Fast fragment and compound screening pipeline at the Swiss Light Source.

Crystallography-based fragment screening is a highly effective technique employed in structure-based drug discovery to expand the range of lead development opportunities. It allows screening and sorting of weakly binding, low molecular mass fragments, which can be developed into larger high-affinity lead compounds. Technical improvements at synchrotron beamlines, design of innovative libraries mapping chemical space efficiently, effective soaking methods and enhanced data analysis have enabled the implementation of high-throughput fragment screening pipelines at multiple synchrotron facilities. This widened access to CBFS beyond the pharma industry has allowed academic users to rapidly screen large quantities of fragment-soaked protein crystals. The positive outcome of a CBFS campaign is a set of structures that present the three-dimensional arrangement of fragment-protein complexes in detail, thereby providing information on the location and the mode of interaction of bound fragments. Through this review, we provide users with a comprehensive guide that sets clear expectations before embarking on a crystallography-based fragment screening campaign. We present a list of essential pre-requirements that must be assessed, including the suitability of your current crystal system for a fragment screening campaign. Furthermore, we extensively discuss the available methodological options, addressing their limitations and providing strategies to overcome them. Additionally, we provide a brief perspective on how to proceed once hits are obtained. Notably, we emphasize the solutions we have implemented for instrumentation and software development within our Fast Fragment and Compound Screening pipeline. We also highlight third-party software options that can be utilized for rapid refinement and hit assessment.

SDU - software for high-throughput automated data collection at the Swiss Light Source.

Recent advances in automation have fostered the development of unattended data collection services at a handful of synchrotron facilities worldwide. At the Swiss Light Source, the installation of new high-throughput sample changers at all three macromolecular crystallography beamlines and the commissioning of the Fast Fragment and Compound Screening pipeline created a unique opportunity to automate data acquisition. Here, the DA+ microservice software stack upgrades, implementation of an automatic loop-centering service and deployment of the Smart Digital User (SDU) software for unattended data collection are reported. The SDU software is the decision-making software responsible for communications between services, sample and device safety, sample centering, sample alignment with grid based X-ray diffraction and, finally, data collection.

Brief Report: Challenges in Obtaining the Informed Perspectives of Stakeholders Regarding HIV Molecular Epidemiology.

BACKGROUND: HIV molecular epidemiology (HIV-ME) is now being used in a variety of ways, including molecular HIV surveillance to help identify and respond to emerging HIV transmission clusters as specified in the Ending the HIV Epidemic in the U.S. initiative. However, HIV-ME in general, and its use for cluster detection and response, in particular, raises significant ethical and social concerns, which have spurred vigorous debates. Nevertheless, there is a paucity of information regarding how these potential benefits and concerns are perceived among people living with HIV and people without HIV at an increased risk. SETTING: Virtual engagement with US participants. METHODS: We rigorously developed a brief informational video about HIV-ME and conducted a series of in-depth interviews with people living with HIV and people without HIV at an increased risk. RESULTS: Through extensive stakeholder engagement during the video development process and subsequent in-depth interviews (N = 24), several preliminary findings surfaced. In contrast to the high level of concern raised by some critics of HIV-ME, our data appear to show broad support for it. In addition, we observed conflation of perspectives about HIV-ME with concerns about HIV public health surveillance more generally. CONCLUSION: Our experiences reveal substantial communication challenges related to the nature of HIV-ME that need to be overcome to ensure that it is properly understood, which is necessary for meaningfully engaging stakeholders in discussions about its use. Moreover, ongoing, responsive, engagement efforts are critical. Additional systematic data are needed to help inform policy making and practice regarding HIV-ME.

Probing ligand binding of endothiapepsin by `temperature-resolved' macromolecular crystallography.

Continuous developments in cryogenic X-ray crystallography have provided most of our knowledge of 3D protein structures, which has recently been further augmented by revolutionary advances in cryoEM. However, a single structural conformation identified at cryogenic temperatures may introduce a fictitious structure as a result of cryogenic cooling artefacts, limiting the overview of inherent protein physiological dynamics, which play a critical role in the biological functions of proteins. Here, a room-temperature X-ray crystallographic method using temperature as a trigger to record movie-like structural snapshots has been developed. The method has been used to show how TL00150, a 175.15 Da fragment, undergoes binding-mode changes in endothiapepsin. A surprising fragment-binding discrepancy was observed between the cryo-cooled and physiological temperature structures, and multiple binding poses and their interplay with DMSO were captured. The observations here open up new promising prospects for structure determination and interpretation at physiological temperatures with implications for structure-based drug discovery.

