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Systemic juvenile idiopathic arthritis (sJIA) and adult-onset Still's disease (AOSD) are considered the same disease, but a common approach for diagnosis and management is still missing., Methods: In May 2022, EULAR and PReS endorsed a proposal for a joint task force (TF) to develop recommendations for the diagnosis and management of sJIA and AOSD. The TF agreed during a first meeting to address four topics: similarity between sJIA and AOSD, diagnostic biomarkers, therapeutic targets and strategies and complications including macrophage activation syndrome (MAS). Systematic literature reviews were conducted accordingly., Results: The TF based their recommendations on four overarching principles, highlighting notably that sJIA and AOSD are one disease, to be designated by one name, Still's disease.Fourteen specific recommendations were issued. Two therapeutic targets were defined: clinically inactive disease (CID) and remission, that is, CID maintained for at least 6 months. The optimal therapeutic strategy relies on early use of interleukin (IL-1 or IL-6 inhibitors associated to short duration glucocorticoid (GC). MAS treatment should rely on high-dose GCs, IL-1 inhibitors, ciclosporin and interferon-γ inhibitors. A specific concern rose recently with cases of severe lung disease in children with Still's disease, for which T cell directed immunosuppressant are suggested. The recommendations emphasised the key role of expert centres for difficult-to-treat patients. All overarching principles and recommendations were agreed by over 80% of the TF experts with a high level of agreement., Conclusion: These recommendations are the first consensus for the diagnosis and management of children and adults with Still's disease., Competing Interests: Competing interests: BF: AbbVie, Amgen, Biogen, BMS, Celltrion, Fresenius Kabi, Galapagos, Gilead, Janssen, Lilly, Medac, MSD, Nordic Pharma, Novartis, Pfizer, Roche-Chugai, Sandoz, Sanofi-Genzyme, SOBI, UCB, Viatris; SM: BMS, Lilly, SOBI; JA: Sobi, Novartis, Roche, Pfizer, AbbVie, Lilly; AB: Boehringer Ingelheim, Novartis, AbbVie, Fresenius Kabi, GlaxoSmithKline; CB: SOBI; TC: AbbVie, Novartis, Roche; LD: AbbVie, Amgen, Astra-Zeneca, Boehringer-Ingelheim, BMS, Lilly, Galapagos, GlaxoSmithKline, Janssen, Kiniksa, Novartis, Pfizer, SOBI; ADB: Lenovo; EF: AbbVie, BMS, Galapagos, Lilly, Medac, Novartis, Sanofi, UCB, Pfizer, Roche, SOBI; DF: Boehringer-Ingelheim, SOBI, Novartis, Werfen Innova; MG, SGL, FM, FOR, SV: Novartis, SOBI; RG, AG: Novartis; IJ: Amgen, Lilly, Novartis, SOBI; PN: BMS, Brickell Bio, Cerecor, Exo Therapeutics, Miach Ortho, Novartis, Pfizer, SOBI, UpToDate, American Academy of Pediatrics; SO: Novartis, SOBI, Pfizer; PQ: Amgen, AbbVie, BMS, Roche-Chugai, Lilly, Novartis, Pfizer, Sanofi, SOBI, Health Events; PR: AbbVie, BMS, Janssen, Novartis, SOBI; SS: Novartis, SOBI, CSL Behring, Takeda, BioCryst, Biotest; M-ET: Boehringer-Ingelheim, Pfizer, Lilly, MSD, SOBI, Janssen, BMS, Fresenius Kabi, Galapagos, AbbVie; CW: Novartis, SOBI, UCB; LC: Meda Pharma, Angelini Pharma, Pfizer, SANOFI-AVENTIS, Fresenius Kabi, Galapagos; FDB: SOBI, Novartis, Apollo, Kiniksa, Sanofi, Roche, Elixiron, Regeneron; ADM, SB, KL, CL, ES, T-CW: none., (© European Alliance of Associations for Rheumatology, EULAR 2024. Re-use permitted under CC BY-NC-ND. No commercial re-use. No derivatives. See rights and permissions. Published by BMJ on behalf of EULAR.)

