Denmark is worried about the number of people infected with
livestock-associated MRSA
Two publications testify to the turmoil caused by the sharp increased in the number of
people infected with meticillin-resistant Staphylococcus aureus (MRSA) in Denmark.
These pathogens belong to the MRSA clonal complex CC398 ST 309, the origin of
which is associated with pig farming. The public health concern and media crisis on
this topic which has been on-going for 2 years stem from the fact that the four people
who died in Denmark from MRSA CC398 since 2012 were not exposed to livestock (all
had a comorbidity factor).
Regarding the different types of MRSA involved in clinical cases (not carriers), the observed
increase of MRSA CC398 is the only one with such a steep slope (see Figure 1). Indeed, there
has been an increase in the number of MRSA infections, all categories combined (+425 %
between 2013 and 2014), and also for CC398 alone (+66 % per year between 2004 and 2011,
doubling the number of cases to 1 276 in 2014). Again in 2014, 43% of these human
infections were linked to CC398.
A retrospective study was carried out, focused on the link between infection by MRSA
CC398 and exposure to livestock, between 1999 and 2011. Of the 7,429 human strains of
MRSA collected during that period, 416 were CC398 (livestock-associated clone) and 51
concerned people without contact to livestock. These data confirm the sharp increase in both
categories (clinical and carrier cases, see Figure 2).
Next, the authors looked at the dates and places of residence of each of the CC398 cases that
had been exposed to livestock and compared these with those of patients without livestock
exposure. Statistical analyses show a “clear temporal relationship between the annual number
of infections in people with and those without exposure [r=0.99, p<0.0001]. Furthermore, the
least exposed people seemed to live closely to patients that had been exposed to livestock”;
they have 2.5 higher risk of infection than if they would have lived in a community without
human cases of CC398.
The authors conclude that “the expanding livestock reservoir of MRSA CC398 [pigs] may
have led to increased spillover [of the germ] into the surrounding community”, probably due
to inter-human transmission (food-borne transmission is considered negligible).
Sources:
- EPI-News, 2015, n° 23.
http://www.ssi.dk/English/News/EPI-NEWS/2015/No%2023%20-%202015.aspx
- Eurosurveillance, 2015, vol. 20, n° 37, p. 30021
http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=21245
Figure 1. Evolution of the number of MRSA cases in humans in Denmark, between 2006 and
2014. After EPI-News, 2015.
Figure 2. Annual number of MRSA CC398 cases in Denmark, according to the type of
infection: clinical (blue) or carrier state (orange). After Larsen et al., 2015.