Chikungunya virus (CHIKV) is an enveloped RNA positive-strand alphavirus, transmitted by Aedes mosquitoes, belonging to the Togaviridae family and to the arthritogenic Semliki forest virus (SFV) serocomplex, which includes the well-studied Ross River virus (RRV) 1. CHIKV is now recognised to target human epithelial and endothelial cells, fibroblasts, dendritic cells, B cells and macrophages 23, as well as human muscle satellite cells 4.

The virus is known to cause a wide range of acute manifestations combining fever, arthritis, arthralgias, myalgias, rash and fatigue 56. Furthermore, CHIKV infection often leads to prolonged joint pain, which may harbour two different tempos; either a continuous burden or relapse attacks that characterize the hallmark of Chikungunya rheumatism (CHIK-R) 6789. CHIKV might also trigger or reveal inflammatory rheumatic diseases (IRDs) such as rheumatoid-like arthritis (RA) or psoriatic arthritis (PsA) 1011. In this context, routine biomarkers (C-reactive protein, CRP; erythrocyte sedimentation rate, ESR), rheumatoid factor, anti-cyclic citrullinated peptide antibodies, HLA-B27 expression, imaging features, or the need for upgraded treatment, usually enable accurate diagnosis. However, for the few CHIK-R patients matching the criteria for IRDs, the differential diagnosis and decision-making for treatment might be even more challenging 121314.

Of note, the pathomechanisms underlying musculoskeletal pain and chronic arthritide after acute CHIKV infection are partially elucidated. It has been hypothesised that these symptoms may come from the early escape of CHIKV from blood monocytes 3 and its subsequent relocation and persistence in synovial macrophages rather than to an autoimmune process 15 as previously observed with RRV infection 16. This hypothesis is supported by a recent macaque model in which Chikungunya disease involves long-term viral persistence in macrophages of lymphoid, liver, joint and muscle tissues 17. Moreover, a positive association between high titres of CHIKV-specific IgG antibodies and long-lasting arthralgias has been observed contemporaneously from a pilot study by our group, and in an Italian cohort, without the pathophysiological significance or clinical impact of such relationship having been sought 1819.

To date, the predictors of the course of CHIK-R have so far been poorly investigated 8. According to Sissoko and coworkers, age over 45 years, severe initial joint pain and underlying osteoarthritis are independent indicators of non recovery 8. These findings are consistent with those reported in the seminal descriptions of CHIK-R 2021, or found in recent case series 2223, which showed a correlation between the age at disease onset and the duration of rheumatic symptoms.

Recently, we conducted the TELECHIK cohort study with the aim to address the topics of the burden of Chikungunya and perceived morbidity in the community after the 2005 to 2006 outbreak 24. In this context, we performed this ancillary study to search for prognostic factors of long-lasting rheumatic musculoskeletal pain (RMSP) indicative of CHIK-R. We took the opportunity of this research to test several hypotheses with regard to a putative involvement of the host immune response in the pathomechanism of CHIK-R. Long-lasting RMSP could be driven by either the initial viral load or by the intensity of viral exposure under the challenge of repeated infective bites and waves of CHIKV in La Réunion, or by ongoing antigenic stimulation due to the persistence of CHIKV in host sanctuaries, using the value of CHIKV-specific IgG antibody titre measured at plateau phase, at the time of the seroprevalence study 25, as a proxy.

Among the 422 subjects included, 50 children (age < 15 years) and 26 adults were ruled out due to fear of poorly characterizing the course of CHIK-R, as well as to enhance benchmarking with some of the rare articles published in the field 678181920213031. The study participation profile is displayed in Figure 1.

Three hundred and forty-six subjects were analyzed for the determinants of CHIK-R on average two years after infection (range 15 to 34 months). The median age was 50 years (range 15 to 91 years). The sex ratio of men to women was 0.61 (131 to 215). All participants were assessed for the CHIK-V-specific IgG value on an average of 232 days (7.6 mo) after acute infection (range 1 to 18 mo).

The characteristics and the localizations of CHIK-R at disease onset in this population are presented in Table 1. In bivariate analysis, women tended to report lingering RMSP more often than men (42.2% vs 31.9%, P = 0.083). The intensity of the CHIK-R outcome increased with age, from the recovery, passing through the relapsing, to the lingering group (P < 0.001). Participants with an underlying comorbidity were older than those without. Among the latter, those with a pre-existent rheumatic disorder were the oldest. Body mass index and previous rheumatic disorders were not linked to CHIK-R outcome. The subjects with two or more comorbidities were more likely to declare chronic RMSP than those without an underlying condition. The presence of MetS was associated with both relapsing (crude OR 2.31, 95% CI 1.04, 5.12) and lingering RMSP (crude OR 3.93, 95% CI 1.35, 8.35) although this link was skewed by adjustment for age (data not shown). Among the criteria defining MetS, type-2 diabetes mellitus and dyslipidaemia were linked irrespectively to the expression of CHIK-R, whereas arterial hypertension was associated restrictively with lingering RMSP. Renal failure was also associated with CHIK-R outcome.