Patients' Reactions to Letters Communicating Collateral Findings of Pragmatic Clinical Trials: a National Web-Based Survey.

BACKGROUND: Collateral findings in pragmatic clinical trials are findings that may have implications for patients' health but were not generated to address a trial's primary research questions. It is uncertain how best to communicate these collateral findings to patients. OBJECTIVES: To determine how reactions to a letter communicating collateral findings relate to who signed the letter, the type of finding, or whether the letter specified that the finding arose from a pragmatic clinical trial. RESEARCH DESIGN: Web-based survey experiment using a between-subjects design in which respondents were randomly assigned within education strata to view and respond to 1 of 16 hypothetical scenarios. SUBJECTS: Adults recruited from an online panel constructed from a probability sample of US-based postal addresses. MEASURES: The primary outcomes were the action the respondent would take next (i.e., contact a doctor immediately or something else) and the respondent's emotional reactions (i.e., all positive, all negative, mixed, or none). RESULTS: A total of 4080 respondents had analyzable data. Although some effects were statistically significant (P < .05), none exceeded a prespecified threshold for policy relevance (15 or more percentage points). Ratings of letter clarity and level of understanding were lower for letters that included a description of the clinical trial. CONCLUSIONS: Signatory and level of detail about collateral findings did not substantially affect people's intentions to take the recommended action of contacting their doctor. Deciding whether to include a description of the pragmatic clinical trial requires a trade-off between transparency and more difficulty understanding the contents of the letter.

Stakeholder perspectives regarding pragmatic clinical trial collateral findings.

CONTEXT: Pragmatic clinical trials (PCTs), which are becoming widespread since they are relatively inexpensive and offer important benefits for healthcare decision-making, can also present practical, ethical, and legal challenges. One such challenge involves managing "pragmatic clinical trial collateral findings" (PCT-CFs), or information emerging in a PCT that is unrelated to the primary research question(s), yet may have implications for individual patients, clinicians, or health care systems from whom or within which data were collected. The expansion of PCTs makes it likely healthcare systems will increasingly encounter PCT-CFs, yet little guidance exists regarding their appropriate management. METHODS: We conducted semi-structured interviews with key stakeholders experienced in the conduct or oversight of PCTs and those in health system leadership. Interviews explored respondents' experience with PCTs and PCT-CFs, and actual or hypothetical reactions to PCT-CF management. We used standard methods of qualitative analysis to identify key themes. FINDINGS: Forty-one stakeholders participated. Four key themes emerged. First, discussions of PCT-CFs are complicated by layers of ambiguity related to both the nature of PCTs themselves, and unanticipated results that emanate from them. Second, management of PCT-CFs is context-specific, and not amenable to a "one-size-fits-all" approach. Third, there was a wide diversity of attitudes regarding the scope of researcher responsibilities in PCTs. Fourth, PCT-CFs had generally not been previously considered by respondents, but there was widespread belief in the importance of prospective planning to anticipate such issues in future PCTs. CONCLUSIONS: PCT-CFs are likely to increase, yet those charged with PCT-CF decision-making and their disclosure are unlikely to have experience with these issues. Further deliberation about the ethical obligations and implementation processes regarding PCT-CFs is needed. To enhance the likelihood of developing sound policies and practices, such deliberations should include the input and perspectives of key stakeholders in PCTs, including professionals, policy makers, and patients.

Identification and management of pragmatic clinical trial collateral findings: A current understanding and directions for future research.

While the embedded nature of pragmatic clinical trials (PCTs) can improve the efficiency and relevance of research for multiple stakeholders, embedding research into ongoing clinical care can also involve ethical and regulatory challenges. An emergent challenge is the management of pragmatic clinical trial collateral findings (PCT-CFs). While PCT-CFs share some features with incidental or secondary findings that are encountered in conventional clinical trials and clinical care, the PCT context differs in ethically relevant ways that complicate PCT-CF identification and management. We report on the results of a two-year multi-method investigation of PCT-CFs. Overall, five core themes emerged: 1) the liminal nature of PCTs and the implications of this for PCT-CFs; 2) the context-specific nature of PCT-CF management; 3) the centrality of institutions; 4) the importance of prospective planning; and 5) patient expectations. Among the central lessons of this work are that prior ethics guidance from other settings cannot easily be adapted to address PCT-CFs, nor can a single approach readily accommodate all PCT-CFs. Moving forward, stakeholders, including researchers, institutions, ethics oversight bodies, and funders, should anticipate and plan for PCT-CFs in the design, conduct, and analysis of PCTs. Future scholarship is needed to examine experiences with PCT-CFs, and the practical and conceptual issues they raise for the future conduct of PCTs.

