The number of persons who had methicillin-resistant Staphylococcus aureus (MRSA) detected for the first time in 2020 was considerably lower than the number observed in previous years. In 2020, a total of 2,883 cases were recorded, compared with 3,657 in 2019, 3,669 in 2018, 3,579 in 2017 and 3,550 cases in 2016, Figure 1. The reduction was expected due to the COVID-19-induced travel restrictions, limitations on social contacts and reduced number of healthcare contacts.
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 (exposure) 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 (2,532, 88%), whereas infection abroad was stated for 351 cases (12%) compared with 651 cases (18%) in 2019. The number of hospital-acquired MRSA cases recorded was only 70 (75 in 2019) and thus continued to comprise only a limited share of the total number of cases (2.4%). Community-acquired MRSA cases with no contact to hospitals or to the nursing sector comprised 1,307 cases in 2020 (45% of all cases), which is lower than in 2019 (1,536 cases, 42%). In 659 of these cases (50%), there was known exposure to a person with MRSA; in the overwhelming majority of cases, a member of the household (99%). The number of MRSA cases of the livestock type (CC398) observed in 2020 (931) was lower than the number observed the four previous years (1,122 in 2019, 1,215 in 2018, 1.212 in 2017 and 1,249 in 2016), and comprised 32% of the total number of cases.
In 1,281 cases (44%), sampling was performed due to clinical infection. This was 318 fewer cases than in 2019. In 38 cases, MRSA was isolated from blood, corresponding to 1.6% of all S. aureus bacteraemia cases, which is in line with recent years. MRSA infection was detected in 40 patients and 11 hospital staff members, which is in line with the average recorded for the past ten years (48). The number of community-acquired infections was 622; thus, 2020 breaks with the increasing trend observed in the preceding years, Figure 2. In 90 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 159 in 2020 compared with 174 in 2019 and 186 in 2018, Figure 2. A total of 215 infections were detected among the imported cases in 2020. This represents a considerable decline, but the number was probably affected by the very low travel activity in 2020 due to COVID-19 restrictions.
MRSA of the CC398 type was detected using specific PCR analysis, which also showed if the strain was of the livestock MRSA type (n = 931) or a human variant (n = 31). Isolates with a characteristic livestock MRSA CC398 profile were typically not spa-typed, whereas all other isolates were typed.
The remaining 1,952 isolates represented 326 spa types from 24 different clonal complexes (CC groups). The ten most frequently observed spa types comprised 45% of the isolates. The five most frequently observed spa-types were: t304 (n = 170; CC6), t223 (n = 133; CC22), t127 (n = 129, CC1), t002 (n = 110, CC5) and t008 (n = 107, CC8).
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. Via the notification forms and typing of the submitted isolates, a total of 31 outbreaks were identified at hospitals, nursing homes and other institutions.
These comprised a total of 129 MRSA cases, including 51 with infection and 78 detected by screening and infection tracing. Among the 31 outbreaks, seven occurred in neonatal departments. The largest outbreak comprised 29 cases (spa-type t127/CC1) at several hospitals in the greater Copenhagen area, and cases related to this outbreak were detected throughout 2020. The majority of the cases were carriers, 19 cases, whereas ten cases had a clinical infection. Another outbreak that affected neonatal departments was limited to a single hospital and comprised 11 cases with spa-type t6100/CC5. Furthermore, an additional five cases were added to a neonatal department outbreak initially observed in 2018. The outbreak therefore reached a total of 89 detected cases by 2020 (t223/CC22). Additionally, seven outbreaks occurred at nursing homes (a total of 17 patients) and eight outbreaks occurred at other (non-neonatal) hospital departments (20 patients).
Livestock MRSA CC398
The number of livestock MRSA CC398 cases was lower in 2020 than in 2019 (931 and 1,122 cases, respectively). Until 2014, when the largest number of cases was identified (1,276), the number increased considerably, but it has since remained stable. A total of 234 (25%) had a clinical infection at the time of their diagnosis, compared with 253 (23%) in 2019, Table 1.
A total of 82% of the cases were found in persons with direct contact with pigs or in household members to these persons. A total of 141 (15%) persons had no known contact to animals. Among these, 95 had a clinical infection. Contact to livestock, particularly pigs, is the primary source of infection with livestock MRSA in humans. The number of infected persons with no known contact with livestock is in line with the number recorded in recent years and therefore does not indicate any substantial new route of infection for livestock MRSA for humans in Denmark. 2020 saw a total of ten 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. 2020 recorded a total of 14 patients with direct contact to mink. Furthermore, MRSA was detected in nine of these patients’ household members. This is lower than in the previous years, and we may expect this route of infection to be reduced considerably in years to come.
Imported MRSA cases
The number of imported cases was 351, which is considerably lower than the corresponding number observed in 2019 (651). This number corresponds to 12% 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. 2020 recorded a total of 24 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) and 2019 (28).
This report is also described in EPI-NEWS 25-26/2021