For the study period, 61,522 S. aureus–associated hospital discharges were identified. The overall averaged 12-year incidence rate for all S. aureus infections was 127 (95% CI 122–133) per 100,000 population per year. The overall incidence rate for S. aureus SSTIs was 108 (95% CI 105–111) per 100,000 population, S. aureus sepsis 14 (95% CI 13–16) cases per 100,000, and staphylococcal pneumonia 5 (95% CI 4–6) cases per 100,000. The incidence rate for sepsis caused by S. aureus and pneumonia caused by staphylococci did not change significantly over the study period; however, the incidence rate for S. aureus SSTIs increased significantly, from 81 (95% CI 78– 84) cases per 100,000 population in 2000 to 140 (95% CI 137–144) cases per 100,000 in 2011 (p<0.001) (Figure 1), which represents an increase of ≈5% each year. In contrast, the rate of acute all-cause hospital discharges in New Zealand fell significantly, from 9,657 (95% CI 9,625–9,689) per 100,000 population in 2000 to 8,701 (95% CI 8,673–8,729) per 100,000 population in 2011 (p<0.001). Consequently, the relative proportion of S. aureus SSTIs to all hospital discharges doubled, from 0.8% in 2000 to 1.6% in 2011.

Figure 2

Figure 2. Average annual ASR (nocases/100,000 population) of staphylococcal sepsis (A) and staphylococcal skin and soft tissue infections (B), New Zealand, 2000–2011ASR, age-standardized rate.

Incidence of staphylococcal pneumonia did not vary significantly by geographic location (p = 0.8); however, incidence of staphylococcal sepsis (p = 0.02) and SSTIs (p = 0.01) did (Figure 2). In particular, there was a distinct north–south gradient for staphylococcal SSTIs; rates in the northern and central regions were ≈3 times rates in the southern region.

Incidence of S. aureus infections also varied markedly by sociodemographic characteristics (Appendix Table). Staphylococcal infections of all forms were significantly more likely to occur among male than female patients; this difference was most marked for S. aureus sepsis (relative risk [RR] 1.9; 95% CI 1.8–2.0). The incidence rates for sepsis and pneumonia were significantly higher among patients >70 years of age (62 and 24 cases/100,000 population/year, respectively) than among patients of other age groups (Appendix Table). In contrast, the incidence rate for S. aureus SSTIs was highest among those <5 years of age (242 cases/100,000 population/year). The incidence of all disease types was highest among Māori and Pacific Peoples (Appendix Table). In particular, Māori were 3 times more likely and Pacific Peoples almost 5 times more likely than Europeans to have an S. aureus SSTI.

Figure 3

Figure 3. Admission rate ratios for Staphylococcus aureus–associated hospital discharges by ethnicity according to level of deprivation, New Zealand, 2000–2011A) Staphylococcal sepsis; B) staphylococcal pneumonia; C) staphylococcal skin and soft tissue infectionsNZDep, New...

The incidence of S. aureus disease also varied significantly according to socioeconomic deprivation; the incidence rates for sepsis, pneumonia, and SSTI were significantly higher among patients residing in areas of high socioeconomic deprivation. This disparity was most marked for SSTIs; patients residing in areas of high deprivation were almost 4 times more likely to have S. aureus SSTIs than were those residing in areas of low deprivation (RR 3.7, 95% CI 3.6–3.8). An independent association seemed to exist between S. aureus disease and ethnicity after socioeconomic status was adjusted for, such that for each tier of socioeconomic deprivation, all 3 types of S. aureus disease were more common among Māori and Pacific Peoples than among those of European or other ethnicity (Figure 3).

In this study, we analyzed the longitudinal incidence and epidemiology of serious S. aureus disease across the entire New Zealand population during 2000–2011. Incidence of S. aureus SSTI increased dramatically while incidence of S. aureus sepsis and pneumonia remained relatively stable. Our finding of a persistent increase in serious S. aureus SSTIs over the past decade is a substantial public health concern, particularly given the overall decrease in acute overnight hospital admissions in New Zealand.