Patients' perspectives on the derivation and use of organoids.

Organoid research is enhancing understanding of human development and diseases as well as aiding in medication development and selection, raising hopes for even more future therapeutic options. Nevertheless, this work raises important ethical issues and there is a paucity of data regarding patients' perspectives on them. We report on 60 interviews with adult patients or parents of pediatric patients from diverse disease populations who receive medical care at a major academic research institution in the United States. Interviewees expressed broad support for organoid development and use. However, patients viewed brain organoids, and sometimes gonadal organoids, as morally distinct; and some organoid research poses moral concerns. Nonetheless, patients generally understood the potential value of such research and approved of it, provided it was aimed at good intent and conducted with ethical oversight and a robust consent process. These data should help inform conceptual and policy deliberations about appropriate organoid use.

Combining High-Throughput Synthesis and High-Throughput Protein Crystallography for Accelerated Hit Identification.

Protein crystallography (PX) is widely used to drive advanced stages of drug optimization or to discover medicinal chemistry starting points by fragment soaking. However, recent progress in PX could allow for a more integrated role into early drug discovery. Here, we demonstrate for the first time the interplay of high throughput synthesis and high throughput PX. We describe a practical multicomponent reaction approach to acrylamides and -esters from diverse building blocks suitable for mmol scale synthesis on 96-well format and on a high-throughput nanoscale format in a highly automated fashion. High-throughput PX of our libraries efficiently yielded potent covalent inhibitors of the main protease of the COVID-19 causing agent, SARS-CoV-2. Our results demonstrate, that the marriage of in situ HT synthesis of (covalent) libraires and HT PX has the potential to accelerate hit finding and to provide meaningful strategies for medicinal chemistry projects.

Patients' Views About the Disclosure of Collateral Findings in Pragmatic Clinical Trials: a Focus Group Study.

BACKGROUND: Pragmatic clinical trials (PCTs) are increasingly being conducted to efficiently generate evidence to inform healthcare decision-making. Despite their growing acceptance, PCTs may involve a variety of ethical issues, including the management of pragmatic clinical trial-collateral findings (PCT-CFs), that is, information that emerges in PCTs that is unrelated to the primary research questions but may have implications for patients, clinicians, and health systems. OBJECTIVE: We sought to understand patients' views about PCT-CF disclosure, including how, by whom, and the nature and extent of information provided. DESIGN: Prospective, qualitative focus group study. PARTICIPANTS: Focus groups were conducted in Baltimore, MD; Houston, TX; and Seattle, WA (overall N = 66), during July and August 2019. APPROACH: All groups discussed a hypothetical scenario involving the detection of a PCT-CF of contraindicated medications. Participants were asked about their reactions to the PCT-CF and issues related to its disclosure. KEY RESULTS: Reactions to learning about the PCT-CF were mixed, ranging from fear of a significant health problem, anger that the contraindicated medications had gone unnoticed and/or for being included in research without their permission, to gratitude for the information. Preferences for how such disclosures are made varied but were driven by several consistent desires, namely minimizing patient harm and anxiety and demonstrating trust and respect. Many wanted their treating clinician to be informed of the PCT-CF so that they would be prepared to answer patients' questions and to discuss treatment options. CONCLUSIONS: The detection of PCT-CFs is likely to increase with further expansion of PCTs. As such, clinicians will undoubtedly become involved in the management of PCT-CFs. Our data illustrate some of the challenges clinicians may face when their patients are informed of a PCT-CF and the need to develop guidance for disclosing PCT-CFs in ways that align with patients' preferences and values.

Feline bilateral inflammatory aural polyps: a descriptive retrospective study.

