Methicillin-resistant Staphylococcus aureus epidemic in Pirkanmaa

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Arvo building, auditorium F115, address: Arvo Ylpön katu 34.

Elina Jokinen

Doctoral defence of Lic.Med. Elina Jokinen

Methicillin-resistant Staphylococcus aureus epidemic in Pirkanmaa

The field of science of the dissertation is Infectious Diseases.

The opponent is docent Mari Kanerva (University of Helsinki). Professor Katri Kaukinen acts as the custos.

The language of the dissertation defence is Finnish.

Methicillin-resistant Staphylococcus aureus (MRSA) epidemic in Pirkanmaa

Aim of this thesis was to study the epidemiology of methicillin-resistant staphylococcus aureus (MRSA) in Pirkanmaa and the clinical characteristics of Staphylococcus aureus (S. aureus) bacteremia. A large MRSA epidemic broke out in Pirkanmaa county in 2001. From 2001 to 2014 4118 MRSA transmissions were detected and 3527 (85.6%) of them were heath care associated. Transmissions were particularly common in long-term facilities. To combat the epidemic, several infection control measures were established including universal screening and measures to improve hand hygiene. More resources were allocated for prevention work. After the year 2011 the number of MRSA transmission started to decrease and from year 2011 to 2014 the number of annual transmissions decreased 57%.

Strain typing enables identification of transmission chains that are related to the epidemic and of sporadic cases that are not related to the epidemic. In Pirkanmaa MRSA epidemic, 79% of transmissions were caused by a single epidemic strain, t067, but its proportion decreased to 45% in year 2013.

Between years 2005 and 2015 the incidence of S. aureus bacteremia increased from 21.6 to 35.8 per 100 000 population. This is probably due to ageing of the population and increase of invasive medical procedures but also due to better diagnostics. The incidence of MRSA bacteremias increased reaching its top, 4.5/100 000, in the year 2011 but decreased thereafter. The decline of MRSA bacteremias occurred simultaneously with the decline of MRSA transmissions and is probably due to improved hospital hygiene. Of MRSA bacteremias, 82.6% were caused by the epidemic strain, t067. In bacteremias caused by methicillin-susceptible S. aureus (MSSA) strain type distribution was more diverse and no dominating strain could be identified.  The majority of MSSA bacteremias were caused by penicillin-resistant S. aureus (PRSA), but the proportion of penicillin-susceptible S. aureus (PSSA) increased significantly from 23.9% to 43.1% between years 2008 and 2015. The rising proportion of PSSA enables the use of narrow spectrum antimicrobials.

In this study the risk of having MRSA bacteremia compared to MSSA bacteremia was greater in patients who were smokers, on glucocorticoid therapy, had previous surgery or whose bacteremia was health care associated. MRSA bacteremia was associated with greater mortality (odds ratio 2.0) than MSSA bacteremia. Of focus, pneumonia was associated with higher mortality but skin and soft tissue infections and osteomyelitis and spondylodiscitis with lower mortality. Unknown focus was associated with higher mortality, which emphasizes the importance of proper investigations to identify infective focus. Previous glucocorticoid treatment, liver cirrhosis, older age and cardiac disease were associated with higher mortality. There was no difference in mortality or clinical characteristics between PRSA and PSSA bacteremia.

Case fatality decreased from year 2005 (21.8%) to year 2015 (17.1%). This may suggest that the quality of care may have improved. It is also possible, that cases of lesser severity were more frequently detected with higher blood screening rates. The highest mortality was associated with S. aureus strain types t002 and t067 (27.6% and 26.9%, respectively). Strain types were further assigned to bigger groups based on their relatedness. SpaCC 067 –group was associated with the highest mortality (25.6%). The results of this study also suggest that spaCC 008 is more often associated with endocarditis, spaCC 012 with foreign body infections and spaCC 2133 with skin and soft tissue infections than other groups. There are, however, several other factors such as patient characteristics, other virulence factors and treatment that determine the disease course of S. aureus bacteremia.


The dissertation is published in the publication series of Acta Universitatis Tamperensis; 2373, Tampere University Press, Tampere 2018. The dissertation is also published in the e-series Acta Electronica Universitatis Tamperensis; 1879 , Tampere University Press 2018.

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