Exposure score was not related to CHIK-R, making obsolete the hypothesis of a relationship between recurrent exposures and the expression of RMSP. On the other hand, there was a significant non linear positive association between CHIKV-specific IgG titres and the expression of CHIK-R. Thus, CHIKV-specific IgG level averages increased from the recovery, passing through the relapsing, to the lingering group (P < 0.001).

In a full multinomial logistic regression model (Table 2) controlling the exposure level, gender and underlying comorbidities (MetS, history of IRDs and other chronic conditions), the predictors of relapsing RMSP were age ≥ 60 years, severe initial rheumatic involvement, and increasing CHIKV-specific IgG titres. Prognostic factors for lingering RMSP were age ≥ 45 years (increasing OR from the group aged 45 to 59 years group to the group aged ≥ 60 years), severe initial rheumatic involvement and increasing CHIKV-specific IgG titres.

The key predictors of relapsing or lingering RMSP were identified in the minimal multinomial logistic regression model adjusted for gender after dropping out the exposure score and underlying comorbidities. As a result, independent factors were age ≥ 45 years (with increasing OR from the group aged 45 to 59 years group to the group aged ≥ 60 years), severe initial rheumatic involvement and increasing CHIKV-specific IgG titres (Table 3). Of note, the OR of the predictors of CHIK-R increased with the intensity of CHIK-R (higher for lingering than for relapsing RMSP). The same conclusion was reached taking relapsing RMSP plus recovery as the reference (Additional file 3).

Interestingly, CHIKV-specific IgG titres were linked to female gender and correlated with age but not with the probability of infection (Table 4). The highest sex-related difference in IgG levels was observed in the reproductive age group (Figure 2a). Concurrently, the highest age difference was recorded in male individuals (Figure 2b). The CHIKV-specific IgG titre values also correlated with the intensity of initial rheumatic involvement through an age-dependent gradation that reached significance in the oldest subjects (Figure 2c). However, in detail, a severe clinical picture contributed to increasing IgG titres only in young women (Figure 2d).

Thus, CHIKV-specific IgG titres correlated with the intensity of CHIK-R but not to the infection load, which argues against the notion that the rise of CHIKV-specific IgG titres may be due to re-infection following new introduction and waves of CHIKV in La Réunion. In another manner, the sex and age-related discrepancies observed across the different clinical pictures at presentation do not favor major involvement of the viral load in the pathomechanism of CHIK-R. We conclude that the host humoral response to the persistence of ongoing antigenic stimulation is a plausible clue to long-lasting RMSP.

Here we report a prognostic survey of CHIK-R based on the subset of participants reporting RMSP at the onset of CHIKV infection, who were extracted from the TELECHIK cohort study, a population based-study aimed at assessing the disease burden and perceived morbidity in La Réunion island community after the 2005 to 2006 CHIKV outbreak 24. The major findings of our survey confirm the incidence at population level of CHIK-R across different variable categories, and the prognostic values of advanced age (≥ 45 years), severe initial rheumatic involvement at presentation, and rise in CHIKV-specific IgG titre through the recovery/chronic phases to predict the severity of CHIK-R over time. Thus, among previously uninfected individuals 25, the subjects ages 45 to 59 years and particularly the oldest subjects (≥ 60 years) were more likely to evolve towards relapsing and lingering chronic RMSP in an age-dependant dose-response manner. In addition, the risk estimate for severity of CHIK-R among the individuals in the 45-59 years age group was even increased towards the categories of outcome (that is, from the relapsing to the lingering feature group) in comparison with younger peers (aged 15 to 29 years). In the same way, the subjects who experienced severe initial rheumatic involvement (six or more painful sites with at least four other symptoms) at the acute stage of infection were more likely to exhibit chronic RMSP on follow up. Concurrently, the subjects who developed a strong CHIKV-specific IgG response shared a similar issue. These results were robust and were observed across different regression models controlling the exposure level to the infection (that is, the infection load) and major confounders such as female gender and underlying comorbidities. Likewise, these two factors have been previously recognized to be associated with poor health-related quality of life in people living with several musculoskeletal conditions, including IRDs, osteoarthritis, low back pain and soft tissue disorders, as reported in an Italian population 29.

