Surveillance for Avian Bornavirus in Colorado and Wyoming, USA, Raptor Populations.

Avian bornavirus (ABV) is known to infect at least 80 avian species and is associated with avian bornaviral ganglioneuritis (ABG). Avian bornaviral ganglioneuritis is characterized by a lymphoplasmacytic infiltration of the nervous tissue, mainly affecting the nerves that supply the gastrointestinal tract of birds. This disease is diagnosed commonly in psittacines under human care and has been demonstrated in wild bird species; however, its occurrence in raptors is largely unknown. Because of the commonality of ABV in the pet bird population, there is concern about the spread of this virus to other companion avian species, such as falconry birds, as well as wildlife. This prospective study used reverse-transcription quantitative PCR (RT-qPCR) to survey free-ranging Colorado and Wyoming, US, raptor populations for ABV. Quantitative PCR was performed on mixed conjunctival-choanal-cloacal swabs collected from live birds (n=139). In dead birds, a combination of mixed swabs (n=265) and tissue samples of the brain (n=258), heart (n=162), adrenal glands (n=162), liver (n=162), kidney (n=139), spinal cord (n=139), and brachial plexus (n=139) were evaluated. All 1,565 swab and tissue samples RT-qPCR results from the 404 birds evaluated were negative. Based on these results and a lack of clinical signs suggestive of ABG, ABV is likely not a prevalent pathogen in Colorado and Wyoming raptor populations at this time.

Detection of skunk adenovirus-1 in domestic ferrets (Mustela putorius).

We describe gross pathology, histopathology, ultrastructure, and molecular characterization of skunk adenovirus 1 (SkAdV-1) in 3 ferrets (Mustela putorius) submitted to the Infectious Diseases Laboratory, Zoo and Exotic Animal Pathology Service at the University of Georgia. Case 1 came from a cohort with a 6-week history of multiple ferrets with respiratory disease and mortality. Cases 2 and 3 were submitted 19 days later; they died 3 days after arrival at the vendor's facility. Histopathological evaluation found evidence of suppurative bronchopneumonia, with intralesional gram-positive cocci in case 1. The others had evidence of subacute multifocal histiocytic and lymphoplasmacytic interstitial pneumonia. Viral isolation, polymerase chain reaction (PCR), and DNA sequencing identified SkAdV-1 in lung tissue. In situ hybridization confirmed adenovirus within the lung lesion, and adenovirus particles were visible under an electron microscope. This is the seventh species in which SkAdV-1 has been identified. Cross-species transmission events have important implications for wildlife management and multispecies facilities.

Naturally occurring psittacid alphaherpesvirus 3 and probable adenovirus coinfection in an Indian ringneck parakeet in the United States.

Psittacid alphaherpesvirus 3 (PsAHV-3) is a rarely reported virus that has been associated with pneumonia in psittacine birds. A 5-mo-old Indian ringneck parakeet (syn. rose-ringed parakeet; Psittacula krameri) was euthanized after developing torticollis and lethargy. Postmortem examination of the parakeet revealed severe pneumonia with syncytial cells containing eosinophilic intranuclear inclusion bodies (INIBs) in the respiratory epithelium. Gene sequencing of total DNA extraction detected PsAHV-3 in the frozen lung. The pancreas had severe lytic necrosis, with the remaining parenchymal cells expanded by large basophilic INIBs. Electron microscopy of the pancreas revealed intranuclear paracrystalline arrays of viral particles morphologically resembling adenovirus. Although we found 3 PsAHV-3 cases in birds in a literature search, we found no cases with adenoviral coinfection.

Angiostrongylus cantonensis Infection in Brown Rats (Rattus norvegicus), Atlanta, Georgia, USA, 2019-2022.

Rat lungworm (Angiostrongylus cantonensis), a zoonotic parasite invasive to the United States, causes eosinophilic meningoencephalitis. A. cantonensis harbors in rat reservoir hosts and is transmitted through gastropods and other paratenic hosts. We discuss the public health relevance of autochthonous A. cantonensis cases in brown rats (Rattus norvegicus) in Atlanta, Georgia, USA.