BACKGROUND: Feline inflammatory aural polyps (IP) have been reported as solitary growths, yet bilateral polyps may occur more commonly than described previously. OBJECTIVES: To identify the prevalence and risk factors associated with bilateral feline IP within a population of cats evaluated at a veterinary teaching hospital over a 10 year period. ANIMALS: Twenty-five cats with histologically confirmed IP. METHODS AND MATERIALS: A retrospective study was performed by searching computerized medical records of cats seen at The University of Tennessee Veterinary Teaching Hospital from 2005 to 2015. Keywords used for the search included "feline polyp", "ventral bulla osteotomy", "inflammatory polyp ear canal" and "nasopharyngeal polyp". RESULTS: Cats were separated into groups with unilateral or bilateral disease. Six of 25 (24%) cats had bilateral IP. Twelve cats had computed tomography performed (four of six with bilateral IP; eight of 19 with unilateral IP). Clinical signs, history of upper respiratory infection, post-treatment complications, and IP recurrence with and without local and systemic steroid administration were studied between groups and no differences were identified. In four of six cats with bilateral disease, the second polyp was identified with advanced imaging performed at the time of referral. CONCLUSIONS AND CLINICAL IMPORTANCE: These findings demonstrate that bilateral IP occur more frequently than reported previously and establish a prevalence for bilateral IP. These findings also highlight the value of advanced imaging techniques in diagnosing bilateral IP.

Selectively Disrupting m6A-Dependent Protein-RNA Interactions with Fragments.

We report a crystallographic analysis of small-molecule ligands of the human YTHDC1 domain that recognizes N6-methylated adenine (m6A) in RNA. The 30 binders are fragments (molecular weight < 300 g mol-1) that represent 10 different chemotypes identified by virtual screening. Despite the structural disorder of the binding site loop (residues 429-439), most of the 30 fragments emulate the two main interactions of the -NHCH3 group of m6A. These interactions are the hydrogen bond to the backbone carbonyl of Ser378 and the van der Waals contacts with the tryptophan cage. Different chemical groups are involved in the conserved binding motifs. Some of the fragments show favorable ligand efficiency for YTHDC1 and selectivity against other m6A reader domains. The structural information is useful for the design of modulators of m6A recognition by YTHDC1.

Exploring the Risk Posed by Animals with an Inconclusive Reaction to the Bovine Tuberculosis Skin Test in England and Wales.

The single intradermal comparative cervical tuberculin (SICCT) test is the primary test for ante-mortem diagnosis of bovine tuberculosis (TB) in England and Wales. When an animal is first classified as an inconclusive reactor (IR) using this test, it is not subject to compulsory slaughter, but it must be isolated from the rest of the herd. To understand the risk posed by these animals, a case-control study was conducted to measure the association between IR status of animals and the odds of them becoming a reactor to the SICCT at a subsequent test. The study included all animals from herds in which only IR animals were found at the first whole herd test in 2012 and used data from subsequent tests up until the end of 2016. Separate mixed-effects logistic regression models were developed to examine the relationship between IR status and subsequent reactor status for each risk area of England and for Wales, adjusting for other explanatory variables. The odds of an animal becoming a subsequent reactor during the study period were greater for IR animals than for negative animals in the high-risk area (odds ratio (OR): 6.85 (5.98-7.86)) and edge area (OR: 8.79 (5.92-13.04)) of England and in Wales (OR: 6.87 (5.75-8.22)). In the low-risk area of England, the odds were 23 times greater, although the confidence interval around this estimate was larger due to the smaller sample size (11-48, p < 0.001). These findings support the need to explore differential controls for IR animals to reduce the spread of TB, and they highlight the importance of area-specific policies.

Antibacterial effect of N-acetylcysteine in combination with antimicrobials on common canine otitis externa bacterial isolates.

