Chronic wasting disease (CWD) and scrapie are transmissible spongiform encephalopathy diseases caused by prions, infectious forms of the prion protein. Currently, immunohistochemistry (IHC) is the sole approved diagnostic method for confirming these prion infections in formalin-fixed tissues. Evaluation of prion IHC requires specially trained veterinary pathologists to assess multiple quality control parameters as well as characteristic chromogen immunolabeling patterns. This manual slide review creates a significant bottleneck for laboratories needing to rapidly scale up surveillance during periods of increased testing demand. Given the repetitive and standardized nature of prion IHC slide review, this assay represents an ideal candidate for computer-assisted diagnostics. To address this challenge, we developed a deep learning-based image analysis approach tailored to review slides from large-scale veterinary prion disease surveillance. Our training dataset included 143 prion IHC whole-slide images containing a total of 3296 annotations. Annotated images were segmented into nonoverlapping tiles and used to fine-tune a pretrained convolutional neural network, enhancing the model's ability to recognize prion-specific quality control parameters and labeling features. When tested on a separate, blinded testing dataset of 50 CWD IHC slides, the model achieved 100% concordance for chromogenic labeling when compared with evaluation by a trained veterinary pathologist. The overarching objective of this project is to automate the initial review of prion IHC slides using deep learning-based image analysis to substantially reduce the time needed for evaluation. Implementation of this technology should enhance diagnostic consistency, improve efficiency, and provide scalable capabilities essential for comprehensive prion surveillance throughout the veterinary diagnostic laboratory network.

Lysosomal storage diseases (LSDs) are a diverse group of inherited disorders characterized by the accumulation of undegraded macromolecules within lysosomes, leading to cellular dysfunction and progressive organ damage. Neuronal ceroid lipofuscinosis (NCL) is a subset of LSDs defined by the accumulation of autofluorescent lipopigments in neurons and, in some cases, peripheral tissues. While naturally occurring LSDs have been documented in domestic and laboratory species, reports in nonhuman primates (NHPs) remain rare. This study describes an LSD in 5 related rhesus macaques (Macaca mulatta) at the California National Primate Research Center, with clinical and histopathologic features supportive of NCL. Affected animals exhibited progressive cognitive and motor deficits, along with an atypical musculoskeletal phenotype including coarse facial features. Histologic examination revealed neuronal cytoplasmic vacuolation, primarily in the cerebellum and brainstem, with storage material exhibiting autofluorescence, positive staining with Sudan black, and luxol fast blue, and a positive periodic acid-Schiff reaction. Transmission electron microscopy identified structures resembling curvilinear inclusions and granular osmiophilic deposits, supporting a NCL diagnosis. This report expands the spectrum of NCL presentations in NHPs, reinforcing their potential as an alternative model for LSD research. The identification of this naturally occurring NCL model in NHPs may enhance our understanding of translational models for therapeutic strategies.

Carcinomas have been reported to metastasize to the globe, specifically the uvea; however, the distinguishing histologic features of intraocular metastatic carcinomas in cats are not well-documented. Eleven cases of feline ocular biopsies with intraocular metastatic carcinomas and history of a pulmonary mass submitted to the Comparative Ocular Pathology Laboratory of Wisconsin from 2019 to 2025 were collected and reviewed. Alcian blue stains and immunohistochemical markers for thyroid transcription factor-1 and vimentin were evaluated. No significant sex or breed predispositions were identified. Uveitis, glaucoma, and the presence of an ocular mass were the most common clinical reasons for enucleation. All cases had a thoracic mass identified by radiographs prior to enucleation. All neoplasms had carpeting to gland-forming growth patterns lining surfaces of intraocular structures, affecting the anterior uvea. Goblet cells were highlighted with Alcian blue staining in 7/11 cases (64%). Neoplastic cells immunolabeled for thyroid transcription factor-1 in 2/11 cases (18%) (neither of which had goblet cells) and vimentin in 4/11 cases (36%). Based on this study, feline intraocular metastatic neoplasms are commonly associated with glaucoma and less commonly with uveitis. Histomorphologic and immunohistochemical features of pulmonary origin neoplasia were not consistently found in these metastatic masses of suspected pulmonary origin; the presence of goblet cells was the most commonly identified histologic feature.