MANAGEMENT OF ATRIAL FIBRILLATION, VENTRICULAR ARRHYTHMIA, LEFT VENTRICULAR DYSFUNCTION, AND SUBSEQUENT CARDIOPATHOLOGIC EVALUATION IN AN ORANGUTAN (PONGO PYGMAEUS × ABELII).

A 37-yr-old male vasectomized hybrid orangutan (Pongo pygmaeus × abelii) was diagnosed with left ventricular dysfunction during a preventative health care examination. Treatment was initiated with carvedilol. The following year, this orangutan was evaluated for intermittent lethargy. Following observation of an irregular cardiac rhythm during an echocardiogram, a lead II electrocardiogram revealed atrial fibrillation and ventricular arrhythmia. Additional treatment included amiodarone, furosemide, spironolactone, clopidogrel, and aspirin. An improved activity level was noted, and follow-up testing showed restoration of a sinus rhythm, reduced frequency of ventricular arrhythmia, and improved left ventricular function. The orangutan died 27 mon after initial diagnosis of heart disease, and a complete necropsy was performed. This article describes successful diagnosis and management of structural and arrhythmic heart disease in an orangutan, emphasizing the role of cardiac disease screening and behavioral training in apes, as well as the value of matching thorough antemortem and postmortem cardiac evaluation.

Application of environmental sampling to investigate a case of avian chlamydiosis in a pet store and breeding facility leading to mass bird exposures.

Chlamydia psittaci is a bacterium that causes chlamydiosis in birds and can cause zoonotic psittacosis in people. In November 2017, we received notification of a suspected case of avian chlamydiosis in a captive cockatiel (Nymphicus hollandicus) that was sold by an online pet bird retail and breeding facility in Washington State. We describe the investigation with emphasis on how environmental sampling was used to guide veterinary and public health interventions. Bird samples were collected either from pooled droppings, pooled plumage or individual nasal and choanal swabs. Environmental samples were obtained by swabbing cleaning mops, tables and cage structures. All samples were tested by polymerase chain reaction and positive samples underwent genotyping. Approximately 1000 birds representing four taxonomic orders were kept within an open-space warehouse. Eight of 14 environmental samples and one of two pooled faecal samples were positive for Chlamydia spp. The contaminating strain of Chlamydia spp. was identified as genotype A. The facility was closed for environmental disinfection, and all psittacines were treated with oral doxycycline for 45 days. Ten of 10 environmental and two of two pooled faecal samples were negative for C. psittaci 11 months after the completion of environmental disinfection and antimicrobial treatment. This investigation highlights the importance of preventing and mitigating pathogen incursion in an online pet retail and breeding facility. Environmental sampling is valuable to guide animal and public health interventions for control of C. psittaci, particularly when large numbers of birds are exposed to the pathogen.

Hepatic genomic assessment of dietary ingestion of 2-aminoanthracene in Sprague Dawley rats.

This research aims to investigate the effects of 2-aminoanthracene (2-AA), a polycyclic aromatic hydrocarbon (PAH), on the liver. PAH is a by-product of the incomplete combustion of fossil fuels. Specifically, the impact of 2-AA on different body tissues in animals has been reported. The liver is an organ central to the metabolism of PAHs, including 2-AA. Sprague Dawley rats ingested a well-defined dose of 2-AA in their diet (0, 50, and 100 mg/kg 2-AA) for 12 weeks. Hepatic global gene expression using Affymetrix Rat Genome 230 2.0 microarray was performed. Overall, more than 17,000 genes were expressed. Approximately 70 genes were upregulated, while 65 were downregulated when control rats were compared with low-dose animals. Similarly, 103 genes were upregulated and 49 downregulated when the high-concentration 2-AA group was compared with the control group rats. This result suggests that the magnitude of gene expression fold change depends on the dose of 2-AA ingested. Several differentially expressed genes are involved in biological processes such as gene transcription, cell cycle, and immune system function, indicating that the ingestion of 2-AA could impact these processes. The over-expression of genes related to liver inflammation, nonalcoholic liver disease, hepatic glucose processing, and PAH metabolism were noted.

