Pigs Could be Reservoir for New Multiresistant Salmonella Strain

A new multi-resistant strain of Salmonella has recently been identified, for which pigs could be the reservoir, according to research by the European Food Safety Authority (EFSA).
calendar icon 4 June 2013
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The multi-resistant Salmonella enterica serovar 1,4,[5],12:i:-, a monophasic variant of S. Typhimurium (1,4,[5],12:i:1,2) emerged only recently and is now among the most common serovars isolated from humans in many countries.

In Greece, monophasic Typhimurium, which was recorded for the first time in human isolates in 2007 - in 0.3 per cent of total isolates - increased sharply after 2007.

Since 2009, it has become the third most frequent serovar.

In the study for EFSA by G. Mandilara, M. Lambiri, M. Polemis, M. Passiotou and A. Vatopoulos, 119 S. enterica 1,4,[5],12:i:- strains of human, animal and food origin, isolated during the period between 2006 and 2011, were examined.

The researchers from the Hellenic Centre of Disease Control and Prevention in Vari, the Hellenic Centre of Disease Control and Prevention in Athens and the Veterinary Reference Centre for Salmonella in Chalkis, Greece found the strains verified as monophasic Typhimurium variants by polymerase chain reaction (PCR) (97 strains), were further characterised by phenotypic (antibiotic resistance and phage typing) and molecular (pulsed-field gel electrophoresis – PFGE) methods.

The incidence of infections caused by different Salmonella serovars and subtypes associated with different animal sources appeared to change considerably over time.

In Greece, between 2006 and 2011, the number of human isolates submitted to NRCS decreased by 40 per cent and the notification rate fell by 47 per cent according to the study.

Annual differences observed in the number of isolates are almost comparable to reported cases, even in 2010, when a sharp decrease of isolates (58 per cent) and notifications (57 per cent) occurred.

This decrease resulted from the proportionally fewer isolates and cases reported by each one of the clinical laboratories (hospitals and diagnostic laboratories).

A decreasing trend was also observed for the Enteritidis serovar; the number of S. Enteritidis isolates decreased by 68 per cent between 2006 and 2011, whereas total number of S. Typhimurium, and other isolates were fairly consistent over time.

Considering together S. Typhimurium and its monophasic variant, their total numbers in 2010 and 2011 approximate those of S. Enteritidis.

The data indicated that the reduction of the total number of isolates can be attributed to the reduction of Enteritidis serovar.

In the EU, the numbers of human salmonellosis confirmed cases declined from 2006 to 2010, by almost 40 per cent, attributed to the marked reduction of S. Enteritidis cases reported in several European countries and the successful Salmonella control programmes in fowl populations.

In the first decade of 2000, an international increase of the monophasic 1,4,5,[12]:i:- serovar was observed. This serovar appeared to be ecologically successful since it has spread rapidly causing numerous human infections in many countries.

In Greece, monophasic Typhimurium ranks third among all serovars in humans since 2009 and in the EU was the fourth most common serovar in humans in 2010.

According to the results of the study, 18 per cent of isolates (22/119) serotyped as 1,4,5,[12]:i:- were not confirmed as monophasic Typhimurium variants by PCR assays.

The researchers said that given that S. Typhimurium is included in the Commission regulations concerning the reduction of the prevalence of certain Salmonella serovars in Gallus gallus , and that the monophasic variant is targeted by measures to control Salmonella serovars of public health concern in laying hens, standard serotyping needed to be combined with PCR for the correct reporting of strains as Typhimurium (monophasic or biphasic).

The results indicated that multiple clones of multi-resistant monophasic Typhimurium are circulating in Greece.

The most frequently encountered clone in humans and pigs was that of phage type DT120, R-type ASSuTSpTm and PFGE profile STYMXB.0010, while in poultry other clones were detected.

The data indicate that pigs may be a reservoir of this clone in Greece.

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June 2013

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