metastatic lymph nodes. The objective of this prospective, cross‐sectional study was to determine if elastographic stiffness differs for malignant versus benign canine and feline lymph nodes that were considered abnormal in gray‐scale ultrasound imaging. Animals scheduled for ultrasound‐guided fine needle aspirates of a lymph node at a single center were prospectively enrolled. Elastography was performed by a board‐certified veterinary radiologist (G.S.) prior to tissue sampling. Softness was scored qualitatively as 1–4 (1 = hard; 4 = soft) on an image depicted by a color scale (blue = hard, red = soft). Quantitative analysis was performed using custom‐made software. Fifty‐one lymph nodes were included in the evaluation, 21 had benign and 30 had neoplastic cytology. Benign lymph nodes were softer (median score of 2.5, range 1–4) than malignant lymph nodes (median = 2, range 1–3), and differences were significant in qualitative and quantitative analyses (P < 0.01). Of the lymph nodes with a score of 2 or below (hard), 19/25 (76%) were malignant, of the lymph nodes with a score over 2 (soft), 24/36 (67%) were benign. Findings indicated that elastographic stiffness scores differed between benign and malignant lymph node groups; however overlapping scores for the two groups limited the use of this technique for routine clinical diagnosis of malignancy.     

Predictive value of hypoglycin A and methylencyclopropylacetic acid conjugates in a horse with atypical myopathy in comparison to its cograzing partners

Hypoglycin A (HGA) was detected in blood and urine of a horse suffering from atypical myopathy (AM; Day 2, serum, 8290 μg/l; urine: Day 1, 574, Day 2, 742 μg/l) and in its cograzing partners with a high variability (46–1570 μg/l serum). Over the period of disease, the level of the toxic metabolites (methylencyclopropylacetic acid [MCPA]‐conjugates) increased in body fluids of the AM horse (MCPA‐carnitine: Day 2, 0.246, Day 3, 0.581 μmol/l serum; MCPA‐carnitine: Day 2, 0.621, Day 3, 0.884 μmol/mmol creatinine in urine) and HGA decreased rapidly (Day 3, 2430 μg/l serum). In cograzing horses MCPA‐conjugates were not detected. HGA in seeds ranged from 268 to 367 μg/g. Although HGA was present in body fluids of healthy cograzing horses, MCPA‐conjugates were not detectable, in contrast to the AM horse. Therefore, increasing concentrations of MCPA‐conjugates are supposed to be linked with the onset of AM and both parameters seem to indicate the clinical stage of disease. However, detection of HGA in body fluids of cograzing horses might be a promising step in preventing the disease.