Fatal Systemic Haemosporidiosis in a Free-Ranging Greater Sage-Grouse (Centrocercus urophasianus).

A wild adult Greater Sage-grouse (Centrocercus urophasianus) from northeastern Oregon, US, was found dead, with multiorgan necrosis and inflammation associated with protozoal schizonts and phanerozoites. Molecular characterization identified these protozoa as Plasmodium homopolare H2 and a novel Leucocytozoon sp. lineage. Fatal systemic haemosporidiosis has not been previously reported in Greater Sage-grouse.

Parent and Provider Perspectives on Primary Continuity Intensivists and Nurses for Long-Stay Pediatric Intensive Care Unit Patients.

Rationale: Primary continuity intensivists and nurses for long-stay patients (LSPs) in pediatric intensive care units (PICUs) are understudied strategies used to mitigate the fragmented care of typical rotating care models. Objectives: To investigate the advantages and disadvantages of primary continuity intensivists and nurses for LSPs as perceived by their parents and PICU providers. Methods: We conducted a prospective cross-sectional mixed-methods study of the perspectives of parents whose children were admitted to a PICU for >10 days and had one or more complex chronic conditions regarding the care provided by their PICU intensivists and nurses. As part of a trial, patients had been randomized to care provided by a rotating on-service intensivist who changed weekly and by PICU nurses who changed every 12 hours (usual care group) or to care provided by the same on-service intensivist plus a primary continuity intensivist and primary nurses (primary group). In addition, PICU providers (intensivists, fellows, and nurses) were queried for their perspectives on primary intensivists and nurses. Novel questionnaires, assessed for content and face validity and for readability, were used. The parental questionnaire involved indicating their degree of agreement with 16 statements about their children's care. The provider questionnaire involved rating potential advantages of primary continuity intensivists and nurses and estimating the frequency of disadvantages. Descriptive statistics and divergent stack bar charts were used; parents' and providers' responses were compared, stratified by their children's group (usual care or primary) and provider role, respectively. Results: The parental response rate was 71% (120 completed questionnaires). For 10 of 16 statements, parents whose children had primary continuity intensivists and nurses indicated significantly more positive perceptions of care (e.g., communication, listening, decision making, problems due to changing providers). The provider response rate was 61% (117 completed questionnaires); more than 80% believed that primary intensivists and nurses were highly or very highly beneficial for LSPs. Providers perceived more benefits for patients/families (e.g., informational continuity, facilitating and expediting decision making) than for staff/institutions (e.g., staff satisfaction). Providers reported associated stress, expenditure of time and effort, and decreased staffing flexibility with primary practices. Conclusions: Perceived benefits of primary continuity intensivists and nurses by both parents and providers support more widespread adoption and study of these continuity strategies.

Topical Imiquimod Therapy for Localized Solar Dermatitis in a Dog.

Chronic sun exposure in dogs can result in clinical changes in the skin referred to as solar or actinic dermatitis. A 5-year-old, neutered male, American Bulldog presented with localized unilateral erythematous macules, plaques, alopecia, comedones and hemorrhagic bullae involving the non-pigmented skin on left lateral ventral flank area. The presence of hyperplastic epidermis with keratinocyte dysplasia, superficial dermal elastosis, and multiple follicular cysts with occasional rupture (furunculosis) on histology, together with history and characteristic skin lesions was consistent solar dermatitis/actinic keratosis. Skin scrapings were negative and treatment for secondary pyoderma was initiated with oral clindamycin for 8 weeks. After 2 months of antibiotic therapy, the hemorrhagic bullae resolved; however, erythematous solar/actinic skin lesions with induration and comedones persisted. Topical application of imiquimod 5% cream 3 times weekly for 8 weeks resulted in the resolution of erythema, but some of the non-inflamed comedones remained. Staining for elastin and Ki67 revealed keratinocyte hyperplasia in hair follicle infundibulums and alterations in the elastic fibers around follicles, which may lead to closure and formation of follicular cysts. Imiquimod has been long suggested as a treatment option for solar dermatitis, but this is the first known case report detailing its efficacy in dogs.

