MRSA - Overviw of Disease Occurrence 2023

MRSA 2023

In 2023, 3,649 individuals were registered with methicillin-resistant Staphylococcus aureus (MRSA). Thus, the number was at the same level as in the years before the COVID-19 epidemic, while during the epidemic, the number decreased by approximately 20% due to restrictions on international travel and social contacts.

Epidemiological Classification

MRSA is classified by the presumed mode of transmission into imported, hospital-acquired, community-acquired, and livestock-associated. Livestock-MRSA in Denmark almost always stems from the MRSA type CC398, which is closely linked to contact with live pigs.

Community-acquired MRSA is further subdivided based on whether the individual had contact with hospitals or the care sector within the past six months, and whether they had contact (exposure) to a known MRSA-positive individual.

Table 1 shows the distribution of reported MRSA cases based on presumed transmission conditions.

The majority of MRSA cases were acquired in Denmark (3,075), while imported cases were reported in 574 instances (16%). During the COVID-19 epidemic in 2020-21, this proportion was lower, while before COVID-19, it ranged between 15% and 20%.

The number of hospital-acquired MRSA cases was 102 (55 in 2022 and 54 in 2021), constituting only a small portion of the total cases (2.8%).

Community-acquired MRSA cases without contact with hospitals or the care sector accounted for 1,803 cases in 2023 (49%). Of these, 808 cases (45%) had known exposure to an MRSA-positive individual.

The number of livestock-MRSA CC398 cases was 856, constituting 23% of the total MRSA cases. The proportion of livestock-MRSA CC398 was 28% in 2022 but has otherwise not been below 30% since 2013, peaking in 2014 at 43% of the total number of new MRSA cases.

The decline in livestock-MRSA CC398 may be due to the surveillance including only new cases, meaning individuals previously found positive are not registered again.

Infections

For 2,021 cases (55%), clinical infection was the reason for sampling. Such a high number of infections has not been recorded before. This increase is particularly evident in community-acquired cases without contact with hospitals or nursing homes, as well as imported cases, Figure 2.

In 39 cases, MRSA was isolated from blood, corresponding to 1.5% of all S. aureus bacteremias, which is consistent with recent years. Among the hospital-acquired cases, 44 patients (43%) were found with infections, while 15 healthcare workers (45%) were diagnosed with MRSA infections. The number of infections among imported cases increased from 279 in 2022 to 434 in 2023. The number of community-acquired infections without contact with the healthcare system was 1,050. This is the highest recorded number after two years of significantly fewer cases due to COVID-19 restrictions. For both these epidemiological categories, this is the highest number of infections recorded since mandatory reporting was introduced in November 2006. For 144 cases of community-acquired infection, there was known exposure to MRSA, Table 1.

The number of community-acquired infections with reported contact to hospitals or the care sector within the past six months was 242 in 2023, also the highest recorded number, Figure 2.

Typing

MRSA type CC398 was identified using a specific PCR analysis, which simultaneously determined whether the strain was the livestock-MRSA type (N=856) or the human variant (N=41).
Isolates with the characteristic livestock-MRSA CC398 profile were generally not spa-typed, while all other isolates were typed.

The remaining 2,793 isolates belonged to 410 different spa types, grouped into 25 different clonal complexes (CC groups). The five most frequent were t304/CC6 (N=263), t127/CC1 (N=188), t008/CC8 (N=161), t223/CC22 (N=149), and t002/CC5 (N=136). These clones have been among the most common for several years.

Outbreaks

Statens Serum Institut offers whole-genome sequencing of all isolates from outbreaks, which allows for a more precise characterization than spa-typing alone. If an outbreak is caused by a clone within a commonly occurring spa-type, whole-genome sequencing can be an essential tool for delineating and containing the outbreak. This allows the precise extent of the outbreak to be better determined.

Through notification forms, collaboration with local MRSA units, and typing of submitted isolates, a total of 41 outbreaks were identified in hospitals, nursing homes, and other institutions. These included 199 MRSA cases, of which 80 were infections and 119 were found through screening and contact tracing. Most outbreaks involved one to five patients. The largest outbreak involved 32 patients and originated in a daycare center. Many of the children had impetigo and were infected with a fusidic acid-resistant clone of MRSA. Among the 41 outbreaks, 12 occurred in neonatal departments. Six outbreaks had started in previous years, and additional patients were identified during 2023. Additionally, there were 15 outbreaks in nursing homes or related to home care (a total of 49 patients) and six outbreaks in other hospital departments outside of neonatal units (15 patients).

Livestock-MRSA CC398

The number of livestock-MRSA CC398 cases (N=856) was only slightly higher than in 2022, when 828 cases were registered. A total of 236 (28%) had clinical infections at the time of diagnosis, Table 1.

Contact with livestock, particularly pigs, is the primary source of livestock-MRSA transmission to humans. A total of 691 (81%) cases were found in individuals with direct contact with pigs or their household members. The remaining 165 (19%) individuals had no known contact with animals, 117 of whom had clinical infections. The proportion of infected individuals without known contact with livestock was consistent with recent years, suggesting no new significant transmission routes for livestock-MRSA to humans in Denmark. In 2023, there were nine cases of bacteremia with livestock-MRSA CC398, two of whom died within 30 days of the sampling date.

MRSA can also be detected in other livestock besides pigs, especially horses and cows, and rarely in poultry. The prevalence of MRSA in these types of herds is much lower than in pig herds. Similarly, hedgehogs are a natural reservoir for the mecC variant of MRSA. Findings of livestock-MRSA CC398 in humans with contact to animals other than pigs were limited in 2023 and included 18 patients with contact to cattle, 12 with contact to horses, and three with contact to poultry.

In 2023, none of the 58 mecC MRSA patients reported contact with hedgehogs or other animals.

Imported MRSA

The number of imported cases was 574, higher than in the two previous years (423 in 2022 and 254 in 2021). This number corresponds to 16% of the total cases. Among the imported cases, 81 (15%) had prior hospital contact abroad.

This overview is also discussed in EPI-NEWS 4/2025.