It is noteworthy that in bivariate analysis female gender tended to be associated with lingering RMSP. Consistently, this finding has been previously reported for persistent arthralgias among CHIKV-infected subjects followed in the Italian cohort in the setting of the 2007 local CHIKV outbreak 19. Of note, the strength of this association was modulated in our cohort by the level of the humoral immune response, which was found to be much higher among female than male patients from both the Réunion (P < 0.001) and Italian Chikungunya cohorts (Figure 2a). Such a gender difference in the CHIKV-specific IgG level was found at the baseline and may be due to the natural estrogen-driven shift towards the Th2 cytokine profile for promoting antibody production in women, or to the inhibitory effect of dihydrotestosterone (DHS) on B cell functioning in men 32. On an other hand, it may also be due to higher production of immunosuppressive IL-10, which has been found to correlate positively with DHS and negatively with serum IgG₃ titres in male patients with other viral infections 3334. These same mechanisms may have driven the discrepancy between IgG titres following CHIKV infection in men and women. Interestingly, the isotypic switch observed in Singapore CHIKV-infected patients referred exclusively to the IgG₃ subclass 35. Though we could not characterize the contribution of specific IgG isoforms, our results are consistent with those of the Italian cohort in which IL-10 levels decreased over time following the acute stage, in parallel with the rise in specific IgG level 36.

In the present study, more than 50% of the subjects ≥ 45 years of age at the time of infection still suffered from lingering RMSP on average two years after acute infection (Table 2). Other contributors have found a similar burden 6782223. The prognostic value of advanced age as a harbinger of CHIK-R at disease onset has gained attention since the seminal description of CHIK-R 2021. Consistently, the positive link between age and the duration of RMSP has been confirmed in recent years by several cohort studies 681923. In line with these authors, we found a strong dose-response between increasing age over 45 years and the severity of CHIK-R, arguing for a causal relationship linking age to the duration and severity of CHIK-R. Moreover, a positive association between high titres of CHIKV-specific IgG in the plateau phase and long-lasting arthralgias has been observed contemporaneously from a pilot study by our group and in the Italian cohort 1819. A possible mechanistic hypothesis may be that an imbalance towards B cell expansion and differentiation, in response to IL-6 secretion following the progression of both immunosenescence 37 and Chikungunya 353638, is triggered by viral persistence in host sanctuaries 151718.

The contribution of underlying comorbidity has long been considered a prerequisite to the subsequent expression of CHIK-R 5678181920212223243132, and a pre-existing rheumatic disorder was reported among 44% of the patients hospitalized after a CHIKV infection in La Réunion 7. A history of arthralgia before the outbreak was associated with persistent joint pain in the Italian cohort 19. Of the rheumatic disorders that could contribute to CHIK-R, RA 101115, seronegative spondylarthritis 1031, soft tissue rheumatism 11, osteoarthritis 78, and gout 8 have been reported to afflict CHIKV-infected patients. Previously, in the TELECHIK study, we were able to link osteoarthritis to long-lasting RMSP in the whole cohort 24, but failed to replicate this result in the subset of the arthralgic participants at disease onset. Indeed, rheumatic disorders were more likely to be associated with chronic RMSP when associated with one or more components of the MetS (that is, obesity, type-2 diabetes mellitus, hypertension or dyslipidaemia). However, this relationship did not resist to adjustment for age. Importantly, arterial hypertension and cumulative comorbidities have previously been associated with CHIK-R 8. Therefore, a high incidence of lingering RMSP was observed in MetS-positive subjects (53.8%). On the one hand, CHIKV has been suggested to trigger auto-immune IRDs such as RA or PsA 1112 and there is a growing body of evidence linking IRDs to MetS 39, especially RA or PsA 40. On the other hand, undifferentiated arthralgias according to rheumatologic guidelines 121314, which represent the hallmark of CHIK-R, should be considered for specific involvement in systemic inflammation and enhancement of MetS, assuming these two risk factors are key issues in the increased incidence of cardiovascular diseases occurring among IRD patients 40. The issues should be considered of importance and deserve further studies to close a putative gap between Chikungunya and MetS.

The prognostic value of severe initial rheumatic involvement for predicting long-lasting RMSP is classical with alphaviruses, and encompasses most of the human pathogens included in the SFV serocomplex plus the Sindbis virus 14142. For CHIKV, it was first reported in La Réunion 8 and subsequently confirmed in other settings 19222330. The intensity of symptoms at disease onset has been linked to plasma viral load, CRP level, and Th1-driven cytokine traffic 31541424344. Indeed, in our study, a positive link between the intensity of symptoms at disease onset and the course of CHIK-R was found beyond the distribution of CHIKV-specific IgG titre values (Tables 2 and 3, and Additional file 3). Indeed, the relationship was found independent of IgG levels, while observing a correlation between the severity of the clinical picture and the IgG level (Table 4), highlighting a proper role of each of these factors on the outcome. This relationship may involve the pivotal action of IL-6 in the recovery phase of infection 353638, which favours the switch from innate to adaptive immunity leading to B cell expansion and IgG₃ production 35. Although controversial, IL-6 secretion at the acute stage may also explain the concomitant increase in CRP level 543 and its predictive value for CHIK-R 1531.