Primary nervous system lymphoma in cats.

Here we characterize the neuroanatomic distribution, neuropathology, and immunophenotype of 10 cases of primary nervous system lymphoma in cats. Cases were retrospectively searched from 2 academic institutions. Selected cases were reviewed and subjected to immunohistochemistry (IHC) for CD3, CD20, and Pax5. The mean age of affected cats was 9.1 y, and no sex or breed predilection was observed. The most common clinical sign was ataxia (8 cases). Gross changes reported in 8 cases consisted of white-to-tan masses (7 cases) or swelling (1 case) within the neuroparenchyma (5 cases) or epidural spaces (3 cases). Histologically, intraparenchymal lymphomas occurred in the gray and white matter or perivascular spaces (7 cases); extraparenchymal lymphomas (6 cases) consisted of neoplastic cell infiltration of the perivascular spaces in the leptomeninges, choroid plexus, or epidural spaces. Nerve lymphomas were diffusely infiltrative. Tumors occurred in the brain (4 cases), spinal cord and nerves (3 cases), spinal cord (2 cases), and brain, spinal cord, and nerves (1 case). IHC was consistent with a B-cell lymphoma in 5 cases and with a T-cell lymphoma in 5 cases.

A Single-Centered Randomized Controlled Trial of Primary Pediatric Intensivists and Nurses.

Background: For long-stay patients (LSP) in pediatric intensive care units (PICU), frequently rotating providers can lead to ineffective information sharing and retention, varying goals and timelines, and delayed decisions, likely contributing to prolonged admissions. Primary intensivists (one physician serves as a consistent resource for the patient/family and PICU providers) and primary nurses (a small team of PICU nurses provide consistent bedside care) seek to augment usual transitory PICU care, by enhancing continuity and, potentially, decreasing length of stay (LOS). Methods: A single-centered, partially blinded randomized controlled trial of primary intensivists and nurses versus usual care. PICU patients admitted for or expected to be admitted for >10 days and who had ≥1 complex chronic condition were eligible. A block randomization with 1:1 allocation was used. The primary outcome was PICU LOS. Multiple secondary outcomes were explored. Results: Two hundred LSPs were randomized-half to receive primaries and half to usual care. The two groups were not significantly different in their baseline and admission characteristics. LSPs randomized to receive primaries had a shorter, but non-significant, mean LOS than those randomized to usual care (32.5 vs. 37.1 days, respectively, p = .19). Compared to LSPs in the usual care group, LSPs in the primary group had fewer unplanned intubations. Among LSPs that died, DNR orders were more prevalent in the primary group. Other secondary outcome and balance metrics were not significantly different between the two groups. Conclusion: Primary intensivists and nurses may be an effective strategy to counteract transitory PICU care and serve the distinctive needs of LSPs. However, additional studies are needed to determine the ways and to what extent they may accomplish this.

Subcutaneous and cardiopulmonary dirofilariasis in a dog.