Objectives: A EULAR task force was convened to develop points to consider (PtC) for patient education in physical activity and self-management of pain in young people with juvenile-onset rheumatic and musculoskeletal diseases during transitional care., Methods: A task force of 26 people from 10 European countries followed the EULAR Standardised Operating Procedures to establish overarching principles (OAPs) and PtC based on a literature review and expert consensus. Level of evidence (LoE), grade of recommendation (GoR) and level of agreement (LoA) were determined., Results: Two OAPs and seven PtC were formulated. The OAPs highlight the importance of personalised transitional care in rheumatology, ideally based on shared decision-making and incorporate interactive education to empower young individuals in managing their physical activity and pain. The PtC emphasise the clinical importance of patient education in these areas to improve readiness to transfer from paediatric to adult care. For two PtC, the GoR was moderate (grade B), based on individual cohort study (LoE 2b). For the remaining five PtC, the GoR was weak (grade D), based on expert opinion (LoE 5). The LoA among the task force was high, ranging from 9.4 to 9.8, except for one PtC that was 8.7., Conclusion: These EULAR PtC establish guidance on best practices for delivering patient education in physical activity and self-management of pain during transitional care in rheumatology. The adoption of these PtC in clinical settings is recommended to standardise and optimise transitional care across European healthcare systems. Additionally, the task force expects that these PtC will drive future research and potentially shape policies across Europe., Competing Interests: Competing interests: JC-I: none declared, RP-M: none declared, EB-V: none declared, PA-V: none declared, SA: none declared, JA: none declared, IB: none declared, DC: none declared, MC: none declared, WC: none declared, DDC: none declared, AD: none declared, LL: payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from Pfizer; AM: none declared, KM: grant/research support from Pfizer, Novartis and Medac, AFM: none declared, AN: none declared, SO: none declared, GP: none declared, ZS: none declared, TV: none declared, NMW: none declared, AZC: none declared, YP: none declared, LC: none declared, FE-L: none declared., (© European Alliance of Associations for Rheumatology, EULAR 2024. Re-use permitted under CC BY-NC-ND. No commercial re-use. No derivatives. See rights and permissions. Published by BMJ on behalf of EULAR.)

Objectives: We report the safety, tolerability and efficacy of tofacitinib in patients with juvenile idiopathic arthritis (JIA) in an ongoing long-term extension (LTE) study., Methods: Patients (2-<18 years) with JIA who completed phase 1/3 index studies or discontinued for reasons excluding treatment-related serious adverse events (AEs) entered the LTE study and received tofacitinib 5 mg two times per day or equivalent weight-based doses. Safety outcomes included AEs, serious AEs and AEs of special interest. Efficacy outcomes included improvement since tofacitinib initiation per the JIA-American College of Rheumatology (ACR)70/90 criteria, JIA flare rate and disease activity measured by Juvenile Arthritis Disease Activity Score (JADAS)27, with inactive disease corresponding to JADAS ≤1.0., Results: Of 225 patients with JIA (median (range) duration of treatment, 41.6 (1-103) months), 201 (89.3%) had AEs; 34 (15.1%) had serious AEs. 10 patients developed serious infections; three had herpes zoster. Two patients newly developed uveitis. Among patients with polyarticular course JIA, JIA-ACR70/90 response rates were 60.0% (78 of 130) and 33.6% (47 of 140), respectively, at month 1, and generally improved over time. JIA flare events generally occurred in <5% of patients through to month 48. Observed mean (SE) JADAS27 was 22.0 (0.6) at baseline, 6.2 (0.7) at month 1 and 2.8 (0.5) at month 48, with inactive disease in 28.8% (36 of 125) of patients at month 1 and 46.8% (29 of 82) at month 48., Conclusions: In this interim analysis of LTE study data in patients with JIA, safety findings were consistent with the known profile of tofacitinib, and efficacy was maintained up to month 48., Trial Registration Number: NCT01500551., Competing Interests: Competing interests: HIB has received research grants from Bristol Myers Squibb, Novartis and Pfizer; is an employee of Cincinnati Children’s Hospital Medical Center; has received consulting fees or other remuneration from AbbVie, AstraZeneca/MedImmune, Bayer, Biocon, Boehringer Ingelheim, Bristol Myers Squibb, Cerecor, Eli Lilly, EMD Serono, Janssen, Novartis, Pfizer, Roche, R-Pharm and Sobi; and is a member of speaker bureaus for Novartis and Pfizer. JDA has received honorarium from Novartis. JFB has received research grants from AbbVie, Bristol Myers Squibb, Janssen, Pfizer and Roche. ALB is a member of speaker bureaus for AbbVie, Novartis and Roche. RC has received consulting fees or other remuneration from AbbVie, Bristol Myers Squibb, Eli Lilly, GSK, Novartis, Pfizer, Roche and Sanofi. GS has received consulting fees or other remuneration from Novartis and Sobi and research support from IpiNovyx. CN has received research grants from AbbVie, AstraZeneca, Bristol Myers Squibb, Eli Lilly, GSK, Pfizer and UCB. MV-DM is an employee of Clínica de Investigacion en Reumatologia y Obesidad, SC. HP, CC, CWh, KeK and SL are employees and stockholders of Pfizer. AW was an employee and stockholder of Pfizer at the time of this analysis. AM has received consulting fees or other remuneration from Aurinia, Bristol Myers Squibb, Eli Lilly, EMD Serono, Janssen and Pfizer. DL’s institution, the Cincinnati Children’s Hospital Medical Center, has received research grants from Bristol Myers Squibb, Janssen, Novartis, Pfizer, Roche and UBC; and has received consulting fees or other remuneration from AstraZeneca, Boehringer Ingelheim, GSK, Roche, Novartis, Pfizer, Takeda and UBC. DL is also a data safety and monitoring board member or chairperson for the National Institutes of Health and the Canadian Arthritis Society. NR has received honoraria for consultancies or speaker bureaus from Ablynx, AstraZeneca/MedImmune, Biogen, Boehringer Ingelheim, Bristol Myers Squibb, Eli Lilly, EMD Serono, F Hoffmann-La Roche, GS, Janssen, MSD, Novartis, Pfizer, R-Pharm, Sanofi, Servier, Sinergie and Sobi. The IRCCS Istituto Giannina Gaslini, where NR works as a full-time public employee, has received contributions from Bristol Myers Squibb, Eli Lilly, F Hoffmann-La Roche, GS, Janssen, Novartis, Pfizer and Sobi; this funding has been reinvested for the research activities of the hospital in a fully independent manner, without any commitment with third parties. EA-A, GC, WDLP, LJ, ÖK, KaK, RR-C, JCRR, LW-W, JEW and CWo have declared no conflicts., (© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ on behalf of EULAR.)