BACKGROUND: Approved treatments for canine otitis externa are limited in variety and may contain ototoxic ingredients. With bacterial resistance an ongoing concern, it would be ideal if non-ototoxic agents combined with antibiotics resulted in a synergistic effect, requiring lower antibiotic concentrations to treat infections. Evidence of synergism and antagonism between N-acetylcysteine (NAC) and various antibiotic classes has been reported; the present research group was interested in examining these interactions. HYPOTHESIS/OBJECTIVES: To determine if NAC, an otoprotective and antimicrobial compound, has synergistic activity when combined with enrofloxacin or gentamicin in vitro against bacterial isolates causing canine otitis externa. ANIMALS: Twenty-two isolates from canine clinical cases of otitis externa were identified and tested, including seven Staphylococcus pseudintermedius, 12 Pseudomonas aeruginosa and three Corynebacterium spp. isolates. METHODS AND MATERIALS: Each isolate was grown on blood agar for 24 h and transferred to Mueller-Hinton broth (MHB), with a final concentration of 5 × 105 cfu/mL. Each well was inoculated with 50 µL of bacterial suspension. N-acetylcysteine was diluted in MHB to a starting concentration of 160 mg/mL. Enrofloxacin and gentamicin were diluted to 64 µg/mL. Individual and checkerboard serial microdilution assays were performed in triplicate with negative controls for all isolates tested. RESULTS: Interactions observed for NAC and enrofloxacin were synergistic (4.5%), indifferent (77.3%) or antagonistic (18.2%). Interactions observed for NAC and gentamicin were synergistic (4.5%), indifferent (45.5%) or antagonistic (50%). CONCLUSIONS AND CLINICAL RELEVANCE: Most interactions between NAC and enrofloxacin or gentamicin were indifferent or antagonistic at the concentrations tested in vitro.

High-Affinity Alkynyl Bisubstrate Inhibitors of Nicotinamide N-Methyltransferase (NNMT).

Nicotinamide N-methyltransferase (NNMT) is a metabolic enzyme that methylates nicotinamide (NAM) using cofactor S-adenosylmethionine (SAM). NNMT overexpression has been linked to diabetes, obesity, and various cancers. In this work, structure-based rational design led to the development of potent and selective alkynyl bisubstrate inhibitors of NNMT. The reported nicotinamide-SAM conjugate (named NS1) features an alkyne as a key design element that closely mimics the linear, 180° transition state geometry found in the NNMT-catalyzed SAM → NAM methyl transfer reaction. NS1 was synthesized in 14 steps and found to be a high-affinity, subnanomolar NNMT inhibitor. An X-ray cocrystal structure and SAR study revealed the ability of an alkynyl linker to span the methyl transfer tunnel of NNMT with ideal shape complementarity. The compounds reported in this work represent the most potent and selective NNMT inhibitors reported to date. The rational design principle described herein could potentially be extended to other methyltransferase enzymes.

Interaction specificity of clustered protocadherins inferred from sequence covariation and structural analysis.

Clustered protocadherins, a large family of paralogous proteins that play important roles in neuronal development, provide an important case study of interaction specificity in a large eukaryotic protein family. A mammalian genome has more than 50 clustered protocadherin isoforms, which have remarkable homophilic specificity for interactions between cellular surfaces. A large antiparallel dimer interface formed by the first 4 extracellular cadherin (EC) domains controls this interaction. To understand how specificity is achieved between the numerous paralogs, we used a combination of structural and computational approaches. Molecular dynamics simulations revealed that individual EC interactions are weak and undergo binding and unbinding events, but together they form a stable complex through polyvalency. Strongly evolutionarily coupled residue pairs interacted more frequently in our simulations, suggesting that sequence coevolution can inform the frequency of interaction and biochemical nature of a residue interaction. With these simulations and sequence coevolution, we generated a statistical model of interaction energy for the clustered protocadherin family that measures the contributions of all amino acid pairs at the interface. Our interaction energy model assesses specificity for all possible pairs of isoforms, recapitulating known pairings and predicting the effects of experimental changes in isoform specificity that are consistent with literature results. Our results show that sequence coevolution can be used to understand specificity determinants in a protein family and prioritize interface amino acid substitutions to reprogram specific protein-protein interactions.

Description and characterization of a hair coat disorder in schipperkes.