Canine subcutaneous dirofilariasis is rare and typically caused by Dirofilaria repens. An adult male, intact, mixed breed dog from South Carolina, USA, was examined because of physical trauma to the right maxilla. The dog died during hospitalization and was submitted for necropsy. Gross post-mortem changes included approximately 150 adult nematodes morphologically consistent with Dirofilaria spp. in the pulmonary artery, right atrium, and right ventricle. Histologically there was widespread proliferative pulmonary endarteritis with intraluminal nematodes morphologically consistent with Dirofilaria spp. Four similar nematodes were present in the subcutaneous tissue of the left medial thigh and tibial area. These nematodes were located within the fascia and skeletal muscles. They were surrounded by epithelioid macrophages and multinucleated giant cells, with some lymphocytes and plasma cells, and areas of mineralization (interpreted as mineralized cross sections of nematodes). Nematodes were morphologically identified as D. immitis. Subcutaneous dirofilariasis caused by D. immitis occurs when migrating nematode larvae develop into adults in the subcutaneous and skeletal muscle tissues. Key clinical message: Subcutaneous dirofilariasis in dogs is rare and most often associated with Dirofilaria repens. This case highlights the pathology findings of subcutaneous D. immitis in a dog.

Dirofilariose sous-cutanée et cardio-pulmonaire chez un chien. La dirofilariose sous-cutanée canine est rare et généralement causée par Dirofilaria repens. Un chien de race mixte, mâle adulte intact, originaire de la Caroline du Sud, aux États-Unis, a été examiné en raison d'un traumatisme physique au maxillaire droit. Le chien est décédé pendant l'hospitalisation et a été soumis à une nécropsie. Les changements post-mortem macroscopiques incluaient environ 150 nématodes adultes morphologiquement compatibles avec Dirofilaria spp. dans l'artère pulmonaire, l'oreillette droite et le ventricule droit. Sur le plan histologique, il y avait une endartérite pulmonaire proliférative généralisée avec des nématodes intraluminaux morphologiquement compatibles avec Dirofilaria spp. Quatre nématodes similaires étaient présents dans le tissu sous-cutané de la cuisse médiale gauche et de la région tibiale. Ces nématodes étaient situés dans le fascia et les muscles squelettiques. Ils étaient entourés de macrophages épithélioïdes et de cellules géantes multinucléées, avec quelques lymphocytes et plasmocytes, et des zones de minéralisation (interprétées comme des coupes transversales minéralisées de nématodes). Les nématodes ont été identifiés morphologiquement comme D. immitis. La dirofilariose sous-cutanée causée par D. immitis survient lorsque les larves de nématodes en migration se développent en adultes dans les tissus musculaires sous-cutanés et squelettiques.Message clinique clé :La dirofilariose sous-cutanée chez les chiens est rare et le plus souvent associée à Dirofilaria repens. Le cas présent souligne les trouvailles pathologiques associées à la présence sous-cutanée de D. immitis chez un chien.(Traduit par Dr Serge Messier).

Dietary ingestion of 2-aminoanthracene (2AA) and the risk for type-1 diabetes (T1D).

Type 1 diabetes (T1D) is an autoimmune disorder caused by the destruction of insulin-secreting ß-cells.T1D is on the rise around the world. Exposure to polycyclic aromatic hydrocarbons (PAHs) including 2-aminoanthracene (2AA) is considered a contributor to TID increase. The contribution of the ingestion of 2AA toward T1D vulnerability is examined. 2AA is found in a variety of household products. Juvenile male Sprague Dawley rats ingested various amounts of 2AA contaminated diet for 12 weeks. Results showed marginal reduction in body weight gain for the 100 mg/kg treated animals. Glucose tolerance test (GTT) indicated no changes at six weeks. However, at week 12, both treated groups had higher levels of blood glucose than the control group. Serum insulin concentration was elevated in the 50 mg/kg group while reduced in the 100 mg/kg animals. Serum lactate dehydrogenase activity was elevated in treated groups. Evaluation of pancreatic inflammatory cytokines revealed overexpression of IL-1B, IL-6, and IL-7. Apoptotic genes in the pancreas of exposed rats were overly expressed. Histopathology and insulin immunohistochemistry data showed the presence of mesenteric vessels surrounded by lymphocyte and enlarged size of islet cells respectively in the high dose group. These results suggest 2AA ingestion may enhance T1D development.

