What drives the higher incidence of psychosis in London compared to Palermo?

Background: Incidence of psychosis seems to be lower in Italy than inother European countries (Tansella et al. 1991; Lasalvia et al. 2012,Tarricone et al. 2012); however there are no studies comparing theincidence of psychotic disorder in Northern and Southern Europe.Methods: Incidence and socio-demographic data on all psychoticpatients presenting for the first time to the mental health services ofPalermo were collected over a period of three years.Palermo incidence rates were compared to South London ratesobtained from the AESOP study (Kirkbride et al 2006). South Londonrates were reanalyzed excluding people aged 16-17 years andsubstance related psychoses. Second generation migrants (peoplewho were born in UK belonging to ethnic minorities) were alsoexcluded to make the sample comparable since migration in Palermoreferred to people who were not Italy born. The term migrants wasused in the present analysis to indicate non-native born British andItalians respectively.Incidence rates of overall psychosis, schizophrenia, affective psychosesand other non-affective psychoses were compared in Palermo and inSouth London by indirect standardization (by age and gender andthen by age, gender and migration) to take into account thedifferences in the population structures between sites. Standardizedmorbidity ratios (SMRs) and their reciprocal of overall psychoses,schizophrenia, other non-affective psychoses and affective psychoseswere calculated.Results: During the study period two hundred and four patientsaffected with a first episode of psychosis (FEP) were ascertained inPalermo. South London cases were 195. Standardized incidence ratesof overall psychoses were 16.9 (95% CI 14.7-19.4) per 100,000 per yearin Palermo and 36.8 (95% CI 31.8-42.3) in South London. Migrants hadan increased risk of developing a psychotic disorder both in PalermoOR: 3.12 (95% CI 1.89-4.93) and in London OR: 2.9 (95% CI 2.15-3.93).After standardizing by age and gender the risk of psychosis wassignificantly higher in South London compared to Palermo for allpsychoses 1/SMR = 2.18 (95% CI 1.98-2.39) and for each diagnosticcategory. After standardizing also for migration the difference in riskof overall psychoses between Palermo and London decreased: 1/SMR = 1.39 (95% CI 1.23-1.56) and no differences in risk were foundany longer between Palermo and South London for schizophrenia andother non affective psychoses; there was however an increased risk ofaffective psychoses in South London compared to Palermo 1/SMR =3.31 (95% CI 2.52-4.21). This result confirms that migration explainsthe majority of the difference in incidence rates between SouthernItaly and London.Discussion: This is the first epidemiological study of psychosis evercarried out in Sicily and one of the few from Southern Europe. The riskof psychoses was higher in South London when compared to Palermo.However no significant differences were found in rates of schizophreniaand other non-affective psychoses after taking into accountthe different proportion of migrants in the two sites suggesting thatmigration might explain the majority of the difference in the risk ofpsychosis. However, it did not explain all the difference as there wasstill an excess of affective psychoses in South London. Further studiesare needed to explore the role of other risk factors (Mediterraneandiet, vitamin D, social fragmentation, drug use) in influencing the riskof psychosis.