The number of persons who had methicillin-resistant Staphylococcus aureus (MRSA) detected for the first time in 2018 was in line with the number observed in the two previous years. In 2018, a total of 3,669 cases were recorded, compared with 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 6 months, and if 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 (3.081, 84%), whereas infection abroad was stated in 588 cases (16%) compared with 581 in 2017 (16%). The number of hospital-acquired cases recorded was 100 (also 100 in 2017) and still comprised only a limited share of the total number of cases (2.7%). The number of MRSA cases of the livestock type (CC398) observed in 2018 (1,214) was in line with the number observed in 2017 (1,212) and in 2016 (1,249) and comprised 34% of the total number of cases. Community-acquired MRSA cases with no contact to hospitals or to the nursing sector comprised 1,457 cases in 2018 (40% of all new cases), which is on a par with the share of the new cases recorded in 2017 (1,402 cases, 39%). In 797 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%).
In 1,478 cases (40%), sampling was performed due to clinical infection. In 37 cases, MRSA was isolated from blood, corresponding to 1.6% of all S. aureus bacteraemia cases, which is slightly lower than the level observed in 2017 (46 cases, 2.2%). In 39 patients and 20 hospital staff members, an MRSA infection was detected; in 2017, the total number was 63. The number of community-acquired infections decreased slightly in 2018 to 636, whereas the preceding years recorded 456 cases in 2014, 500 in 2015, 607 in 2016 and 654 in 2017, Figure 2. In 109 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 6 months increased slightly in 2018 (186) compared with 2017 (161), 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 = 1,215) or a human variant (n = 35). Isolates with a characteristic livestock MRSA CC398 profile were typically not spa-typed.
The remaining 2,419 isolates (66%) represented 362 spa types from 26 different clonal complexes (CC groups). The two major groups, CC22 (n = 383) and CC5 (n = 355), comprised 16% and 15% of the 2,419 isolates, respectively; and represented 42 and 34 different spa types, respectively. The 10 most frequently observed spa types comprised 44% of the isolates. Among these, t304 (n = 234; CC6) and t223 (n = 213; CC22) were the most frequently observed spa types.
Via the notification forms and typing of the submitted isolates, a total of 16 outbreaks were identified at hospitals, nursing homes and other institutions. These comprised a total of 111 MRSA cases, including 18 with infection and 93 detected by screening. Among the 16 outbreaks, six occurred in neonatal departments. The largest outbreak comprised 59 cases (spa-type t223) and spread to a total of four Danish neonatal departments. The majority of the cases were carriers, 53 cases, whereas six had become infected. The remaining outbreaks were minor, counting 2-8 cases.
The Staphylococcus Laboratory at 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 in 2018 was in line with the level observed in 2017 with 1,215 and 1,212 cases, respectively. Until 2014, when the largest number of cases was identified (1,276), the number rose considerably, but it has since remained stable. In 2018, a total of 256 cases (21%) had a clinical infection at the time of their diagnosis, compared with 272 (22%) in 2017, table 1.
A total of 85% of the cases were found in persons with direct contact with pigs or in household members to these persons. A total of 26 persons had indirect contact to animals through their occupational activities, e.g. butchers or barn builders. A total of 158 (13%) persons had no known contact to animals. This is slightly below the level observed in 2017 (172 persons, corresponding to 14%). Among these, 87 had a clinical infection. Contact to livestock, particularly to 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 2018 compared with 2017 therefore does not indicate any new route of infection for livestock MRSA for humans in Denmark. 2018 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. 2018 recorded contact with mink in 34 cases, which is somewhat lower than in 2017 (41 cases), but higher than in the 2014-2016 period during which the annual number of cases detected was 15, 15, and 26 cases, respectively.
Imported MRSA cases
The number of imported cases was 588, which is in line with the number of cases observed in 2017 (581). This number corresponds to 16% of the total number of cases. Findings of MRSA in asylum seekers has been stated separately since 2015 as they frequently arrive from areas with a high MRSA incidence. 2018 recorded a total of 44 new MRSA cases among asylum seekers and people who were family reunified with asylum seekers, which is a considerably lower number than in 2016 (417) and 2017 (119).
This annual report is described in EPI-NEWS 26/19.