Sparganosis due to Spirometra sp. (cestoda; Diphyllobothriidae) in captive meerkats (Suricata suricatta).

We report three cases of sparganosis due to plerocercoids of the tapeworm Spirometra sp. in captive meerkats (Suricata suricatta) from a zoo exhibit in the southeastern United States. Two meerkats were euthanized, one due to an uncontrollable seizure and the other due to trauma, and at necropsy cysts containing cestode larvae were observed. A third meerkat had a subcutaneous nodule surgically removed, which contained similar larvae. The third animal died years later, and had numerous cestode larvae in the pleural and peritoneal cavities. The larvae were morphologically identified as plerocercoids of diphyllobothriidean cestodes. On necropsy, multiple nodules, ranging in size from 2.5 to 3.0 cm, were observed in the subcutaneous tissue and muscles. Multifocally, separating skeletal muscle fibers were longitudinal and transversal sections of cestode larva. Histologically, parasitic cysts contained large numbers of neutrophils and macrophages, admixed with proteinaceous material. Molecular and phylogenetic analyses confirmed that specimens from one of the meerkats belonged to the genus Spirometra and was closely related to Spirometra plerocercoids isolated from a snake from the United States and wild felids from South America. Meerkats likely became infected by ingesting infected second intermediate hosts, such as amphibians and reptiles that may have entered the exhibit. Management practices that minimize access of meerkats and other susceptible hosts to intermediate hosts should be implemented.

MYONECROSIS AND DEATH DUE TO PRESUMED MICROCYSTIN TOXICOSIS IN AMERICAN WHITE PELICANS (PELECANUS ERYTHRORHYNCOS).

Over a period of 5 mo, seven out of eight American white pelicans (Pelecanus erythrorhynchos) housed on a spring-fed pond at a zoo died or were euthanized. Clinical signs included inability to stand, anorexia, and weight loss. Clinicopathologic findings included heterophilic leukocytosis and elevated creatine kinase and aspartate aminotransferase. Histopathologic findings on all pelicans demonstrated severe, chronic, diffuse rhabdomyofiber degeneration and necrosis, making vitamin E deficiency a differential diagnosis despite routine supplementation. Based on tissue and pond water assays for the cyanobacterial toxin, microcystin, toxicosis is suspected as the inciting cause of death in these cases. We hypothesize that vitamin E exhaustion and resultant rhabdomyodegeneration and cardiomyopathy were sequelae to this toxicosis.

Cladophialophora encephalitis in an alpaca.

A 4-year-old Huacaya hembra was evaluated for acute neurologic signs including recumbency and a left head tilt. Cranial nerve examination revealed a left ear droop, muzzle deviation to the right, mydriasis of the left eye, an absent menace response, bilateral absent pupillary light reflex when light was directed into the left eye, and bilateral horizontal nystagmus with fast phase to the right. Multifocal intracranial lesions were suspected. Computed tomography revealed an intracranial mass. Postmortem examination, histopathology, and sequencing of a polymerase chain reaction product confirmed a diagnosis of phaeohyphomycotic meningoencephalitis caused by Cladophialophora bantiana. Key clinical message: Advanced diagnostic imaging (computed tomography) was useful in achieving a diagnosis of an intracranial mass in an alpaca with acute neurological signs, later confirmed to be central nervous system (CNS) phaeohyphomycosis. Although uncommon, intracranial fungal infection should be considered as a differential diagnosis in camelid patients exhibiting CNS signs, particularly if they do not respond to initial antimicrobial and anthelmintic therapy.

