The epidemiology of psoriatic arthritis in Israel – a population-based study

Psoriasis is a chronic inflammatory skin condition affecting 2–3% of the general population in Europe and North America [1, 2]. Psoriatic arthritis (PsA) is a chronic, potentially destructive inflammatory arthritis affecting people with psoriasis. There is limited data about the population-based epidemiology of PsA. Researchers have estimated that the prevalence of PsA in the general population ranges from 0.05% to 0.67%, whereas the incidence rates range from 6.1 to 41.3 [3–7] per 100,000 population. The majority of these studies were conducted in Europe and North America among people of European origin. Researchers in recent studies in East Asia reported significantly lower prevalence and incidence rates of PsA among Asian individuals [8–10]. There is very little data about the epidemiology of PsA in Middle Eastern populations from various ethnic groups [11, 12]. Assessing the epidemiology of PsA in such populations could contribute to the understanding of the worldwide burden of the disease. Furthermore, because PsA susceptibility is strongly affected by environmental and genetic factors, studying the epidemiology of the disease in ethnically diverse populations from different geographic regions could potentially contribute to the understanding of underlying mechanisms of the disease. The aims of this study were to estimate the prevalence of PsA and its variation among different age, sex, and ethnic subgroups in the general population in Israel and to assess the temporal trends in the population-based prevalence and incidence of PsA in Israel over the past decade.

The prevalence of PsA was slightly higher in females (0.159%; 54% of patients) than in males (0.147%; 46% of patients). The prevalence of PsA peaked in the seventh decade of life, at 0.327%. The age at diagnosis of PsA was higher than reported in previous cohorts from Europe and North America, because over half the patients were diagnosed after the age of 50 years (Fig. 1).

Most patients with PsA had a recorded diagnosis of psoriasis (3869 patients [86.2%]). In the majority (2371 [61.3%]), the diagnosis of PsA was recorded following the diagnosis of psoriasis. In 1061 patients (27.4%), the diagnoses of PsA and psoriasis were recorded in the same year, and in the remaining 11.3% of patients, the diagnosis of PsA preceded the diagnosis of psoriasis.

Temporal trend in the prevalence and incidence of PsA from 2006 to 2015

The crude prevalence of PsA doubled during the study period, from 0.073% in 2006 to 0.153% in 2015 (Table 2). A similar increase was observed in the age- and sex-standardized PsA prevalence: from 0.067% in 2006 to 0.137% in 2015 (204% increase). The crude and the age- and sex-standardized incidence rates remained stable from 2006 to 2015 with (approximately 11 per 100,000 population). The prevalence and incidence rates increased in similar degrees in both sexes over time (data not shown). The temporal trends in sex-standardized prevalence and incidence of PsA by age groups are shown in Fig. 2. A more pronounced increase in the prevalence of PsA was observed in the older age groups (61–70 and > 70 years). A gradual upward trend in the incidence rate was observed among middle-aged individuals (51–60, 61–70 years).

Year	Prevalence of PsA				Incidence of PsA
	Total population (N)	PsA (n)	Crude prevalence (%)	Standardized prevalence (%) (95% CI)a	New PsA (n)	Crude incidence#	Standardized incidencea (95% CI)b
2006	2,620,136	1910	0.073	0.067 (0.065, 0.070)	277	10.6	10.4 (9.5, 11.4)
2007	2,648,067	2132	0.081	0.075 (0.072, 0.077)	251	9.8	9.3 (8.4, 10.3)
2008	2,676,404	2425	0.091	0.084 (0.081, 0.086)	308	11.5	11.4 (10.4, 12.4)
2009	2,705,397	2663	0.098	0.091 (0.088, 0.093)	278	10.3	10.1 (9.2, 11.1)
2010	2,734,705	2938	0.107	0.099 (0.096, 0.102)	303	11.1	11.1 (10.1, 12.1)
2011	2,777,279	3237	0.117	0.107 (0.104, 0.110)	330	11.9	11.7 (10.8, 11.8)
2012	2,816,452	3531	0.125	0.114 (0.111, 0.117)	382	12.5	12.3 (11.3, 13.3)
2013	2,861,934	3866	0.135	0.122 (0.119, 0.125)	354	13.4	13.1 (12.0, 14.1)
2014	2,905,530	4223	0.145	0.131 (0.128, 0.134)	404	13.9	13.5 (12.5, 14.6)
2015	2,931,199	4490	0.153	0.137 (0.134, 0.141)	322	11.0	10.6 (9.7, 11.6)

