MRSA - disease prevalence report 2021

MRSA 2021

In 2021 as in 2020, the number of persons who had methicillin-resistant Staphylococcus aureus (MRSA) detected for the first time was considerably lower than the number observed in the previous years. In 2021, a total of 2,714 cases were recorded, compared with 2,883 in 2020, 3,657 in 2019, 3,669 cases in 2018 cases and 3,579 cases in 2017, Figure 1. The reduction was expected because of the continued COVID-19-induced travel restrictions, limitations on social contacts and reduced number of healthcare contacts.

Epidemiological classification

Based on presumed mode of infection, MRSA is divided 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.

The majority of the MRSA cases were acquired in Denmark (2,460), whereas infection abroad was stated in 254 cases (9%). Prior to COVID-19, the share of infection abroad was 15-20%. The number of hospital-acquired MRSA cases recorded was only 54 (70 in 2020 and 75 in 2019) and thus continues to comprise only a limited share of the total number of cases (2.0%). Community-acquired MRSA cases with no contact to hospitals or to the nursing sector comprised 1,193 cases in 2021 (44%). In 569 of these cases (48%), there was known exposure to a person with MRSA; in the overwhelming majority of cases, a member of the household (99%). The number of livestock MRSA CC398 cases was 959, which is slightly higher than in 2020 (931) but lower than in the previous years (1,122 in 2019, 1,215 in 2018 and 1,212 in 2017) and comprised 35% of the total number of MRSA cases.

Infections

In 1,212 cases (45%), sampling was performed due to clinical infection. This was fewer cases than in the period before COVID-19 (1,599 in 2019, 1,478 in 2018 and 1,471 in 2017). In 40 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 25 patients and 13 hospital staff members, which is lower than the average number recorded for the past ten years (48). The number of community-acquired infections was 571. Until 2019, the number of these infections followed a consistently increasing trend (Figure 2), which was interrupted in 2020; and the decline continued in 2021. In 73 cases of community-acquired infection there was known exposure to MRSA, Table 1.

The number of community-acquired infections with reported contact to a hospital or to the nursing sector within the previous six months was 181 in 2021 compared with 159 in 2020, 174 and 2019 in 186 in 2018, Figure 2. A total of 151 infections were detected among imported cases in 2021. Not since 2011 (n = 145) has the number of imported infections been this low, which is probably owed to COVID-19-related travel restrictions.

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 = 959) or a human variant (n = 18). Isolates with a characteristic livestock MRSA CC398 profile were typically not spa typed, whereas all other isolates were typed.

The remaining 1,838 isolates represented 317 spa types from 28 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 = 168; CC6), t127 (n = 164; CC1), t223 (n = 119, CC22), t002 (n = 106, CC5) and t008 (n = 76, CC8).

Outbreaks

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 30 outbreaks were identified at hospitals, nursing homes and other institutions.

These comprised a total of 109 MRSA cases, including 48 with infection and 61 detected by screening and contract tracing. Among the 30 outbreaks, five occurred in neonatal departments. All five outbreaks started in previous years, and more patients were then detected in the course of 2021. The largest number of cases, n = 27, was recorded for an outbreak of spa type t127/CC1, comprising several hospitals in the Greater Copenhagen Area. Furthermore, an additional ten cases were added to a neonatal department outbreak which started in 2018. The outbreak thereby reached a total of 100 detected cases in 2021 (t223/CC22). Additionally, 13 outbreaks occurred at nursing homes (a total of 30 patients) and seven outbreaks occurred at other (non-neonatal) hospital departments (16 patients).

Livestock MRSA CC398

The number of livestock MRSA CC398 observed was in line with the level observed in 2020 (n = 931) and lower than the number recorded in 2019 (n = 1,122). 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 271 (28%) patients had a clinical infection at the time of their diagnosis, compared with 234 (25%) in 2020, Table 1.

A total of 84% of the cases were found in persons with direct contact with pigs or in household members to these persons. A total of 149 (16%) persons had no known contact to animals. Among these, 101 had a clinical infection. Contact to livestock, particularly pigs, is the primary source of infection with livestock MRSA in humans. The share 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. In 2021, a total of six cases of bacteraemia with livestock-MRSA CC398 were observed. Two patients died in the 30 days following their sampling date.

MRSA may also be detected in other household animals than pigs, including horses and cattle, in particular, but also in sheep and goats. The MRSA prevalence in farms of these types is much lower than in pig farms. Furthermore, hedgehogs are a natural reservoir for the mecC variant of MRSA. Detection of MRSA in humans with contact to other animals than pigs was limited in 2021 and comprised 11 patients with contact to cattle, seven with contact to horses, seven with contact to mink and only two mecC MRSA patients with reported contact to hedgehogs.

Imported MRSA cases

The number of imported cases was 254, which is lower than the corresponding number observed in 2020 (351) and considerably lower than the number recorded in 2019 (651). This number corresponds to 9% 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. 2021 recorded a total of 17 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) and 2020 (24).

This report is also described in EPI-NEWS no. 42-43/2022.