Encéphalite à Cladophialophora chez un alpaga. Une femelle alpaga de race Huacaya âgée de 4 ans fut évaluée pour des signes neurologiques aigus incluant un décubitus et une inclinaison de la tête à gauche. L'examen des nerfs crâniens a révélé un affaissement de l'oreille gauche, une déviation vers la droite du museau, une mydriase de l'oeil gauche, une absence de réponse à la menace, l'absence bilatérale de réflexe pupillaire lorsqu'une lumière était pointée dans l'oeil gauche, et un nystagmus horizontal bilatéral avec phase rapide vers la droite. Des lésions intra-crâniales multifocales étaient suspectées. Un examen par tomodensitométrie révéla une masse intra-crâniale. L'examen post-mortem, l'histopathologie et le séquençage d'un produit de réaction d'amplification en chaîne par la polymérase confirmèrent un diagnostic de méningo-encéphalite phaeohyphomycotique causée par Cladophialophora bantiana.Message clinique clé :L'examen par imagerie diagnostique de pointe (tomodensitométrie) fut utile afin d'arriver à un diagnostic de masse intra-crâniale chez un alpaga avec des signes neurologiques aigus, plus tard confirmé par une phaeohyphomycose du système nerveux central (CNS). Bien que peu fréquente, une infection fongique intra-crâniale devrait être considérée comme un diagnostic différentiel chez des camélidés présentant des signes du CNS, particulièrement s'ils ne répondent pas à un traitement initial avec des antimicrobiens et des anthelmintiques.(Traduit par Dr Serge Messier).

Neuropathology and diagnostic features of rabies in a litter of piglets, with a brief review of the literature.

Porcine rabies is exceedingly rare worldwide. We describe herein the neuropathology and the diagnostic features of an outbreak of rabies in a litter of piglets attacked by a skunk in Georgia, United States. Rabies viral infection was confirmed in 2 of 3 piglets submitted for testing. Inflammatory and degenerative changes were more prominent in the brainstem and consisted of lymphoplasmacytic meningoencephalitis with glial nodules, neuronal necrosis, and neuronophagia. No viral inclusions (Negri bodies) were observed in multiple sections of brain. A fluorescent antibody test on fresh samples of brainstem and cerebellum was confirmatory for the eastern United States raccoon rabies virus variant. Immunoreactivity for rabies virus was detected across all brain sections in both cases but was more prominent in the thalamic and brainstem nuclei, as well as in the medial lemniscus. Rabies is an important differential diagnosis in pigs with neurologic disease.

Effect of Meperidine on Equine Blood Histamine, Tryptase, and Immunoglobulin-E Concentrations.

Objectives: To evaluate changes in immunological parameters following subcutaneous (SC) and intramuscular (IM) administration of meperidine in horses through quantitative analysis of plasma tryptase, histamine, and IgE levels. Methods: Six adult horses were enrolled in a prospective randomized crossover design. Horses were administered one treatment per day, with a seven day washout period: (a) meperidine 1 mg/kg IM, saline 6 mL SC; (b) saline 6 mL IM, meperidine 1 mg/kg SC; (c) saline 6 mL SC, saline 6 mL IM. Blood samples were obtained for plasmatic histamine (baseline, 5, 10, 15, 30, and 60 min) via LC-MS/MS and plasmatic tryptase (baseline, 15, 30, 60, 120, and 240 min) quantification with enzyme-linked immunoabsorbent assays. Serum immunoglobulin E (IgE) concentrations prior to any meperidine treatment and 7-14 days following the first meperidine treatment were evaluated with enzyme-linked immunoabsorbent assays. Histamine and tryptase concentrations were evaluated with a mixed-effect analysis of variance. The levels of IgE at baseline (before the administration of the first dose of meperidine) were compared with the IgE values at 60 min following the second meperidine administration with the Paired t test. Biopsies of localized injection site reactions from subcutaneous meperidine administration were collected from two horses. Results: No statistically significant elevations from baseline in histamine (p = 0.595), tryptase (p = 0.836), or IgE (p = 0.844) were found in any of the horses in this study. There were no differences between treatment groups. Administration of SC meperidine caused a localized vasculitis and thrombosis with regional edema and hemorrhage. Conclusion: No evidence of anaphylactoid or anaphylactic type reactions occurred following IM or SC meperidine administration.