MRSA 2017

The number of persons who had methicillin-resistant Staphylococcus aureus (MRSA) detected for the first time in 2017 was in line with the number observed in 2016. A total of 3,579 cases were recorded in 2017 compared with 3,550 in 2016, Figure 1.

Epidemiological classification

Based on presumed mode of infection, MRSA is categorised into the following categories: acquired abroad, hospital-acquired, community-acquired and livestock-associated. 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 (2.998, 84%), whereas infection abroad was stated in 581 cases (16%) compared with 712 in 2016 (20%). The number of hospital-acquired cases increased to 100 from 43 cases in 2016, but still comprises only a limited share of the total number of cases (2.8%). The number of MRSA cases of the livestock type (CC398) observed in 2017 (1,212) was in line with the number observed in 2016 (1,249) and constituted 34% of the total number of cases. Community-acquired MRSA cases with no contact to hospitals or to the nursing sector comprised 1,402 cases in 2017 (39% of all new cases), which is a slightly larger share of the new cases than recorded in 2016 (1,280 cases, 36%). In 744 of these cases (53%), there was known exposure to a person with MRSA; in the overwhelming majority of cases a member of the household (92%).

Infections

In 1,471 cases (41%), sampling was performed due to clinical infection. In 46 cases, MRSA was isolated from blood, corresponding to 2.2% of all S. aureus bacteraemia cases, which is in line with the level observed the previous year (41 cases, 2.1%). A somewhat higher number of hospital-acquired infections was recorded in 2017. In 44 patients and 19 hospital staff members, an MRSA infection was detected, whereas the total number in 2016 was 43. The number of community-acquired infections increased slightly in 2017 to 654, whereas the preceding years recorded 456 cases in 2014, 500 in 2015 and 607 in 2016, Figure 2. In 144 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 has remained nearly identical since 2014 (155 in 2016, 133 in 2015, 140 in 2014), Figure 2.

Typing

MRSA of the CC398 type was detected using specific PCR analysis, which also showed if the strain was of the livestock MRSA type (n = 1,212) or a human variant (n = 39). Isolates with a characteristic livestock MRSA CC398 profile were typically not spa-typed.

The remaining 2,328 isolates (65%) represented 340 spa types from 26 different clonal complexes (CC groups). The two major groups, CC22 (n = 366) and CC5 (n = 354), comprised 16% and 15% of the 2,328 isolates, respectively, and represented 34 and 33 different spa types, respectively. The 10 most frequently observed spa types comprised 50% of the isolates. Hereof, t304 (n = 249; CC8) and t223 (n = 198; CC22) were the most frequently observed spa types.

Outbreaks

Via the notification forms and typing of the submitted isolates, a total of 14 outbreaks were identified at hospitals, nursing homes and other institutions, comprising a total of 105 MRSA cases, including 21 with infection and 84 detected by screening. Among the 14 outbreaks, four occurred in neonatal departments and three in surgery departments. Two outbreaks in neonatal departments comprised 26 (spa-type t005) and 24 (spa-type t690) cases, respectively. Hereof, the majority (24 and 22, respectively) were carriers. The remaining outbreaks counted 2-9 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 is caused by a clone within a commonly occurring spa-type, whole genome sequencing may provide an important contribution towards identifying which isolates form part of the outbreak, and which do not. Thereby, the exact extent of the outbreak may be determined more precisely.

Livestock MRSA CC398

The number of livestock MRSA CC398 cases decreased slightly from 1,249 in 2016 to 1,212 in 2017. Until 2014, when the largest number of cases was identified (1,276), the number rose considerably but has since remained stable. In 2017, a total of 272 (22%) had a clinical infection at the time of diagnosis compared with 218 (17%) in 2016, Table 1; and 1. 84% of the cases were detected in persons with direct contact to pigs or in persons whose household members had direct contact to pigs. A total of 21 persons had indirect contact to animals through their occupational activities, e.g. butchers or barn builders. A total of 172 (14%) persons had no known contact to animals, which is slightly below the level observed in 2016 (209 persons, corresponding to 17%).

Among these, 92 had a clinical infection, corresponding to 34% of all detected infections with livestock MRSA CC398. A total of four cases of livestock MRSA CC398 bacteraemia were seen in 2017.

All four patients remained alive 30 days after their sampling date.

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. In 2017, contact to mink comprised 41 cases of livestock MRSA CC398, which was an increase compared with 2014-2016 when 15, 15 and 26 annual cases were detected, respectively.

Imported MRSA cases

The number of imported cases decreased from 712 in 2016 to 584 in 2017, corresponding to 16% of the total number of cases. One of the causes explaining this decrease is the lower number of refugees arriving to Denmark in 2017 compared with the preceding years. Finding of MRSA in asylum seekers was reported separately as from 2015 as these cases are often from areas with a high MRSA incidence, and in Denmark this group is offered treatment only if the logistic conditions permit so. In 2017 a total of 119 new MRSA cases among asylum seekers and people who were family reunified with asylum seekers were recorded. This is a considerably lower number than in 2016 (417).