This study aimed to evaluate the density, subtypes, and potential functional roles of mast cells in canine transmissible venereal tumors (TVTs) during tumor progression (pretreatment) and regression following vincristine therapy. Biopsy samples were collected from dogs at 3 different time points: day 0 (n = 20), day 7 (n = 20), and day 14 (n = 10). Total mast cell counts were assessed using toluidine blue (TB) staining, while immunohistochemistry for chymase and tryptase was performed to classify mast cell subtypes. Masson's trichrome staining was used to evaluate the severity of fibrosis during regression. In addition, immunohistochemical expression of stem cell factor (SCF) and protease-activated receptor 2 (PAR2) were examined to explore the possible roles of mast cells in the tumor microenvironment. In the late regression group (day 14), the number of TB-, tryptase-, and/or chymase-positive mast cells increased by 3.46, 2.49, and 3.4 times, respectively, compared with the progression group (day 0). In addition, the number of SCF-positive cells and the expression of PAR2 were 6- and 2.38-fold higher in the day 14 group compared with day 0, respectively. While mast cell accumulation is a known feature of various tumors, its precise role in tumor biology remains controversial. The findings of this study suggest a potential role of mast cells and their associated mediators in tumor regression and tissue remodeling processes of TVTs.

In 1895, the German pathologist Dr S.M. Wischnewsky described black, plaque-like discolorations of the gastric mucosa in humans whose cause of death was associated with hypothermia. The pathogenesis of these gastric lesions, known as Wischnewsky spots, is thought to be associated with blood redistribution mechanisms as the body attempts to preserve core body temperature in hypothermic conditions. However, the complete pathogenesis is not fully understood. The relationship between Wischnewsky spots and hypothermia in nonhuman species has also been documented, but again, such lesions in veterinary pathology have an unclear pathogenesis. This study investigated the presence of Wischnewsky spots in several veterinary forensic cases involving 89 emaciated dogs and 46 emaciated cats submitted to the Department of Veterinary Pathology, University of Liverpool, from 2016 to 2023. The gastric lesions were categorized, and collected data were used to compare the characteristics of those lesions found within animals deemed to have died from starvation compared with those having died via cachexic mechanisms. This study aimed to further understand the pathogenesis and diagnostic significance of Wischnewsky spots in veterinary forensic cases. The results indicated that Wischnewsky spots have a significant association with cats and dogs deemed to have died by starvation compared with cases of cachexia. Although comprehensive pathogenesis remains unclear, veterinary forensic cases with Wischnewsky spots are likely to be linked to the combination of stress and hypothermia experienced by cats and dogs that die because of starvation.

Four dogs with neutrophilic cholangitis were identified with intracholangiocytic apicomplexan protozoa. Two cases were biopsies, and both cases had gallbladder involvement. The other two cases were unexpected findings on autopsy. Immunohistochemistry against Toxoplasma gondii on all cases labeled the zoites. Internal transcribed spacer-1 polymerase chain reaction (PCR) performed on formalin-fixed paraffin-embedded samples was successful in the 2 biopsy cases, and subsequent sequencing identified >99% sequence identity to Hammondia sp. Of the biopsied dogs, 1 dog recovered successfully after antiprotozoal treatment while the other dog was euthanized due to poor response. The 2 autopsy cases were given immunosuppressive medication. Hammondia are within the Sarcocystidae family of apicomplexan parasites and are closely related to Neospora caninum. This study and the current literature support that Hammondia, specifically Hammondia heydorni, an organism that was previously considered nonpathogenic or minimally pathogenic, can rarely exhibit tropism to the biliary system and cause neutrophilic cholangitis, cholangiohepatitis, and cholecystitis in dogs.