PsA Psoriatic arthritis

^aAge- and sex-standardized to Israel population in 2006

^bCrude and standardized rates are per 100,000 population

The population-based epidemiology of PsA has been investigated mostly in European populations. Limited information exists about the epidemiology of PsA in other ethnic groups and geographic regions, in particular in Middle Eastern populations. Researchers in a few small clinic-based studies assessed epidemiologic features of PsA in Iranian and Turkish patients [11, 12]. To the best our knowledge, this is the first large-scale population-based study that provides information about the prevalence of PsA in a Middle Eastern population including two ethnically diverse subpopulations. Our estimates of the prevalence of PsA in Israel (0.153%) fall well within the prevalence range reported in other population-based studies in Southern European countries, including Greece (0.06–0.17%) [22, 23] and Turkey (0.05%) [11], but tended to be lower than estimates in the United States (0.25%) [24] and Northern European countries such as Sweden (0.25%) [25], Norway (0.19–0.67%) [6, 26], the United Kingdom (0.19%) [3], and Iceland (0.16%) [27]. Moreover, the prevalence of PsA was significantly higher than the reported prevalence in East Asian populations, which is less than 0.10% [9, 10]. Fewer studies assessed the incidence of PsA. Overall, our estimated incidence rate of 10.9 cases per 100,000 population fell within previous estimates in other European populations, which ranged from 6.1 to 41.3 per 100,000 population [4, 6, 7]. The variation in prevalence rates may be attributed to genetic and environmental factors but also to variations in study designs and case definitions. In general, surveys that were based on self-reported diagnosis of PsA found higher prevalence rates than studies that relied on medical records and health administrative data.

The prevalence of PsA among patients with psoriasis in our study was 4.07%, which is lower than the estimated prevalence in other population-based studies (ranging from 4.6% to 11%) [3, 24, 28]. Possible explanations may include the low prevalence of the HLA-B27 antigen in the Israeli population [29], the older peak age of diagnosis of PsA observed in our cohort (range 60 to 70 years) [3, 26, 27], or possibly other genetic and environmental factors that are unique to our population.

We have demonstrated that the crude and age-adjusted prevalence of PsA doubled during the study period. These findings are in line with recent studies from Europe and Asia in which researchers reported an increase in the prevalence and incidence of psoriasis and PsA over time [7, 9, 30, 31]. The observed increase in prevalence may be partially attributed to an increase in the incidence rate among individuals aged 51 to 70 years, which is the peak age of diagnosis of PsA in Israel. Increased awareness of PsA among physicians over the past decade, possibly promoted by the publication of the Classification Criteria for Psoriatic Arthritis (CASPAR) criteria in 2006, educational activities among specialists, the introduction of effective biologic medications for PsA, and the increased use of sensitive diagnostic modalities (e.g., ultrasound) likely contributed to the observed time period effect. Additionally, with the increasing awareness of the disease, it is possible that rheumatologists are more likely to classify a disease as “PsA” rather than “spondyloarthritis,” which may explain some of the increase in occurrence over the past decade. Other potential causes, such as increased occurrence of known risk factors for PsA (e.g., obesity) at the population level or changes in the population demographics (e.g., immigration of high-risk groups), are less likely to contribute to the observed increase over such a short time period.

Several demographic factors were associated with the occurrence of PsA. As in other studies, PsA was almost equally distributed between women and men (female/male ratio 1.08) [3]. Other factors that influenced the occurrence of PsA were ethnicity, SES, and BMI. The prevalence of PsA was almost twofold higher in the Jewish population than among Arab individuals. This difference in disease occurrence may be related to variations in genetic structure, environmental exposures, or issues related to healthcare use [21]. With regard to impact of genetic heterogeneity within the Jewish population on the occurrence of PsA, information about family origin is unavailable in the CHS database.

This study has several limitations that should be taken into consideration. First, we could not capture patients with PsA who did not seek medical attention or those who were undiagnosed as having PsA. Nonetheless, we estimated the proportion of the population that places a burden on the healthcare system. Second, the case definition is based on diagnostic coding from electronic medical records that are based on physician diagnoses. This may have led to misclassification of cases. This risk, however, was minimized by selection of an algorithm with high accuracy that used data based largely on a diagnosis by relevant specialists. The primary strength of the study is the use of a large representative sample of over half the Israeli population. Consequently, the results of the study could be generalized to the broader population beyond those patients included in our study.

In summary, the epidemiology of PsA is much less studied than in many other rheumatic diseases. High-quality real-world data about the epidemiology of PsA is of clear importance for understanding the burden of the disease on society. Our study, which included over 4000 patients with PsA, is one of the largest population-based studies to date and is the first to include an evaluation of the prevalence of PsA in Middle Eastern individuals. The prevalence of PsA in the adult Israeli population in 2015 was 0.153%. The reported prevalence of PsA has doubled over the past decade, highlighting the rising medical needs and economic burden of this patient population. Further studies are needed to continue monitoring this trend and understand its underlying causes.