The number of persons who had methicillin-resistant Staphylococcus aureus (MRSA) detected for the first time in 2019 was in line with the number observed in the three previous years. In 2019, a total of 3,657 cases were recorded, compared with 3,669 in 2018, 3,579 in 2017 and 3,550 cases in 2016, Figure 1. Thus, it seems that the increase observed in the number of MRSA cases, mainly since 2007, has now largely receded.
Based on presumed mode of infection, MRSA is categorised into the following categories: acquired abroad, hospital-acquired, community-acquired and livestock-associated MRSA. In Denmark, livestock MRSA is nearly always caused by the CC398 MRSA type, which is closely associated with live pigs.
Community-acquired MRSA is further divided according to whether the person has had contact to hospitals or the nursing sector (nursing homes, etc.) within the past six months, and whether the person has come into contact with a known MRSA-positive person.
Table 1 shows the distribution of notified MRSA cases by presumed mode of transmission.
The majority of the MRSA cases were acquired in Denmark (3,006, 82%), whereas infection abroad was stated for 651 cases (18%) compared with 588 in 2018 (16%). The number of hospital-acquired MRSA cases recorded was only 75 (100 in 2018) and thus continued to comprise only a limited share of the total number of cases (2.1%). Community-acquired MRSA cases with no contact to hospitals or to the nursing sector comprised 1,536 cases in 2019 (42% of all new cases), which is slightly higher than the share of the new cases recorded in 2018 (1,457 cases, 40%). In 844 of these cases (55%), there was known exposure to a person with MRSA; in the overwhelming majority of cases, a member of the household (97%). The number of MRSA cases of the livestock type (CC398) observed in 2019 (1,122) was lower than the number observed the three previous years (1,215 in 2018, 1,212 in 2017 and 1,249 in 2016) and comprised 31% of the total number of cases.
In 1,599 cases (44%), sampling was performed due to clinical infection. In 46 cases, MRSA was isolated from blood, corresponding to 2.1% of all S. aureus bacteraemia cases, which is slightly higher than the level observed in 2018 (37 cases, 1.6%). In 36 patients and 22 hospital staff members, an MRSA infection was detected; in 2018, the total number was 59. The number of community-acquired infections increased to 721 in 2019, and thus follows the increasing trend observed in the preceding years: 636 in 2018; 654 in 2017; 607 in 2016 and 500 in 2015, Figure 2. In 143 cases of community-acquired clinical infection, there was known exposure to MRSA, Table 1. The number of community-acquired clinical infections in which there was known contact to a hospital or to the nursing sector within the previous six months was 174 in 2019, compared with 186 in 2018 and 161 in 2017, Figure 2. A total of 393 infections were detected among the imported cases in 2019, a number which has followed a steadily increasing trend over the years, Figure 2.
MRSA of the CC398 type was detected using specific PCR analysis, which also showed if the strain was of the livestock MRSA type (n = 1,122) or a human variant (n = 41). Isolates with a characteristic livestock MRSA CC398 profile were typically not spa-typed.
The remaining 2,494 isolates (68%) represented 366 spa-types from 26 different clonal complexes (CC groups). The 10 most frequently observed spa types comprised 47% of the isolates. The five most frequently observed spa-types were: t304 (n = 230; CC6), t223 (n = 181; CC22), t008 (n = 153, CC8), t127 (n = 151, CC1) and t002 (n = 134, CC5).
These MRSA spa-types have also been reported from other countries, which is occasionally helpful in outbreak tracking work.
Via the notification forms and typing of the submitted isolates, a total of 23 outbreaks were identified at hospitals, nursing homes and other institutions. These comprised a total of 157 MRSA cases, including 47 with infection and 110 detected by screening and infection tracing. Among the 23 outbreaks, seven occurred in neonatal departments. The largest outbreak comprised 26 cases (spa type t3595/CC59) and spreading to two Danish neonatal departments. The majority of the cases were carriers, 18 cases, whereas eight had become infected. Another outbreak affecting neonatal departments was associated with an outbreak from 2018 and concerned 25 cases of spa type t223/CC22. This outbreak counted patients from a total of three neonatal departments. Hereof, five patients had become infected with MRSA, whereas the others were carriers. A third outbreak which was linked to a neonatal department comprised a total of 19 patients (two infected with spa type t386/CC1). An additional eight outbreaks comprised nursing homes (counting a total of 38 patients) and two outbreaks included residential schools (11 patients).
Statens Serum Institut offers whole-genome sequencing of all outbreak isolates, i.e. characterisation of the isolates beyond spa type determination. If an outbreak was caused by a clone within a commonly-occurring spa type, whole-genome sequencing may be an important means for identifying and limiting the outbreak. Thereby, the exact extent of the outbreak may be determined more precisely.
Livestock MRSA CC398
The number of livestock MRSA CC398 cases was lower in 2019 than in the preceding years. Until 2014, when the largest number of cases was identified (1,276), the number rose considerably, but it has since remained stable. A total of 253 (23%) had a clinical infection at the time of their diagnosis, compared with 256 (21%) in 2018, Table 1.
A total of 86% of the cases were found in persons with direct contact with pigs or in household members to these persons. A total of 30 persons had indirect contact to animals through their occupational activities, e.g. butchers or barn builders. A total of 129 (11%) persons had no known contact to animals. This is slightly below the level observed in 2018 (158 persons, corresponding to 13%). Among these, 86 had a clinical infection. Contact to livestock, particularly pigs, is the primary source of infection with livestock MRSA in humans. The slightly lower number of infected persons with no known contact with livestock in 2019 compared with 2018 and 2017 therefore does not indicate any substantial new route of infection for livestock MRSA among humans in Denmark. 2019 saw a total of eight cases of livestock MRSA CC398 bacteraemia.
Livestock MRSA CC398 has also been recorded in persons with contact to mink, which is consistent with MRSA findings in mink and their fodder. Contact to mink is therefore included as a special risk situation in the 2016 MRSA guideline. 2019 recorded a total of 20 patients with direct contact to mink. Furthermore, MRSA was detected in five of these patients’ household members. This is lower than the numbers observed in both 2018 (34 cases) and 2017 (41 cases).
Imported MRSA cases
The number of imported cases was 651, which is slightly higher than the number of imported cases observed in 2018 (588). This number corresponds to 18% of the total number of cases. Findings of MRSA in asylum seekers have been stated separately since 2015 as they frequently arrive from areas with a high MRSA incidence. 2019 recorded a total of 28 new MRSA cases among asylum seekers and people who were family reunified, which is a considerably lower number than in 2016 (417), 2017 (119) and also lower than the number recorded in 2018 (44).
This report is also described in EPI-NEWS 23/20.