Negri bodies (NBs) are cytoplasmic inclusions associated with rabies virus infection and are a characteristic histopathologic feature of rabies. Although their morphology has been described in humans and experimental animal models, detailed characterization in naturally infected dogs remains limited. The objective of this study was to characterize the histopathologic, immunophenotypic, and ultrastructural features of NBs in naturally infected dogs from the Philippines. Brain tissues from 24 dogs were examined by routine histology, special stains, immunohistochemistry, and indirect immunofluorescence. Transmission electron microscopy was performed in 6 selected cases. In the cerebrum, hippocampus, and cerebellum, NBs were predominantly composed of abundant matrix material and occasionally contained inner bodies (classic NBs), with variable association with bullet-shaped virions and ribosomes. These inclusions were immunoreactive for rabies virus nucleoprotein and phosphoprotein and stained positively with luxol fast blue. Inner bodies were immunoreactive for glycoprotein and matrix protein and were positive with silver impregnation, Nissl, and Bodian stains. In contrast, small inclusions (lyssa bodies) identified in the thalamus and brainstem consisted mainly of immature viral particles with minimal matrix material. Collectively, these findings indicate region-dependent ultrastructural variation in rabies virus-associated inclusions in naturally infected dogs, while supporting a shared viral origin among the different morphologic forms. This characterization further supports the diagnostic value of these inclusions in veterinary neuropathology.

Understanding the origin, distribution, and biology of different cell populations in chimeric mice is critical for interpreting the pathological changes developed in these models. To this aim, the methodological work presented here illustrates the validation and application of a collection of labeling techniques to differentiate between specific mouse and human tissue/cell components in formalin-fixed paraffin-embedded samples from chimeric mice, especially those bearing human tumor and immune cells. First, broad approaches to identify cells of human origin using ubiquitous immunohistochemical targets such as HLA-A, Ku80, and human mitochondrial 60 kDa protein (hMito) were established using specimens from humanized mice and a human tissue microarray including both normal and neoplastic samples. Due to its crisp membranous immunoreactivity, HLA-A was the most useful marker for visual human cell identification; however, Ku80 and hMito may be suitable options when HLA-A is not expressed in the cells of interest. Importantly, using one or more of these markers provides a broad range of coverage for the vast majority of human-derived cells in chimeric mice. Second, tailored immunohistochemical or in situ hybridization methodologies to distinguish specific human or mouse cell subsets are presented, focusing on immune/inflammatory cells and human chimeric antigen receptor (CAR) T-cells. These diverse approaches are accompanied by descriptions of case examples highlighting practical diagnostic and experimental applications in the context of various humanized mouse models. While not comprehensive, this work represents a valuable starting reference for pathologists and investigators working with humanized mouse models and seeking to add spatial resolution to the complex landscape of chimeric tissues.

In October 2020, adult male and female NSG (NOD. Cg-Prkdcscid Il2rgtm1Wjl/Sz) mice were reported for diarrhea within a mouse barrier facility. Other immunodeficient strains harboring the SCID (Prkdcscid) or Rag (Ragnull) mutations together with the IL2rg (Il2rgnull) mutation were affected. At its peak, over 20 laboratories in 10/16 (62.5%) barrier rooms were affected. Mortality was rare except in lactating females (≥ P11). Grossly, nonlactating adult female and male mice (n = 16) had mild to moderate, small and large intestinal distension with corresponding individual cell death and sloughing of superficial enterocytes in the cecocolonic mucosa. Lactating NSG dams (n=6) had moderate to severe gastrointestinal distension and/or segmental, dark red to gray, small intestinal discoloration. In addition to the same histologic lesions seen in nonlactating female NSG mice, lactating NSG dams often had severe ulcerative inflammation affecting the jejunum, ileum, cecum, and colon. Traditional ancillary diagnostic tests including aerobic and anaerobic cultures (blood, liver, spleen, and intestines), fecal PCR, and fecal floatation failed to yield a causative organism. Further cohousing and oral gavage studies determined neither immunocompetent CD1 (Crl:CD1 [ICR]) mice nor immunodeficient NOD scid (NOD.Cg-Prkdcscid/J) and Rag2 KO (C57BL/6. Cg-Rag2tm1.1Cgn/J) mice were susceptible to clinical disease. Extensive control barriers were implemented including a veterinary-managed NSG breeding barrier, alterations in husbandry practices, and strategic environmental disinfection, allowing for continuity of experimental studies while avoiding widespread depopulation of the barrier. Subsequent strain-resolved metagenomics and qPCR assay development identified Clostridium cuniculi and its enterotoxin exclusively within diarrheic mice.