Objectives: To assess the ability of baseline serum biomarkers to predict disease activity and remission status in juvenile idiopathic arthritis (JIA) at 18-year follow-up (FU) in a population-based setting., Methods: Clinical data and serum levels of inflammatory biomarkers were assessed in the longitudinal population-based Nordic JIA cohort study at baseline and at 18-year FU. A panel of 16 inflammatory biomarkers was determined by multiplexed bead array assay. We estimated both univariate and multivariate logistic regression models on binary outcomes of disease activity and remission with baseline variables as explanatory variables., Results: Out of 349 patients eligible for the Nordic JIA cohort study, 236 (68%) had available serum samples at baseline. We measured significantly higher serum levels of interleukin 1β (IL-1β), IL-6, IL-12p70, IL-13, MMP-3, S100A9 and S100A12 at baseline in patients with active disease at 18-year FU than in patients with inactive disease. Computing receiver operating characteristics illustrating the area under the curve (AUC), we compared a conventional prediction model (gender, age, joint counts, erythrocyte sedimentation rate, C reactive protein) with an extended model that also incorporated the 16 baseline biomarkers. Biomarker addition significantly improved the ability of the model to predict activity/inactivity at the 18-year FU, as evidenced by an increase in the AUC from 0.59 to 0.80 (p=0.02). Multiple regression analysis revealed that S100A9 was the strongest predictor of inactive disease 18 years after disease onset., Conclusion: Biomarkers indicating inflammation at baseline have the potential to improve evaluation of disease activity and prediction of long-term outcomes., Competing Interests: Competing interests: CM: Speakers bureau: Novartis, Sobi. Support for attending meetings: Sobi. VR: Committees: Secretary of the JIA Working Party in the Pediatric Rheumatology European Association (PReS). MR: Data safety monitoring board: STARS trial. CK: Grants: Novartis, German Research Foundation. Consulting fees: Sobi, Novartis. Speakers bureau: Sobi. DF: Grants: Novartis, Sobi. Consulting fees: Boehringer, Novartis. Speakers bureau: Novartis, Sobi, BioNTech. Advisory boards: Novartis, Sobi. Boards: Paediatric Rheumatology European Society council, EULAR council., (© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)