The factors underlying the increase in such infections are unknown, but risk factors for the development of S. aureus SSTI are multifactorial and probably include household crowding, delayed or inadequate access to health care, and issues associated with household hygiene (13,14). The reasons for the relatively unchanged rate of S. aureus sepsis and staphylococcal pneumonia in New Zealand are unclear; however, recent studies highlight the decreasing incidence of invasive S. aureus infections in other geographic settings, particularly among those patients recently exposed to health care facilities or receiving health care (15,16). Although improvements in infection prevention practices probably contribute to the decrease of invasive infections (17), other possible unexplored factors include changes in host susceptibility to S. aureus infection (e.g., improved management of concurrent conditions such as cardiovascular disease and diabetes) or temporal changes in the virulence profiles or transmissibility of circulating S. aureus strains.

Consistent with the findings of other studies of infectious diseases in New Zealand (13,14), we found notable sociodemographic disparity in the incidence of S. aureus infections; incidence of all S. aureus infections was highest among Māori or Pacific Peoples and among those residing in areas of high socioeconomic deprivation. Even after adjusting for socioeconomic deprivation, we found that the incidence of all S. aureus disease was significantly higher among Māori and Pacific Peoples than among patients of European and other ethnicities; this pattern is seen for infectious diseases generally in New Zealand (14). The underlying reasons for this apparent ethnic disparity in staphylococcal disease are uncertain. Unexplored possibilities include a higher prevalence of S. aureus colonization among Māori or Pacific Peoples or differences in the circulating S. aureus strain types among distinct ethnic groups, as previously described for our setting (4). However, an alternative possibility is that the area-based NZDep score used to record socioeconomic deprivation does not fully represent those facets of poverty that contribute to the development and prevention of serious S. aureus disease. These unmeasured risk factors include aspects of health literacy relating to early management of insect bites and skin infections, availability of household amenities such as hot water, and affordable and timely access to health care. Specific individual-level and household-level studies are required for determination of the relative contribution of such potentially modifiable risk factors.

We also observed significant geographic variation in the incidence of S. aureus SSTI, with a distinct north–south gradient. This finding can probably be explained by the distribution of population groups in New Zealand; the groups most affected by S. aureus SSTI reside predominantly in the North Island (13). However, other possible contributory factors include geographic differences in access to and provision of health care and climate differences; the climate in the upper North Island is relatively warmer and more humid than that in the southern regions.

A limitation of our study was our use of hospital discharge data for case ascertainment. Use of these data meant that we were unable to determine the proportion of cases occurring in the community versus in the hospital setting, although previous studies have demonstrated that most S. aureus infections in New Zealand originate in the community (4,7,8). However, our aim was not to provide detailed information on individual S. aureus infections but rather to provide a broad overview of the trends and demographics of serious S. aureus infections across the entire New Zealand population. In addition, our data represent only those patients whose hospital discharge was associated with S. aureus disease; they do not represent those patients who sought care from a primary care physician or who sought care at a hospital but were not admitted. For example, a recent study of children with SSTIs in 1 New Zealand region found an estimated 14 primary care cases for every 1 hospital admission (18). Furthermore, these data represent only those instances in which an etiologic agent was described and recorded in the discharge diagnoses. It is therefore highly likely that the overall prevalence of staphylococcal disease in our setting is substantially higher than that estimated here.

In summary, our study provides valuable longitudinal data on the prevalence of serious S. aureus disease in the New Zealand population and represents one of the few studies that systematically assessed the epidemiology and demographics of staphylococcal infections across an entire nation. The steady and significant increase in serious S. aureus SSTI coupled with notable sociodemographic disparity in disease incidence is a disturbing national trend. A concerted multimodal public health intervention is urgently required to tackle this problem.