Salmonella enterica serovar isolated in broilers in Argentina

Broilers are reared intensively and with a high concentration of birds, for such reason there is some risk of contamination with paratyphoid serovar of Salmonella enterica subsp. enterica that quickly spread in all the birds in the infected batch. The processing of chicken at slaughter plants is also very intensive and a few infected chickens can contaminate the whole batch to slaughter. Moreover, there is the possibility of contamination during transport of poultry in cages by the stress and overcrowding experienced, favoring horizontal transmission of infection via the fecal-oral route.

As the contamination of carcasses usually exceeds the contamination of chickens raised on farms and those entering the slaughterhouse, it is very important to know if some of these animals are carriers of Salmonella spp. and more importantly, determine which serovars they belong to. Although almost all serovars of Salmonella enterica subspecies enterica are considered zoonotic and potentially cause infections in humans, only some of them are the ones causing the most frequent infections and have been associated with foodborne outbreaks. According to the different serovars that are found in the birds, the different epidemiological implications of each may be inferred. In general, the paratyphoid avian serovars that are able to invade the internal tissues of poultry and persist in the reproductive organs are importante, or those capable of producing vertical infections in birds, such as S. Enteritidis (Humphrey, 2006). There are other serovars that do not invade the tissues of the bird and that usually enter the farm via contaminated food, these serovars are usually quickly eliminated in the following batches of birds when biosecurity is improved on the farm and contaminated food ingredients (Humphrey, 2006)

This work was financed by the projects "Support to the Development of Biotechnologies in the MERCOSUR-BIOTECH No. ALA/2005/017/350" and INTA-PE - AESA-203 941 "Poultry Production Diseases" for the presence of Salmonella spp. and the corresponding serovars from broiler slaughter plants in Argentina.

Between 2009 and 2010, cecae and livers and spleens  were collected in the slaughter line of three broiler slaughter plants located in the provinces of Buenos Aires, Entre Rios and Cordoba in Argentina. From each batch, 0-15 pairs of cecae were collected together in the same package 1 and 10-15 livers with gallbladders with their spleens in another separate container. These samples were sent refrigerated by land to the bacteriology laboratory of INTA EEA at Balcarce, delaying transport up to 7 days. Once the samples arrived into the laboratory, feces were extracted from the cecae of each lot and placed in a sterile bottle. Separately, 10-15 livers and 10-15 spleens of each group of birds were macerated together with the addition of bile. The contents of the cecae for 128 batches of chickens and macerated organs of 95 lots were analyzed. In the province of Buenos Aires, 47 samples of contents of the cecae and 42 samples of macerated organs were examined, in the province of Entre Rios 61 of cecae and 33 macerated and in the province of Cordoba 20 cecae samples and 20 macerated. All samples were analyzed using the horizontal technique described in ISO 6579:2002 for the isolation of Salmonella spp. Those colonies with morphological characteristics similar to those of the genus Salmonella were selected and the appropriate biochemical tests (Barrow & Feltham, 1993, Ewing, 1986) were conducted to verify the identity of the genre. The classification at serovar level was performed with the serotyping technique with specific antisera produced at INPB - ANLIS "Dr. Carlos G. Malbrán".

Salmonella spp was isolated in 3.9% (5/128) of the cecae samples examined, and 17.8% (17/95) of macerated liver and spleen. We obtained 22 isolates identified by biochemical tests as Salmonella enterica subsp. Enterica. Nine strains were characterized as Salmonella infantis, 3 as S. Thompson, 3 as S. Rissen, 2 as S. Agona, 2 as S. Brandenburg, 2 as S. Enteritidis and 1 as S. Oranienburg. Table 1 summarizes the percentage of isolates and serovars by province and by type of sample.

S. Infantis is among the 10 most common serovars in humans in Europe since 2001. As from the late 1970s, this serovar has been increasing gradually in countries such as Argentina, Australia, Brazil, Canada, Finland, Holland, Hungary, Japan, New Zealand and Russia. The main reservoir of S. Infantis are mainly poultry and broiler chickens (Miller et al., 2010). The remaining serovars are rare, although more studies are necessary Argentina in slaughter plants to determine the prevalence of Salmonella spp. and of the related serovars.

Table 1. Isolates of Salmonella enterica subsp. enterica and serovars from samples of cecae and macerated organs in broiler slaughter plants located in three provinces of Argentina (Buenos Aires, Cordoba and Entre Rios).

According to the National Reference Laboratory of the Institute of Enterobacteria Carlos G. Malbrán, from 2000 onwards, S. Enteritidis, S. Infantis and S. Agona were among the top 10 most frequently isolated serovars in humans, besides being S. Enteritidis and S. Thompson being involved in human outbreaks of foodborne disease of food from various sources (Caffe et al. 2007 & Caffe et al., 2010).

-      Industrially it is possible to produce broilers lots free of salmonella and it was shown that 96.1% of feces and 83% of the macerated organs were not contaminated.

-      The finding of different serovars of Salmonella enterica subsp. enterica in several of the lots examined points to the difficulty of the poultry industry to prevent these infections and completely eradicate these bacteria from the food chain.

-      The isolation of some serovars from macerated organs, in lots of chickens whose cecae contents were negative, underscores that some of these strains have a high septicemic capacity, but a low capacity for colonization or persistence in the enteric tract.

-      While all Salmonella enterica subsp. enterica are considered zoonotic and potentially capable of causing infections in humans, the finding of serovar Enteritidis must be strictly controlled in batches of broilers and their parent stock, given the capacity of transmission of this bacterium and its significance as an agent pathogenic to humans.

-      The application of strict biosecurity in farms, during transport and at slaughter will reduce the contamination of the next batch of chickens that are processed.

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