Background: The COVID-19 pandemic has significantly impacted individuals with chronic conditions. This investigation assessed the quality of care provided to pediatric and adolescent patients with juvenile idiopathic arthritis (JIA) during the pandemic in Thailand., Methods: This cross-sectional analysis enrolled JIA patients aged ≤ 18 years at an academic tertiary care facility from April 2022 to March 2023. Retrospective reviews were performed, complemented by patient and caregiver questionnaires to assess the pandemic's impact on care quality., Results: Seventy JIA patients (37 males, 33 females) with a mean age of 13.5 ± 3.1 years were included. A total of 41.4% of the caregivers reported negative impacts on JIA care due to the pandemic and the lockdown, and 31.4% of the patients experienced pandemic-related anxiety. A comparison between the pandemic and prepandemic periods revealed a higher incidence of active disease, although the difference was statistically nonsignificant (37.1% vs 14.2%, p = 0.106). Nonadherence significantly predicted active disease status (adjusted OR 15.04, 95% CI 2.48-91.15, p = 0.03). COVID-19 vaccinations were administered to 85.7% of patients; 52.8% of whom contracted mild COVID-19. Most patients (71.4%) postponed clinic visits; 36% due to lockdowns and 28% due to concerns about COVID-19 exposure in healthcare settings. The majority of patients received telephone JIA management advice from rheumatologists during the lockdown (91.4%)., Conclusions: The COVID-19 pandemic and associated lockdown measures affected the care of JIA patients, impacting both physical and mental health. Nonadherence was a critical factor in disease flare-ups. Telemedicine is indispensable for patient care., (© 2024. The Author(s).)

Background: Juvenile Idiopathic Arthritis (JIA) is the most common rheumatic inflammatory disease in childhood. Optimal management requires clinicians to be up to date with the rapidly evolving evidence base. 'Living' evidence-based clinical practice guidelines, which integrate new evidence as soon as it is available, are a novel method to enhance the translation of research into practice. To determine the most relevant questions that should be prioritised in national Australian JIA living guidelines, we invited Australian and New Zealand paediatric rheumatologists and other relevant health professionals to identify and rank their most important questions in order of priority. Methods: All 47 members of the Australian Paediatric Rheumatology Group (APRG) were invited to participate in a modified Delphi study comprising two rounds. The first round identified demographic information of respondents, current attitudes to guideline use and invited submission of priority management questions. The second round asked respondents to rank 27 collated and refined questions identified in round one in order of priority. Results: There were 29 (62%) and 28 (60%) responses to the first and second survey rounds respectively. About two thirds were rheumatologists or trainees (66, 68%), nearly half had more than 10 years of experience (45, 46%) and practice setting was largely hospital (79, 86%) and urban (86, 75%). Most respondents used clinical guidelines in their practice (72% sometimes, 24% often), most frequently American College of Rheumatology (ACR) (66%) and European Alliance of Associations for Rheumatology (EULAR) (59%) guidelines. Reported barriers to guideline use included that they are not up to date and access difficulties. Most respondents (83%) considered Australian guidelines were necessary and two-thirds indicated they would use them if integrated into practice software. The highest ranked topics were down-titration and discontinuation of disease modifying anti-rheumatic drugs (ranked first), best outcome measures (second) and treatment targets in JIA (third). Conclusions: There is strong clinician support for the development of Australian living guidelines for JIA. Consensus was reached on the ten top-ranked priority questions. Our guidelines will develop evidence-based recommendations for these high priority questions that will be updated in real time as needed to facilitate rapid translation of evidence into clinical practice. [ABSTRACT FROM AUTHOR]

Abstract Background We aimed to characterize etanercept (ETN) use in juvenile idiopathic arthritis (JIA) patients enrolled in the Childhood Arthritis and Rheumatology Research Alliance (CARRA) Registry. Methods The CARRA Registry is a convenience cohort of patients with paediatric onset rheumatic diseases, including JIA. JIA patients treated with ETN for whom the month and year of ETN initiation were available were included. Patterns of ETN and methotrexate (MTX) use were categorized as follows: combination therapy (ETN and MTX started concurrently), step-up therapy (MTX started first and ETN added later), switchers (MTX started and then stopped when or before ETN started), MTX add-on (ETN started first and MTX added later), and ETN only (no MTX use). Data were described using parametric and non-parametric statistics as appropriate. Results Two thousand thirty-two of the five thousand six hundred forty-one patients with JIA met inclusion criteria (74% female, median age at diagnosis 6.0 years [interquartile range 2.0, 11.0]. Most patients (66.9%) were treated with a non-biologic disease modifying anti-rheumatic drug (DMARD), primarily MTX, prior to ETN. There was significant variability in patterns of MTX use prior to starting ETN. Step-up therapy was the most common approach. Only 34.0% of persistent oligoarticular JIA patients continued treatment with a non-biologic DMARD 3 months or more after ETN initiation. ETN persistence overall was 66.3, 49.4, and 37.3% at 24, 36 and 48 months respectively. ETN persistence among spondyloarthritis patients (enthesitis related arthritis and psoriatic JIA) varied by MTX initiation pattern, with higher ETN persistence rates in those who initiated combination therapy (68.9%) and switchers/ETN only (73.3%) patients compared to step-up (65.4%) and MTX add-on (51.1%) therapy. Conclusion This study characterizes contemporary patterns of ETN use in the CARRA Registry. Treatment was largely in keeping with American College of Rheumatology guidelines.