In the absence of evidence of early-onset autoimmunity or circulating immune complexes 15, the proper role of IgG on CHIK-R remains elusive. From our study, the fact that non-linearity characterizes the positive association between CHIK-R and specific IgG levels argues for the contribution of other factors. The possibility of mixed cryoglobulinemia has been suggested 45 but not replicated in observational studies 18. Long-term persistence of IgM has been reported with CHIKV 27, sometimes associated with erosive arthritis 46, so that it is the timing of antibody commutation rather its significance (putative persistence of CHIKV antigens) that would be of critical importance.

We could not draw insights from the absence of a link between the exposure level and the course of CHIK-R, or between the infection load and CHIK-specific IgG titres. This could be due to the fact that our exposure scoring system did not capture the host behavior after infection throughout the epidemic, or that of the risk of being infected changed over time due to vector control measures. With the potential contribution of memory B cells to the level of CHIKV-specific IgG titres and subsequent related immunopathology, this negative finding does not imply failure to take care to prevent the burden of CHIK-R over the course of long-lasting CHIKV outbreaks. Indeed, the people at risk for CHIK-R should now be targeted using anti-Ae. albopictus saliva IgG abs 47.

This study has some strengths and limitations. The limitations inherent in the TELECHIK study have been reviewed extensively elsewhere 24. Regarding the rheumatologic aspects of Chikungunya, one of the main drawbacks to inference would be that in a declarative survey, patients were not examined physically by a clinician to rule out differential diagnoses and confirm the incapacity. However, we had previously demonstrated that the role of subjectivity was negligible in the TELECHIK study 24. Here, we have shown that the spectrum of early clinical manifestations declared at disease onset are slightly overestimated compared to those described in the setting of medicine or rheumatology clinics 56. This discrepancy could be explained in our study by the selection of more female and older participants, highly susceptible to CHIK-R, as well as by the enlargement of the window of disease onset due to the retrospective assessment of complaints, rather than to significant recall bias. Indeed, clinicians may also be inattentive to subtle complaints in the context of being overwhelmed by the number of consultations during epidemics. In that case, we also demonstrate in another sensitivity analysis that our findings are robust and valid for the most pejorative expression of CHIK-R (Additional file 3). Thus, we are confident that declaration bias was unlikely to have changed the scope of our findings.

The TELECHIK cohort study provides valuable clues in the field of rheumatology by contributing to strengthen the recognition and interpretation of the predictors of CHIK-R. Our findings also demonstrate the putative prognostic value of the IgG host level assessed in the recovery/chronic phase times. Thus, the standardized assessment of the engagement of the humoral response and the timing of production of neutralizing IgG₃ might help to identify CHIKV patients at risk for long-lasting RMSP. This issue might also change the paradigm from almost exclusive interest in the innate immune response to adaptive immune trafficking, and thus pave the way to new avenues of research into Chikungunya 3536. Consistently, these perspectives should focus on an integrative approach of exploring the timing of the IgG switch and B cell signalling, innate/adaptive collaborations, chronic infection and aging 37, or potential roads to late-onset autoimmunity 48 and lymphoproliferative disorders 49.

abs: antibodies; AS: ankylosing spondylitis; CHIK-R: Chikungunya rheumatism; CHIKV: Chikungunya virus; CIR: cumulative incidence rate; CNIL: Commission Nationale Informatique et Libertés; CPP: Comité de Protection des Personnes; CRP: C-reactive protein; DHS: dihydrotestosterone; ELISA: enzyme-linked immunosorbent assay; ESR: erythrocyte sedimentation rate; HLA: human leucocyte antigen; IFN: interferon; IgG: immunoglobulin G; IL: interleukin; Insee: Institut National des Statistiques et des Etudes Economiques; IRDs: inflammatory rheumatic disorders; IRR: incidence risk ratio; MetS: metabolic syndrome; OR: odds ratio; PsA: psoriatic arthritis; RA: rheumatoid arthritis; RMSP: rheumatic musculoskeletal pain; RRV: Ross River virus; SFV: Semliki Forest virus.

The authors declare that they have no competing interests.

PG1 analyzed the data, drafted and reviewed the manuscript; AF helped to design the study, analyzed the data, reviewed the data for consistency and errors, as also the manuscript; AM, EB, GB1, BAG, DM, GB2 and FF reviewed the manuscript, each in his field of expertise for consistency and perspectives; CM performed telephone interviews, and entered the data in the computer file; KB performed data management; OR helped to analyze the data; AM, PG2 and SK performed serologies; FF was the principal investigator of the SEROCHIK and TELECHIK surveys. All authors read and approved the final manuscript.