BACKGROUND: Schipperkes develop bilaterally symmetrical alopecia preceded by lightening of the hair coat not associated with systemic clinical signs. The alopecia is presumptively diagnosed as Alopecia X but has never been investigated. HYPOTHESIS/OBJECTIVES: The purpose of this study was to describe clinical features, histopathological and laboratory abnormalities associated with symmetrical, noninflammatory alopecia in schipperkes. ANIMALS: Three healthy and eight affected schipperkes. METHODS AND MATERIALS: Complete blood count (CBC), serum chemistry panel, urinalysis (UA), urine cortisol: creatinine ratio (UCCR), total thyroxine (T4 ), free thyroxine by equilibrium dialysis (fT4 ed), thyroid stimulating hormone (TSH) and pre- and post-stimulation cortisol with sex hormone analyses were performed for all dogs. A minimum of two skin biopsy sites were sampled for histopathological evaluation. RESULTS: The CBC, serum chemistry panel, UA, T4 , fT4 ed, TSH and post-ACTH cortisol concentrations were normal for all dogs. Androstenedione concentrations were increased for all dogs; two of three healthy and five of eight affected dogs had increased post-ACTH androstenedione concentrations. Estradiol concentrations were increased for all healthy and four of eight affected dogs, with post-ACTH estradiol concentrations increased for all healthy and five of eight affected dogs. Progesterone concentrations were increased for two of three healthy and four of eight affected dogs. Post-ACTH progesterone concentrations were increased for one of three healthy and four of eight affected dogs. For one of three healthy and two of eight affected dogs, 17-hydroxyprogesterone concentrations were increased; post-ACTH 17-hydroxyprogesterone concentrations were increased in three of eight affected dogs only. Histopathology results from affected dogs were consistent with hair cycle arrest. CONCLUSIONS AND CLINICAL IMPORTANCE: This disorder resembles Alopecia X clinically and histologically.

An early warning tool for predicting at admission the discharge disposition of a hospitalized patient.

OBJECTIVES: To develop an early warning discharge disposition prediction tool based on clinical and health services factors for hospitalized patients. Recent study results suggest that early prediction of discharge disposition (ie, whether patients can return home or require placement in a facility) can improve care coordination, expedite care planning, and reduce length of stay. STUDY DESIGN: Retrospective analysis of inpatient data; development of multiple logistic regression model and an easy-to-use score. METHODS: We used retrospective data from all patients who were admitted in 2013 to the general medical service at the Veterans Affairs Boston Healthcare System and discharged alive. A derivation-validation approach was used to build a multiple logistic regression model, which was transformed into a score for potential implementation. RESULTS: Of the 4760 patients discharged in 2013, 485 (10.2%) were discharged to a facility other than home. Correlates of discharge to a facility included a primary admission diagnosis of neoplasm (odds ratio [OR], 2.71; 95% CI, 1.73-4.25), diseases of the nervous system (OR, 2.53; 95% CI, 1.26-5.08), and musculoskeletal diseases (OR, 2.55; 95% CI, 1.52-4.27), as well as discharge to a facility during previous hospitalization. Patients with a prior primary diagnosis of circulatory disorder and those with comorbidity of hypertension, either complicated or uncomplicated, were less likely to be discharged to a facility. A value of 5 or greater on the 20-point scale indicated discharge to a facility with 83% sensitivity and 48% specificity. CONCLUSIONS: A validated, easy-to-use score can assist providers in identifying upon admission those patients who may not be able to go directly home after hospitalization, thus facilitating early discharge planning and coordination, potentially reducing length of hospital stay and improving patient experience.

The prevalence of Dermatophilus congolensis in horses with pastern dermatitis using PCR to diagnose infection in a population of horses in southern USA.

BACKGROUND: Dermatophilus congolensis is a facultative anaerobic actinomycete that causes papular to exudative dermatitis with crusting in horses. This organism is frequently implicated as a cause of pastern dermatitis, but few data are available validating the organism's association with this disease. HYPOTHESIS/OBJECTIVES: The aim of this study was to evaluate if D. congolensis is associated with pastern dermatitis in horses utilizing RT-qPCR. ANIMALS: Fifteen client-owned horses diagnosed with pastern dermatitis and eight client-owned unaffected control horses were utilized for this study. METHODS: A cross-sectional study was performed. History and physical examination findings were recorded, and samples were collected and tested for D. congolensis utilizing cytological evaluation and RT-qPCR. Dermatophyte culture and superficial skin scrapings were also performed. RESULTS: Ten of 15 horses with pastern dermatitis had feathered pasterns. Dermatophilus congolensis was identified by RT-qPCR from one nonfeathered horse but none with feathered pasterns. Cytological evaluation identified bacteria in all horses but failed to identify organisms resembling D. congolensis in any horse. Four of 15 horses, all feathered, were positive for Chorioptes mites. Fungal culture was negative for dermatophytes in all horses. All test results were negative for the eight control horses. CONCLUSIONS AND CLINICAL IMPORTANCE: Dermatophilus congolensis was uncommonly associated with pastern dermatitis in horses in this population. However, chorioptic mange was commonly associated with pastern dermatitis in feathered horses and represented an important differential diagnosis for this clinical presentation.