Abstract Background Recognition of temporomandibular joint (TMJ) involvement in children with juvenile idiopathic arthritis (JIA) has gained increasing attention in the past decade. The clinical assessment of mandibular range of motion characteristics is part of the recommended variables to detect TMJ involvement in children with JIA. The aim of this study was to explore explanatory variables for mandibular range of motion outcomes in children with JIA, with and without clinically established TMJ involvement, and in healthy children. Methods This cross-sectional study included children with JIA and healthy children of age 6–18 years. Mandibular range of motion variables included active and passive maximum interincisal opening (AMIO and PMIO), protrusion, laterotrusion, dental midline shift in AMIO and in protrusion. Additionally, the TMJ screening protocol and palpation pain were assessed. Adjusted linear regression analyses of AMIO, PMIO, protrusion, and laterotrusion were performed to evaluate the explanatory factors. Two adjusted models were constructed: model 1 to compare children with JIA and healthy children, and model 2 to compare children with JIA with and without TMJ involvement. Results A total of 298 children with JIA and 169 healthy children were included. Length was an explanatory variable for the mandibular range of motion excursions. Each centimeter increase in length increased AMIO (0.14 mm), PMIO (0.14 mm), and protrusion (0.02 mm). Male gender increased AMIO by 1.35 mm. Having JIA negatively influenced AMIO (3.57 mm), PMIO (3.71 mm), and protrusion (1.03 mm) compared with healthy children, while the discrepancy between left and right laterotrusion raised 0.68 mm. Children with JIA and TMJ involvement had a 8.27 mm lower AMIO, 7.68 mm lower PMIO and 0.96 mm higher discrepancy in left and right laterotrusion compared to healthy children. Conclusion All mandibular range of motion items were restricted in children with JIA compared with healthy children. In children with JIA and TMJ involvement, AMIO, PMIO and the discrepancy between left and right laterotrusion were impaired more severely. The limitation in protrusion and laterotrusion was hardly clinically relevant. Overall, AMIO is the mandibular range of motion variable with the highest restriction (in millimeters) in children with JIA and clinically established TMJ involvement compared to healthy children.

Abstract Background Alterations in the composition of the fecal microbiota in children with juvenile idiopathic arthritis (JIA) have been observed in several studies, but it has not been determined whether the standard treatment for JIA changes the composition or function of the microbiota. The first-line disease-modifying anti-rheumatic drug for treatment of JIA is usually methotrexate, followed or supplemented by anti-tumor necrosis factor alpha drugs, such as etanercept. The aim of this study was to investigate the effects of methotrexate and etanercept treatments on the fecal microbiota and the fecal short-chain fatty acids (SCFAs) in children with JIA. Methods In this multicenter study, the composition of fecal microbiota from 45 treatment-naïve children with JIA was compared with that from 29 children treated with methotrexate and 12 children treated with etanercept. We also made pairwise comparisons of 15 children sampled before and during methotrexate treatment and 7 children sampled before and during etanercept treatment. The microbiota was determined using sequencing amplicons from the V3 and V4 regions of the 16S rRNA gene. Alpha-diversity, community composition, and relative abundances of bacterial taxa were analyzed in all comparisons. Analyses of fecal SCFAs, using a high-performance liquid chromatograph, were performed for the pairwise comparisons. Results We did not find any significant differences in α-diversity or community composition of microbiota. However, principal coordinate analysis indicated a change in community composition in 7 of the 15 paired samples before and during methotrexate and 2 of the 7 paired samples before and during etanercept. Comparisons of the relative abundance of taxa revealed minor differences before and during treatment with methotrexate or etanercept, but they were not significant after correction for multiple analyses, and the unpaired and paired analyses did not show similar changes. There were no significant differences in levels of fecal SCFAs before and during treatment with methotrexate or etanercept. Conclusions Treatment with methotrexate or etanercept had minor, but no significant or consistent changes either on composition of microbiota or on levels of SCFAs, suggesting that these changes are not related to the therapeutic effects of methotrexate or etanercept.

Biological agents are widely used in the treatment of autoimmune rheumatic disorders. We report on serious adverse events during treatment with anti-tumor necrosis factor antibody in two of our patients with juvenile idiopathic arthritis. One patient was treated with a biological agent due to juvenile idiopathic arthritis complicated by uveitis, developing miliary tuberculosis during treatment. After treatment with antituberculotics, she recovered completely. Her underlying disease is currently in remission. Another patient was treated for juvenile spondyloarthritis and developed an inflammatory process of the central nervous system with serious neurological deficits. He was treated with high-dose corticosteroids, followed by slowly tapering doses of corticosteroids. His neurological deficits improved, but are still present. Similar cases have been described previously, but there are no recommendations how to treat arthritis afterwards in such patients. We would like to emphasize the need of developing guidelines for further treatment of arthritis after the occurrence of serious adverse effects during treatment with biological agents.

While most countries provide safe and effective influenza vaccines for at-risk groups, influenza vaccine coverage among children with rheumatic diseases remains uncertain. This study investigated influenza vaccination rates in children with juvenile idiopathic arthritis (JIA) during the 2019-2020 season and assessed the knowledge and attitudes of caregivers of children with JIA regarding influenza vaccination. The secondary aims were to identify barriers to vaccination and explore strategies to improve vaccination rates. A multi-centre, cross-sectional anonymous survey was conducted in 7 countries during the 2019-2020 influenza season to assess the uptake history of influenza vaccination. Among 287 participants, only 87 (30%) children with JIA received the influenza vaccine during the 2019-2020 season. Children who were more likely to be vaccinated were those with systemic juvenile idiopathic arthritis (sJIA), a history of previous vaccination and those aware of the vaccination recommendations. Conversely, children who previously experienced adverse vaccine-related events reported the lowest uptake. The primary reason for non-vaccination was lack of awareness about the necessity of influenza vaccination.  Conclusion: Despite variations among countries, the uptake of influenza vaccines remains low in children with JIA. Improving awareness among families about the importance of influenza vaccination may increase vaccination rates in children with rheumatic diseases. What is Known: • Rheumatic children are at increased risk for influenza infection due to immunosuppressive therapy and immune dysregulation. • Influenza vaccine is formally recommended to children with rheumatic diseases. What is New: • This multicentre study showed that influenza vaccine uptake rates remain suboptimal among children with Juvenile Idiopathic Arthritis despite formal recommendations. • Factors like previous experience with vaccination and information provided by medical professionals via different ways play essential roles in increasing vaccination rates and can contribute to improved health outcomes for these vulnerable children., (© 2024. The Author(s).)

Objective: Given the multifactorial pathogenesis of juvenile spondyloarthritis (JSpA) and evidence of a protective effect in phenotypically similar diseases, we aimed to test whether breastfeeding is associated with the development and disease activity of JSpA., Methods: This single-center retrospective case-control study included children with JSpA and age- and sex-matched controls with a 1:1 ratio. Univariable and multivariable conditional logistic regression modeling for matched pairs was used to test the association of infant factors with the development of JSpA, including infant nutrition and form of delivery. Linear regression was used to assess the association of JSpA disease activity (JSpA Disease Activity Index with 6 elements [JSpADA6]) at presentation with breastfeeding exposure, form of delivery, and antibiotic exposure., Results: For the 195 case-control matched pairs, the mean age was 13.0 years and 47.7% were female. For breastfeeding, 88.7% of controls and 69.2% of JSpA cases were exposed to breastfeeding of any duration, respectively ( P < 0.001). In the multivariable model, exclusive breastfeeding > 6 months was independently and significantly associated with a lower chance of JSpA development (odds ratio 0.47, 95% CI 0.30-0.72; P < 0.001). The median JSpADA6 was not significantly associated with breastfeeding for > 6 months. However, vaginal delivery was significantly associated with a lower JSpADA6 (B = -0.65, 95% CI -1.13 to -0.17; P = 0.008)., Conclusion: This study suggests that infant factors that affect the microbiome may be associated with the occurrence and disease activity of JSpA at presentation., (Copyright © 2024 by the Journal of Rheumatology.)